1969 Knight R-195 Communications Receiver Kit - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: June 16, 2021, 02:20:14 am
content: I finally found an older radio that I can build from a kitIt's made by Knight and is dated October, 1969. The way I see it, it's been sitting in someone's closet for over 50 years waiting for me to work on it!I consider this a repair too because some of the boards were pre-built by Knight but they used carbon composite resistors that may now be out of tolerance. I'm going to take the time to bring this radio into perfect shape with the best of parts from Digi-Key.I can hear my neighbors now, "what's that tower for?"I'll post pictures as I un-box it and scan the schematic tomorrow.

author: james_s
author_rank: Super Contributor
timestamp: June 16, 2021, 02:24:47 am
content: That's totally cool, I'd love to build an old kit like that. I'm sure some people will freak out that you're "ruining" an unbuilt kit by building it but what else is an unbuilt kit good for? Just do a good job, your other work that I've seen has been pretty meticulous. I'm pretty sure my uncle has exactly that same radio, he found it at a garage sale about 30 years ago when it was already old. I remember playing with it sometimes when I was a kid.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 16, 2021, 03:12:29 am
content: ...and the Nippon Chemi Con caps are dated '69 too:

author: Smoky
author_rank: Frequent Contributor
timestamp: June 16, 2021, 03:44:26 am
content: The gold cad plating on the metal is near perfect. The kit also includes plastic tuning tools!The boards are in the same shape, but I plan to replace the resistors and electrolytic capacitors. I have a nice selection of Vishay Dale CMF resistors that should work good.

author: james_s
author_rank: Super Contributor
timestamp: June 16, 2021, 08:21:17 pm
content: That's the shiniest tuning capacitor I've ever seen. I've never encountered a new one before.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 17, 2021, 12:38:40 am
content: Shiny it is. I would say it's because they put the parts in plastic bagsHere are the scans of the schematic and parts list:SchematicPartsAnd while searching the internet about this radio, I found this schematic by Bill Meacham. He went through his radio and found some differences from what he seen on his schematic compared to how his radio was built.He annotated some voltages and values that he discovered didn't match.I'll keep an eye out for what I measure too.As for all of these images, they are large, so right-click to see them in full size.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 17, 2021, 01:03:45 am
content: So far, the only thing that doesn't look good are the packets of wires. Something in them turned to goo and now everything is stuck together. I ordered new 22 AWG stranded silver-plated Teflon wire but it looks like the tuning line cord is screwed. The line cord measures .03" in diameter so I'll be searching for some fresh stuff:The resistors measured as expected too. 60 new resistors coming upAs for the rectifier diodes CR4-9 on the schematic (DS-16C's), they appear to be in good condition but I couldn't find a data sheet for them:I read an article by Mark Johnson where he tested and ranked 48 semiconductor diodes based on the smallest dI/dt at switch-off. The Vishay SBYV28 type came out on top so I'm going to buy a few of the 100v version:...and a fresh batch of Mil-spec wire and cable has arrived tooThe pages of instructions are laid-out pretty well. On this page, the instructions say to cut the bare spool of 24 AWG tin-plated copper wire to size and slip it into the decomposing sleeving. I'll better that by using some 22 AWG silver-plated PTFE stranded stuff:I bought these Vishay/Sprague Atom axial capacitors to replace the new/old Nippon Chemi Con electrolytics that came with the kit. I read that these Spragues were just a hollow can with a smaller "cheap" cap inside so I tested the 1000uf/50v Atoms against the Nichicon UHE 1000uf/50v's on my Sencore LC53. Btw, the Atom's are described on their datasheet as "low-leakage" caps, and sure enough, the results proved it. The Atoms dropped to zero leakage but not the UHE's. Anyway, they'll fit the point-to-point wiring area better while looking "retro" too:Since the majority of the parts that I need are now sitting on the bench, I started to rebuild the audio board. The original caps were tested for leakage and capacitance. All of them tested high:The resistors also read high. Here's R49 (4.7K):I'm going to use 1%, .5%, and .1% Dale CMF60's and Vishay SFR25 resistors in this radio:I also made one change on the audio board. I replaced C52, a 1uf/25v electrolytic, with a 1uf/50v PPS capacitor:Whoa!So I'm going through and replacing resistors on the audio board and I find this:It's a short of the base and collector of transistor TR10, a 2SC838.This is a factory-built board from the kit. Sure enough, after I cleaned the pad with solder-wick, you can see how the stencil for the conformal coating wasn't laid over the pads correctly. I'm going to go through these boards with a fine-tooth comb:Dodged a bulletThe audio board is now finished...and now onto the I.F. board.Before:After:Here's what I'm finding on the boards assembled by Knight. I imagine a few of these kits were returned due to issues caused by cracked solder joints, shorts, and dirty boards (solder balls in resin).Soon I'll be turning to page 1 of the "assembly" instructions

