Is this feasible ? And I apologize for the vagueness. - Page 1
Forum section: electronics / repair

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 12:01:39 am
content: I am a raw beginner to the electronics world and am studying anything that looks to be worthwhile.In my closet is a Nad 402 am-fm tuner. Well it dont work and I want to make it work. I thought that it would be a good beginners project.I checked the minimal stuff that I know how to do. Checked for an open circuit by putting my vom leads on the end of the power cord. Yep, shows as open. The tuner is fused so I tested the 2 fuses and one was burned. If memory serves me, they were 415ma, 250v. They were odd in that they had a small resistor in the fuse. A ham buddy of mine said that they were slow blow. That was the replacement that bought. One capacitor looked odd, kinda swollen so I replaced it. Another site, somewhat kiddingly said that it would be a diode. So I replaced 2 of them. 1N4001 with 1N4007 cause that's all I had.The tuner was acting very strange. After I replaced the fuse the 1st time, the control panel lit up and I tried to select a station. Poof, lights out and another burned fuse. Replaced the fuse and now it won't even light up.Its beyond my skills to repair this thing so I want to hire a knowledgeable person to repair it as part of a teaching session with me as the student.How do I go about doing this ? I am willing to pay $nn per hour to do this, whatever the market deems fair. I have the Nad service manual and it has a schematic so thats a help.Is this anyway possible.I live in Houston TX if anybody wants to try it. :=)

author: james_s
author_rank: Super Contributor
timestamp: July 16, 2018, 01:35:27 am
content: Well I'm nowhere near Houston or I'd offer to help, I've been known to do this sort of thing in exchange for pizza & beer. Good call knowing when to stop though, one of the most common noob mistakes is forging ahead without realizing until it is too late that they're in over their head and have turned a small problem into a big problem.In the meantime you might look around on trash day or post on craigslist looking for broken electronics to practice on, and then take a few days to study electronics fundamentals, just basic stuff like linear power supplies. Start on stuff that's beat up or otherwise not worth much because you'll probably break a few things beyond repair before you get the hang of it. Also if nobody replies here you might look around to see if there's a hackerspace or other geeky gathering nearby.

author: Relayer
author_rank: Contributor
timestamp: July 16, 2018, 01:53:10 am
content: Hello Hextejas,It would be ideal if you can scan the power supply section of the radio and post itup in here, as it will make it easier to diagnose your problem.In the mean time though, you can check with your VOM set to low ohms, the mainfilter capacitor. It is usually the biggest cap in the power supply section and shouldbe rated at least 200VW (VW = Voltage Working or just Working Voltage).If you find that its reading very low ohms and stays in that state for several seconds,then you have a short circuit. But it would be extremely doubtful that the capacitor isthe cause of the short. It will most definitely be semi-conductor component.If you find that the VOM reads low ohms to start off with, then it starts climbing up toinfinity, then the main primary voltage rail is OK. You need to look elsewhere.Either way, try checking any diodes, transistors etc, to see if any of them have shorted.Please let us know how you get on.Regards,Relayer

author: coromonadalix
author_rank: Super Contributor
timestamp: July 16, 2018, 02:58:10 am
content: here's the service manual:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwjRiPzVz6LcAhUJ3YMKHeAaCqQQFggoMAA&url=https%3A%2F%2Fwww.vintageshifi.com%2Frepertoire-pdf%2Fpdf%2Ftelecharge.php%3Fpdf%3DNad-402-Service-Manual.pdf&usg=AOvVaw3XkxulnRb7VW2I7vjnuh0dNormally you check for shorts on all the power supply (ies) in it, when its possible you isolate all the circuits, ex: preamp, power amp, tuner section, and work your way thruPictures would help or tell us witch part number you have changed, you wrote having changed an swollen capacitor, did you put the same capacitance value and or put it in the right polarityThe 32v power supply section act as an voltage doubler of somekind, strange way to save an transformer secondary output.The fuse is the last thing to blow, damage has been made, changing fuses wont help.You have to find the cause ... this is a very basic tuner, you can measure a few things if you have an basic electronic knowledgeIs the transformer is set to the right main ac voltage selectionYou have 4 regulator sections, do they have an short at their outputs ? dead capacitors may cause or can cause a short, capacitor in reverse polarity can do the same. Blown rectifiers diodes can do harm ...Do you have an variable current limited power supply ? you can substitute some voltages in it, check if your power supply ex: limited to 50 milli amps goes into overloadSometimes you remove some parts to get an good measurement, the measured values can be changed or interfered with the parts around ...good luck

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 11:56:02 am
content: Wow ! Good stuff here and I will proceed as suggested.Another thought I had while waiting in the dentists office would be to contact an engineering school and see if they have a smart student that wants to earn a few $$$.Onward

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 16, 2018, 02:13:23 pm
content: Quote from: Hextejas on July 16, 2018, 11:56:02 amI grew up in Houston and learned electronics while taking Vocational Radio and TV class in high school. We routinely repaired stuff for teachers, friends, etc. We needed broken stuff to learn with. We did it for free. Alas, they dont have those anymore.How important is this thing to you? I am not in Houston, but I am relatively close. Am tempted to help.

