Fluke 332D DC Voltage Standard Restoration - Page 1
Forum section: electronics / repair

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 06, 2019, 11:31:18 am
content: So I got me an old Fluke 332D.Paid 10,000yen (about US$90) all up including shipping for it, and it was listed as kind of working but intermittently doing weird things with the output, something that screamed 'dirty contacts' to me seeing as this thing is full of rotary switches.Got it home and it fired up fine.a bit scratchy on the dials here and there, so I spent some time on the lower dials with the deoxit and a light scrub with the fiberglass pen on the switch contacts and deoxit fader in the two pots. Still have to tackle the voltage selector switches.The output is a little out, but I'll have to get my hands on an 8.5 digit multimeter to really dial it in, my best multimeter is a 6.5 digit Keysight 34461A.I've also read that the caps in these tend to go bad, so I took inventory and made a full list on the Mouser website of caps to replace.I won't be touching the tantalums as they are dry slug axial and are very reliable and generally fine unless obviously bad (and they are uber expensive to buy these days), but I did find all the 30D series electro caps to match perfectly (with only a little increase in rated voltage here and there). All the other standard electro caps will be replaced with Vishay axial, using the high temp and lower ESR versions where possible.Also, what should be the Y caps on the input aren't marked with any specifications, so they will be replaced with new caps of the correct rating, along with a new mains cord so it's all shiny (The brass contacts are a bit tarnished and I have a limitless supply of mains cords).The only thing I'm a little stuck on is the main filter capacitors.They are 125uF, 450V Sprague caps, part number 106336 and have a really funky 5 pin footprint that no other modern caps use.I don't know if they even need to be replaced, but if I do need to, I could either solder in some standard radial caps and just bend the leads a bit to match, or make a secondary board to piggy-back the new caps on and use jumper wires to attach it to the main PCB.Last pic is of this type of cap.Any thoughts?

author: tggzzz
author_rank: Super Contributor
timestamp: July 06, 2019, 03:33:45 pm
content: Quote from: TERRA Operative on July 06, 2019, 11:31:18 amThey can be awkward to remove; care is required,The best way is to make a secondary PCB for the new caps. One technique to avoid jumper wires is tomeasure the width of the existing lugs and diameter of holes in the existing PCBon the secondary PCB, put two holes slightly smaller than that distance apartsolder a bog-standard 0.1" header into the two holes, then insert the two headers into one of the PCB holes, and solderEnsure gaps between tracks on the secondary PCB are sufficient to avoid HV breakdown!

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: July 06, 2019, 03:38:13 pm
content: I have the 332B and I had to go through and replace virtually all of the caps, many had leaked. Those large Sprague caps can be replaced with newer radial caps but be aware that (at least in the B model) they used the cans as part of the interconnect circuits, you will have to put shorts in place of the cans or you will have open circuits. Not difficult to see where to put the wire shorts in on the PCB. No modifications to the PCB were needed.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 06, 2019, 04:05:10 pm
content: Yeah I thought that might be a thing to watch for, but I just took a look and it seems like I'm safe, they aren't using the case of the cap as an interconnect on this one.One thing I'll have to figure out is how I'll fix an adapter board in place so it doesn't stress the PCB etc.I think I'll have to squint at it intently for a bit and maybe frown a little too until I come up with an idea.The problem is, it's so clean inside that I want to try to keep it looking as untouched as possible.Maayybbeee I could restuff those caps, but maybe I can't be bothered..... hmmm.

author: bitseeker
author_rank: Super Contributor
timestamp: July 06, 2019, 05:02:37 pm
content: Yeah, apart from the bit of corrosion on the controls, it's immaculate inside. Nice find, TO.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 07, 2019, 04:14:28 am
content: Ok, I think I have a plan for those big capacitors.I think I'll use something like this to restuff them:https://www.mouser.jp/ProductDetail/871-B43547A5157M000I'll 3D print some spacer rings to hold the new capacitor inside the shell of the old too, then wire the new capacitor to the old capacitor pins, matching their original footprint configuration.Then I'll put a little label on the top with the details of the change, and reuse the cardboard insulator tube thingos as fitted from factory to make it all look original.I think it's the easiest way to mount the new caps, and will look completely original too.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 08, 2019, 09:14:45 am
content: So, got all the capacitors ordered from Mouser, not so cheap those axial caps....I'd have saved a few bucks if they were radial, but it is what it is...Also, I started cutting the big PSU caps open to stuff the new caps inside when they arrive and found the blade in my pipe cutter is completely blunt and chewed up somehow. I can't remember what I would have cut to make it so bad but I'll have a new blade soon.

author: Zucca
author_rank: Unknown
timestamp: July 08, 2019, 01:43:09 pm
content: Love it, congratulations TERRA!

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 08, 2019, 01:57:27 pm
content: Gots me a new blade for the pipe cutter and cut open the caps, made a good job of it too.Now I just gotta get my 3D printer online and start printing the 3 sabots to hold the new caps centrally inside.

author: drknob
author_rank: Contributor
timestamp: July 08, 2019, 02:03:00 pm
content: Check this source:https://hayseedhamfest.com/They'll do custom caps.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 15, 2019, 02:30:10 pm
content: I replaced all the capacitors in the unit now, pretty painless with all the removable cards, I didn't even have to remove the HV motherboard to replace the caps there either.I ended up gutting the old big PSU filter caps and 3D printing a sabot to hold the new ones centrally inside.They slip in just nice and with a little bit of aluminium tape to seal the cans back up and a label on top, they'll work nicely I think.

author: Zucca
author_rank: Unknown
timestamp: July 16, 2019, 08:08:14 am
content: and you didn't fire up the beast?

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 16, 2019, 10:31:16 am
content: Gotta keep you guys wanting more!Actually, I had to go to work, but I will fire it up real soon.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 16, 2019, 04:07:17 pm
content: Ok, so I'm starting to play around with tweaking dials and getting this thing working right and I have realised that the extender board is missing....Looks like I need the PCB pins and an edge connector which are both obsolete... yay..Does anyone have a 332(x) parts unit they can sell me some bits from by chance?I found this info, but it'll be a long shot to find anything on the ebay/etc....Details of part numbers:https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg761177/#msg761177Pic of a home-made extender card, showing the pins and edge connector:https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg445755/#msg445755I also found this which looks interesting...:https://www.eevblog.com/forum/testgear/fluke-332a-adjustmentverification/msg685631/#msg685631

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 18, 2019, 04:52:09 pm
content: Alrighty, got all the capacitors replaced and no output....After some poking around, I found a bunch of out of spec carbon composite resistors inside and a busted Zener on the Aux PSU board. It's the precision 6.3V one in position CR6. I'll replace it with a 1N4735 zener for testing then swap to a super low temp coefficient 1N829A once it's working.I think I'll rip out all the carbon composite resistors and replace them all with metal film resistors too (I can get Japanese 1% tolerance, 50ppm metal film resistors for a good price).I figure if some are bad (6 out of 14 are bad on the Aux PSU board alone), I may as well replace them all to be sure they are all good so I don't end up chasing my tail then replacing them later anyway.The resistance checks in the service manual at the start of the troubleshooting section indicate there is a problem with the Aux PSU board, so replacing these bits might just get it working enough to test the rest of the unit...Also, I am restoring and upgrading my variac to allow me to test stuff more easily. Prompted by the need to use one for some of the tests on this 332D.It got a full disassembly, a sandblast and repaint in hammer finish paint, then a 3D printed box with a good amount of plugs and sockets to make it's use much easier than the old basic binding posts that were on it.Just sent an email off to the manufacturer to see if I can get new handles and a dial faceplate. Fingers crossed as I had luck a while back sourcing spare carbon brushes from them.Next step is to hit Akihabara tomorrow for resistors and diodes, then it's time to give the desoldering gun a good workout.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 23, 2019, 03:40:55 am
content: Well, I'm going to have to have a sit down chat with my 2 year old son.... I caught him poking around in the 332D and after telling him 'no!' and giving him a Tek scope to tweak dials on instead, I didn't think much more of it.Turns out he broke a wire on the power switch.....No big deal in the end, it was a quick repair.I wish I saw it before I started replacing resistors though, I planned on testing the unit before and after and assumed I tweaked something during the capacitor replacement, so went ahead replacing ALL the things! to eliminate as much as possible. I didn't count on a simple broken wire....So now I have almost all the carbon composite resistors replaced, just a few to go.It turns out at least a third are out of spec, so not a complete waste of time...I just have to source two more. The store in Akihabara didn't have a 2.7\$\Omega\$ 1/2W resistor in the type I wanted, and no-one seems to make a 1.1\$\Omega\$ resistor in the specs I want too. Digikey only list them in 5% tolerance 350ppm or so.I'll probably stick a much more common 1.2\$\Omega\$ resistor in, unless someone tells me it's a bad idea?....The resistor in question is R20 to the far center-right of the first schematic shown below, feeding into the second schematic.

