HP 3440a digital voltmeter repair/troubleshooting: moving on to PSU ripple  - Page 1
Forum section: electronics / repair

author: valley001
author_rank: Frequent Contributor
timestamp: January 11, 2019, 06:22:31 pm
content: Hi all,I am new here, been working on repairing this old HP volt meter. I also have a thread going on antique radio forum.Short story on this meter, it is a parts meter I bought to repair my once (and now again) working HP3440A, that I damaged while adjusting the -8.000v reference (screwdriver slipped and touched the back of A2). The meter then stopped functioning. I used A2 from this parts unit and installed it in my good meter and it is now functional, so I know the problem is in A2, but there are other problems with the parts unit. I would like to get it going if possible.I have verified the voltages out of the PSU per 5-17 in the manual.I have verified 400hz out of the oscillator per 5-18.Ido notget a reset pulse per 5-32.Ido notget any waveform out of A2TP1.Ido notget a "narrow positive pulse" out of A2CR5B on my scope (I am also not certain what a "narrow positive pulse" is but I get nothing on the scope.Voltage readings per table 5-6:A2Q1: -.672A2Q2: -1.33A2Q3: -.519A2Q4: -.321A2Q5: -.734A2Q11: -.378A2Q12: -.832A2Q13: -20.5This is the extent of my testing so far, and has revealed some problems. A2Q1-5, all voltages are low. I removed Q4 and 5 and tested them out of circuit with the diode check function on my DMM and they seem fine. I wonder if some other issue might cause all the voltages to be low on these transistors? A2Q11-13 seem to read mostly in spec.I am interested in ideas on where to go next. Am I at the point of just subbing in transistors and see if things change? Is there a more thoughtful way to approach these problems?

author: rhb
author_rank: Super Contributor
timestamp: January 12, 2019, 10:29:53 pm
content: I suggest you ask here:HP-Agilent-Keysight-equipment@groups.iothe guy who designed the meter way well reply. I've seen that happen in several discussions on the list. It was on yahoo groups, but finally people got fed up and David Kirkby moved it to groups.io.

author: Armadillo
author_rank: Super Contributor
timestamp: January 12, 2019, 11:10:37 pm
content: Yoo!, I think we are sufficiently able here.The reset pulse is a train of synchronized reset pulses.[5-32]I would advise you to;Clean ALL the contacts with contact cleaner and eraser if you can reach the surface, particular wiper of R6Change C5.If there are still no pulses then;Then with respect to Power Ground, measure DC Voltage of;Q5 Emitter =Base =Collector =Q6Q2Q3

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 01:32:24 am
content: OK, took some measurements. Have not yet changed C5. The neon responds to turning to the sample pot.Q5 Emitter = 1.6 to 2.2vBase = Sawtooth -2.08 to -4.4Collector = Square wave +13.6 to -14.4vQ6 Emitter = -1.2 to -1.6Base = Square wave -1.72 to -.64Collector = Squareish wave -24.8v to -.4vQ2 Emitter = Squareish wave -24v to -13.6vBase = Squareish wave -24.8v to 0vCollector = Sawtooth -35v to -13.6vQ3 Emitter = -21.6 no wave patternBase = -10.8 no wave patternCollector = -10.4 no wave pattern

author: rhb
author_rank: Super Contributor
timestamp: January 13, 2019, 02:26:10 am
content: Quote from: Armadillo on January 12, 2019, 11:10:37 pmI certainly agree, but there is a lot to be said for getting advice from the engineer who designed the instrument. And very few of them hang out here.

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 02:55:20 am
content: Darnit now the count gate neon is no longer flashing. I may have grounded pin 9 or 10 while testing

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 04:00:45 am
content: Change Q3. Its defective.

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 04:16:39 am
content: Now? Why is Q2 collector a sawtooth? Could be affected by shorted Q3. But as a caution;Can you check what capacitor is C1? After you take out Q3  and before you replace with new one, measure and check C1. It should be very very high ohms "OL". [With POWER OFF].

