Power Transistors for old HP-6920b - Page 1
Forum section: electronics / repair

author: RichardM
author_rank: Frequent Contributor
timestamp: March 16, 2023, 12:48:46 am
content: Hi AllI could not resist purchasing this old HP calibrator for <$50 AUBefore turning it on I pulled the series power transistors which are PNP germanium hp part 1850-0168. I think they may be Bendix or HLAB branded according to the manual but after several nights searching I cannot find any specifics about them, somone indicated they were 70v high gain 5A. Two seem ok, one is dead and one looks really leaky. I was looking for advice on a suitable replacement. There looks to be a shortage of TO-3 transistors or perhaps they are fading away naturally. I have some MJ15004 or 2N2955 I could use as replacements ? They are Q8, Q9, Q14 and Q15 in the attached schematic.Any help appreciated as always.RegardsRichard

author: Kim Christensen
author_rank: Super Contributor
timestamp: March 16, 2023, 01:51:02 am
content: The best I could find wasthis infowhich is pretty close to what you found.A bit pricey, but they have2 at Sphere.cain stock. I have ordered stuff from them about 7 years ago but nothing since but all was OK that time.

author: RichardM
author_rank: Frequent Contributor
timestamp: March 16, 2023, 08:27:05 am
content: ThanksI have 2 versions of the service manual and it appears a 2N1545 may be interchangeable for the 1850-0168, at least on the lower pcb board anyway. I will try to search based this transistor also.

author: factory
author_rank: Super Contributor
timestamp: March 16, 2023, 06:30:51 pm
content: The change notes seems to suggest they used a matched pair for Q33/34 from s/n prefix 1105A, these are being used as output rectifiers.Q14/15 are the inverter switching transistors, the inverter may be damaged if the external AC reference signal is lost, the manual does mention this.David

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2023, 06:44:29 pm
content: The Q14 / Q15 part could be a bit tricky as the transistors will also see reverse operation of some kind.The Q8 and Q9 part looks like a more normal linear operation and chances are good that normal silicon types (e.g. 2N2955) should be OK too.

author: RichardM
author_rank: Frequent Contributor
timestamp: March 16, 2023, 10:24:57 pm
content: ThankyouThe manuals are a little confusing. One manual indicated Q8,9,33,34 are Bendix B1217 (upper PCB) and Q14,15 are Bendix B1203 (lower PCB). The other manual shows all transistors are HLAB 1850-0168. In circuit Q33,34 seem ok so fingers crossed.Richard

author: factory
author_rank: Super Contributor
timestamp: March 18, 2023, 09:13:00 pm
content: You need to check the serial number prefix and use the appropriate manual and/or change sheets.The 1965 manual has the Bendix parts, the 1967 manual has the HP part numbers, change sheets give the matched pair from change 7 (1105A).I've had a look in mine (1201A prefix) the matched pair (HP #06920-80093) are Motorola 2N1545, looking at the solder joints these look to be originals, note the error on the circuit diagram, the symbol should be PNP.The solder on the four 1850-0168 on the top board has been disturbed in the past, I have three Motorola 1850-0168 and one TI 1850-0168 fitted to mine.I would suggest checking the larger capacitors, some of mine are showing high ESR and/or cracked seals and could do with replacing. I seem to remember from another thread, someone mentioning the voltage rating was a bit low on one of the capacitors.Edit: more errors in the later manual, it lists C31 & C32 as ceramic caps, older manual is correct with electrolytics listed.David

author: RichardM
author_rank: Frequent Contributor
timestamp: March 26, 2023, 11:37:27 am
content: I replaced all 4 of these transistors with MJ15004, rewired a new IEC power socket and turned it on with my dimb bulb. Amazingly it works for DC volts and amps. Just need to test AC now. I purchased all new axial capacitors so I will systematically replace those now. I have noticed the ten turn pot appears to need a clean. Can this be opened up and cleaned ?Richard

author: coppercone2
author_rank: Super Contributor
timestamp: March 27, 2023, 10:50:09 am
content: post picture of potentiometer, I have not seen a HP with a ten turn on it that is unsealed IIRC. Unless there are screws on the back you are SOL I think. I have thought about cutting them open and gluing them back together with a fine saw, but that is just conjecture. If the housing is thick enough you might be able to press fit pins in there with a bit of glue and that might hold for a good while, or bend something out of sheet metal that wraps around it, like the part used to gang potentiometers, or like wafer switches (that is a sandwich between two plates, spacers with screws). I have a bad ten turn that I might cut open, but I suspect it might be difficult to reassemble.Would be nice to figure out if this is feasible actually, because they kinda add alot of price to a project compared to say two screws, some brass tubing and sheet metal.I did replace the germanium transistor on some HP supplies, it all seemed to work OK.I looked at my spare ten turn pots, one of them has three screws on the back (spectrol),the ducan brand one has 4 melted plastic studs (possibly drillable for nice disassembly), and the 4 turn bourns (that turns without hiting a stop forever) looks like the back was just glued on some how, but I suspect this has some kind of intend in it, that is if you cut it off, you would need to either fabricate a new part or add a spacer to glue the old one back on, like I mean I think it has seat for the shaft on the back maybe. But even so, you could make a mold of this part out of like JB weld and put a ring on top then file or machine it down to the correct height if you cut it flat.