author: tggzzz
author_rank: Super Contributor
timestamp: August 23, 2019, 07:00:12 am
content: Quote from: TERRA Operative on August 23, 2019, 03:40:55 amI used two techniques to teach my daughter that electricity can bite.The first was to give her a "lightning strike" from an old piezoelectric Zerostat used to dissipate dust-attracting static electricity from vinyl LPs.The second was to have her touch an electric fence. Before doing that I made sure she knew to touch it with the back of her hand so there was no chance muscles would contract and grip it. I also demonstrated how to do it.In her early teens, we backpacked around India without knowing where we were going to sleep at night, and before driving a car she was making forced landings as a solo pilot.In other words, I showed her how to recognise danger and avoid the consequences.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 23, 2019, 07:20:49 am
content: Yep, I'm making sure he plays safe. He knows his boundaries, he just likes to push them.No access to energized equipment of any type though, he's gotta be older for that. For now he just pushes buttons and turns dials and sometimes doesn't break stuff.

author: tggzzz
author_rank: Super Contributor
timestamp: August 23, 2019, 08:06:23 am
content: Quote from: TERRA Operative on August 23, 2019, 07:20:49 amYou want to encouragesafeexploration; you will never ever get the balance completely right!It does no harm for him to realise that safety boundaries are there for good reasons, and that violating them can have unpleasant consequences.Early teens is a good time for people to read Tom Godwin's classic short story "The Cold Equations",https://photos.state.gov/libraries/hochiminh/646441/vantt/The%20Cold%20Equations.pdf

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 17, 2019, 11:57:49 am
content: Alright, this one has fermented on the to-do pile long enough, so time to get back into it.All carbon resistors and capacitors are replaced, so that's looking good now.My next problem is that the service manual is a little self-referential in that I apparently need another one of these units to properly test/calibrate this unit..... hmm.Also, I apparently need a differential voltmeter, anyone know if my Keysight 34461A 6-1/2 digit multimeter is a good enough replacement, or should I look at getting a proper differential voltmeter? (which means a whole nother project too....)So next step is to run it through the tests in the service manual as far as I can with the gear I have here to see how it looks and see what dials I have to tweak. I had better get my lucky socks out and practice holding my tongue in just the right position.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 17, 2019, 01:33:55 pm
content: Hello,all linearity calibration, and also the range calibrations are done by differential method, which is not necessary, if you get access to a higher precision DMM, like 3458A, 8508A, or so. Therefore, Null Detector is also not necessary. The 34461A anhow would be ok as a Null detector in the standard setup, instead of the 845AR, but not for the 1kV range, as that would exceed the common mode voltage limit.The linearity calibration requires a DMM with at least 0.1ppm linearity, that is 1µV on 10V scale, so your 34461A with 2ppm linearity is not enough.The absolute calibration for 10, 100V and 1kV can be done in good precision with these mentioned 8.5 digit DMMs directly, but the 3458A has a very bad 1kV spec., so you would need another one, or a 752A.Frank

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 17, 2019, 01:43:17 pm
content: Hmmmmm, I'll have to track down a better meter somewhere.I might just do my best with the 34461A to at least see if I can get it in the ballpark (With all the replacements I've done, it's not like I'll make it worse than it is now...) and if it checks out, then I can find a better meter to give it a real adjustment.