author: Old Printer
author_rank: Frequent Contributor
timestamp: July 16, 2018, 06:26:48 pm
content: Many good suggestions here, I'll add another. Is the main goal to get the radio working, or is learning a bit of electronics more the case. If you want to learn electronics hop on YouTube and poke around. You won't have to look hard, there is a ton. Two good channels with shortish videos aimed at basic knowledge are by GreatScott and Afrotechmods. There are hundreds and our host here, Dave has an excellent assortment but tend to be longer and more advanced. If you want more let me know, I have watched a lot. I download them onto my iPhone and listen commuting to and from work, don't look at the screen while driving

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 16, 2018, 06:35:46 pm
content: I see from the schematics, that the 5V supply is always powered once you plug it in. The other power rails are controlled by the power switch.If the unit does not blow a fuse with the power switch off but plugged in, then the 5V rail and load is probably OK.You can verify this by plugging it in with the power switch off and measuring the 5V rail.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 08:14:12 pm
content: Quote from: Old Printer on July 16, 2018, 06:26:48 pmPrimarily, I want to get the radio working and secondarily learn a bit of electronics along the way. Thank you for the youtube suggestions. I already subscribe to Dave's patreon service and I read here often.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 08:16:20 pm
content: To any responder, how do I go about checking voltages at a capacitor when they are wired so tightly to the pcb ? Absolutely no room at all so as to get under it.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 08:23:05 pm
content: Quote from: coromonadalix on July 16, 2018, 02:58:10 amYes, on the capacitor, same value, i paid attention to polarity.Xformer is set correctly, as came from factory.4 voltage regulatorsI dont know how you determined this. Remember my newbieness.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 16, 2018, 08:40:36 pm
content: Quote from: Wimberleytech on July 16, 2018, 06:35:46 pmWimberley, for a teachable moment, how did you determine that the 5v was always on ? The 2 diodes, D8 and D9 (I think) 1N4001 are the ones that I replaced with 1N4007. I hope that the substitution was ok.Oh yeah, with the power on and my meter probes on each side of the diode, I get 0v readings.Thanks again

author: coromonadalix
author_rank: Super Contributor
timestamp: July 16, 2018, 08:51:46 pm
content: to check capacitors you can find any thru hole part who's been connected to each capacitor side ... without going under the board, you have to understand the electronic schematicnot to be rude, you seem to have very very low knowledge ... to check diode you do not power the tuner, you have on the meter a diode check:here's some infos :https://www.fluke.com/en-us/learn/best-practices/test-tools-basics/digital-multimeters/how-to-test-diodes-using-a-digital-multimeterHow is a diode marked?The positive side is called the anode, and the negative one is called the cathode. The diode circuit symbol, with the anode and cathode marked. Current through a diode can only flow from the anode to the cathode, which would explain why it's important for a diode to be connected in the correct direction.youtube:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwj9tOXYvqTcAhWj5IMKHRxtBPgQwqsBCGUwDA&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DmMXDa5hVzXA&usg=AOvVaw3EHzaY8_FVGf5_uMVNk9RThttps://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=14&cad=rja&uact=8&ved=0ahUKEwj9tOXYvqTcAhWj5IMKHRxtBPgQwqsBCGgwDQ&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dgzh8ICLMXzQ&usg=AOvVaw2FRddKQf43-uwNQCvluBVWCan you test a diode in a circuit?The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. The Diode Test procedure is conducted as follows: Make certain a) all power to the circuit is OFF and b) no voltage exists at the diode. ... Set the multimeter to measure ac or dc voltage as required.I know there is 4 voltages regulators since i have 30 years of experience in this.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 16, 2018, 08:59:37 pm
content: On page 10 of the service manual you have 3 legged to-220 casing fixed regulators named : lm7812, lm7912 and lm7806lm7812 is a positive 12 volt regulatorlm7912 is a negative 12 volt regulatorlm7806 is a positive 6 volt regulatorand the fourth is a transistor regulator made with a few parts around it : q17 printed on the board, part number is bc64 xxx something?your 1n4001 diode can be replaced by any number higher than 1n4001, ex : in 4002, 1n4003, 1n4004, 1n4005, 1n4006 and 1n4007, the 4007 is an 1000v diode

author: jmelson
author_rank: Super Contributor
timestamp: July 16, 2018, 09:35:09 pm
content: Quote from: Hextejas on July 16, 2018, 11:56:02 amYou might check out TX-RX labs, a makerspace in Houston. They have some REAL electronic geeks there. They also have some classes in electronics, where you might fix this unit as a class project.Jon

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 17, 2018, 06:12:08 pm
content: Coromon, I take no offense at your statements, and I will admit my lack of knowledge re electronics "stuff". I have a lot of experience designing and writing computer programs and the appearance of the Raspberry PI, Arduino, Etc., has caused me to try and learn enough electronics so as to be able to design (HAH) and wire devices so as to be able to attach to the aforementioned processors. Believe me that from my perspective, electronics are a whole lot more difficult. And I really appreciate all the help. I will get involved in the TX-RX group here at their next meeting plus a lot of youtube viewing,Now back to the tuner.Wimberly with his noticing that the 5v rail was on without the switch being pressed was eye opening to me ( Thank you Wimberly ).*****What I just discovered was that the fuses that seemed to be good were measuring 35M ohms. Sheesh, what the heck could have happened to them ? *******Anyway, I put 2 more known good fuses in,did NOT turn it on. and it still blew the fuses. I think that I better have a look at the power supply board.Another curious thing was that along with the TP (test points ?) that I saw scattered around the board there was also one named "earth" along with one named "gnd".Gnd I understand but Earth ? Are they different ? I guess they could be in that earth refers to the middle wire coming out of the wall and gnd was the boards ground. I sure dont know.As I was taking these voltage readings, for gnd, I was using the case. Is that a safe protocol or is there something else to look for in the event that the designer had a difference reference for gnd. My finding the "earth" post prompted this question.In any respect a new rectifier is on order so I cant do much more until it arrives.Thanks