author: alm
author_rank: Super Contributor
timestamp: March 27, 2023, 11:15:37 am
content: Quote from: coppercone2 on March 27, 2023, 10:50:09 amI've seen ten tun pots that had a metal clip you could bend to open (Bourns), and some (not in HP equipment) that had the front part with the shaft press fit in the plastic shell, and could be separated by inserting a sharp object between the front and the shell.

author: RichardM
author_rank: Frequent Contributor
timestamp: March 30, 2023, 11:40:17 am
content: I finally got back to the HP-6920b today. DCV and DCA output is fine but I have no real output in AC mode and the output bulb does not illuminate. Back to my hobbyist troubleshooting skills. What circuit or components are key to the AC output ? The 1V AC reference seems fine.RegardsRichard

author: factory
author_rank: Super Contributor
timestamp: March 30, 2023, 07:27:09 pm
content: Difficult to say, switch 1 middle position (AC) is connected to many places, the demodulator seems only to be used in AC (bypassed in DC) though.David

author: RichardM
author_rank: Frequent Contributor
timestamp: March 30, 2023, 09:45:38 pm
content: Thanks David. Yes I find the schematic and use of so many transformers hard to follow. Also knowing where circuit ground is. I generally start to check diodes and transistors. Are Q33/34 used for both AC and DC ? I thought so.

author: factory
author_rank: Super Contributor
timestamp: March 31, 2023, 02:46:51 pm
content: No Q33/34 are only used as rectifiers for the DC mode, again note the middle position of switch S1 bypasses these transistors, lower half of the output transformer winding is left open circuit.It would have been nice if the Harrison division had added extra diagrams for the AC & DC modes, with the relevant parts highlighted, similar to the flow diagrams found in some other manuals.The ground reference for measurement looks to be -S on the diagram, this appears to be the chassis ground and in the first four positions of the range switch S2 is also connected to the - output jack.David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 04, 2023, 02:53:11 am
content: I really don't know what I should be observing from the demodulator circuit ? DCV and DCA working fine. All small transistors check ok in circuit. I do see a square wave at collectors of Q14 and Q15 but not 10V amplitude that is shown in one version of the service manual. Love to know what another owner of a 6920b may observe. Maybe the replacement MJ15004 transistors are not suitable for the AC ? Although I see 1V AC reference is fine.Cheers

author: RichardM
author_rank: Frequent Contributor
timestamp: April 04, 2023, 10:10:47 am
content: Another observation. The AC output is completely different (still incorrect) depending if I power the 6920b on (a) 115V directly from the variac or (b) through my dimb bulb tester (buld not lit) which is powered from the same variac ?

author: coppercone2
author_rank: Super Contributor
timestamp: April 04, 2023, 11:41:00 am
content: did you try to load the output, it says it does 5 amps on AC.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 04, 2023, 12:04:14 pm
content: No I haven’t. What would loading the AC output show ?

author: coppercone2
author_rank: Super Contributor
timestamp: April 04, 2023, 12:04:53 pm
content: not sure, but if changing the input impedance does something maybe the output impedance might do something more obvious

author: coppercone2
author_rank: Super Contributor
timestamp: April 04, 2023, 12:06:00 pm
content: I wired one of those germanium transistors backwards in something HP (the one that have wire soldered to the lead) and it was doing funky stuff related to power levels IIRC, when the output got over 10mA or so.

author: factory
author_rank: Super Contributor
timestamp: April 04, 2023, 05:57:37 pm
content: Quote from: RichardM on April 04, 2023, 02:53:11 amYes I could check waveforms at Q14/15 (dependant on spare time at weekend), any suggestion of front panel settings? to have them set the same for comparison.I'm sure the AC output is a crude sinewave, as it's referenced to the mains waveform.Quote from: RichardM on April 04, 2023, 10:10:47 amHow many volts are being dropped across the bulb? even if not visibly lit it will still drop voltage, hopefully not too much though.David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 04, 2023, 11:11:28 pm
content: Thanks DavidI will compile a table with measurements and if you were able to check just a few against your unit that would be hugely appreciated.I get a nice square wave on the collectors of Q14/15 in DC mode but the amplitude is less than half the 10V outlined in the service manual. I did change Q14/15 to MJ15004 transistors as one was dead. If I then switch to AC mode then I see a distorted square wave with the same amplitude that looks nothing like the sine wave in the manual. The amplitude at Q28/29 is also down in amplitude.My unit has the 005 option for 50hz mains but is still wired for 115V. 50Hz is compatible with Australia but we have 230V mains hence why I am using a variac. When the AC output is ON and I adjust the variac output by as little as 1VAC I see an immediate and sometimes larger change at the output ?Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: April 05, 2023, 11:57:00 am
content: Some DC readings to begin withIf I set the output dial to 1000, Voltage range 1V, DC selected I get the following(1) Q14/15 Collector, Output=off, square wave -0.5V to +0.5V or 1V p.p 100Hz(2) Q14/15 Collector, Output=on, square wave -5V to +5V or 10V p.p 100Hz(3) Q30/31 Collector, Output=off, sine type wave +2.5V to +7.5V or 5V p.p 50Hz(4) Q30/31 Collector, Output=on, sine type wave +7V to +12V or 5V p.p 50Hz(5) Q28/29 Collector, Output=on, square wave +4.5V to +9.5V or 5V p.p 100HzI have also noticed after a touch test that Q21 gets scorching hot. It is a metal can but Q19/20 are plastic transistor TO-92 style. Should it be getting that hot ? Bit suspicious ?Richard