author: valley001
author_rank: Frequent Contributor
timestamp: December 17, 2019, 04:58:38 pm
content: It is good to see you have not given up hope on this.I have been working with my 332D for some time, and I believe I tracked most of its issues. The most recent problem was intermittent failure of the chopper (solid state version) to function correctly. It would work and then quit, with the voltage output becoming screwy. In an attempt to readjust the chopper (per 4-65 in the manual) barely touching R43 would result in the output suddenly becoming stable. I guess that pot is on its way out perhaps.When working I can get 1.000000 volt output within a few counts on my 3456A (dithers between 1.000001 and .999999), which is still a bit out of spec if my understanding of the manual is correct.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 17, 2019, 05:23:01 pm
content: Quote from: valley001 on December 17, 2019, 04:58:38 pmWhich manual do you mean, that of the 3456A, or the 332D ?The Fluke 332D specification indicates 10 ppm setting + 10µV for the 10V range, plus 20µVrmsof noise, therefore your observed +/- 1µV is unbelievably better than that. The 3456A specification might even be worse, 8ppm @ 24h, 23ppm @ 90d, so everything is well inside any spec, I think.Frank

author: valley001
author_rank: Frequent Contributor
timestamp: December 17, 2019, 07:22:41 pm
content: Hello Dr. Frank,I was referring to the 332D manual but I still do not fully comprehend how these specifications work in terms of output accuracy vs stability, I need to study them more. Are you saying that at 1.000000 setting one could observe fluctuation of 1.000005-.999995 and that would be in spec for the 332D?I have not calibrated my unit I am simply running it and observing how stable the output is over a period of time. My 3456A is in calibration and after warm up I can set the dials on the 332D to achieve a 1.000000 volt output (1.000001-.999999 observed) but I have to set the 332D at .999876 or so on my un calibrated 332D. I do not have access to a 8.5 digit meter for calibration.Quote from: Dr. Frank on December 17, 2019, 05:23:01 pm

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 17, 2019, 10:41:04 pm
content: I found this link that looks a bit interesting, I'd give it a go (once my unit is working) if I could find some spare header pins...It might be a a way to upgrade the chopper amp if yours is faulty anyway maybe.http://www.friedrich-messtechnik.de/index.php/bauanleitungen/87-fluke335-332

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 02, 2023, 07:08:20 pm
content: Time to stick this one back on the bench... It's been in hibernation for a little while.So the latest is that the unit would trip out as soon as you turned the power dial to the 'operate' position. But if I took the lid off and left the safety interlock switch open, turned the dial to 'operate' and then pressed the interlock to activate the unit, it would come alive and kick out a super stable voltage.So, after looking at the schematics it seems to be something in the Current Limiter PCB A5A6.Luckily I have a spare (with completelytrashedcapacitors), and if I swap it in and let the caps reform a little, the unit starts to work.Ok, I have my culprit...After squinting at the PCB, and with the spare next to me, I soon found some problems....It seems I am an idiot....I installed a 22K resistor in place of the old 2.2K carbon composite (durrrrrrr), andsomehowI used 470uF capacitors in place of the original three 250uF parts....Ok, well the resistor is easy to get in Akihabara, and I'll add the capacitors to my next Digikey order.More to come...(I also now have a much older 335A to look at next too)

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 04, 2023, 12:38:46 am
content: Well, looks like the rabbit hole does far deeper than initially expected, as usual..So it turns out that Current limiter board is actually ok. When I actually paid attention to the schematics, the capacitors are just bulk smoothing caps after the bridge rectifiers and vreg section etc, the DC coming out is perfectly stable and all is well.I replaced the 22K resistor with a spare one for now (It's actually too big, I only have a 1W, where the spot is for a 1/2W, new correct resistor coming soon) and that PCB is working fine as far as I can tell.Now I can consistently make the unit fail by swapping out the Series Pass Driver PCB A5A2.I'm not convinced it is actually at fault as such though. I have checked all the semiconductors on my Peak Atlas component testers and they all test ok, capacitors and resistors are good (no swapped values this time!) and everything seems fine...My thinking is that the old board I am testing with the old components may have drifted enough to get a bit loosey-goosey and more tolerant of an actual fault somewhere else.Although, I did notice that the schematic calls for two 12V zener diodes, but my PCB has two 10V zeners instead as per the parts list... The part markings in my PCB match for 10V parts, and were original parts, and they test bang on in the Zener diode tester too.I've noticed a number of errors and ommissions from the schematics so far, so I've been updating it in the PDF file as I go. I'll release the file once I have been through it enough to catch what errors I see (and generally tidy up, bookmark, etc, the PDF file too)I tested the range switch (all 1 million decks of that 3 position rotary switch!) just to make sure it is all ok, and after a small blast of Deoxit to clear up one contact that was reading a couple ohms, all contacts are now reading around 0.1 ohms (not accounting for test lead resistance) so it is now perfect.I did notice that if I try to turn the power knob from Standby to Operate, the current meter on the front panel shoots up then the unit trips out. Like the unit is hitting over-current.But if I leave the internal interlock switch open to inhibit the output when I turn the power knob to Operate, then press the interlock switch to enable the output after a second or two, the unit works perfectly.Hmmm, could be an inrush current problem with the new capacitors somewhere maybe?

author: TheDefpom
author_rank: Frequent Contributor
timestamp: January 04, 2023, 08:34:37 am
content: Quote from: TERRA Operative on January 04, 2023, 12:38:46 amAs good as the Peak testers are they don't pick up high voltage breakdown, I had an issue on my Fluke 5200A where one of the high voltage output transistors tested fine but when high voltage was put on the part it would collapse and go to low resistance...(I showed it in the video series I did), check it on a curve tracer that can make sure it meets its spec for voltage breakdowns.Don't rule out that transistor, my money is on that :-)

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 04, 2023, 10:08:30 am
content: Yeah, true that.All the smaller transistors are socketed, so I was able to swap them between the old spare good board and bad board from this 332D, still the bad board was bad. Although there is a small TO-66 packaged transistor that is soldered in so I didn't try swapping it yet.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 04, 2023, 03:08:36 pm
content: And the Defpom called it.Turns out that Q3 on the Series Pass driver is actually bad, even though the Peak DCA75 Pro thinks it looks ok.I swapped in the transistor from the Series Pass Driver board in my 335A that is next up on the bench and now the 332D is nowmostlyworking!So, time to go find a '40250' TO-66 transistor, or equivalent thereof.Now, the next issue.I can consistently get the machine to operate when in 100V or 1000V mode, but if I start in 10V mode it trips out.BUT, if I start in 100V or 1000V mode, then switch to 100V mode it operates flawlessly besides not being able to regulate output current in 10V mode so if I try to run the current limit adjustments (put an ammeter directly across the output) the dial on the inbuilt gauge immediately shoots right up and then the unit trips out. The 100V and 1000V ranges operate correctly and flawlessly.For a laugh, I ran through the adjustment procedure, just to make sure all the dials and levers were in the right place, to make sure I hadn't screwed up an adjustment somewhere so bad that it was the problem, but nope.I was able to fully adjust the unit and it's stable as bedrock, besides the two issues on the 10V setting.So, next problem to look at is the 10V setting. I'll swap a couple more boards back and forth to see if I can narrow it down a bit further.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 04, 2023, 04:46:34 pm
content: Ok, looks like the Series Pass Element Board is the one playing games now.I swapped in a spare board and it operates fine when switching from standby directly to the 10V range.Current limiting is still a bit blegh, but that may be just down to adjustment to take into account of the old Series Pass Element board I'm testing with, I'll deal with that if needed once the original board starts on the 10V range setting..I also swapped across all the socketed transistors with my fingers crossed it would be that easy, but the fault remains on the original board, so I gotta look deeper at the eight 2N3739 transistors on that board...I gotta get my Tek 577 curve tracer working (damn tantalums!).Too much stuff to fix!

author: TheDefpom
author_rank: Frequent Contributor
timestamp: January 04, 2023, 07:52:05 pm
content: Excellent, I hope you are recording a video for the repair.Don’t forget to re-cap, and check all the carbon composite resistor values have not drifted too far from their original values.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: January 04, 2023, 08:40:26 pm
content: I second that statement for a video.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 04, 2023, 11:09:50 pm
content: Yep, I think I have some video from years back, will finish it off.All electrolytic capacitors have been replaced, and all carbon composite resistors have also been replaced with 1%, 100ppm-ish metal film resistors back when I started the restoration, so they are a known good element to my troubleshooting.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 05, 2023, 11:07:54 am
content: Ok, so after poking away and testing this and that, I'm down to testing the pass element transistors (seven of 2N3739) and all eight 200V zener diodes.The Peak Zener tester only goes to 50V, so I'll have to dig out a power supply capable of 200V and some resistors for current limiting to test the zeners.My only power supply is a huge heavy rack-mount HP thing... yaaaay...As for the transistors, not sure. I'll have to figure out an easy way to test them. I have no space to pull out the Tek 577 curve tracer to fix it.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 05, 2023, 02:09:12 pm
content: For zener testing you can use cascade voltage multiplier (behind some transformer) + resistor to limit the current if needed. You can also use 2 multi-voltage transformers with same output voltage and connect secondary windings and with Japanese transformers you will have plenty options how to connect the outputs :-). What I have seen they are 2x isolated 120V with 100 V and 110 V taps.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 05, 2023, 03:09:58 pm
content: I ended up using the 332D itself.I stuck the working series pass board in and set the output to 210V. The zener diodes are 1N3051A rated to 200V, 1W, so I strung two 22K resistors in series to limit the power to about a watt and gave them a test.They were all within their 10% tolerance, the worst being -7.5-ish%, most hovering around -5%, so they are all good.Onto testing the transistors under close to full working voltage and load, gotta figure out a rig for that now.So, I an the transistors on the Peak DCA75 Pro again just for fun, and took more notice of the numbers this time.Turns out there's one that has a HFE of half of the others, and half of the rated specs... I think that is most likely to be the bad one.Some new transistors have been ordered from Europe via ebay... Now to play the waiting game...

author: factory
author_rank: Super Contributor
timestamp: January 05, 2023, 07:40:56 pm
content: For higher voltage Zeners & some diodes, I've used an insulation tester (aka Megger) with a DVM connected in parallel to the output, this is how I found some incorrect diodes fitted in place of 100V Zeners at work, breakdown voltage of the diodes was at least double.As well as being useful for checking capacitors for leakage, the Megger has also been used to find failing neons & number tubes, the failed ones require higher voltage to make them light.Since upgraded to this Sullivan Megohmmeter, not sure what the output current is, but it's more than the wee Megger can manage, shown here testing a suspect batch of tubes (most were cracked), I added a decade resistor and the DMM is measuring the volt drop across the tube.David

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 05, 2023, 11:20:12 pm
content: Oh! I didn't even think of that!I have a Fluke megger here even.It outputs from 50V to 1000V, not sure on the current, but it would probably work for a zener.

author: Roman oh
author_rank: Regular Contributor
timestamp: January 29, 2023, 08:13:05 am
content: Hi Terra Operative.Interested in your repair. I had a catastrophic failure with me series pass element - don't know what happened, but came in in the morning after the unit had been on overnight, and it was stuck with no output. Turns out two of the 200V zeners ( the first two) and five of the 2N2739s (all except the first two!) had gone to meet their maker. What excited me in your post was "I have ordered some from Europe via ebay". Can you tell me where? I have spent hours searching for suitable replacements. Thought it would be simple, as there are zillions of HV transistors around in power supplies, CFLs, etc. But these all (leaving aside the package differences, which I was willing to try to work with, if needs must) have very low hFE in the 10-30 range or lower, so are probably unsuitable.I shall watch your repair story with interest, and would really appreciate a pointer to a 2N3739 replacement source...Roman

author: factory
author_rank: Super Contributor
timestamp: January 29, 2023, 01:32:39 pm
content: Probably this listing in Europe;https://www.ebay.co.uk/itm/221751361830David