author: Smoky
author_rank: Frequent Contributor
timestamp: July 04, 2021, 01:06:10 am
content: I'm moving along with the radio pretty good but I have a question about these coils and transformers:On the schematic, it shows that within the dotted lines of T3 (I.F. transformer), T4 (BFO Coil), and T5 (I.F. Transformer) are unmarked capacitors.I watched a video where Mr. Carlson says that they are, on average, 100pf capacitors. He also stated that in these types of coils, the capacitor isn't soldered-in. It's a press-fit sandwich with a piece of Mica spacer that has a layer of conductive metal applied to it or something like that, and over time, tarnish sets-in and disrupts the signal path and causing noise to be created. He shows how to bypass the capacitor with new ones but he doesn't mention any sort of test that I can do first to see if there is a problem.looking at these two types of schematics within this radio, is there a test that you can help me set up to test these 50 year old/new coils for degradation/noise?I.F transformerBFO Coil

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2021, 04:17:25 am
content: Why test the coils? Do you have reason to suspect they're bad? I don't think I've ever seen a coil like that which had failed. The best test is probably the radio itself.

author: Smoky
author_rank: Frequent Contributor
timestamp: July 04, 2021, 04:50:25 am
content: Thanks James.I've no reason to believe that they're bad. It's just that I'm beginning to realize why that small Leader oscilloscope repair attempt I made a year ago was so difficult. The Japanese way was to populate all of the boards with components first, then add a gazillion jumper wires to the boards. Then after installing the boards, connect the gazillion jumper wires to all of the switches and other boards in the chassis.This radio is just like that.And now after watching Paul Carlson's video on these coils, I thought, why not be proactive and save myself some future pain and suffering

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2021, 05:01:23 am
content: Do a basic ohm test to confirm that they are not open circuit and that windings are not shorted to one another and that will catch the vast majority of things that could reasonably go wrong.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2021, 08:59:17 pm
content: "a new modular concept where most parts are already soldered in and all critical adjustments have been made at the factory so that the builder merely interconnects the boards"The transformers are fragile and irreplaceable so I would not do much with them. The Litz wire may corrode and break inside so a continuity test is good enough. Inside I usually see tubular capacitors that don't fail, Mr. Carlson might be talking about older, higher impedance (tube) radios where those caps go bad? I did not agree with his tuning technique because of the loading error of his spectrum analyzer.You have to have the right alignment tool and if the slugs are too stiff they tend to crack apart. A little graphite might work. Most important is to not put the transformers in backwards or mix them up T-4, T-5 but I think the factory has them already in?Knight-Kit R-195 priced at $89.95 in new productsHam Radio magazineJune 1970 pg. 74 is $624 in today's dollars.Small text mentioned inPopular Electronics magazineJune, August, Oct. 1970 but Knight did not take out ads.It looks like an era where Japanese radios and kits, i.e. Radio Shack DX-150 were prevalent and Heathkit was the most expensive.

author: Tom45
author_rank: Frequent Contributor
timestamp: July 05, 2021, 10:46:56 pm
content: When I was in college in the 60s I helped a number of guys that had built audio Knight Kits coming to me for help. From >50 year old memory I remember replacing a lot of power supply electrolytic capacitors in order to fix problems with excessive hum.These were recently completed kits. So either the supplied capacitors were faulty, or they were under specified. Given how often it happened, probably the latter.So watch out for excessive power supply ripple.

author: Smoky
author_rank: Frequent Contributor
timestamp: July 06, 2021, 03:57:29 am
content: Yes FloobyDust, the transformers were installed by Knight. I tell you, the copper on and in them shine like new and they look really good.I may try just a drop of dry Teflon lube into each core since I don't have any graphite. Blaster Dry Lube w/Teflon that I have is also safe for plastics.I imagine that I'll be needing to align this radio when it's done since the resistor values are probably no where near where they were in 1969? I need to find a copy of that Popular Electronics magazine tooAnywho, I measured the pots and the 2K one measured around 1.7KOhms. The 5K pots measured 4.71KOhm and 4.83KOhm. I put a small shot of DeoxIT D5 into them and their movements are smooth and the numbers were fluid while looking at the DMM. They're brand new pretty much:What I like the most so far about this radio is that it's all metal. From the brass standoffs and bushings to the metal case. Most of the holes in the metal are tapped and spaced very accurately too. Parts fit perfectly. Kits like this would probably cost quite a bit if made today:One change I did was to upgrade the tie strips to these NOS Sprague ones. They're much beefier:Tom45, thanks for letting me know about the capacitors. Maybe I should set up the rectifier circuit on the small breadboard that I have and do some experiments before the final install? I could set it up with the original parts/values first then modify it for the best result. I wouldn't know how much of a load to put on it though.Also while looking through the paperwork I found this modification from Knight. It has two wires stapled to it. So, I imagine, this change isn't present on the schematic? or, maybe, it's just an error in the instructions and the schematic is correct. I haven't gotten to that page in the instructions yet. I do know this, the parts list is more accurate than the schematic when it comes to resistor values:So far, so good but I'm not even 2/3rds of my way through page 1 yet! and there are 25 more pages to go

