Thorn 3000 PSU Repair Blog

First of all so as to help understand the operation of the PSU, here’s what I’ve learnt from others and the manual.

At switch on
  • The half-wave rectifier W601 produces the 240V rail.
  • As a result, the 30V zener W605 comes into operation, biased by R608.
  • The 30V series regulator transistor VT601 can then come into operation.
  • Once the 30V rail has been established, the line oscillator will start up supplying the trigger pulses to drive the mono-stable circuit. So the last supply to appear is the ‘Chopper’ regulated rail.
  • No 30V rail means no ‘Chopper’ drive, therefore the delay switch transistor VT602 makes this doubly certain. Unless W605 conducts, VT602 remains ‘cut-off’ and the mono-stable cannot operate since VT603’s emitter is virtually open circuit.
  • Therefore if the 30V zener or VT602 or VT601 are o/c or C609 s/c there will be no ‘Chopper’ drive, no ‘EHT’, no ‘Field scan’ or ‘sound’
The 240V supply rail

W601, C602, C603, R601, R602 (fusible), R608

All these bar the last one create the 240V supply. R608 feeds the zener that makes the 30V supply

The 30V supply rail

R606, R615, VT601, C627, R608, W605, VT602, C607, C609

All these create the 30V supply

The 58-65V Supply rail
  • The 58-65 supply is achieved via the chopper transistor VT604 with its inductive load reservoir L603
  • VT604 is switched on and off at line frequency by a square-wave generated by a mono-stable multi-vibrator VT603/VT606
  • When VT604 is switched ‘ON’, energy is stored in L603
  • When VT604 is switched ‘OFF’, the efficiency diode W616 switches on and the current flow in the load is maintained.
  • Stabilisation is achieved by varying the on/off time of the chopper transistor, varying the mark-space ratio of the waveform produced by the mono-stable
  • The output voltage is sensed by the feedback amp VT608, whose collector voltage is used to adjust the time-constant of the mono-stable circuit

Let battle commence

Up on the bench first is one I started a while back but gave up on when I could not get the 60V rail up. Having just fixed the similar fault on the 2703 with it all fresh in my mind, here goes the next one.

First I will map some voltages, scope the Mono-stable and the Chopper Drive. 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?

Thorn 3000 PSU Repair Blog 1


Transistor & Function Emitter Base Collector
VT602 (delay switch) .001 .847 .027
VT603 ( mono-stable) .766 1.38 9.87
VT604 (Chopper) .033 .003 296
VT605 (Chopper Drive) .696 1.37 .696
VT606 ( mono-stable) 1.37 2.01 2.17
VT608 ( feedback amp) -.164 .058 .402

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.

Transistor & Function Emitter Base Collector
VT602 (delay switch) .001 .843 .027
VT603 ( mono-stable) .768 1.31 10
VT604 (Chopper) .048 .009 286
VT605 (Chopper Drive) .055 .722 .057
VT606 ( mono-stable) .721 1.52 1.35
VT608 ( feedback amp) .060 .002 .206

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

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.

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.

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.

Transistor & Function Emitter Base Collector
VT602 (delay switch) .001 .832 .038
VT603 ( mono-stable) 1.03 1.83 1.69
VT604 (Chopper) .041 .004 306
VT605 (Chopper Drive) 0 .120 53
VT606 ( mono-stable) .778 26.5 25.7
VT608 ( feedback amp) .005 0 0

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 next 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.

Transistor & Function Base Emitter Collector
VT602 (delay switch) .872 .002 .086
VT603 ( mono-stable) 1.4 .778 9.68
VT604 (Chopper) 54.9 54.9 278
VT605 (Chopper Drive) 1.7 .799 3.98
VT606 ( mono-stable) 2.4 1.7 2.7
VT608 ( feedback amp) 31.1 31.7 .003


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.

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.


  • 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 dmm_gif

  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 nowbash_gif

Transistor & Function Base Emitter Collector
VT602 (delay switch) .872 .002 .086
VT603 ( mono-stable) 1.4 .778 9.68
VT604 (Chopper) 54.9 54.9 278
VT605 (Chopper Drive) 1.7 .799 3.98
VT606 ( mono-stable) 2.4 1.7 2.7
VT608 ( feedback amp) 31.1 31.7 .003

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


The next PSU Module up for repair

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 grin_gif

The module is definitely tripping instantaneously and R609 feels VERY warm, so six possible scenarios to consider.

  1. Chopper transistor VT604 short-circuit. Also check W606,W609, W616
  2. Crowbar SCR W621 short-circuit
  3. 72V Zener W617 short-circuit
  4. Feedback amplifier VT608 faulty, causing M/S ratio of monostable to allow 58V to 65V rail to increase above strike voltage of W617
  5. W620 open-circuit
  6. Mica washer on chopper transistor broken down.