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 05:06:42 am
content: OK, Q3 is HP 1851-0024 or 2N383 I will need to place an order. I will remove and test the cap in the mean time.Now that the sample rate neon is dead, I should look at A3Q10 and 11 correct?

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 05:14:11 am
content: Please carefully probe them for pulses and read the Base, Collector, and Emitter voltages.Also probe A3Q1.[150v on Q1 Note!] Safety for you and equipment.

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 06:33:25 am
content: As someone who is not trained in electronics but is very interested (20 years ago I could have pursued this option) what am I looking for when testing transistor voltages? What does a good transistor look like compared to a bad transistor? I have used the diode check function on my DMM in the past but perhaps that does not tell the whole story?

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 06:40:33 am
content: Hi;Buy the component tester, you will love it.Diode Mode, for NPN, red on base, black on emitter and black on collector should reads like diode. Then red to collector, black to emitter, should be open. Then red to emitter and black to collector should be open.Do Reverse polarity for PNP.But please see Youtube for how to test.https://www.ebay.com/itm/All-in-1-LCR-Component-Tester-Transistor-Diode-Capacitance-ESR-Meter-Inductance/322762949895?hash=item4b262bc507:rk:1:pf:0

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 05:49:53 pm
content: Well, good news. I only blew the .75a -35v fuse, so the count gate is intact.

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 06:01:20 pm
content: Back to the A2 board (the one that received the screwdriver slip). A2Q1-5, should I see a wave form out of the base and collector on these transistors? Probing a few I get a voltage but no wave pattern on the scope. Shall I post these base, collector, emitter readings or is this premature pending repair on A3?

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 06:26:40 pm
content: Hi, we should get A3 working first.is A3Q1, Q10, Q11, Q12, Q16, Q17 producing the pulse? If not we need the voltages.Why don't you extract the component from the spare board first and start procurement when 1 set is working?

author: valley001
author_rank: Frequent Contributor
timestamp: January 13, 2019, 10:02:02 pm
content: Results:Q1-Emitter = .4 to .2v no waveBase = Squareish wave 3.2 to -.8vCollector = Square wave 58 to -55v (sometimes a spike to 100v happens at the edge of the square)Q10Emitter = 40mv to -80mv no waveBase = Square wave 6.6 to -.6vCollector = Square wave .6 to -12.6vQ11Emitter = Square wave 14.4 to -13.6Base = no wave -12vCollector = squareish wave 18.4 to -13.6vQ12Emitter = .8 to -1.6v no waveBase = squareish wave 1v to -13.2Collector = square wave 36 to -1.6vQ16Emitter = sawtooth 12 to -12vBase = Sawtooth same as Emitter 12 to -12vCollector = no wave -11.6 to -13.2vQ17Emitter = no wave 22.23vBase = no wave 21-22vCollector = sawtooth 18 to -6v

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 10:24:22 pm
content: Are you able to get A3Q3 replaced? This Q3 reset the plug in unit, the voltage comparator and the decade counter. Man this chip is important.Without this Q3, best to wait for now.Oh, I glance your measurement, looks fine to me. I will take a closer look waiting for your Q3.By the way, don't you have the spare board to temporary borrow the Q3?

author: Armadillo
author_rank: Super Contributor
timestamp: January 13, 2019, 10:31:13 pm
content: After you replaced the Q3, measure the voltages, capture the "short" pulse. "short"!!If you can get reset pulse;temporary assemble up and try it out the unit and observe what's up?. If still not working then we go A2.

author: valley001
author_rank: Frequent Contributor
timestamp: January 14, 2019, 12:15:04 am
content: I do have the other 3440A functional, but I would rather not remove parts from it now that it is working and calibrated.I checked through my stock of used transistors and do not have a suitable replacement so I will have to wait until the replacement 2N383 arrives this week.