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 06, 2021, 05:29:26 am
content: QuoteUh, don’t do that. The inductor cores are the equivalent of a waxed paper drinking straw. The ferrite core should have a light wax coating on it. Any blaster products would likely do irreparable harm.Stop with the “improvements” and just build the thing.

author: james_s
author_rank: Super Contributor
timestamp: July 06, 2021, 06:38:22 pm
content: For what it's worth, the one my uncle has works just fine, it's never had any work done on it at all, at least not in the ~30 years he's owned it. I would not expect to have to make any adjustments and would not mess with the coils at all. Monkeying with the coils is a good way to make sure it never works right.

author: SeanB
author_rank: Super Contributor
timestamp: July 06, 2021, 07:08:20 pm
content: That wire with the green goo is because the original formulations of PVC wire from the 1960's used PCB oil in the pvc as a plasticiser, to make it flexible. However with time the oil reacts chemically with the copper wire ,forming the green goop you got. Yes replacement wire is in order, and if you got the proper MIL spec cable it will be either tinned or silver plated, so will not react with the PVC and PCB oil to form that kind of reaction. Common in older buildings to open a switch or socket and see the drips of green goo on the wire and metalwork, if it has not gotten to the point of dripping down the wall already.

author: Smoky
author_rank: Frequent Contributor
timestamp: July 08, 2021, 03:02:55 am
content: Thanks for the warning about the coils. I had no plans of touching them and nothing has been squirted inside of them either. The boards were restored, cleaned, and put back in their boxes.SeanB, the wire is M22759/11. I believe it's silver-plated copper. I buy it from an Ebay seller going by the name of EmpireDon. He claims to have over 40 years of experience in aerospace assembly. He sells some nice cableI've installed one of the variable capacitors on the front panel and I'm using RG-316 shielded cable. The original stuff was going bad (sheath was stiff) and the conductors and shielding is bare copper. My magnet stuck to the center conductor so it was steel based and I thought RG-316 would be a good substitute. I slipped some fiberglass/silicone sleeving over it too since the metal edges were sharp. As for solder, I'm using Kester 62/36/2 leaded silver solder (24-7150-0018):As for the "improvements" that I do, well, I like doing them. I have between 9pm and 1am to fiddle with electronics and since it took 50 years for someone to open this box, I'm going to take my good, sweet time putting it togetherI like using these NOS Sprague tie strips. They sit higher above the chassis than most other strips and it allows me to use the holes below the eyelets. I populate the tie-strip as much as I can before it's installed. As for this one, there are still four components and four transformer wires yet to be attached to it! I've done this several times before in other projects and it saved me from some frustrating soldering inside of the chassis, especially if it's deep:On another strip, I drilled a small .04" hole in the ground post for a jumper. The "open" eyelets can now be used for the big caps all by themselves:The signal meter, lamps, and the tuning capacitors were next:The radio is starting to take on some form too. Lots of work has been done so far. Still, the band switch has yet to be wired. I'm on page 12 of the instructions where the IF and RF boards are pre-wired and installed:This is the current state of the underside of the chassis. The panel light wiring and the transformer/rectifier circuit is complete and the audio board is installed. I'm on page 14 where the factory modifications are to be inserted. It appears that the modifications are adjustments to the grounding points:So, with the band switch mounted, it's time to solder 50 wires to it!One thing is for sure, if one had to use an old-fashioned soldering iron to assemble this, it could easily become a mess. Plus, they didn't have Teflon sheathed wire back then either!The first two wafers are done:The third wafer was easier. The small blue cap is C-33 (.047uf NP0):The last wafer is almost finished. The small blue cap is C-32 (.0022uf NP0). Ten more wires to go:And now the band switch is D-U-N!Up next is the tuning shaft, the flywheel, and the mode switch...*I pulled the trigger on an older HP spectrum analyzer on Ebay. It was being sold for parts but I feel some good "vibes" about it. Hopefully it works and can be used for radio repair projectshttps://www.eevblog.com/forum/repair/hp-3588a-spectrum-analyzer-gamble/