author: james_s
author_rank: Super Contributor
timestamp: July 17, 2018, 06:29:49 pm
content: Yes TP usually means test point. Gnd or Ground often means the ground reference point of the circuit, while earth ground is usually the metal frame connected to the ground pin on the plug that literally connects to the earth. These may or may not be tied together, in some cases earth is the safety ground while the circuit ground is just the negative side of the power supply.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 18, 2018, 04:23:28 pm
content: I am thinking that this voltage regulator is bad. Its a LM7805, the 5 volt rail. I desoldered it and wired it up in a breadboard and it lets a lot more than 5V through. it seems like it passes the input voltage straight through.Would that cause the fuse to blow ?

author: james_s
author_rank: Super Contributor
timestamp: July 18, 2018, 04:27:48 pm
content: That could cause all sorts of things to blow, are you sure you wired it up correctly?

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 20, 2018, 02:47:38 am
content: Quote from: Hextejas on July 16, 2018, 08:40:36 pmJust got back to this thread...I see from the comments that follow, you are blowing fuses even without turning on power. The problem is in the 5V rail--at least.Yes could be the 7805, but if it is a short input to output, all the other chips downstream are probably fried as well...in which case...you have a nice boat anchor.

author: Hextejas
author_rank: Frequent Contributor
timestamp: July 20, 2018, 03:43:46 am
content: Yikes, I hope not.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 05, 2018, 07:22:38 pm
content: Just back from a long road trip and back at this beast.I replaced the 7805 and the rectifier and the fuses.It powers on now and doesn't blow the fuses. The front panel lights up but nothing is displayed.So where do I look next ? Measure where ? I was thinking of trying to input a 1khz test tone and measure somewhere.Plus I would like someone to answer a question about my soldering of the 7805.I have a picture with a toothpick pointing to the area in question.When I took the old 7805 out I damaged the PCB. When I put in the replacement I only soldered one of the pins as it seemed to be the only one that went anywhere on the back of the board.See the attached picture and thanks.

author: james_s
author_rank: Super Contributor
timestamp: August 06, 2018, 01:37:19 am
content: Um, all three of the pins go somewhere, how could it possibly work with only one pin soldered? You can see traces leading to other components right in that photo.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 06, 2018, 02:07:13 am
content: Seeing the damaged traces are giving me goosebumps, sorry i have to say this, you're damaging the tuner more and more instead of repairing it ... its a messy jobTo help you, i have to be frank, that's not intended to insult, nor offend you, that is a learning process.You should practice on dead boards, practice and develop soldering skills IE: good iron temperature, working with copper wick, or an vacuum pump to de-solder parts, learn to read schematics and symbols, parts functions, read books, see some youtube videos ... learn to use tests equipment's, understanding read values etc ...Sure in life you have to start somewhere, but not on equipment you want to make work as a start.Even with tons of experience (30 years in my case) you may damage some parts or equipment, i had this happened to me recently. We tend to be overconfident sometimes. Doing things too fast, overlook things or don't look well enough, we automate our judgment, do mechanical / repetitive movements without thinking etc etc... the list goes on and on.Members here are trying to help you, it is not easy, we may use technical terms or see thing differently EX: it would take maybe 10 minutes or less to see what is the problem in your tuner for experienced people. This is a very basic tuner, nothing fancy.Even the only pin soldered may be bad, the pcb trace may be cut where the solder is ...Do you have a friend who's knowledgeable in electronics, or some professors at school, an nearby shop who could help you understand

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 06, 2018, 03:17:47 am
content: Quote from: james_s on August 06, 2018, 01:37:19 amThank you James. My confusion with my soldering question is based upon this. Using the back side of the board and measuring the continuity from my unsoldered pin to a small distance along the trace, and it shows no continuity, ie no connection.Measuring from the same pin to a pin at the end of the same trace shows continuity.Hence my reasoning that the circuit is satisfied by the connection on the front of the board.Where am I wrong.I was trying to avoid having to solder a jumper? onto the back.

author: james_s
author_rank: Super Contributor
timestamp: August 06, 2018, 03:52:49 am
content: Are you trying to measure the trace through the solder mask? If so then that won't work, the green (or whatever color) mask over the copper is an insulator, you won't be able to measure anything without scraping off a spot of mask.Also I'll reiterate my earlier advice to scrounge some electronic junk from the trash, post an ad on craigslist, nextdoor, etc looking for junk to practice on. Spend a day watching youtube videos and practicing soldering and desoldering and it will make everything much easier.

