BRC/Thorn 3000/3500 PSU Module Repairs

As reported above the 6.3 & 30V rails are present, however the 58-65V rail is again missing. On the previous PSU the voltage at the junction of R607/C607 was low ( 4V), on this PSU it is high at 48V

Voltage checks on the following transistors show the delay switch and monostable look OK but the Chopper drive is again suspect. The subsequent scope confirms this too, do we have another VT605 E-C short?

 VT603

VT605

Found W612 was o/c replaced, this however did not cure the lack of 58-65V rail.

With W612 replaced I decided to remap the voltages, no effect to VT602, VT603 & VT604, the remain much the same. However as expected VT605, VT606 and VT608 voltages all change. 

I then isolated the Base of VT605 to take a voltage reading expecting it to rise to 12V, it actually rose to 25.4V.

Next was a quick recheck with the scope of VT603 ( b/c) and VT605 (b/c)

VT603

VT605

Something strikes me as odd with regards to VT605, I'm going to remove it to test, also see what the collector pad voltage sits at with VT605 out of circuit

Well I wasn't expecting that, VT605 tests OK.

With VT605 removed from circuit, the collector pad measured .022V 

I have 51V on R607/C607 Junction so I guess I need to remove and check W610, VT606 and VT603 to eliminate from suspicion.

You'll be able to repair these power supply slabs with your eyes shut soon! 

Is R607 overheating? check the junction of R607, C608 if you have 40 odd volts here and almost nothing at VT605 collector with it removed then T602 has an o/c primary which I've never had or you have a print/wiring issue. Check this before doing anything else.

John.

This evening I removed VT606 and tested, it was OK.

However what I did find were two of its pads( base & collector) had lifted and were unreliable.

The pads were removed and I decided to fit a transistor socket. The leads were extended so as to make a good mechanical fix and insulated where necessary.

I then scoped the base of VT606 to see what was going on.

Looking good, so now to see what was happening on the base of VT604, Ooops 400mV that's not good

Looking around the panel and as John rightly surmised a wiring fault had occurred, the wire from R607 - T602 had broken. Once soldered back the base of VT604 looked much better.

The 58-65 Rail restored and another PSU is working.

That concludes this one just to close up and test.

Good work Chris but please get rid of the green Plessey C607 and upgrade it to 1500uF before you put that board into service. 

John.

That'll teach me...... its official, these PSU's have it in for me!

I removed the PSU working from the test jig to close it up. PCB located carefully into position, soldered the connections back to the side. Then reconnected to the jig assembled and the little git's 58-65V rail has gone again.

So its opened up yet again spatch-cock style on the test Jig.... Ground Hog Day!!!!!!!

As they say "Gutted".

Back to square one, a quick scope onto the base of VT606 and there's no waveform. As VT606 is now socketed a test of this shows its OK. So it looks like I'm back to starting at the mono-stable yet again.

The Readings

Well the following table shows things have gone pear shaped.

The delay switch VT602 looks OK

Things go wrong at the mono-stable VT603. Also R607 is back up at 52.5V, also the Chopper drive VT605 collector, has high voltage.

I need to remove VT605 to check for o/c and VT603 will have to be removed to test. I know the other part of the mono-stable VT606 is OK as stated above its easy to remove now socketed, it tested OK. While at it I should also check W610. Hopefully the fault lies in these few components.

For now I think I will call it a day and get back to this tomorrow.

Bench time again for the cantankerous PSU

OK so despite this PSU working happily last night for half an hour or so, it decided to stick its fingers up at me once I had closed up and resoldered the PCB down. As can be seen from the table above the clue starts at VT603 with very low voltage on the collector. Scoping the mono-stable this afternoon confirms in good old Dr McCoy fashion, "its dead Jim". 

Turns out W610 a 1N4002 diode connected to the emitter of VT605 (Chopper Drive) transistor, was o/c. Installing a new diode restored normal service to the 58-65V rail.

Just when you thought it was safe to peek out from the covers

There's more BRC/Thorn PSU modules yet!

Next beasty up on the bench is an odd ball, this one has all the rails but despite this when installed in a set, the most EHT I get is 8-10kV. SET EHT fault? If I remember rightly the EHT does not track with the adjustment of R629.

Let battle commence with the third PSU

A little bench-time so the customary spatch-cocking PSU time.

This one as I mentioned above, is at least presenting a different scenario than the first two PSU's, in that all the rails are present. However as also stated and confirmed with quick check of the meter at the 'Chopper' output fuse (F602), it shows the 58-65V rail sitting at just under 55V, this is incorrect.