author: Smoky
author_rank: Frequent Contributor
timestamp: July 28, 2021, 03:49:21 am
content: The mode switch was next and it was better wired while it was loose in the chassis than after it was mounted as the booklet required:But what made things confusing was that there are more soldering tabs on the "real" switch than what was printed on the instructions, plus, look how they numbered the tabsYou literally had to go by the little circles that represented the rivets to be sure you were in the right spot!Anywho, the switch is installed and now it's off to installing the speaker, the line cord, and the tuning string.Four more pages of instructions to goI followed a few of FloobyDust's links too and downloaded the catalog page from Allied's 1970 catalog:

author: floobydust
author_rank: Super Contributor
timestamp: July 29, 2021, 09:05:14 pm
content: You have loads of patience, I couldn't do the band switches with dial cord nearby?Looking at the old magazines, newfangled transistors appears to have fostered a kit radio boom. These are $500-600 in today's dollars and wages were tiny back in 1969. Heathkit you would need a bank loan to buy one. I guess radio was the killer app for electronics hobbyists.It doesn't look like you did the grounding mod to prevent hum in the AF amp? I would say volume control VR-45 ground wht/red does not go to the chassis GND turret, but instead connects to AF Board point 68. If the AF Board 65 is grounded on the other side of the filter caps, closer to the bridge rectifier it would give 120Hz hum. The radio chassis should be grounded not at CR-7 but instead after the filter caps, at say C-37 or C-38. This is to avoid the C-37 ripple currents from making 65 have hum.

author: Smoky
author_rank: Frequent Contributor
timestamp: July 29, 2021, 10:30:14 pm
content: FloobyDust, the angle of the picture makes it look like the mod wasn't done but the white/red wire that you see going to the grounding tab is actually coming from pin 1 of VR52, which is one pot over.Here's a better shot of that area:I haven't purchased a dial cord yet but the search is onThe original dial cord was shipped in with the original wire pack and whatever was leaching-out ruined itBtw, the last pages of the assembly manual has the specs and alignment steps for the R-195:...and if I have any issues, I'll just mail them a letterWith the line cord and the speaker connected, the manual reads, "this completes all the wiring and soldering of your kit."Over the next few nights, I'm going to go back and check every stepAnd even though the dial cord is on order, it doesn't stop me from powering-up the radioI tested the 14V rail with the oscilloscope probe in the location of R-41 and C-38:...and the ripple:Under 20mV and looking good!No antenna is installed but the local AM radio station came in strong!The dial cord came in so I started with tying loops on the ends using super-small Bowline knots. I put a drop of super glue on the knot to make it solid. One end hooks over a metal tab on the pulley and the other loops over the spring's eye which puts tension on the cord.Now with VC-2 tied up, the dial cord work is doneThe Bandspread Dial and the aluminum faceplate was next. I replaced the aluminum washers with plastic ones that go behind the switch nuts so not to scratch the faceplate. The back of the faceplate is bare aluminum so it makes good contact with the chassis. All of the faceplate parts were polished with Mother's carnauba wax too:The pointer was next. It has a couple little flaws in the paint but it still looks great:Next was to drop in the dial scaleI still have a few small things to do. I want to make sure it tunes to a station accurately before I slip it into the chassis. I'm currently searching for a good RF signal generator. I also need to make or buy an antenna. I'll keep updating the progress as it happens

author: Smoky
author_rank: Frequent Contributor
timestamp: August 14, 2021, 08:45:06 pm
content: If anyone felt that this radio didn't belong in the "repair" category of EEVblog, let me easily put that thought to restI had the radio almost together and decided to turn it on to see how well the dial pointer aligned to the dial scale and to see how far off our local AM radio station was (680 WPTF).So I turn on the radio and all I hear is faint static through the speaker! Pretty much nothingUp to this point, I did nothing other than turn the band switch, the volume control, and the tuning knobSo I went back and checked all voltages throughout the radio and everything measured near perfect compared to what is printed on the schematic.What did I do? short something? Jeez!I went back and checked everything and began to look closely at the band switch and all of the wires and all of a sudden something caught my eye!Do you notice something odd in this picture?