author: Zucca
author_rank: Unknown
timestamp: August 06, 2018, 08:05:22 am
content: Hextejas,please post a lot of pictures and print the schematics on paper and write on them your measurements. Then post them here like he did (was insicure like you) see belowThat's the most effective way to get the help you need.Look at the happy endhttps://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg1669490/#msg1669490we can solve everything just take your time and post as much details you can.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 06, 2018, 10:48:30 am
content: Zucca thank you very much. Somehow the idea of marking up the schematic like that never occurred to me.I can see how that will be very useful and it is exactly those kind of suggestions that I want.I will see if I can figure out how to do it.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 06, 2018, 10:59:50 am
content: James, thank you for your suggestions. I will look around for practice material.I actually think that my soldering technique is not so bad, the de-soldering is another horror story.I have some solder wick and will give it a try. Somehow I I thought it was to be used to clean up the PCB after the component was removed. For the life of me I cannot get a de-soldering pump thingie to work.That's how I messed up the board in question. By prying up the component and trying to pull it out while the solder was soft.

author: Zucca
author_rank: Unknown
timestamp: August 06, 2018, 11:15:05 am
content: Quote from: Hextejas on August 06, 2018, 10:59:50 amPCB are like pretty young girls, if you are using force on them they will not make you happy anymore....and you will spend a lot of money and time to fix the situation.

author: james_s
author_rank: Super Contributor
timestamp: August 06, 2018, 03:10:02 pm
content: Solder wick is for removing solder, it can be used after removing the component but in this case you should use it prior to that too. It helps a lot to dip it in liquid flux, especially if it has been sitting around for a while. I never had any luck with the plunger type solder suckers either. For years I got by with one of those suction bulb desoldering irons from Radio Shack then I finally got a vacuum desoldering gun and there is nothing else like it. For occasional use though wick usually works pretty well. Never forcefully pry anything, that will almost always destroy the board.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 06, 2018, 09:46:38 pm
content: Quote from: james_s on August 06, 2018, 03:10:02 pmSome success so far after I wired up the 2 legs that I ignored earlier, though not all the controls do what they are supposed to do.The voltages coming out of the 7812 and 7912 are lower than what I thought so I am going to replace them paying particular attention to their removal.I will give the flux trick a try. The solder wick worked pretty well though and I was fortunate in that the legs of the VR are pretty beefy.

author: james_s
author_rank: Super Contributor
timestamp: August 06, 2018, 11:29:58 pm
content: What are the voltages? I expect the 7812 and 7912 are power rails for op-amps used in the audio section, and not likely to have any effect on the controls. Do you have a bad connection to the front panel?

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 03:36:11 pm
content: Well, this is not so good. I think that I messed up. I replaced the AN7812 and the AN7912 with a LM7812 and LM7912.I thought that they were functionally equivalent but I guess not, because as soon as I turned it on it blew both fuses.I am not so good at comparing datasheets So, then I used the internet to find functionally equivalent parts. Or so they say.Anyhow they are on the way.As an aside, I am getting better at de-soldering. The flux trick worked well.Frustrating.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 03:45:32 pm
content: The AN7812 and AM7912 appear to be directly compatible with the LM7812 and 7912, just from a different manufacture. Are you sure you didn't get the locations swapped? This is part of the reason I suggested mentioning the voltages you measured so we can suggest whether it's necessary or not. I doubt there was anything wrong with the regulators because +/-12V will be for analog audio stages and have nothing to do with the digital controls and now you've created an additional problem while likely barking up the wrong tree.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 09:26:04 pm
content: Quote from: james_s on August 07, 2018, 03:45:32 pmThank you James and yes I am sure that I got the right ones in place. I marked them with a marking pencil and double checked them. Since I need the practice, 😁 I can take out the LMs and put the ANs back in. Then take the voltage readings.More as I progress,

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 09:30:06 pm
content: Do they mount to a common heatsink? If so then make sure you retained the insulating hardware, otherwise the tabs are at different potentials and will create a short if they are electrically connected.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 11:17:43 pm
content: Quote from: james_s on August 07, 2018, 09:30:06 pmYes they are all 3 mounted to the common heat sink and when I took them off, 1 of them had what looked like a piece of felt behind it. I guess that is what you mean by insulating material so I will need to read up on it. I thought that I would want to have all 3 of them tightly connected to the piece of metal for good heat dissipation. Let me see if I can get a picture of it. Ok got it.Yikes, I dont remember which one had the felt pad, also one of them had what looked like a plastic insert for the mounting screw. Apparently to keep it from touching the metal.So much to learn. And here I thought they were using the metal as a super heat sink. Why the heck did they use such a big piece if the VRs were not to be connected to it ?No wonder it is blowing the fuses. I hope the service manual shows which ones are to be insulated.Thanks again.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 11:24:50 pm
content: That's a silicone insulating pad. You'll notice the screw also has an insulating shoulder washer isolating the tab electrically. With a TO-220 package the tab is almost always connected to the center pin, and if you look at the datasheet you'll see that the center pin is ground on the 78xx regulator and on the 79xx regulator the middle pin is the input. So if you connect both tabs together you've connected input directly to ground and what do you suppose will result from that?Something to remember with anything like this is that *every* part has a specific function, and if any one of them didn't need to be there, it wouldn't be there. Details count, taking pictures along the way as you've done is a very good idea. The more stuff you work on, the more you'll start to recognize the important details like this, and the more stuff will start to look the same, with the same half dozen or so faults showing up over and over in all sorts of different things.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 08, 2018, 08:40:09 pm
content: Well, that was a lesson well learned, about the insulating layer and screw protector for the 1 regulator. As I looked at the again I noticed that for 2 of the 3, the tab was to be connected to ground whereas the 7912 twernt so. So, I got that squared away and the panel lights up again. I wont make that mistake again.I started reading voltages with it plugged in and turned on, GULP, and I saw what seemed to be an oddity.For 3 of the rails, and I am not sure I am saying that right, they seemed to read spot on. +5, +12, and -12.The + 32 was a different story.I took a reading at Q17 expecting to see +32 but no. I got 2.1V on all 3 legs of the transistor. Picture attached. I thought that it might be the rectifier/bridge RB152 but it is new. So now I don't know what to do, what to replace, what to read,The diode at D12 also read 2.1V.Maybe see what the readings are coming out of JP5.I apologize for the quality of the picture. The thing is so darn small that when I try and blow it up it gets very fuzzy.Do you have any suggestions as to what software to use so as to be able to cut and paste from schematics ?Maybe I can redraw the portion that I am working on, that might work.MAny thanks

