Power Transistors for old HP-6920b

[Kleinstein]

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.

[factory]

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.

The 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-B235AD7EA54D

But 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-B235AD7EA54D

Looking at a picture from my later unit, it has plastic transistors for Q19/20 and a metal can for Q21.



David

[RichardM]

Thanks David. Does Q21 get hot in your unit ?

Richard

[factory]

Thanks David. Does Q21 get hot in your unit ?

Richard

Yes 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

[factory]

Some DC readings to begin with

If 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 100Hz

I 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

The 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 50Hz

Some 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

[RichardM]

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 noise

My 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

[RichardM]

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

[factory]

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

[coppercone2]

if you take those electrolytic caps apart, sometimes its nice and off white and sometimes its brown inside, so I like replacing them

[RichardM]

Thanks

The 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.

[factory]

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

[RichardM]

Ok thanks

I 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.

[coppercone2]

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

🧽 🧼

[RichardM]

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.

[factory]

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

[factory]

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

🧽 🧼

Needs 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/#msg3788180
Though part of me wants to get it going without distrusting the rust & spider remains, like shango066 does. 

David

[coppercone2]

you can probobly sell that to a vietnamese restoration channel for more then its worth.

[RichardM]

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.

[RichardM]

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.

[RichardM]

Hi David

I 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 output

Richard

[factory]

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

[RichardM]

Thanks David

No, 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 !!

[RichardM]

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?

[RichardM]

Q14/15 transistors are fine. I replaced them with orginals and the result is the same. No output light and R4 has no effect.

[RichardM]

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