Why am I not surprised everyone of those suspects tested OK nuts_gif

Its looks like I’m facing a dead short scenario, opened up the other side to see if there’s anything untoward going on with wiring underneath.

Think I have just located the fault, it looks like the mains transformer primary is o/c. It should read around 35R this one reads 3.2M

Tested a spare and as the photo shows this is what it should read. I also tested two known working models in case in circuit has a baring, it did not, they too measured 35R. Looks like a TX transplant is now up.

The  replacement TX was installed, as I say luckily it was the only spare TX I happened to have. Glad to report this has resolved the ‘Tripping’ and what’s more I have all the rails.

This concludes the BRC/Thorn PSU module repairs………. for now!


Yet Another PSU

OK don’t all groan at once, one day this blog will prove very useful to someone looking for 3K psu repair ideas. wink

Whats wrong with this one? Not a lot really, all the rails are present except the 58-65V is high and cannot be adjusted at the SET EHT preset. In its present state I would have high EHT if it were installed in a TV. I’ve checked R629 and its OK, I replaced R631 the skeleton Factory EHT pre-set. The voltage swing is 70.5V – 69.6V depending on the position of the two pre-sets.

Also noted smoke emanating from R607. The voltage at the junction of R607/C608 is correct-ish ( 2V up) at 47V, the other end of R607 at the junction of C607 is only 7.1V and should be 12-15V. It seems to be getting very hot.

For starters guess I need to check and take readings at the delay switch (VT602) and W607, the mono-stable (VT603/VT606) and feedback amp (VT608).

Time to ‘spatch-cock’ the PSU

Well this ones got me for the moment.  I’ve removed all the likely suspects (F602, W605, VT602, VT603, VT605, W607 and W610) for no 58-65V rail, they alll tested OK. duno_gifits going to be ones of those PSU’s then!

Well when all the culprits check out OK it leaves but one thing, faulty traces/pads and it was. Ran jumper wires for the emitter and base, the 58-65 rail is now restored. I’m now back to where I was a few hours ago however I have a few less components to check now. grin_gif

So back to the high 58-65V rail adjustment problem

The mono-stable is not working the voltages confirm this as does the collector waveform and this is due to the collector volts of the feedback amp being missing.

VT606 Mono-stable

Base 2.3V should be .85V
Emit 1.5V should be .4V
Coll 2.5V should be 23.5

VT603 Mono-stable

Base 1.2V should be 1.6V
Emit .651V should be 1VColl 9.9V should be 6.5V

VT608 Feedback Amp :collector volts are missing

Base 40V should be 30.5V ( the base voltage can be varied up and down with the use of R629/R631)
Emit 33.2V should be 31.2
Coll .006V should be 10V

The absence of collector volts will mean the time constant of the mono-stable will not be adjusted. So need to see why the collector volts are awol. Well I’ve got nowhere with tracking why I haven’t any collector volts on the feedback amp. I’ve removed and checked all I thought that might be causing this fault. Until I do the time-constant of the mono-stable shall remain kaput! Which means the chopper transistor will not be turning on and off correctly. Think this needs  a fresh mind and a new day, so will start again tomorrow. In addition to the missing collector volts in the feedback amp a new fault has developed now, the PSU is tripping as soon as the 58-65V rail starts up. I found that if I remove W615 ( isolating the feedback amp) the new fault is isolated the 58-65V rail comes up and it does not trip. Something in the feedback amp is not happy causing the crowbar to fire, this must be where all my problems lie, as I say tomorrow, wood for trees at the mo.

Fixed the 58-65V rail trip, it was an electrolytic in the feed back amp C621. Replaced and now I have the 58-65V rail up and not tripping, whilst at it I also replaced C622 & C620 for good measure. The rail is too high though at 71V and I still have the missing collector volts and 40V on the base with 33V on the emitter for the feedback amp. So that’s the two problems for today.

The low volts at R607/C607 were a mystery, to document where things are here is the voltage map










B=.7/ C=.4

























Clearly the chopper drive is not in a good place

The scenario following on from last night and what I’m faced with now is:-

The PSU no longer remains up.

Upon power up 240V is established along with 6.3V, 30V and 58-65V rails. As the 58-65V rail comes up the PSU trips with R609 overheating. I believe the chopper output voltage has now risen above 72V which means W617 conducts and fires the ‘crowbar’ W621, thereby shorting the 240V line and operating the cut-out. R609 gets very hot too whilst this occurs in the very short space of time so the current draw is large.

I checked W617 it tested fine but replaced any way, tested again, tripped again.

I checked VT604 (chopper transistor) it was perfectly fine. I replaced the mica washer and applied fresh heat-sink compound.

Checked and confirmed OK W616, C610, C634. C629 (Mullard tropical) looked a little worse for ware so replaced. Tested again, tripped again.

Replaced the crowbar W621, tested again, tripped again.