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2018, 09:03:09 pm
content: That transistor is fed by a charge pump voltage doubler circuit consisting of those two diodes and capacitors. What do you get across each of those diodes? A voltage doubler like that cannot supply very much current so if anything is overloading the output it will be dragged down. Do you know what that voltage is used for? 32V is unusually high for a piece of equipment like that but it may be for the varactor diode that does the actual tuning, I vaguely recall they sometimes require a fairly high bias voltage.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 09, 2018, 01:38:10 am
content: Quote from: james_s on August 08, 2018, 09:03:09 pmI found where the 32V is being used. I hope you can make sense of it.Dang but its not so clear. I will try and get a better picture tomorrow if you think it will do any good.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 09, 2018, 02:05:05 am
content: normally the 32v is for the tunner section

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 03:59:48 am
content: I can't really see what's going on there because it's a bit blurry, but it looks like it may indeed be bias for the varactor diode in the tuner. It should have no effect on the controls, those will all be 5V related.If the +32V is actually way off though it won't tune right and it may detect that it's not locking on the desired frequency.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 09, 2018, 09:51:32 am
content: Quote from: james_s on August 09, 2018, 03:59:48 amThat's what's happening, or rather not happening. The tuner is acting weird. Plus I can't seem to select AM vs FM.Oh yeah, a basic question that I have is about using my meter. When I measure the voltages it is in reference to chassis ground, correct? And when you ask for voltage across a diode, for example, one probe is on one side of the diode and the other probe on the other side of the diode, correct ?There are a few test points on the board and one of them is labeled "earth". Never saw that before.Do you need to see the actual part numbers or can you tell whats going on just by looking at the lines, circles, and squares, or rectangles. In the schematic, ? Some of the part numbers are so small that i cant read them. I have to find them in the listing of the part numbers.I don't know if that question makes sense.but I guess that I am asking if I would need to redraw by hand, portions of the schematic so that you can see the actual part numbers.Thanks again.

author: Zucca
author_rank: Unknown
timestamp: August 09, 2018, 10:44:31 am
content: Quote from: Hextejas on August 09, 2018, 09:51:32 amThe meter always shows you the voltage difference between RED and BLACK probe.If you want to see if the +5V from the schematics are there in your device, you should ask yourself what is the reference for the +5V on the schematics. 99% it is the ground.Normally there is a difference between EARTH and GROUND. On top of that you can find circuits with multiple grounds so it become easly a trap for young players.See also here, another beginner had the same kind of questionhttps://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg1578022/#msg1578022Quote from: Hextejas on August 09, 2018, 09:51:32 amyes.Quote from: Hextejas on August 09, 2018, 09:51:32 amPictures?Those could be there for make it easy the life in repairs or they where there to check the board during production.PS: Very good with this:

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 04:16:01 pm
content: Since you've gotten sensible looking voltage measurements from the 5 and 12V lines you've obviously found a suitable ground point for the meter. The low voltage on the 32V line is definitely wrong, can you remove that transistor and see if the voltage at the pads for it rise up? That transistor is set up as a voltage regulator using the nearby zener diode as a reference. There are not too many things that could cause the voltage to be low.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 09, 2018, 07:55:04 pm
content: I took the 32V regulator out and replaced it with a newer one and the voltages look good now. Actually I did not screw it down to the metal plate where the other 2 are. It sticks up in the air so I guess it is cool enough. I may stick its own heat sink on it,I have taken another picture of one of the 32V user circuits and it looks like the 32V is not getting there.I need to look some more to see where its going.thanks

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 08:06:05 pm
content: What do you mean by 32V regulator? Q17? So with that transistor replaced you have 32V coming out of it, but nothing on the 32V point in the other schematic section? Can you follow the trace across the board? I would expect it to be a pretty much straight shot.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 09, 2018, 11:10:16 pm
content: Quote from: james_s on August 09, 2018, 08:06:05 pmThat's what Texs Instruments calls the LM78nn and LM79nn. Fixed voltage regulators.Re following the trace, I will look and see how it I can find it. It might be a lot easier on the backside. The front side components have no room under them so as to be able to insert a probe. I am hoping that there is another 32v entry point, if that's the correct phrase.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 09, 2018, 11:24:17 pm
content: Re the 32v into the main circuit .Do designers typically only design one entry point ? I am going to go hunting for another but it would be nice to know that typically there will only be one.As to what I will do in order to debug this beast, I will try to find the 1/2 way point in the circuit and pull out a component so I can take some measurements.If you have a better idea, and I am sure that you do, I would love to gear it.Thanks