author: Kleinstein
author_rank: Super Contributor
timestamp: April 05, 2023, 04:53:59 pm
content: IN the old days they did run some transistors quite hot - at least silicon transistors, not so much with germanium types.If you can still touch them without burning the finger it is probybly still OK.It is hard to tell from the schematics fragment how much power Q21 would normally get, but it could be quite a bit as the voltge ratings of the capacitors look high.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 06, 2023, 03:36:25 am
content: Ok, thanks. It is hot enough to burn if you held your finger on it. It's in the AC reference circuit and I can see the 1VAC output fine so I guess that Q21 must be OK. Does anyone know what transistor could replace a 4JX16B533 ? Cannot seem to find a datasheet for this NPN which is supposed to be for Q19,20 and 21. Q19 and 20 are plastic packages in my unit.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 06, 2023, 06:34:39 am
content: Q19-Q21 don't look that critical. It is a bit unclear at what voltage they operate. This also determines the power that Q21 sees. So one could be OK with small signal types like 2N3904 or better use slighlty larger ones like 2N5551 or 2N2219 or maybe even MJE340 or BD139. With only 1 VAC there should also be no issue with temporary highly negative VBE.It is possibly they changed (at the factory or as a repair) Q21 because of the heat issue.

author: factory
author_rank: Super Contributor
timestamp: April 06, 2023, 05:55:15 pm
content: Quote from: RichardM on April 06, 2023, 03:36:25 amThe 4JX16B533 are HP part #1854-0027 according to the later manual, some data here;https://www.parttarget.com/5961-00-924-4465_5961009244465_5961009244465.html/-79EDB718-0DA2-48B7-9516-B235AD7EA54DBut Q21 has also been changed in the later manual for HP part #1854-0244, oddly there is no mention in the backdating/change notes for this part.https://www.parttarget.com/5961-00-431-6829_5961004316829_357139.html/-79EDB718-0DA2-48B7-9516-B235AD7EA54DLooking at a picture from my later unit, it has plastic transistors for Q19/20 and a metal can for Q21.David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 07, 2023, 01:22:25 am
content: Thanks David. Does Q21 get hot in your unit ?Richard

author: factory
author_rank: Super Contributor
timestamp: April 10, 2023, 04:10:54 pm
content: Quote from: RichardM on April 07, 2023, 01:22:25 amYes it does, I left it on for 5 minutes and it's too hot to touch.I've taken some readings, will post them later.David

author: factory
author_rank: Super Contributor
timestamp: April 10, 2023, 06:12:27 pm
content: Quote from: RichardM on April 05, 2023, 11:57:00 amThe same tests on mine, also a 230V 50Hz one, it came from Israel over 10 years ago, couldn't find any in the UK at the time, not seen many since either.(1) Q14/15 Collector, Output=off, square-ish wave -0.1V to +0.1V or 0.2V p.p 50Hz(2) Q14/15 Collector, Output=on, square wave -5V to +5V or 10V p.p 50Hz(3) Q30/31 Collector, Output=off, sine-ish type wave +2.4V to +8.6V or 6.2V p.p 50Hz(4) Q30/31 Collector, Output=on, sine type wave +7.6V to +13.8V or 6.2V p.p 50Hz(5) Q28/29 Collector, Output=on, square-ish wave +3.4V to +10.4V or 7V p.p 50HzSome slight differences and where you had 100Hz, I had 50Hz for all measurements.Some extra measurements of test 2 & 5 in AC output mode, last picture is the 1V output on the front panel.P.S. this will be recapped as some point in the future, just got a bit of a wait for some that are on backorder.David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 11, 2023, 10:06:22 am
content: Thanks very much David. I will have a look at my waveforms and compare them to yours. One thing I do know is I see nothing like that at the from panel. Just random pulses and noiseMy unit has the 005 50hz option but runs at 115VAC. Perhaps I should rewire the transformer for 230V, probably not the issue I guess.Do you thinks any of the film caps are at risk of failure. Many measure fine but several 0.01uf measure oddly in circuit.Cheers