Rotating R629 the SET EHT preset should allow you to set this rail, the optimum setting being 61V, it does not. The preset has no effect whichever way it is rotated, this will need looking into. R629 duff? I can't be certain but I'm sure I checked this before I will remove to confirm.

Before that and to get to know the status of this PSU, I'm going to map all the voltages and scope all the important parts of the circuit. This will at least base-line where this module is.

All is not quite as its seems

Here is the voltage map of the third PSU, despite all the rails being present, all is not as well as it seems. 

Initial observations, the Chopper is down 60V though at 278V. The mono-stable ( VT603/VT606) looks out of salts. The fault again looks to be the Chopper drive (VT605), the collector is down which would explain the low chopper (VT604) voltage. 

Time to scope and test, I will isolate the base of VT605 and see what happens to the collector. Depending on that result remove and check the collector pad. There is a lot of track lifting around VT605 with some repairs.

Update:

Back at the PSU repairs after a minor diversion in the BBC test tone generator project.

I've just isolated the Base of VT605 ( Chopper Drive transistor) to see what would happen to the collector voltage, it was sat at 51V. Not what I was expecting, so it looks like the drive transistor is OK.

To be sure I removed it to test. The pad voltages were C=51V, B=25V and E -.845V. The transistor as suspected passed but the Hfe - gain of the transistor is down at 60. This was compared to a couple of new old stock whose Hfe were 84 & 99.

I wondered if the lack of gain in the drive transistor might be responsible for the missing 7V on the collector? The answer was no, replacing VT605 the voltages remained the same. For good measure I scoped the Base and Emitter and they looked like the expected waveforms in the manual.

Having established the chopper drive transistor is not at fault I'm suspecting the feedback amp VT608 and I guess from looking the table above that was evident. The collector volts are non existent and as this is used to adjust the time constant of the mono-stable its looking guilty. 

That's for tomorrow now.

Need a strategy re-think as I'm down rabbit holes chasing my own tail.

VT608 (feedback amp) is fine, I removed VT603 and VT606 (mono-stable) which were also OK so along with VT605 at least I've eliminated the silicon as the problem.

Facts:

• 58-65V rail is sat at 54V
• Feed back amp collector volts are non existent and as this I mentioned this is used to adjust the time constant of the mono-stable.
• R629 the SET EHT preset should allow you to adjust the 58-65V rail, it does not.

These are the list of candidates 

1. EHT preset R629 open-circuit
2. W618 low resistance
3. C614 open-circuit
4. Dynamic Trip control R622 open-circuit
5. R610 1R open-circuit
6. W622 Dynamic Trip outside tolerance.

Will scuttle off and report back later, hopefully its one of those 

I removed and checked all the above, all were OK. I replaced the low powered Thyristor W622 just in case it was out of tolerance as my DCA55 tester just declares its pin-outs and see it as a Thyristor. The 60V rail just stubbornly sits at 54V and cannot be adjusted.

Eliminating the above listed potential causes of low 58-65V rails, leads me back to the glaring problem of low collector volts on the 'chopper drive' transistor, the non existent collector volts of the 'feed-back amp'. Which means the time-constant of the mono-stable is out, which in turn means the 'Chopper transistor' VT604 cannot be turning on and off correctly.

But I'm out of ideas for now

Junction W615/C614 - Mark Space

VT603 - Base/Collector

VT605 - Base/Collector

VT606 -Collector

Well this module is now fixed, the low 60V rail repaired. What was it? R631 the skeleton EHT preset.

I can't take credit for this discovery as a visitor turned up today in the shape of John. I described to John what I had done thus far with regards to the fault and showed via the scope and meter. We made a few checks on suspect diodes but still no joy, the rail remained at 54V and could not be adjusted. John then enquired if I had checked the Skeleton preset, I said that I had not and had been loathed to mess with it, I could tell by the smirk on his face that was a "Big" mistake, it was o/c   :doh: replaced and the rail was up to 60V and could be adjusted.

PSU Module Number Four

OK so that's three modules repaired, on to the next.

This next one I cannot remember anything about other than the Crowbar Thyristor W621 was missing and funnily enough R631 the Skeleton EHT preset ( not  taken by me) was missing.

It looks like I had made a start on replacing some of the caps and a couple of resistors, I will install a new W621 find a 4.7K preset and then introduce it to the Rig to see where its at.

This ones going to be fun

Powered up and all the rails dead! even the 6.3, I think its tripping, I'm sure I saw the 30V line light on the Rig flash, was so quick though. Saved the best one for last