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 11:30:21 pm
content: I'm not entirely sure what you mean, but if I understand right, yes, designers typically only provide one path for a given signal. Sometimes this path will snake all over in order to get to the destination, sometimes it will switch multiple times from top to bottom on a double sided board, or pass through jumper wires to cross over other traces but generally there is only one. Having multiple paths for the same signal creates a loop and in some cases that can have undesirable effects, plus there is no real advantage.Removing a component that is in line somewhere is a perfectly valid troubleshooting technique. When possible I usually desolder just one end and bend it up so the part doesn't run off or get mixed up with others.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 10, 2018, 03:06:33 am
content: Quote from: james_s on August 09, 2018, 11:30:21 pmYes James, you deciphered my meaning exactly and I hadn't thought about leaving one end still connected. I will get back at it tomorrow.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 10, 2018, 09:32:42 pm
content: I dont know of this is right or not but the voltages coming out of the xformer are 12v AC and 20v AC. As soon as I plug in the 3 pin cable coming from T-1, to the PCB and turn the power on, one of the fuses starts to get really really bright. I turn it right off since I am running out of fuses. The xformer is an EL-48.I dont know what to do now. I wanted to put power to it so I could start reading voltages but when I do that, I blow fuses.Maybe I should give it up.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2018, 09:37:16 pm
content: Search for 'Dim bulb tester'.Basically if you wire a mains lamp in series with the mains supply, it should limit the current enough for you to debug without blowing fuses. Start with a low wattage bulb.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 10, 2018, 09:59:01 pm
content: Quote from: Gyro on August 10, 2018, 09:37:16 pmThank you Gyro.When the fuse lights up like that, is it an indication that something is drawing a lot of current ?Would you or anyone else like to predict which component it is ?

author: james_s
author_rank: Super Contributor
timestamp: August 10, 2018, 10:20:43 pm
content: Yes, it most certainly means something is drawing a lot of current. When did this new symptom start? Are you sure the insulating hardware for the regulator has not been damaged or displaced?

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 10, 2018, 11:59:48 pm
content: Quote from: james_s on August 10, 2018, 10:20:43 pmI can't quite pinpoint the time it started. I don't remember doing anything different. I think that tomorrow I will turn it on and see if it blows.The regulator is isolated and not touching the metal plate like before. I can't answer what is different.What I did do was to push some components apart so as to be able to see their numbers.It's a puzzle.

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 12:03:17 am
content: Well, something has shorted out somewhere. I would check continuity between that regulator tab and the heatsink, then look around for a solder bridge between two pads.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 11, 2018, 03:53:59 pm
content: I found 3 wires from a component that had not been cut off and might have been touching the chassis so I snipped them.Turned it on and still got the hot fuse but it did not blow.Are fuses supposed to look like this ?What do you think now ? Is it still drawing too much current ?

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 05:12:03 pm
content: Are you sure that's a fuse and not a light bulb? There are some bulbs with the same form factor as fuses. A fuse most certainly should not be glowing.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 11, 2018, 08:25:01 pm
content: Quote from: james_s on August 11, 2018, 05:12:03 pmNah, it's a fuse so that means that I have to look harder. Grrrrrrr.

author: Zucca
author_rank: Unknown
timestamp: August 12, 2018, 08:52:18 pm
content: the more you work the more you can learn and be quicker in the next reapair. It's not time wasted at all.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 13, 2018, 01:37:10 am
content: I built a dim bulb tester and I am not so sure what to expect when I use it.I think that it will still let 120V to the NAD but limit the current ?Will it affect the DC voltage readings at various places on the board ?I have never used one before so I didnt know what to expect. The 1st time I tried it, with a 100W bulb and the NAD light up. I pulled 1 side of the glowing fuse out and put my ammeter in series and it was reading 3.5 A !! WOW. I dont know how I could have gotten such a high reading with a fuse rated at 315MA.Next was a 60W clear, but the fuse still glowed but not so brightly. Curious that when I started to turn the NAD around so I could look at the front, it blew one of the fuses, the glowing one.Oh well.So, I took out the 60W clear light and replaced it with a 60W yellow and the fuse did not start to glow. I again pulled 1 side of the fuse so as to read the current and it read 41ma. I think. Then I left it alone and started taking readings.After about 30 minutes of taking readings, I think that I might have shorted a transistor.Very frustrating.More on Tuesday when more fuses get here.And I need to start taking better notes. As I read what I just wrote, some things seem impossible.Peace and ciao.George KG5TKY

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 04:07:13 am
content: There is no real need for the dimbulb tester here, you already know something is pulling WAY too much current. If you do use the bulb, 100W is too large, 60W is probably still larger than you need, but it doesn't really matter. Again you already know something is pulling way too much current so you need to figure out where the short is. This wasn't happening before, you created a short somewhere.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 13, 2018, 10:39:43 am
content: Quote from: james_s on August 13, 2018, 04:07:13 amIs there a better way to do this or do I just eyeball it ?Thanks

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 03:20:54 pm
content: Well, use some deductive reasoning. Look at which fuse is getting hot, look at the schematic and find paths between that fuse and ground. Look at parts of the board that you have disturbed, the fault is almost always in an area you have messed with. It was working before, now it isn't, you changed something somewhere that caused the fault condition.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 13, 2018, 06:31:52 pm
content: Quote from: james_s on August 13, 2018, 03:20:54 pmJames, this thing never did work. At one point it became able to power on, and I started checking things, but it never worked.When the next load of fuses get here I am going to take another look at the 5V rail.Thanks again

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 07:07:00 pm
content: It didn't work, but it wasn't making the fuse glow was it? I don't see how the voltage rails could be anywhere near correct with something drawing that much current.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 14, 2018, 02:50:41 am
content: Quote from: james_s on August 13, 2018, 07:07:00 pmThe fuse glow came later in the game. In the beginning it would blow fuses as soon as I plugged it in. When i 1st got it out of the closet it had 2 blown fuses.

author: james_s
author_rank: Super Contributor
timestamp: August 14, 2018, 03:51:01 am
content: Well the fact that the 32V output from the power supply is effectively zero and the fuse is glowing certainly suggests a shorted component related to that.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 19, 2018, 07:28:46 pm
content: Progress I think.I think that it is not blowing fuses now, at least it hasn't for the last hour. Truthfully, I am not sure what I did to fix it. I noticed some electrolytic capacitors that looked a bit swollen so I replaced them along with a few more capacitors and 2 diodes. Anyway it stays on and is drawing only about 10ma across the fuses.Which prompts a question.When I am reading voltages do I read it in ref to ground or is it across the component in question, or maybe both ?So now, the front panel stays lit and shows nothing. I took a few more voltage readings but I dont know where to look next.There is an off page connector that shows 5V+ but I dont know if it is an input or output 5V. In any respect it is 000.2 photos attached showing the 5V in question along with the LM7000. It seems like there should be more activity there, but I have no clue. I would probably replace the LM7000 and pray.thanks

author: james_s
author_rank: Super Contributor
timestamp: August 19, 2018, 08:00:24 pm
content: Swollen capacitors is a pretty good indication that they are bad, the top of an electrolytic cap should never appear bulged at all, if it does then it is bad.Voltage measurements are normally to ground, if the schematic lists a voltage at a particular node then that is always referenced to ground. Sometimes there is reason to measure the voltage across a component but in that case it will be specified, if not mentioned then assume it is a measurement to ground.If you are missing a voltage somewhere, figure out where you do have 5V if anywhere, start at the regulator, if 5V is present on the output then follow the circuit, checking for 5V at other nodes that should be connected and find out where it is missing.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 19, 2018, 10:46:02 pm
content: Quote from: james_s on August 19, 2018, 08:00:24 pmThank you James and following the schematic is where I am having a problem. From the voltage regulator, +5, +12, and -12, all go to one thick line, which goes to the upper level schematic. The +32v just goes to an arrowhead. Do the engineers really mean to have those 3 voltages on one wire ?You can guess that I am new at this ?Do schematics typically read left to right ?

author: james_s
author_rank: Super Contributor
timestamp: August 20, 2018, 12:42:55 am
content: The thick line on a schematic is called a bus, it can be thought of as a bundle of separate wires that have similar function or go from one general area of a schematic to another. It is done purely for visual clarity, which may not seem intuitive if you don't know what it means but consider this. Say you have a 32 bit address bus on a microprocessor which connects to 10 other ICs. Imagine what it would look like if you tried to draw 32 separate wires in parallel, each connecting to all those ICs, you would have an absolute rat's nest that would be hard to follow. Instead you draw a thick line or in some cases a wide stripe and have the individual signals go in and out of that stripe, labeled where they enter and exit, the result is much more clear.That's what's going on with the voltages, all of the power supply voltages here are drawn as a bus, with each wire identified by its label.Schematics are not read in any particular direction, they are normally organized by functional blocks, often with little or no relation to the physical layout of the circuit. The schematic is there to describe the way the circuit works and group together components that are part of a certain function, like the power supply, the front panel buttons, the RF front end, and so on. Sometimes the parts of a subsection are physically clustered together but they could be spread all over the board if there were reasons for it to be done that way.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 23, 2018, 07:11:11 pm
content: Further along reading voltages and 1 current that seemed odd since the unit was off.And it lead me to some questions as to how to take the readings.I desoldered L3 and put the ammeter in series at the connection. It read -22MA even with the unit turned off. Is that usual ? And I dont know where to put the + and - probes, hence the -22MA. It could be +22MA.And when I take the voltage readings on the top of the PCB, I dont know at which end of the component to place the +probe or the -probe with the other at ground. So sometimes I have been doing it both ways and indicating both of the readings and separating them with a comma. Like on the schematic that shows the transformer, C61 is 19 , .08 etc.Where the 32V should be, it is reading 38V. I dont know if that is within tolerances or not. I cannot read datasheets to see wassup. The resistor R10, just before Q2 transistor 2SC1815, reads 38,8 with a drop of 29.4. I guess the sum=38 so all is well.Q2 transistor 2SC1815 reads a 0000V on the base. Does that seem right ?I took the readings at each pin of the IC, LM7000 and wrote them onto the hand drawn LM7000. The LM7000 is way obsolete so if it is faulty it will be difficult to get a replacement.So, are these readings what are to be expected and where should I start looking for faults. I have been trying to trace the schematic, left to right, top to bottom, as that is what another site recommended. However that scheme seems to fall apart quite often.Oh yeah, when I was looking at the FE407, titled U1 Front end, its a sealed box so its tough to get many readings although I took a reading at pin 6 and it was 2.85V. Pin 6 says OSC so is that supposed to be a sine wave and should I be able to read it using AC volts ?Since the blowing fuses situation seems to be stable and I am getting the correct voltages out of the power supply,except 32=38, do you think that I should start another thread since this one seems to be getting quite lengthy and cluttered with my early fumblings ?Thanks again

