BRC/Thorn 3000/3500 PSU Module Repairs

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.

A few tasks to be getting on with there, I think that's it for this evening.

Why am I not surprised everyone of those suspects tested OK 

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 has shorted turns. It should read around 35R this one read 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.

crustytv said
Think I have just located the fault, it looks like the mains transformer primary has shorted turns. It should read around 35R this one read 3.2M 

Ahem... with those values ITYM 'open-circuit'!

Hi Mike,

Nice to see you, err yes I knew what I meant, sometimes I don't arf type bollox 

Anyway, spending a week on PSU repair has been invaluable in getting to grips with the faults they can present. I'll get this TX transplanted in and then see what else this one offers up. This is the first TX failure in a Thorn I've come across, it really took me by surprise as I thought they were bullet proof. Luckily I had one in stock.

Those transformers were electrically very reliable, rare but not unknown to see one with shorted turns. The usual fault requiring replacement was buzzing from the core. Never seen one with the readings you have there.

John.

Jayceebee said
Those transformers were electrically very reliable, rare but not unknown to see one with shorted turns. 
John.

Hi John,

That's what I had been reading in a number of sources, hence it being on my mind. I guess there's a first time for everything, with the sets at almost 50 years I suppose we are going to see all sorts of new problems arising.

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! I will get back to the HMV 2703 repair. After that I will remove the Marconiphone 4711 PSU and see what state it is in.

The replacement transformer appears to have only 7 solder tags Chris, is that correct?

John. 

Hi Chris et al!

Mike Bennett had some intermittent tripping on his Fergie 3000 (3703). It turned out to be the mains TX. It was running VERY hot. We suspect a solitary shorted turn. A replacement solved the problem

Tas

On the failed PSU the end solder tag was unused, it had never even be soldered as can be seen from the photo. I transposed the wiring over to the replacement TX exactly as found. In fact I checked all the other PSU's I've been fixing and all those are the same, the end tag on the secondary is unused.

The replacement TX tap tags were in poor condition, so I reinforced it with hot-melt. Interestingly the end tag was snapped off so I removed what was left and left buried in the melt seeing as its not needed.

The additional tag was connected to a few extra turns on the heater winding as a "Boost" to maintain the heater at 6.3v when the set was working on a 210v main supply. Unscrupulous engineers used this tag to "up" the heater volts when running on 240v to get a bit more emission out of a worn CRT.

Always check the end tag is unused, you don't want to overrun a good CRT.

Jayceebee said
Unscrupulous engineers used this tag to "up" the heater volts when running on 240v to get a bit more emission out of a worn CRT.

Always check the end tag is unused, you don't want to overrun a good CRT.

I was not aware of this "Unscrupulous" activity and you got me worried so I had to go and check my treasure Baird 8724. Thankfully the tag was unused, whilst at it I checked the Baird 8749, that too was untouched. Now I know I will ensure the Marconi 4711 and the Fergy 3703 when I get a chance, are also safe from this dastardly deed. 

At a bit of a loss what to do this afternoon, was toying with starting a TV repair but couldn't be bothered to lug one up to the bench, decided to keep on ploughing through the stock of Thorn 3K psu modules. OK don't all groan at once, one day this thread will prove very useful to someone looking for 3K psu repair ideas.

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

No progress thus far and as is usual two steps back, I've now lost the 58-65V rail.

Will report back when I've discovered the cause if this latest PSU poop!

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. its going to be ones of those PSU's then!

You complain when they are too easy. 

Frank

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.

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.

Collector of VT606

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.

Good idea Chris, I will have a look at the circuit properly tomorrow too.