author: Armadillo
author_rank: Super Contributor
timestamp: January 14, 2019, 12:24:06 am
content: Oops! my mistake. I thought that was a spare not working unit.You're right, wait, then.

author: valley001
author_rank: Frequent Contributor
timestamp: January 14, 2019, 12:46:38 am
content: I probably did not word it well. The unit we are troublshooting is the parts unit with the damaged A2 board from the other unit. So I know A2 is damaged but the parts unit was not functional when received so there are problems with it, I am hoping the Q3 will fix the A3 board on the one we are troubleshooting and then we can move on to A2.I am getting better with my scope and have been able to verify some of the wave forms per 5-11. 5-11 A and C are correct. 5-11 B, D, E and not correct for the A3 board.

author: valley001
author_rank: Frequent Contributor
timestamp: January 14, 2019, 01:03:16 am
content: I should probably also mention we are troubleshooting with the 3441A range selector plugin installed.

author: Armadillo
author_rank: Super Contributor
timestamp: January 14, 2019, 05:57:10 am
content: 5-11 B - Waiting for your Q35-11 D - In your previous reply, you said you have got 12 to -12 sawtooth at the emitter of Q16, if it is not the same now, please picture the curve over so that I can understand it what you saying. Otherwise do a remeasurement of BCE of Q16 again.5-11 E - Is dependent on 5-11D above, please picture a curve over.Regarding the A2, in your original 1st message, it appears there are no power coming into the board or I have no idea where you measured from as per your writing.So you can start making sure the board edge connector is making good connection, clean those contacts and measure all the power are coming into the board correctly at the mean time.

author: Armadillo
author_rank: Super Contributor
timestamp: January 14, 2019, 09:02:41 am
content: Also for the A2 board, with the power supplies incoming checked OK, then you can test all the TPs and Wave Points, for any that is wrong, then the upstreams Transistors BCE needs to be measured.

author: valley001
author_rank: Frequent Contributor
timestamp: January 14, 2019, 06:49:07 pm
content: Quote from: Armadillo on January 14, 2019, 05:57:10 amInteresting. Emitter of Q16 shows the ramp per 5-11D, but I do not get the same wave pattern at the indicated test point A3 (6) (pin 6 correct?)Quote from: Armadillo on January 14, 2019, 05:57:10 amOK, I am going to confess that I am very new to this. I had the scope on AC coupling...I switched to DC coupling and now I see a mostly correct 5-11E. Same for 5-11DQuote from: Armadillo on January 14, 2019, 05:57:10 amWill do.

author: valley001
author_rank: Frequent Contributor
timestamp: January 15, 2019, 04:34:58 am
content: Now I have an intermittent fault on the -35v rail. It will sometimes read -20v.Also, when the -35 rail is malfunctioning to -20v the count neon stops flashing.

author: Armadillo
author_rank: Super Contributor
timestamp: January 15, 2019, 10:15:11 am
content: For the A9 board, Check if F2, C2 , is OK,then if fault is not found,measure Q1, Q1, Q2 BCE voltages as last resort.Edit: this kind of intermittent you should also careful for cold joints, contacts etc...EDIT: [PARTICULARLY R12 WIPER CONTACT - CONTACT CLEAN, CHECK OR REPLACE]

author: valley001
author_rank: Frequent Contributor
timestamp: January 15, 2019, 05:39:06 pm
content: Tapping on C2 causes the voltage to come back intermittently.On my component tester I get:Vloss 2v499ufESR .32 ohmsChange C2 anyway?