author: james_s
author_rank: Super Contributor
timestamp: August 23, 2018, 10:22:58 pm
content: Generally unless otherwise noted, all voltage measurements will be referenced to ground, so put the negative probe on a node indicated by the ground symbol and leave it there for measuring voltages unless you have a specific reason not to.When you take current measurements the polarity on the display just depends on which way you've got the probes, generally you want to have it whatever way gets you a positive reading. There is not really any such thing as negative current but the indication can be helpful in that it tells you the polarity of the circuit.38V seems high for the 32V rail, as I recall from the schematic it should be regulated. You might check and see if the pass transistor that is supposed to regulate that has shorted, or if you forgot to solder something back that you removed.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 23, 2018, 10:35:56 pm
content: Hoping the tunner section is not fried over 38V

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 24, 2018, 01:27:39 am
content: Does it or can it make sense that the 22MA of current is present with the until switched off ?Are some parts of tuners designed this way ?Thanks

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 24, 2018, 10:20:02 am
content: Quote from: coromonadalix on August 23, 2018, 10:35:56 pmCorom, is there a fairly easy way/reading that i can take that would tell me if the tuner is fried.As I look at the display and try the buttons, the only one that doesn't work is the mode which I think selects AM/FM.The others when I tried them seemed to do what I remember, except for the "tune". It picks something, I forget what, but seem to remember 90.5.Also if I hook up an antenna, is there a way that I can see if a signal is getting in ?

author: james_s
author_rank: Super Contributor
timestamp: August 24, 2018, 03:00:41 pm
content: It's possible there can be current somewhere with it switched off, especially if it has a remote that can turn it on or stores preset stations in memory.From what you describe now, I think the problem is probably in the 32V supply being high. If that voltage is too far out of spec it won't be able to lock on the frequency because the 32V rail is used to bias the varactor diode which is the active element in selecting the frequency. As I recall, this voltage is regulated by a classic linear regulator circuit with a zener diode reference.

author: Hextejas
author_rank: Frequent Contributor
timestamp: August 30, 2018, 06:20:28 pm
content: Getting closer.The AM side works and I have noise coming out of the speakers. Now for the FM.It seems like the FM signal will share the same amplifier circuit as the AM, at least I hope so.I am trying to solve the FM problem in the same way that I did the AM.I injected a 1khz test tone into the circuit at where I thought it would do some good, and followed it until it got to the speakers.So, I was trying to find a source of a 88mhz signal for injecting into the FM circuit. So far no luck.From the pictures I just posted it seems like I should be able to trace the FM signal starting at pin 21 of chip LA1266.Now some questions.At pin 21 of LA1266, should I be able to see a FM signal on a scope ?Where should I be able to see the beginning of the FM signal chain ?LA1266 is the AM/FM tunerLA3401 is this VCO Non-Adjusting PLL FM MPXStereo Demodulator with FM Accessoriesand I mostly do not know what it does.Thanks

author: james_s
author_rank: Super Contributor
timestamp: September 01, 2018, 07:22:15 am
content: So does the AM tuning tune the correct station corresponding to the frequency shown on the display? Are all of the power supply voltages correct now? That is the first thing to check before doing any other troubleshooting, if the voltages are wrong it won't work.

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 05, 2018, 06:05:39 pm
content: Thank you again all for the help. I am getting closer and think that I want to start another thread.The voltages are correct, not blowing fuses, and I can tune an AM station and actually listen to it. FM is another critter though.So, I am going to start another thread for the FM .Bye and thanks again.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 05, 2018, 08:18:15 pm
content: you did not have to start another thread ...

author: james_s
author_rank: Super Contributor
timestamp: September 06, 2018, 04:23:02 am
content: Why would you start another thread?! That just makes it confusing.

author: Sceptre
author_rank: Regular Contributor
timestamp: September 15, 2018, 03:22:38 am
content: Quote from: Hextejas on September 05, 2018, 06:05:39 pmIncluding the 32V rail? What was wrong with it?

author: Hextejas
author_rank: Frequent Contributor
timestamp: September 15, 2018, 09:05:24 am
content: Quote from: Sceptre on September 15, 2018, 03:22:38 amIf I remember correctly, it was too high, almost 40v. Also, I have given up on this for now.

author: m3vuv
author_rank: Super Contributor
timestamp: September 17, 2018, 09:52:37 pm
content: why not put a 100watt light bulb in series with the power cord to limit the current,bridge the fuse and see what component gets hot?