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 29, 2023, 03:17:14 pm
content: I'm going through the return process with them at the moment, bought 7 of them and half had hfe lower than the lowest spec in the datasheet, they were a mixed bag of differently stamped parts (some didn't match the others). They were also shipped stuck straight into plain foam in a plastic bag, no ESD protection at all....

author: factory
author_rank: Super Contributor
timestamp: January 29, 2023, 03:30:17 pm
content: I bet they would be 10x the price if they had tested them & chucked the duds, that's the trouble with third party suppliers, never know what you are going to get.Is there a modern part that could be used instead?David

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 29, 2023, 04:43:18 pm
content: That's my next step if I don't decide to cannibalise my spare Series Pass PCB...

author: Roman oh
author_rank: Regular Contributor
timestamp: January 31, 2023, 12:49:47 am
content: David,Thanks for the link. First thing I notice is "Doesn't post to Australia". Then TerraOperative's experience...Hmmm...might just keep looking.Thanks againRoman

author: Roman oh
author_rank: Regular Contributor
timestamp: January 31, 2023, 01:19:11 am
content: Hi again Terra Operative,Fyi, I have made it a project over several years to "improve" this beast mainly with a view to reducing its power consumption, so I wouldn't have to wait hours for it to stabilise. (No load power consumption after ref heater warmup is now around 11W, instead of the 30-40 that is was). As part of that, some years back, I "improved" the series pass by making the series string of 2N3739s into a darlington configuration using some 2N6517s in a TO92 package. This enabled me to increase those hot 22K resistors(R28-R34) feeding the bases to 220K and save a few of those watts. So the dependence on hFE of the 2N3739s was reduced. Worked reliably for a number of years, so probably not a factor in my recent mishap.Out of curiosity (since you have yours opened up) I notice that my board has, for for R6,8,10...18 and R21-26, uniformly 150K 1/2W CC resistors fitted whereas the schematic shows a series of stepped values from 56K -100K for the former, and 100K for R21-26. The board does not appear to have been modded, but hard to tell. Spice model of my configuration (both with and without the darlington mod) suggests a pretty good distribution of voltage up to 1200V in across the string, nowhere approaching the 300Vceo limit of the transistors, so I'm not too concerned. But I would be interested to know whether your boards are similarly configured.Roman

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 31, 2023, 04:51:54 am
content: I am definitely interested in your mods!I have some PCB's made way back when I started looking at this thing to replace the chopper amp with modern silicon which will be installed once I get it all working in it's original factory configuration, and I was thinking to replace the zener regulators on the right-most board (Current Limit board) with good quality linear regulators too for another easy upgrade.I have a couple versions of the series pass board, so I'll check tonight and let you know. Will take a few photos roo.

author: kawal
author_rank: Frequent Contributor
timestamp: February 03, 2023, 02:38:15 am
content: Quote from: TERRA Operative on January 05, 2023, 11:07:54 amI had a lot of issues with the same unit. The old transistors would test good in diod mode but had gain close to 1. Check the gain of the transistors!

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 04, 2023, 08:43:38 am
content: Ok, updates.I mixed and matched the pass transistors with what I had on hand to select the best ones.Now the fault is still there! It will start and work fine in the 100V and 1000V modes, but itdoesn'twork in the 10V mode....When switching from standby to operate in the 10V mode, it trips out, and if starting in 100V or 1000V mode then switching to 10V mode, it doesn't trip but it gives no output.....I really need to find a modern equivalent for these 2N3739 pass transistors so I can buy brand new parts that I know and can trust are 100% good...Ugh, I'm getting tired of this thing. I'll poke it some more before I throw it on the 'for sale as-is' pile.....

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 04, 2023, 09:51:35 am
content: Also, pictures of the two types of series pass boards I have.First is an older untouched one from my 335A, and the newer one is what I am currently using in my 332D, all carbon resistors and capacitors are replaced with new.

author: tanarchy
author_rank: Newbie
timestamp: December 29, 2023, 08:19:28 pm
content: Did you ever find a modern replacement for the 2N3739?The prices have gone up significantly for NOS