author: Armadillo
author_rank: Super Contributor
timestamp: January 15, 2019, 05:42:38 pm
content: Yes, I would replace C2.[EDIT: NOT ANYWAY. IS A NECESSITY. - 499uF when cold, intermittent when hot caused by raise ESR. When cap out of spec, is a necessity to change it. Don't let it pass.] While your soldering iron is on, look for more cold joints.Quote from: valley001 on January 15, 2019, 05:39:06 pm

author: valley001
author_rank: Frequent Contributor
timestamp: January 15, 2019, 06:27:49 pm
content: OK, understood. This is very educational.Now to find a reasonable source for the replacement, my stock on had is for repairing tube radios so I dont have any 500uf caps but I will check through my parts bins for used. New seem pricy. I assume I have some leeway on ratings this being an electrolytic right? 510uf 80v OK for example? Voltage needs to be at rating or higher and capacity +/- 20% or so is OK?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 15, 2019, 06:53:49 pm
content: The voltage rating can be higher, even much higher, though this is more expensive.The capacitance has quite a lot of tolerance, and these can be asymmetric: like +50/-20%. So for an original 500 µF the standard 470 µF can be OK. depending on the use even more than +-20% can be OK. If in doubt (and high tolerance) one might want to measure the new caps, after they have been exposed to the new working voltage for some time.510 µF 80 V sound like an odd, old cap, that might be already half dead.

author: valley001
author_rank: Frequent Contributor
timestamp: January 15, 2019, 07:12:33 pm
content: The original is 500uf -10 +50% 75v, so probably better to go higher right? I do see that 470uf 100v is common and cheap but with its 20% tolerance could fall well below the rating of the original.

author: Armadillo
author_rank: Super Contributor
timestamp: January 15, 2019, 07:52:17 pm
content: 100V type is OK. If it can fit in, I think will be bigger? or old type is even bigger than modern ones? possible.[Edit : 1000uF, 680uF all should be find, 470uF....well yeah...it will still work]

author: valley001
author_rank: Frequent Contributor
timestamp: January 16, 2019, 08:47:49 pm
content: OK, no C2 sub in my parts bins but new part arriving Friday.Q3 is here for the A3 board, I plan to remove old Q3 and test the associated cap. Is there anywhere else I need to look so I dont fry the new Q3?

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 09:03:12 pm
content: The last time did you already check the coupling capacitor?If the coupling capacitor is OK, then Q3 will not be fried.When you probe, just make sure no slipping. Don't probe when power is on. Best is do hooking only when power is OFF.EDIT: As a precaution, when the Q3 is removed, measure each empty pads with respect to power ground, non of them should be shorted to ground, particularly at the collector, since is routed to other boards. The other thing that can fried Q3, is installing it wrong way around..... joking.

author: valley001
author_rank: Frequent Contributor
timestamp: January 16, 2019, 09:54:37 pm
content: Ill measure that cap today. You mean to measure resistance between pad and power ground correct?I did institute a policy of switching off between probing points, after my earlier mishaps.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 10:01:22 pm
content: Yes, the empty B, C, E pads to Ground using nearby associated leads for hooking, with the board in circuits to other boards, just as a precaution check for double assurance.You sure you didn't buy 2 or 3 spares, since usually delivery and packing is more expensive than the transistor?

author: valley001
author_rank: Frequent Contributor
timestamp: January 16, 2019, 10:13:13 pm
content: I have two Q3 available, original HP part numbers. Also bought a bunch of other original semiconductor parts along with these.

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2019, 10:32:56 pm
content: Since you got a spare, then I say that from the previous BCE reading of Q3 is unlikely Q3 is shorted to ground.It must have died due to ageing.

author: valley001
author_rank: Frequent Contributor
timestamp: January 16, 2019, 10:40:35 pm
content: Ill recheck BCE on it before starting, just to be sure. I can now also save my waveforms, so I now plan to make a file I can go back and reference rather than remeasuring every time.

author: valley001
author_rank: Frequent Contributor
timestamp: January 16, 2019, 11:56:02 pm
content: Remeasure Q3:C: -11.2 to -10vE: -11.2 to -10vB: -23, to -20vNo wave forms that I can discern.Maybe I mixed up emitter and base on the initial measurements. Im trying to slow down so I dont make those sorts of mistakes.