author: RichardM
author_rank: Frequent Contributor
timestamp: April 11, 2023, 11:12:59 am
content: This is how T3 is wired in my unit for 115Vac. The jumpers should be removed between 1A and 1B, and 2A and 2B, and 1B and 2A connected for 230Vac. Is this what you see David. I would hate to destroy my transformer

author: factory
author_rank: Super Contributor
timestamp: April 11, 2023, 07:28:30 pm
content: Yes that is how mine is wired, I've changed quite a few other items of TE with split primary transformers from 115V to 230V, sometimes without the help of a manual, not had any problems so far, measure the winding resistances if not sure, don't forgot to change the fuse to 500mA for 230V operation.It's the electrolytic capacitors that are due changing in mine, most of the large ones have cracked seals, some also have poor ESR, some of the caps were out of stock, hence it might take a few months to get them. The small axial tantalum caps with be kept, they are usually very high quality & quite pricey. The film caps shouldn't need replacing unless you find a bad one.David

author: coppercone2
author_rank: Super Contributor
timestamp: April 11, 2023, 09:36:53 pm
content: if you take those electrolytic caps apart, sometimes its nice and off white and sometimes its brown inside, so I like replacing them

author: RichardM
author_rank: Frequent Contributor
timestamp: April 11, 2023, 10:13:28 pm
content: ThanksThe capacitors I am referring to are the ones in orange shrink wrap. Are these tantalum ? My manual says they are film caps, look like axial Sprague Type192p Pacer polyester film caps ? Excuse my lack of knowledge.

author: factory
author_rank: Super Contributor
timestamp: April 12, 2023, 06:56:14 pm
content: No those orange caps are definitely film, I've not had any problems with those in other HP TE.The small axial tantalum are the type with a metal casing & glass seal, these are usually very reliable, very rarely do I find one that has shorted.As for the electrolytic caps, these are being changed because of the cracked seals, but only one had actually leaked out when I acquired it (C10, replaced years ago). Currently only two test a little high in ESR (in circuit), they are C17 & C27.I will take ripple measurements before & after replacing the rest (when they arrive). As I want to know if the modern ones test better or worse. It was the latter for another piece of HP TE that I own two of, the new Japanese caps tested worse than the 45+ year old ones, YMMV.I don't normally do full recaps, others do though.Attached are some pictures of the Harrison spec transistors, that are used in the older 6920B, these are made by Bendix & Motorola, pictures from a very poor condition Harrison PSU I have.David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 12, 2023, 10:38:58 pm
content: Ok thanksI did a recap of most electrolytics on the top board. I am kicking myself I did not take a ripple measurement beforehand. Some of the removed caps had >100ua leakage but I am never sure whether that is a bad number of not. I guess I could try to find an old datasheet. I suspect a number are still fine. I am interested in your ripple measurements for sure. I see quite a bit when in the >1V range.I will post a few images of my Q14/15 measurements tonight AUS time. I do not get a sine wave on the collectors like above. Just a distorted square wave similar to the one in DC mode.

author: coppercone2
author_rank: Super Contributor
timestamp: April 13, 2023, 12:55:55 am
content: Quote from: factory on April 12, 2023, 06:56:14 pm🧽 🧼

author: RichardM
author_rank: Frequent Contributor
timestamp: April 13, 2023, 11:07:51 am
content: First 2 images are what I see on the base of Q14/15 which are for AC (1) and AC (2). 3rd and 4th images are for AC (5 off) and AC(5 ON), and final image is AC (5) Output on from terminal. Pretty ugly.

author: factory
author_rank: Super Contributor
timestamp: April 13, 2023, 06:52:35 pm
content: Could this be a case of the new MJ15004 transistors oscillating? i.e. the high frequency part shown by the scope, also the scope is getting confused with the frequency readout, a 20ms period does not equal 100Hz, should be 50Hz.But it does seem to be more of a problem with Q15, could do with seeing both Q14/15 at the same time.David

author: factory
author_rank: Super Contributor
timestamp: April 13, 2023, 07:10:07 pm
content: Quote from: coppercone2 on April 13, 2023, 12:55:55 amNeeds much much more than that, more like shot-blasting & a repaint for a start, see pictures here for more;https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg3788180/#msg3788180Though part of me wants to get it going without distrusting the rust & spider remains, like shango066 does.David

author: coppercone2
author_rank: Super Contributor
timestamp: April 13, 2023, 09:23:27 pm
content: you can probobly sell that to a vietnamese restoration channel for more then its worth.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 13, 2023, 10:14:39 pm
content: I do have 2 of the 4 original transistors working so I could swap Q14 and Q15 back to these and see what that does. Just a pain to desolder. I am starting to suspect Q5. In AC mode shouldn’t Q5 and Q6 provide the same collector output ? My Q5 collector output is different to Q6. Q6 is a square wave but Q5 has the negative square portion dipping back to 0VDC.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 17, 2023, 12:25:00 am
content: After a bit of additional probing and turning the outputs on and off I now have no correct output in any mode..... sigh..... DCV and DCA were working bit now the output light does not illuminate.I will check all supply rails again but maybe it's the output toggle switch that's the problem. I note after a google search someone else had an issue with oxidation on this toggle switch.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 17, 2023, 10:06:49 am
content: Hi DavidI am hoping you may be able to take another measurement for me. I am trying to establish if I have an issue with the power toggle switch. If I measure the resistance between point 3 and 4 on the opposite sides of the top pcb edge (same as between junction of R15/16 and R94/95) I get 130k ohms. I was expecting to measure as open ?I may have shorted Q14/15, no idea how that could have happened. If I measure between chassis ground and the emitters of Q14/15 of < 1 ohm it appears ?I just wasn’t Happy with with no functioning AC, now I have no sensible outputRichard