Checked the fly lead from W609 to L603 was not shorting to chassis, it was not.

Removed C618, checked OK right value, low ESR and no leakage at rated voltage.

Removed W620, tested OK replaced regardless, tested again, tripped again.

With all the above verified as OK it leaves the possibility as the article states, the mono-stable is holding the chopper on to long. I therefore lifted W615, tested again tripped again. WTF! So with the feedback amp isolated surely It leaves the fault in the mono-stable and the feedback amp was unable to nullify the rise. But where or what in the mono-stable is causing this? Last clutch at a straw W607, If forward resistance of this diode is greater than 2K it is stated the mono-stable will not function. I replaced, tested again, tripped again. I’ve run out of things to check.

Finally lifted the supply to the chopper,  the PSU powers up 240V,6.3V & 30V happily running.

So that’s it ……..Gawd knows what is causing this 58-65V rail rise so high like a hippy on acid causing the tripping away

The Troublesome Final PSU, the end of this saga & what a surprise!

After a restless night with the 3K power supply circuit rattling around my head, I awoke late for me. A swift cuppa and then back into the workshop for another day’s battle with the PSU from hell. Today was a full of doubt day, one where I was convinced it must be either an own goal or I’ve replicated a man made error that was already present.

Painstakingly I went over every change I had made, every component I had introduced. Were the diodes orientated correctly, were the transistors installed the correct types and were the pin-outs mapped accordingly, nothing was wrong. I then compared the circuits to the physical taking note of any variances, again nothing that would cause the fault condition this PSU was exhibiting.

Around three O’clock with virtually nothing left in the bag of ideas I was scoping the anode of W615, it looked so wrong and I was starting to embark on yet another rabbit hole when the door bell rang. Who could that be disturbing my train of thought? Off I scuttled to the door and to my surprise it was Dr. Thorn, none other than our very own John (Jayceebee). I’ve never been more happy to see anyone and my pained expression must have said it all. I think days of this PSU had aged me as I proclaimed, “John great to see you, come on in” dragging him off to the workshop. I explained I had spent the morning and early afternoon beating a rather nice dent in the bench with my head, John pulled up a chair “Right lets be having a look at this”.

Over the next couple of hours he went over the entire board lifting components, taking meter readings and every now and then going, “hmmmm, odd, very odd”. Out came the scope, waveforms checked followed by further head scratching. Now at this point I was a little relieved as I fully expected John to walk in sit down and within a short period of time either find the fault or point out where I had introduced one or two of my own. As this had not happened my misery loved to find some company but what was this damned elusive fault?

As a last ditch desperate measure T602, the chopper drive transformer came under the glare of our suspicion, I reminded John that R2B had mentioned this part the other night but I had stated, “They never go bad”. It was removed but when it was ohm’d out against a stock item, it matched. We replaced it anyway just to be sure, it didn’t change a thing.

At this point John looked at me and said, “you’re not going to like this but can we please try this module in a set”. The disbelief on my face must have said it all, John asked what troubled me about that. I explained two things, one that the PSU has to have 1500pF for C614 to stop the crowbar from firing, that the collector volts on the feedback were so far off book. Two without C614 changed the SET EHT presets have to be wound to their maximum opposites and even then it does not always stop the crowbar from firing.

John explained that he believed I had fixed the original fault days ago and there was in fact nothing wrong with the PSU, it just didn’t like the jig! I asked how could that be I’d fixed seven dead thorn PSU’s in the jig, five of them this year, they all ran happily, why would this one decide it was not going to work in the jig.

John said he was convinced it was the case, I however remained unconvinced. The spatch-cocked PSU was duly put back together and C614 was reverted to its original value of 2000pF, off we went to the TV room where all the TV sets reside. I decided to use the HMV 2703, its PSU module was removed and this troublesome PSU installed in its place. What happened next?………….

Nothing untoward! No tripping, the set came up the rail was checked at 58V, it was adjusted to 60V, all was good……..fright

The PSU was removed put back on the jig and it tripped, the crowbar firing….. So folks for some unknown reason one PSU out of eight I’ve repaired on this jig was having none of it. Why this is the case I have no understanding of, neither does John. The only difference being John had the foresight to not believe what he was seeing, I however did not hence the days of torture.

Earlier in the afternoon in relation the T602 and long before this had been ascertained, I mentioned a phrase of Sherlock Holmes, “Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.” Something I will remember and when put through a trial such as this PSU has given me, one lesson I’m likely never to forget.

As the legendary Les Lawry-Johns once wrote about when doing battle with a Thorn 3K PSU in a customers home. How it defied logic, sapped his usual diagnostic powers and how he rushed off home to tell “Honey Bunch” how awful it had been to him.

Only now do I truly understand!

Hope this blog proves to be of future use to those who find themselves knee deep in BRC/Thorn 3000/3500 PSU repairs.