author: Smoky
author_rank: Frequent Contributor
timestamp: August 14, 2021, 10:23:33 pm
content: When I turn the Band switch to A, notice how the notch in the brass disc is centered around pin 1 on wafer S-1B, but when you look at the brass disc on wafer S-1C, it isn't! It's actually making contact with both pins:Wafer S-1C selects between coils L-6, L-7, L-8, L-9, or L-10:On the other side of S-1C, you can see that a brass disc with a tab selects one of the five coils. The tab appears to perfectly align itself to the pin:So when you turn the Band switch and select band A, B, C, D, or E, one side of wafer S-1C selects the correct pin (coil) and isolates it while the other side of the wafer is supposed to group the remaining pins bringing those coils to ground.That's not happening.For a quick test, I removed the wire from pin 2. This should totally isolate pin 2 from pin 1. It doesn't. I used my DMM and I get continuity to pin 1. The notch in that brass disc should be around pin 1 only and not touching it:Lovely! The brass disc wasn't indexed correctly when it was made. What worries me is since the radio doesn't have the same "gain" as in the Youtube video, was damage done to the three FETs on the RF Board?I'm going to pull them and do some basic tests on all three FETs on the bench to see if one isn't responding like the rest. They are MPF-102's.

author: Smoky
author_rank: Frequent Contributor
timestamp: August 15, 2021, 12:16:05 am
content: TR-1, TR-2, TR-3:The back of the transistors have paint markings so they were graded or matched to some degree:I watched a Youtube video as to how a JFET can be tested using a DMM.Using a Fluke DMM, here are the three transistor's resistance results between:TR-1-source and drain is 130 Ohms-gate and drain is 1.48 KOhm-gate and source is 1.48 KOhmTR-2-source and drain is "open"-gate and drain is 1.28 KOhm-gate and source is "open"TR-3-source and drain is 160 Ohms-gate and drain is "open"-gate and source is "open"Something isn't right with one or two of those transistors. Btw, someone gave me their Triplett analog meter but I don't have leads for it. Why? because the leads are required to have "female" plugs on their ends! Yes, those are "male" banana pins on the inputs of the meterAnywho, I found two sellers on Ebay that are selling new National and Motorola MPF-102 FETs. I'm going to buy a dozen or so.Btw, I believe I can notch the brass disc on the S-1C wafer on its left side to give it enough clearance without taking the entire Band switch apart

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2021, 03:40:32 am
content: I would do surgery on the rotary switch contacts, they're supposed to point straight to the center and a few look bent. The camel hump needs to be there too.Sometimes the rotor nails the side and mashes and bends the contacts. Using a magnifier or microscope, I just straighten the top and bottom leafs, and have them leaving a small air gap the rotor can swing into.I usually clean them dragging a small piece of printer paper through. In the TV repair shop, we'd also lube the rotor with Lubriplate but Fluke 87I uses the same style rotary switches and they recommend Nye Lubricants 813S which is a fluorocarbon gel. I don't think they work completely dry.What the JFETs just died, performance anxiety or all that awful music on AM shocked them lolWhile measuring ohms D-S, the gate cannot be floating or you will get mysterious readings.

author: Smoky
author_rank: Frequent Contributor
timestamp: August 15, 2021, 04:44:12 am
content: Got it FloobyDust.As the Band switch is turned, that notch in the brass disc splits all of the contacts down the row so it's not just at pin 1. The contacts look to be aiming to the center in real life. I'd hate to loosen that rivet that pinches those contacts together then, if they're hit sideways, they'll spin for sure.I'll gap all of the contacts like you said so that the brass disc has an easy start slipping under them. I looked with a microscope at them already and I can see green tarnish under some of them too!As for the FET resistance tests using my DMM, the results settled down solid. I just checked them again.Why those FETs failed is beyond me. I do recall hearing snaps and crackles through the speaker when the Band switch is turned. Too bad there isn't a way to shut down the power to the switch until the selected band rests into its pocket/detent. It's probably slightly arcing while the Band switch is slowly turned across the contacts and maybe that's enough to shock the FETs?Anywho, we'll get this radio rightThe 15 new National MPF102 transistors came in:What looks promising is that the back of the National transistors have paint markings too:I moved over to the Band switch to notch the brass disc on one of the wafers. The disc wasn't indexed correctly by the factory so I needed to clearance the left edge of the notch to keep the disc from shorting two terminals at the same time. What a job! I used a Lenox Gold utility blade to make over 200 slices across the brass disc. I first clamped the disc so that the notch rested in-between two terminals. I then slipped a thin piece of aluminum under the disc to stop the utility blade from cutting the wafer material as it crossed-over the edge after each pull of the knifeHere you can see the small amount of brass starting to lift:I removed the small burrs and polished the disc with 1000 grit sandpaper. This picture shows the Band switch at position 1. The terminal is now isolated from the other four, and that also goes for the rest of the terminals down the line when the Band switch is turnedMy new DCA75 arrived today too! Time to measure-up some JFETs and see if we can't resuscitate this R-195...and while the radio is down, I decided to refinish the pointer. When I tried to adjust the pointer to the chassis, the paint would just crack or peel off entirely. Sure enough, I dunked it into some methyl chloride stripper and found out the base metal is brass. I'm going to scuff it and mist it with white epoxy primer and top it with a gloss white urethane