author: factory
author_rank: Super Contributor
timestamp: April 17, 2023, 08:08:59 pm
content: Yes I can check that for you.The output light being off is possibly related to a shorted Q14 or Q15 and the current limit circuit has triggered.Did you figure out where the high frequency noise, on the switching transistors was coming from?David

author: RichardM
author_rank: Frequent Contributor
timestamp: April 17, 2023, 09:11:21 pm
content: Thanks DavidNo, I was probing around trying to trace the oscillations when the unit stopped working. I hope all the transformers are ok. If Q14/15 are shorted I will be worried as I don’t know how that could have occurred, unless the probing recommended in the manual has led to this. Anyway, I will pull them out and may as well put the original transistors in. TO3 transistors are expensive in AUS !!

author: RichardM
author_rank: Frequent Contributor
timestamp: April 17, 2023, 10:07:47 pm
content: I also note the output level Pot R4 makes no difference to the output. It does not appear shorted though. Perhaps it is bypassed when there is an over voltage or over current condition?

author: RichardM
author_rank: Frequent Contributor
timestamp: April 18, 2023, 09:45:59 am
content: Q14/15 transistors are fine. I replaced them with orginals and the result is the same. No output light and R4 has no effect.

author: RichardM
author_rank: Frequent Contributor
timestamp: April 18, 2023, 11:05:31 am
content: OK, Q10 was dead. I am back to where I was with DCV and DCA both working but ACV and ACA are not working. I now have no ocillations on Q14/15 when in AC mode hooray, that may be because I swapped the orginal transistors back on or because Q10 is now replaced.So, back to the question as to why my ACV is not working.Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: April 20, 2023, 10:23:28 am
content: I am a not having much luck getting ACV workingWhat is primarily responsible of turning the square wave (DVC Mode) on the Collectors of Q14/15 into the sine wave that should be observed when swutching S2 to ACV Mode ?Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: April 23, 2023, 12:29:43 pm
content: A few more scope meaurements all in AC mode1. Capacitor C6 Negative side, output off2. Capacitor C6 Positive side, output off3. Capacitor C6 Positive side, output on4. Q1A/B Base, output off5. Q1A/B Collector, output off6. Q1A/B Emitter, output off7. Q1A/B Collector, output on

author: RichardM
author_rank: Frequent Contributor
timestamp: May 19, 2023, 03:07:19 am
content: OK, I have been suffering in silence trying to repair my 6920b. I am hoping my last desperate post will provide some assistance.Originally it was working for DCV & DCA and not working for ACV/ACA after replacing Q8/9/14/15 with MJ15004. Unfortunately it stopped outputting anything sensible at some point and I narrowed it down to Q10 which decided to fail. Q10 was an old GE transistor N272 (not 2N272). One replacement possible is a 2N3417 which I have used as a replacement, but the unit now has the following symptoms:- The DCV/DCA output (unloaded, apart from multimeter inputs) is correct on all ranges when the front panel dial is greater than about 0300 (scale goes up to 1000). Below 300 the output remains unchanged (ie 3V on 1V range, 30V on 10V range approx) and the output globe dims before fading completely.- I can extend the range somewhat below 300 if I place a load on the outputs, although its not consistent in the lowest volatge range (0.1V).- I have tried all manner of transistors for Q10 with no luck, same behaviour.- I have replaced Q14/15 with 2 working N272 transistors, no luck, same behaviour.- I have tried removing Q13 which is the current limiting circuit, no luck same behaviour.- I have removed and checked Q1/Q2/3/4/5/6/11/12/30/31 in a curve tracer, all appear fine.- R4 also appears fine and linear across the 500ohm range- I replaced CR1/2- CR3 is fine- I replaced R31-35, some were out of spec somewhat.- Q8/9 are still MJ15004 but I do not suspect these as the unit was working previously with these in place.- I can see the 1V Ac and DC reference voltages, other voltages also appear fine.- I notice the following(1) When the output dial is > 300, output is OFF (and correct value output of 0V), the base of Q10 is approx 3V, collector is about 2.3V, and the Emmitter approx 2.3V. When output is then enabled (and correct voltage output), the base of Q10 is pulled from 3V to 0V approx, collector remains about 2.3V, and the Emmitter is pulled to -0.7V approx.(2) When the output dial is < 300, output is OFF (and correct value output of 0V), the base of Q10 is approx 3V, collector is about 2.3V, and the Emmitter approx 2.3V. When output is then enabled (and incorrect value output), the base of Q10 is not fully pulled from 3V to 0V approx and it is a square wave flicking between 0-3V, collector also is not pulled fully down and is a square wave flicking between -0.7 to +2.3V, and the Emmitter is incorrect also. Well below 300 on the front dial (output is unchanged) and the base (ie. 100) the base of Q10 stays at 3V constant, the collector & emmitter remain at 2.3V constant.(3) When the front dial is set to 0000, I see about 3V on the positive side of C6 when referenced to the chassis. As I understand this should be zero with the dial at 0000 ? Why is there this 3V offset? Also note, my first C6 replacement appears to have died which is suspicious. It was brand new. If I remove C6 I still see some voltage at C6+ (<0.4V) and I can then turn the front dial below 0300 and see a change in the output, its not that accurate but at leats it changes as R4 is turned.I am sure to have missed something or have a typo above but any help would be greatly appreciated.CheersRichard