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 12:49:07 am
content: C2 is:Vloss 1.224.09ufESR 1.6 ohms

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 12:59:38 am
content: Edit :This re-measurement indicate that Q3 may not be defective but C2 could be leaked.Take out C2 and reconfirm measurement again.Edit:Q3 is still defective with current flow thru despite it should be cut off. Are you re-measuring with the faulty original transistor?**Double Check R13 making sure it is 68K ohm.Quote from: valley001 on January 16, 2019, 11:56:02 pm

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 01:06:11 am
content: marginal but usable.Quote from: valley001 on January 17, 2019, 12:49:07 am

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 01:17:09 am
content: OK, ill change c2 if I have a suitable replacement. Those measurements were with one leg lifted.Q3, with the diode function on my meter gives .350ish volts minus lead on base, and .160ish volts with positive lead on base. The transistor tester says its two resistors.

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 01:20:38 am
content: You are talking about the orignial spoilt Q3 yes /No?

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 01:27:27 am
content: Yes, original.Testing some of my new electrolytic caps (22uf 160v) and most are reading:Vloss 1%21ufESR 1.6 ohmsThats pretty high ESR right? Same as the old cap. Maybe I should just leave the original?

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 01:31:08 am
content: Yes, please.You can always dressed it up later.

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 01:33:10 am
content: OK.Resistance between pads and pin 15 with card inserted:E 4880 ohmsB 1500 ohmsC 743 ohmsHmm..

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 01:36:24 am
content: OK you are safe to install the new Q3.

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 03:49:50 am
content: New Q3 installed now looks like I have proper wave form per 5-11 B at A3 (19).Also:Ramp looks good per 5-11 D5-11E looks like the correct shape but it is -1v to -13.2v where if I am interpreting 5-11 correctly it should be 0v to -12ish volts. EDIT: If I switch to AC coupling it suddenly looks correct?EDIT again: I have not been paying a lot of attention to the pulse timing and that it matches, is this important? For example, I get a mostly accurate shape of 5-11E with the scope set to 200us, but if I am correct is should be showing at .1 msecs (100us right?)

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 04:48:33 am
content: Both the Volts/Div and Time/Div settings should be same as the 5-11.When you set in DC mode, firstly set "oscilloscope" Channel input to GND and adjust the horizontal line to be same as the 5-11, then adjust the Channel input back to the Volts/Div in accordance to 5-11. Then do the measurement, so the shape and the time should be the same, almost as the figures in 5-11.You should be alright there.EDIT: But for functional troubleshooting now, as long as there are pulses, you should be good to proceed to other troubles. So what are the issues next at A2?

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 06:13:25 am
content: OK, A2 wave forms per 5-11:5-11 F A2TP1- No wave form that I can find.5-11 G A2TP2- Pulse present but does not match exactly.5-11 H A2Q7 Collector- Nothing, no wave form that I can see.5-11 J A2Q9 Collector- Nothing, no wave form -15v or so.5-11 K A2TP3- Wave present but distorted.5-11 L A2TP4- Wave present looks good.5-11 M A2Q15 Collector- Square wave -25 to +1v or so.

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 06:39:20 am
content: Let's approach it one by one.Depress S3, and confirm that J7 [pin at R1], there is voltage coming in when you press S3.Take out C4 and check it outside of circuit to make sure it is OK because C4 is important part.then;Install or Replace C4 back and measure BCE voltage at Q1 to Q5.That will be quite a handful for now.Edit: Also while you checking C4, also check C3 and C13.Check cathode of CR7 that the Ramp Waveform is coming in.

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 03:17:09 pm
content: Depressing S3 yields -7.9ish volts at J7 (referencing power supply ground). Releasing S3 I get voltage readings -5ish volts fluctuating.That's all I could do this morning. Will check the caps and CR7 tonight, should have C2 Friday will replace and test Q1-5.