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2021, 07:10:44 am
content: That's some switch surgery alright. I would have a nervous breakdown doing thatDoes it pick up WWV?The JFETs have a 10:1 IDSS variance 2-20mA as well as transconductance 2,000-7,500 umhos.It makes a kit hell unless the JFETs were hand selected in the first place. You had one bogey (three stripes) different than the other two (two stripes).RF amp TR-1 looks around 4.3mA, mixer TR-2 with 2.0V/10k is only 0.2mA, and TR-3 local osc about 5.6mA. So mixer TR-2 seems to be running at very low drain current. The DC voltage reading at TR-2 might be misleading due to RF present from the local osc. TR-3. I guess you could jam the LO by shorting R-7 or adding a big cap and see if the reading on the mixer changes.Knight might have had issues with the local osc. overloading the mixer? I note in the original post handscribbled schematic from Bill Meacham, the guy found R-4 was 50k and only 1.0V there, which is really small drain current, as well as R-7 as 47k. It is a puzzle.

author: Smoky
author_rank: Frequent Contributor
timestamp: August 23, 2021, 10:40:49 am
content: Thanks for your help FloobyDust!Here are the voltages of the two original Motorola MPF102's and the new National in each band. And just like you said, RF noise may be present. Also note, Knight interchanged the Source and Drain connections of the JFETs in this radio too:For an experiment, I matched three new National JFETs with the DCA75. They have nearly the same Vgs "on" and "off" voltages and so I installed them to see how they affected the performance of the radio.Well, the performance got worse. The local radio station still comes in but it's edgy and gritty and the station was harder to lock onto. Here are the new voltage measurements below. Are the new JFETs pushing the radio into distortion/clipping?The voltages moved outward to the higher and lower extremes but stayed in proportion. There is a pattern here, unfortunately, there are no notes anywhere on the schematic as to what the conditions of the radio should be in when taking voltage readings.I'm not sure if this drawing applies to how the JFETs in this radio operate, but it depicts the Drain voltage resting mid-point between the Source voltage and the supply voltage while at idle:Once the JFETs are brought to their idle state, couldn't the surrounding resistors be tweaked to bring the JFET to its correct bias?It reads that the lower Source voltage should be no less than 2v at idle, and knowing that the supply voltage in this radio is 8.70v, the Drain voltage should be set somewhere around 5.35v.Or am I'm on the wrong track and maybe just swapping JFETs would be a better approach?Who knows? and what are the odds that the printed schematic voltages were taken while a signal was applied to the antenna terminals? wouldn't that bring the upper and lower voltages of the JFETs to their near-maximum limits before distorting*We have plenty of options and my goal is to get the radio working the best it can. The radio works. The new resistors are surely more accurate than the old ones and the old resistors wandered in value. Maybe, the two original/working JFETs should go back in and we just deal with "grading" another for TR-2's slot? The copper traces are super-tough so component swaps are not a problem. In another article, someone made a MPF102 tester using a breadboard and simulated the circuit it needed to work in.And about Bill Meacham's findings, He described the (Drain resistor?) R4 change as "slightly noisier." Didn't that resistor adjustment also push the Source voltage even higher (I heard that resistance changes make the D&S voltages raise or lower in unison) and closer to the supply voltage causing the signal to go into clipping? Shouldn't have R6 been adjusted too to counteract that?Up to this point, the only thing we have touched is the JFETs. I have not turned any coils or touched/replaced any other parts except for the new resistors....and I'm in no rush in getting this radio doneThe more I look at the numbers of the matched JFETs, they don't look so bad compared to the voltages on the schematic, except for TR-1's voltages under the "Source" column. Hmm. We got Bill Meacham's issue*One other thing, the Band switch is on "A" in the schematic so look only across that line when comparing the JFETs voltages.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2021, 07:56:36 pm
content: JFET's wide variance on three parameters is something that really confuses people, makes it messy and people lose patience.Matching JFET's is usually only necessary in say an audio diff amp pair, and threshold or cutoff voltage VGS(off) is just an offset, matching that is not the gain or IDSS so I wouldn't expect it to help here. You might have just ended up swapping in parts that have lower gain.What I'll do is just breadboard a simple common-source amp with the same values, inject 1kHz sine-wave and measure input/output voltages to rank the parts for gain and drain current etc. This ends the madness of guessing what's going on. Does the DCA75 give those numbers? It could. Should fire off an email to them.I think what you want is the highest (gain) JFET transconductance (input voltage controls output current) YET with IDSS and VGS(off) that won't cause the stage to bias off at one end and clip/saturate/distort with the circuit's component values. But across the board higher gain, TR-3 greater LO amplitude- could overload the mixer. I don't know what a design goal for that is, for radio. Might be a balance with R-7 and R-4 setting LO amplitude to the mixer, but R-4 also affects many things. I thought you'd want R-4 low as possible for high mixer gain but that scuttles running the stage at very low current.Maybe consider using transistor sockets for now. Another approach is too look at schematics of later Knight kits and see if they made changes. Or simply try change a resistor value.W. Marshall Leach Georgia Tech course: ECE3050 The JFEThas math for those interested.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2021, 09:18:45 pm
content: Quick note on the MPF102's. They are garbage FETs generally. Wildly varying in Idss and half of them didn't even work to start with. Their popularity was down to availability at the time and cost. This and usually being in stock by Radio Shack caused a somewhat cult following in ham radio circles and the inability for people to understand the circuit and to substitute parts raised the associated cost.It's best to look at the bias conditions (just chuck the circuit in LTspice) and grab a better JFET for the job really. The "graded" parts from the same process are actually now known as 2N5484/2N5485/2N5486 which are far more reasonably priced and avoid the whole selection and measurement processes the MPF102's tend to require.

author: Smoky
author_rank: Frequent Contributor
timestamp: August 23, 2021, 09:35:15 pm
content: This is embarrassingFloobyDust, I did absolutely that, I swapped in lower gain parts!So I put one of the original Motorola MPF102 JFETs on the DCA75:IDSS=10.51mARds=148.5 OhmsNext, I removed one of the three matched National JFETs from the radio:IDSS=7.25mARds=137.4 OhmsNow get this. I went back to the remaining 12 National JFET's and I tested them again, but this time, I looked for the highest IDSS:IDSS=12.00mARds=106.2 OhmsI found two at 12.00mA and one at 11.7mA. What's interesting too is that their resistance readings are between 106.2 and 109.1 Ohms.These babies will be installed in about an hourWait a minute!Look how the symbol changed after IDSS on the DCA75 analyzer. It went from equal (=) to greater than (>). This means the gain is even more than 12! Hell yeah

author: KE5FX
author_rank: Super Contributor
timestamp: August 24, 2021, 12:42:14 am
content: If you haven't hooked your DCA75 up to a PC and played with the curve tracer feature yet, I'd recommend it. It's very nifty, and very educational.

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2021, 01:54:39 am
content: WAIT another minute! IDSS is not the gain - Look for highest "gfs" as Peak calls it.Quote from: bd139 on August 23, 2021, 09:18:45 pmOf that late 1960's era, MPF102, BF245, 2N3819, 2N5457, 2N4416 etc. were out there and I can't see anything that says they are poor semi's.JFET's look almost all the same in datasheets. Huge variance and nobody stomps the competition. Only Toshiba refined them in the 80's.I think any low performance might be due to simple age of the part, NOS isn't always new or the best of the lot.

author: Smoky
author_rank: Frequent Contributor
timestamp: August 24, 2021, 03:47:48 am
content: Thanks for the help FloobyDust.I put the two original Motorola JFETs back in their original locations (TR-1 and TR-3) and a new National in TR-2's spot and that's all the radio needed to bring it back to lifeWhen you said that the transistors may have been hand-selected, I wasn't going to take any chances.All five bands pick up stations even though I'm using just an 8' wire as an antenna!

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2021, 04:22:36 am
content: You mean all these years I've been tuning in WWV Colorado 10MHz 1,100 miles away instead of"CHU"? It sounds bland in comparison and it ends up farther away. Atmospherics wipe out the signal some days and I'm just using a jumper lead for an antenna.With the S-meter near pegged I think it's working wellAre you going to tweak the IF or anything. I have no lab room for a service receiver... although they would make alignment easy, and eBay has them low cost... and winter is coming around soon...

author: Smoky
author_rank: Frequent Contributor
timestamp: August 24, 2021, 04:54:18 am
content: It's amazing how many stations there are! Tune the dial and then the Bandspread and Antenna knob and they come out of the woodwork. Every little nick and tick is a station.Tons from South America it seems, or Cuba, or both!Anywho, I was tuned into CHU Canada at 7850KHz. Ottawa is about 900 miles due north of Raleigh, North Carolina.I recall when I had the front dial scale on the receiver a week or so ago, our local AM station 680 was playing almost an inch off from the numbers. I image there's plenty of tuning to do. I do have that HP spectrum analyzer now so I'll soon be looking for some advice...and shifting to another topic, I went back and measured the 9v rail in the radio at CR-10. It measured low because the initial batch of JAN1N757A-1 Zener diodes I purchased measured (~8.65v). The 1N757A's are 5% tolerance parts and aren't available any tighter unless they're "special" ordered, so it's a crap-shoot:I wanted to see if I could get the 9v rail closer in line to the schematic or slightly higher so I purchased another batch of Zeners. This time I bought JANTX1N757A-1's:Sure enough, the voltage climbed a little higher!After the diode was installed, I measured the 9v rail again:I hear a noticeable difference in the reception/clarity. One band doesn't sound as saturated as before and I can see that the needle pegs the S-meter more sharply. There is a meter adjustment pot on the back of the radio if I need to tone the "swing" down.Anywho, I'm getting to really like this Peak DCA75 analyzer, it has saved me a ton of time alreadyPlus, the pointer is re-finished and looking goooood!I added new rubber feet to the case too. The front pair of feet are slightly taller than the rear to improve the view of the front panel. I bought them fromwww.RubberFeet.usand they are very nice quality and made in the USA!As soon as the dial and faceplate are back on the radio, it'll be time for an alignment.I just don't have the proper RF signal generator yet.I didn't know that the Siglent SDG1032X is also an RF signal generator too! So when I'm back from the Shelby, NC Hamfest this weekend to find some old radios to fix, it'll be alignment time

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2021, 10:05:12 pm
content: You might lower the RF gain or check the AGC action if it's distorting on strong signals, or maybe the S-meter cal is a bit off.Strange the schematic lists RF Gain voltage (39) at 0V but RF gain pot set to -4V and supposedly +8V other side of R-45. The wiring to meter driver TR-9 makes no sense, there must be errors in the schematic, something has to connect to AGC (55) like R-18. R-35 is a C-35? lol, and it should have reverse polarity + to GND (you prob. used a 1uF film cap there anyhow). AGC seems to be node (55).Back in the day I can imagine communicating with draftspeople in Japan being a major difficulty, to fix errors in the schematics or documents.P.S. Appreciate the high quality pics you post.

author: Smoky
author_rank: Frequent Contributor
timestamp: September 07, 2021, 01:57:14 am
content: Knowing now that the Siglent SDG1032X does AM modulation, I realized that the waveform generator only goes to 30MHz. When modulating at 1KHz, wouldn't the Siglent need to go beyond 30MHZ? 30MHz + 1KHz.I haven't started any part of the alignment procedure yet but one of the steps requires the signal generator to be set to 30MHz modulated 30% at 400cps or 1000cps (the highest setting required in the alignment procedure for the Knight R-195).So I set the Siglent to 30MHz, modulated 30% at 1KHz:Having picked up a used HP3588A Spectrum Analyzer recently, I wanted to see if I could capture the signal and have it tell me if the Siglent will work. I set the analyzer up the best I could to get a clear shot. I put the marker on each peak of the modulated signal. Here's a shot with the marker at the top of the center peak (30MHz):Next was the marker over the left peak (30MHz - 1KHz):And now the marker over the right peak (30MHz + 1KHz)All of this is just a learning experience and probably not absolutely required but it's good to get a "visual" of what's going on and to see that the Siglent will go a tad over 30MHz accurately and evenly.

author: KE5FX
author_rank: Super Contributor
timestamp: September 07, 2021, 03:53:15 am
content: Note that it doesn't have to tune past 30 MHz to generate an upper sideband above 30 MHz. You'll get the sidebands for free when you apply either AM or FM modulation (or any other kind.) That's literally the idea behind modulation.

author: bd139
author_rank: Super Contributor
timestamp: September 07, 2021, 06:42:29 am
content: Quote from: floobydust on August 24, 2021, 01:54:39 amNot that at all. MPF102 wasn’t binned basically. The 5456/57/58 were the same part but binned. My point was pick the right binned part rather than the dubiously specified MPF102 so you don’t have to select it yourself. Sometimes when you throw an MPF102 in circuit the bias will be shot due to the Idss variation.3819/4416 were same part but 4416 was selected and packaged with grounded TO18 case.Age isn’t really an issue with these I’ve found. I’ve got a 1969 dated bag of 2n4416’s here and they’re all fine.Regarding “modern” MPF102’s, the past is throughly discontinued so you have no idea what you’re getting. The binned variants are still available though I understand but because they don’t have the magic MPF102 written on them and people don’t know how to select and substitute parts they command less value.