author: factory
author_rank: Super Contributor
timestamp: May 19, 2023, 07:49:34 pm
content: Some equivalents to the Harrison/HP PSU transistors listed here;https://www.antiqueradios.com/forums/viewtopic.php?p=3644649#pN272 = 2N1304 or 2N445A are possible alternates, if using silicon then biasing needs looking at. Change 19 refers to the Si 2N3417, changing R34 and an extra capacitor.Neither side of C6 is connected to the chassis, could the no load voltage be exceeding the capacitor voltage rating? I've seen this before. Could check the voltage across mine with a DMM, need to check the ripple with a scope but will use the Tek THS as it has fully isolated inputs, don't want to short Q8/9 to ground via C6 negative side.I've got a PEG227 1600uF at 63V waiting to replace C6, all the other caps I ordered are still on backorder, which keeps getting extended, currently at least mid August to get all the others.I'm wondering where the higher frequency noise is coming from in the scope trace in your previous post (trace #3), could this be oscillating Si power transistors? I note they never changed any of those from Ge, the changes go to 1981, but some of the small transistors did get changed to Si, along with other parts.Seems to be a typo here, should be 0.3V with 1V range and 3V with 10V range.Quote from: RichardM on May 19, 2023, 03:07:19 amThe lamp dims with the overvoltage protection according to the instructions. I can't comment how dim it should be in normal operation because mine had the lamp assembly completely missing, I had to fit what I could find, which is quite dim in normal use.David

author: RichardM
author_rank: Frequent Contributor
timestamp: May 21, 2023, 04:12:45 am
content: Thanks DavidI would like to know if you measure zero volts approx at the positive side of C6 to chassis with dial set to 0000. I see about 3V.I have removed the oscillations I believe, I was playing around with different transistors for Q2,3,4,5 but have put the originals back now.I am unsure how to approach the changes listed in thr 1965 and 1967 manuals. Do you just pick and choose the changes around the component you are looking to change?The fact that the unit works when the dial is above about 300 has me pulling hair out. Perhaps it is all related to Q10 and biasing changes needed. I have some 2N3417 to try.Richard

author: factory
author_rank: Super Contributor
timestamp: May 22, 2023, 07:16:23 pm
content: OK I checked the positive side of C6 with ref to the chassis, in AC mode, set to 0000, with the output off I get -856mV and with the output on I get 0V.Also checked the voltage & ripple on C6 with the Tek THS720A scope (fully isolated inputs). DC voltage was 25V, ripple with output off was 30mV and output on was 900mV.I've also checked the other capacitors for voltage & ripple, don't have enough time to add all the results tonight, but I can tell you the results agree with the Peak ESR meter, C17 is dead, it has 18V of ripple.C16 & C17 have approx 52V across them, this is just above their rated voltage.David

author: RichardM
author_rank: Frequent Contributor
timestamp: May 23, 2023, 11:39:13 am
content: Thanks very much David. That confirms my suspicion about the 2-3V offset measured on the positive side of C6. I have ordered some 2N1304 at great expense from overseas but I don’t think that is the issue. I will have a look at the ripple and your other measurements. I may also try another power transistor in place of the MJ15004. It is lime I have something leaky connected to the positive trace connected to Q6.Richard

author: RichardM
author_rank: Frequent Contributor
timestamp: May 24, 2023, 11:27:44 am
content: A couple of measurements from my 6920b using my Siglent scope. I am not convinced I know how to really measure ripple accurately.Only DC mode at the moment as AC is not working at allDC Mode:Picture 1 is negative side of C6, 0000 setting, DC off, AC Coupled. Shows 1.28V ripplePicture 2 is positive side of C6, 0000 setting, DC off, AC Coupled. Shows 464mv ripplePicture 3 is negative side of C6, 0000 setting, DC off, DC Coupled. Shows -21.8V minI see about +2 - 2.5 VDC on positive side of C6, 0000 setting, DC off as presviously mentionedRichardDC Mode:Picture 1 is negative side of C6, 0000 setting, DC off, AC Coupled. Shows 1.28V ripplePicture 2 is positive side of C6, 0000 setting, DC off, AC Coupled. Shows 464mv ripplePicture 3 is negative side of C6, 0000 setting, DC off, DC Coupled. Shows -21.8V minI see about +2 - 2.5 VDC on positive side of C6, 0000 setting, DC off as presviously mentionedRichard