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 03:30:27 pm
content: Good, Sign of Life at least.No hurry, relax.[Edit: Releasing S3 and there is a readout does "suggest" that CR5 is not doing its comparison job and Q1-Q5 did fired over the single pulse created by the releasing action. Cross-Finger On CR5 and CR27, Dynasaur age device, I am sure its hard to find.]Priority Changed:Check cathode of CR7 that the Ramp Waveform is coming in.Check cathode of CR5A anode that the Ramp Waveform is coming in.Check C4.

author: valley001
author_rank: Frequent Contributor
timestamp: January 17, 2019, 04:11:32 pm
content: OK, meeting was cancelled so I get to have some more funCathode CR7 I see the ramp waveformCathode CR5A I see the ramp waveformAnode of CR5A I see the ramp waveformFor fun I checked: Anode of CR5B I see a sawtooth ish or "pointed square" wave form. -.8v to -4.4v.EDIT: Also Checked CR27Cathode- Rampish waveAnode- Nothing .12 to -.040vAnode- Ramp wave form

author: Armadillo
author_rank: Super Contributor
timestamp: January 17, 2019, 04:39:17 pm
content: Please take out C4 and check carefully.Because.;If C4 don't pulse, TP1 see nothing.C4 must pass the CR5B anode pointed square wave thru to the base of Q2 to be amplified by the trains of transistors.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 04:58:03 am
content: OK, I lifted the leg opposite the diodes on C4, component tester results:Vloss 0.0%20.5nfIt does not give an ESR reading. Do I need to remove it completely to get proper readings? Just worried about heating the diode leads.I also tested it with an analog ohm meter set to 1K, no deflection of the meter. I don't have further testing abilities unfortunately. I do have a supply of .022UF 630v 5% metalized polypropylene film capacitors, perhaps I should sub one in?

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 05:59:31 am
content: Let assume C4 is good, so you should get wave at collector of Q1, collector of Q3, Q4 and TP1.If you can't get then all upstreams BCE, we need the voltage measurement.Any wave get through any of them, we are happy, because then we eliminated the CR5 from the list.If non get through to Q1, then we only hope C4 by replace it and try again until lastly - CR5.I think that should be the safe procedure to approach.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 06:07:53 am
content: Ill check those points.Just now, for fun, I took wave forms on either side of C4, on the diode side I get a wave, on the other side I get no pulse. Does this fact tell me C4 is bad?

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 06:14:55 am
content: Q1 C - NothingQ3 C - I get a sharp pointed wave, about 5v pk-pk.Q4 C - More or less the same as above, sharp pointed wave.TP1- Nothing

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 06:29:36 am
content: Can you then measure BCE voltage of Q5, let hope it's defective there.Press "Auto Setup" depending on your scope, in case you don't see anything cause the pulse may be sharp and short.EDIT: USE AC MODE of the scope to see pulse.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 06:42:25 am
content: Q5C -.760 to -.700B sharp point wave -.480 to 5.5v.E -.760 to -740Q5 scope on AC modeC -.040 to -.080B nothing -.040 to .080vE -.040 to -.080

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 06:47:39 am
content: My apology;I mean use AC mode to see the pulse;But I still need the DC voltage at BCE to know if the Transistor is Dead. You can measure by voltmeter hook to it instead of scope if that's easier for you.Cause only with the DC voltage, we can determine if the Transistor OR external bias problem.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 06:51:44 am
content: Q5- Multi meter on DCC -.737B -.326E -.736I definitely get a wave on the base with the scope set to DC mode, if I switch to AC it disappears.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: January 18, 2019, 06:58:09 am
content: Quote from: valley001 on January 11, 2019, 06:22:31 pmJust as a side note, a bipolar transitor "diode" test can read good b-c and b-e and test shorted c-e which should read high both ways. This is probably a more common failure with power transistors. A cheap beta tester is probably more useful. Even the Harbor Freight freebee meter has this function or better yet one of those ATmega 328 testers that can be had for a few bucks.Sent from my SM-G900V using Tapatalk

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 07:00:03 am
content: This warrant that Q5 be taken out to check it. Suspect defective. Expect Collector to be -31V So, if you take out Q5, the R25 leg should read -31 ~ 35, right?Component Test Q5.Quote from: valley001 on January 18, 2019, 06:51:44 am

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 07:20:41 am
content: No voltage at R25 testing with my multimeter on DC, with Q5 removed.Component tester calls Q5 "two diodes .365v on both sides.

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 07:25:56 am
content: Replace Q5.But you need to check R25 is 6800 ohms and why -35 power rail is or not available to it. Confirm power rail problem or R25 problem.You can then proceed to Ground comparator and Ground Amplifier.Same deal as before.I will be out for a while. Let's communicate 2 hours later. It's getting interesting and close.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 03:58:59 pm
content: R25 should be 220K no? Did you mean R26? I do get -36v/-38v there.Installing replacement Q5

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 04:12:28 pm
content: Oh my mistake.... can't see clearly if the writing is R25 or R26, is connected to the collector of Q5 to -35V rail.R25 is 220 ohms not 220K ohms.OK great to hear that R26 is not fried.After Q5 installed, please measure TP2 as well.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 04:17:09 pm
content: UPDATE:Replaced Q5 and now displays are counting normally, settling at .006v reading. Will check test points now and see what things look like.

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 04:23:56 pm
content: WOW! man that's GREAT news.I thought it's going to drag with further multiple faults.... to TEST OUR resolves at troubleshooting..... heheheheehe and actually a bit voodoo feeling with my big mouth with that "YOO, WE ARE ABLE HERE" [see beginning post]... LOL...And to think that the dinasaur diodes actually "SURVIVE"!.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 04:30:07 pm
content: I could not have done it without your help and another on the forum who helped me procure parts. This place is super cool.The funny thing is I DID find those two matched pair diodes, im going to buy them anyway.A2TP2 shows a pulse but it does not match 5-11 G, could still be me being green at using the scope, or I need more than 70mhz to see it.I still need to replace C2, arriving today.Then I can attempt calibration.

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 04:38:47 pm
content: I have a question for you.If I have a signal that is DC biased at somewhere, let's say, -35Vdc carrying "a" pulse, just "ONE" pulse of 200mV Peak to Peak,What setting of the oscilloscope would you set to capture the pulse clearly?I thought, the answer could be very educational, in the hope that members who read this post of "yours" would miss "no pulse" in future.

author: valley001
author_rank: Frequent Contributor
timestamp: January 18, 2019, 04:47:03 pm
content: I am not sure I fully understand the question but will attempt a shot at it.Would it need to be AC coupling with 100mv scale?I answer this way because when I set to AC coupling it centers the signal on the scope. I am really showing how green I am I know.

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 04:55:50 pm
content: Don't worry about Green, I was also green, everyone are also green once. The important is, you don't miss the important pulse ever.Your answer is essentially right, but you miss that "One" pulse, it's not a train of pulses. My take is you miss the pulse.No worry, I see 1328 people read your post as of now. Incredible.So let's others have a go at it so that we all learn a common something essential from here. I am not sure either.... so I better keep my mouth shut.

author: Armadillo
author_rank: Super Contributor
timestamp: January 18, 2019, 10:04:07 pm
content: Imagine a simple posting like on the "Repairing a Logitech Z906 Speaker" can hit 39,541 views, just amazing and incredible.May soon even beat the Pin-Up Posts, above. LOL

author: valley001
author_rank: Frequent Contributor
timestamp: January 19, 2019, 12:00:47 am
content: C2 replaced, the new one is so much smaller. It actually fits under the chassis directly under the old cap. Old cap is out of circuit now. Now I dont have any problems with the -35v rail.I went through the calibration procedure and it seems to be holding linear and within 5 counts or so. Reading my precision reference it reads 9.996, 7.496, 5.000, and 2.501I think there is an issue with the binding posts because if turn them, fiddle with the earth ground jumper, etc the count will dither a bunch.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 19, 2019, 04:21:13 pm
content: maybe some deoxit stuff could do some cleaning and remove surface oxydation

author: valley001
author_rank: Frequent Contributor
timestamp: January 20, 2019, 04:38:07 pm
content: Going through all of the adjustments/calibration on this thing I decided to attempt to look for PSU ripple on -35v rail.On the scope, I see -34.6ish average with 1.3voltspeak to peak! That seems excessive.I had the scope set to DC coupling with the 20mhz limit on. If I switch to AC coupling I could detect a small signal in the 8-9mhz range (harmonic of some sort?) I do have the regular ground lead attached which I have learned is not ideal when trying to measure psu ripple, I may try making a shorter ground probe with a paperclip or something. I suppose I will check the filtering caps on A9.

author: Armadillo
author_rank: Super Contributor
timestamp: January 20, 2019, 04:48:28 pm
content: There was a guy that asked the same question on his SMPS ripples using full 100 mHz, he didn't come back with his question there after.But Oh, I say you are very correct with the BW limit and with the short measuring leads. Must have done a lot of homework there.Increase ripples = increase resistance implying possible failing capacitors, cold/corrode solder joints, poor contacts, failing components that draw extra currents etc....But I am sure you are all able to solve the problem.Please share your interesting findings. thanks.

author: valley001
author_rank: Frequent Contributor
timestamp: January 20, 2019, 05:06:51 pm
content: Which are the smoothing caps on the -35v side? Its not C6-8 or is it?

author: Armadillo
author_rank: Super Contributor
timestamp: January 20, 2019, 05:18:07 pm
content: Any capacitors that are tied to the -35V rails are the smoothing capacitors.So it can be the local capacitors like C2 at Q1, C3 or any remote ones that are tied to the -35V line.[Edit: Particularly at the Q2  and Q1 Driver where any ripples get amplified to the Series Transistor Q1 there. See A9 Schematic].Improve the C2 and C3 there.

author: valley001
author_rank: Frequent Contributor
timestamp: January 21, 2019, 02:01:07 am
content: Another thing I am working on simultaneously is the 400khz oscillator measurement.I have been using my scope to take this reading, and when I initially checked I saw it was 400Khz +/- but reading the manual I see it should be 400Khz +/- 400hz. I see it fluctuating between 401.5Khz ish and 399 Khz ish which falls outside the specification.Is my 70mhz scope sufficient for taking this reading accurately or do I need a proper counter?

author: Armadillo
author_rank: Super Contributor
timestamp: January 21, 2019, 02:29:02 am
content: Your scope at 70mHz is well adequate to measure the 400khz frequency.But I think your meter as measured is accurate enough. The wide tolerance of 400 hz indicates that this is NOT the critical parameter because the accuracy of measurement is on the Dual Slope Measurement technique example both the start to count and stop to count are dependent on the slope regardless of the slight variant. Don't be too pietyBut as you progress, you need to change to better scope definitely.But 70mHz scope and your knowledge technique already can solved 99% of the many practical daily electronics of today.

author: valley001
author_rank: Frequent Contributor
timestamp: January 21, 2019, 02:38:26 am
content: OK, sounds good.This brings up another question, but is it possible to visualize the ramp and incoming signal comparison on the scope using two channels?

author: Armadillo
author_rank: Super Contributor
timestamp: January 21, 2019, 02:53:48 am
content: Channel 1 scope and TRIGGER on the Rising Edge of the Transfer Ampl at Q12 collectorChannel 2 scope on the Ramp Output at the Collector of Q16.Channel 1 scope on the Ramp, trigger on startpulseof RampChannel 2 scope on the stop pulsesYou will see the count portion of the Ramp or T0 and T1 effectively speaking.As long as you find joy in doing that.