author: RichardM
author_rank: Frequent Contributor
timestamp: June 07, 2023, 11:00:46 am
content: Ok, my germanium 2N1304 arrived from the other side of the world. Unfortunately, and unsurprisingly, it made no difference to the DCV output issues described above. At a dead end

author: RichardM
author_rank: Frequent Contributor
timestamp: June 20, 2023, 10:54:33 am
content: Hi AllVery grateful for the help so far. It looks like I am at a bit of a dead end. Am I able to post my issue on another outside forum (ie one specifically for this brand of test gear) ? Or that considered "cross-posting".Richard

author: factory
author_rank: Super Contributor
timestamp: June 20, 2023, 06:23:07 pm
content: It's probably a good idea to try another forum, more help is needed. Nothing in the rules about cross-posting, it seems to be the opposite to normal here;QuoteQuote from: RichardM on June 07, 2023, 11:00:46 amYou've at least ruled out that transistor being a problem. Once you do find out the fault & repair it, you could always revisit the later component mods if you wanted to.David

author: RichardM
author_rank: Frequent Contributor
timestamp: June 20, 2023, 09:44:46 pm
content: Thanks David. And thanks for helping me get this far. I will report any changes here if I get lucky 😀

author: RichardM
author_rank: Frequent Contributor
timestamp: June 27, 2023, 12:36:19 am
content: Ok, so once again my issues were self inflicted.I rewired the T3 transformer for 230V in Australia and that seems to have caused my DC issues. My unit has the 005 Option for 50Hz so I figured it should be fine to change over. Not the case.I am not sure what else needs to be changed to enable 230VRichard

author: factory
author_rank: Super Contributor
timestamp: June 27, 2023, 07:20:54 pm
content: I've read the comments on HPAK groups as well, the wiring for 230V seems the same as mine, can't quite read the T3 transformer part number of your 6920B (mine is 9100-2161), strange how it only works with low line voltage (lower than specced in the manual), seems you still have two separate faults. Line voltage is typically quite high at 245V here.Alignment for 50Hz operation shouldn't be needed, unless it was set for 60Hz operation when running on 110V, but then again we don't know the history of it, or if it's been incorrectly adjusted in the past.The voltages on C16/C17 have been found to be exceeding the 50V rating on several units now, mine are both over, at 52V for C16 and 51V for C17. Back-ordered replacements (Kemet PEG with 63V rating) are now on the way to Mouser, estimated delivery to me is July 13th.David

author: RichardM
author_rank: Frequent Contributor
timestamp: June 27, 2023, 11:45:30 pm
content: Thanks DavidMy transformer has the same part number, 9100-2161.I feel like this circuit is very "sensitive" to parts and inputs, just my hobbyist speculation.Its funny, if the DCV output is turned on, I can just back off the variac voltage a few volts (100VAC to 95VAC) and "BAM", the output corrects itself and stabilises. The same was true when wired for 110V or 230V. When in 230V mode the input voltage needs to be around 170V minimum to see the correct DCV output.Richard

author: factory
author_rank: Super Contributor
timestamp: July 02, 2023, 09:45:41 am
content: If you change Q14/Q15 back to silicon, compare the output waveforms for distortion/oscillation before & afterwards, there has to be a reason way HP didn't change those to silicon, in all the production changes, that go into the 1980s.David

author: RolandK
author_rank: Regular Contributor
timestamp: November 16, 2023, 06:56:12 pm
content: I am just digging in my 6920b:For the 6920B there are 2 manuals available:one from 1965 in good quality incl. schematics, but parts are missing, eg. troubleshooting and oscillograms.the other from 1967 from keysight in bad quality, but with troubleshooting and oscillograms. These parts are worth reading. The schematic is bad to read. I found only a difference for the external reference connection.Under manual changes in the 1967 is the wiring for the 110/220V transformer. When changing from 110V to 220V only change the bridges at the main transformer from 1a-2a and 1b-2b to only 1b-2a. Leave the other connections where they are. The transformer T1 uses the 110V from the main transformer.If the 6920 has the A1, A2, A3 clamps on the backside check A2 is connected to A3. (1965 style)If the 6920 has A1 and A2 check the switch beside is to internal. (1967 style) Else you don't have the AC Reference. Between A1 and A2 should be 15 V AC.Only the AC Reference circuit and the demodulator seems exclusive for AC Mode. All other parts are used for AC and DC. If your DC works, look at these 2 first.The AC mode is a bit very tricky to understand:1. there is the AC Reference CKT. Its Output is a positiv and negativ going sinus with 1V.2. the error amplifer compares this with the output. It has normal and inverted output. They are sampled alternate by the demodulator for each half sinus. So the control signal from the demodulator is a rectified signal proportional to the difference (its a error signal, not a rectified sinus).3. the series regulator delivers a rectified sinus power signal. This has the advantage, that the series regulator does not need high- and lowside drivers. But you need to switch from AC to rectified AC in front (demodulator) and from rectified AC back to AC behind (inverter).4. this rectified sinus is sampled by the inverter to the transformer T4For the understanding:the error amplifier directly compares the output to the AC reference and steers the series regulator to produce exactly the same output signal as the AC reference. There is no amplitude control of an oscillator or anything.The signal changes from AC to rectified AC before the series regulator. After the series regulater it is changed back to AC in T4 by the inverterThe steering signals for the demodulator and inverter are the same rectangle signals (referenced to the + side of the series regulators power supply)Measurement in the series regulator is not easy, as the system GND is connected to the output of the regulator, which makes controlling the output transistor much easier.But: the inverter drive ckt is reference to the + side of the series regulator, for easy switching of the inverter transistors. This is the reason for the transformer T2 in the demodulator, as the demodulator samples the error amplifier signals, which are GND referenced.

author: RPP
author_rank: Contributor
timestamp: February 01, 2024, 02:58:52 am
content: I just started working on a pair of 6920Bs so I'm soaking up the collective wisdom of multiple forums. As I read through this thread, I found a couple of references to oscillation that I think was address by "GDEMU" in the following link:https://gdemu.wordpress.com/2020/10/04/project-time-2/He found that adding a couple of capacitors in the right place eliminated the oscillation he was experiencing. I'm not deep enough into mine to understand what he did and why it worked, but something to keep in mind, I suppose. I'll post as I make progress. I'm still cleaning up the boards and switches and will go through the transistors, diodes, capacitors and resistors to make sure they measure OK. More to follow. . .Rich

author: factory
author_rank: Super Contributor
timestamp: February 01, 2024, 06:52:39 pm
content: The OP fixed the oscillation issue, it was related to replacing some Ge power transistors with Si, Q8 & Q9 need to be Ge;https://groups.io/g/HP-Agilent-Keysight-equipment/topic/99673944#137442David

author: RPP
author_rank: Contributor
timestamp: February 01, 2024, 09:20:05 pm
content: Thanks for the link. . . More info to consume and another Groups.io group to joinRich

author: RPP
author_rank: Contributor
timestamp: February 08, 2024, 03:40:16 pm
content: Quick update to close the loop. I now have a fully functional 6920B. I found that CR4 tested open, so I replaced it with a 1N4004. All of the other transistors and diodes tested good in mine so I fired it up on a dim bulb tester and DC worked, but AC did not. As it turns out, the DBT was reducing the voltage to a point where the AC circuit wouldn't start. Once i increased the bulb to 100W, it came to life. A couple of thoughts for anyone working on one of these in the future:- My unit is a newer one with a C13 connector for AC mains. The dataplate had been removed so I don't know what the serial number was but the changes through 11 have been applied.- CR51 tested as shorted in circuit but was fine when removed. Try it with the function switch in a different position if yours tests as shorted before you pull it.- The multi-turn pot was intermittenly working on mine, so I removed it, opened the cover where the wires go into the body and sprayed it with DeOxit D5. After cylcing it a few times, it was reading correctly, so I blew it out with compressed air and reassembled it.- All of the large electrolytic caps in mine had cracked/bulging end caps even though they tested OK for capacitance and ESR, so I replaced all of them. The tantalums stayed.- C6 (1400uF/30V) C10 & C24 (1200uF/20V) were replaced with 1400uF/50V axial capacitors.- C13, C16, C17, C21 & C27 were all replaced with 100uF/50V.- C14 & C20 were replaced with 22uF/50V.- C22 & C30 were replaced with 4.7uF/450V (that's what i had . . . the voltage rating doesn't need to be that high).- C31 was replaced with 220uF/16V.- C32 was replaced with 4700uF/16V.- I added the 2 capacitors mentioned in my 1 Feb 2024 post and AC voltages are stable/reliable so maybe they are beneficial. I didn't test without them to compare, though.Calibration was done using a Keithley 196 and everything was fairly straight-forward. I used the procedure from the 1967 manual and the only vague part was the first step - inverter phasing. It's not a very precise adjustment but it seemed to dial in OK. The Balance Amplifier adjustment has you measuring between test points 11 and 12 which aren't specifially called out on the schematic but the circled 'external wire' numbers seem to also be the test points from what I can determine since 11 and 12 are part of the balance amplifier circuit. I may go back and try the inverter phasing adjustment from the 1965 manual since that seems to be a better method.The Calibrator is surprisingly accurate across all ranges and functions and this was a fun project. I can't seem to upload photos so I'll try that separately . . . .Rich

author: RPP
author_rank: Contributor
timestamp: February 08, 2024, 04:09:05 pm
content: Photo 1:

author: RPP
author_rank: Contributor
timestamp: February 08, 2024, 04:13:56 pm
content: Photo 2:Photo 3: