Thorn TX90 Chassis
Model: Ferguson 20E2
Original List Price : £000.00 ( to be ascertained)
Integrated Circuits: Yes
Update Day 1
Powered up and the TV came into standby mode with a dash “-” showing on the LED display, only problem, I cannot get it out of standby. I tried pushing the channel up and down button behind the flap, assuming this should, like most TV’s of this period, bring the set out of standby, but no!
I don’t have the mote for this set, but I have purchased one from eBay which looks to be the one that would work with the TV, unfortunately that has not yet arrived. Even so, I have a sneaking suspicion it would not sort this problem, the TV should turn on without it.
Looking at the service manual, it suggests that a possible candidate is TR902, the standby switch transistor. It’s a ZTX753, which is a PNP 120V. I’ve not come across that type before, and sods law I’ve none in stock. I guess before jumping to conclusions, I will first remove and test, It might be OK, and it’s something else cause the TV to be stuck in standby.
The set not coming out of standby difficulty was found to be a PL108 not seated due to a broken retaining clip. Plug fully installed powered on, pressed set channel and the TV jumped into life. Vast plumes of smoke emanate from the rear, quickly switched off. Tried again, this time paying attention as to where it was coming from. It was R201(82R) smoking big time. This resistor is in the bottom end of RV202 the height 100R preset. R190(4.7K) had gone high 144K.
Replaced R190, R201 and RV202 which had been butchered by this smoke up, it was reading only 33R. Once these were all replaced, I naively thought that the set would happily spring back to life, no! Plumes of smoke. Had a chat with my mentor John, and he suggested checking C171, D106, C172, the scan coils etc.
The scan coils checked out oK, C171 was indeed short but C172 was innocent, that’s when I discovered its +ve pad on the PCB, had a short to ground. John suspected C116 or IC102. Isolated pin 3 of IC102 (TDA5400), short has now gone, so IC102 has died.
Question is why, what caused it?
Easier to work on with all the circuits boards removed, there’s so much to remove and check.
Well, the output pair on the TX90 are OK. The new 100R RV202 survived too. I’ll replace the TDA4500, the caps and resistors. I wonder if that’s the end of the smoke escaping?
Removed the TDA4500, and replaced R190, R201, C171 & C172. To play safe and stop component getting fried if there is other problems, R190 was made a 4W. The field output pair were left out of circuit to first see if the line stage is functioning without faults. If it is OK, then I should end up with a field collapse.
Excellent, now time to put the field output transistors back in circuit and see if that stage works or still lets the smoke escape.
Field output transistors installed, powered on and I have a partial field collapse. I tuned into a signal, test card F from my generator. I can see the card, and I have colour, now to find what is causing the partial collapse.
R203, a 33R fusible resistor, was found to be open, I don’t have any in stock so a 33R 1/4W was installed. Powered on a FumpH!! Flames shooting out of it and up the heatsink of TR105, no tie to switch of teh TV did that itself. OMG!!!!
Well, that was not expected, though I did momentarily ask myself as I was switching on, “I wonder what caused R203 to go open”. Like a fool I did not head my inner voice. Not happy as now I have a burnt area on the board.
R203 cooked, the crusty ball beneath the transistor clamp is what’s left of R203. The following photo is the carnage it wreaked! Lost the output pair (TR104 & TR105) too! So pissed off with myself for proceeding, I even thought as I was powering on, “question is why did R203 go open”.
Paid the price for my own stupidity! First time I’ve made such an idiotic mistake, resulting in damage.
Grrrrrr! It’s burnt a hole right through, quite crusty and carbonised now.
A few days later, all the parts have been gathered, and the board fixed as best I can under the circumstances. The burn-up not only burnt a hole in the PCB, it also destroyed some traces. Coupled with the board coming from a time when cost-cutting was well underway, the quality of the PCB and print not as it once was. In places, it has not taken to having parts installed and removed frequently.
So, If you remember, I got the TX90 up with a partial collapse of field. I found the 33R fusbible R203 was open, and that’s when I installed a normal 33R to see if I would get a full field scan, but instead got the burn up. The assumption being that one or both of the output pair, although testing good on the DCA55, when under load must have been faulty.
Well, today I installed all the replacement parts, new TR104, TR105, D106, D197, D137, C171, C172, R201, RV202, R203(fusible) & R190.
Powered up and nothing, it was dead!
I found the fuse over on the auxiliary module, a T2A was open. Fitted another, and now the TV was up in standby. Tentatively pushed the channel to activate, the TV came up without any smoke or drama. Unfortunately, I’m back to where I was before, a partial field collapse.
A quick in circuit check of R203 (47R) and its reading in the high megohms, I suspect the R203 fusible has gone open again! Something is killing R203, it’s doing its job and why I have a partial field collapse. The assumption that one or both the original output pair being faulty under load, appears incorrect.
The question is, what is killing R203?
After speaking to John, as at this point I was flummoxed, it was decided to check the drive out of pin 2 of the newly installed TDA4500.
That is what I could see, certainly looks like nothing but to be sure instead of using my all singing 200mHz Siglent which is very sensitive, I checked with my 50mHz TeK, that indeed confirmed there really was no drive.
This was followed by some resistance checks on Pin2 and pin 3 of the TDA4500. These were 1.7K and 25+K respectively, looks to be OK. I then took stock of what I had checked and replaced. R200 was still original, as was R192, R194, R195, R254,C207, C175.
John said I should certainly check R200 and R193, as either of those resistors failing would turn off the transistors, certainly not bias them full on. Even so, John felt none of those, if failed, should be the cause of R203 the fusible resistor blowing.
I remember I’d checked R193 in circuit, and it read 1K as expected. However, I remembered I’d not been able to locate R200 so had moved along, subsequently totally forgetting it. This time I went hunting and eventually found it buried down inside the LOPT area. Really hard to see as it was obscured by the LOPT, and the heat sink for the field output pair. Also, it was tiny, and sitting in a screen print symbol for a diode with the +ve symbol showing one end. I think I must have seen it but not seen it, if you get my drift.
Anyway, the cct states 390R but in circuit tests show megohms, Ah-ha, think I may just have found something significant. Once removed it was indeed found to be open, in with another and the partial field collapse expanded a little. I then adjusted the height pot.
The collapse is still present because for safety reasons, I had left the R203, the fusible resistor in cct whilst I tried to diagnose the fault, I didn’t want another burn up of the field transistors. Now I can replace it.
Replacing the o/c R203 fusible resistor gives me full field scan.
As can be seen from the resultant test card, the tube is down, this will also explain why I was unable to get any G2 cut-off when I tried to test the CRT.
OK, so out with the B&K again to see what we can do about this. This time I had a little blue cut-off but still nothing for Red & Green. An emissions test revealed the state of things (see below).
Next I decided to use the superb feature of the B&K, namely the Clean and Balance. This is not like a rejuvenate, but far gentler and has worked wonders on many tired CRTs. I ran it across all guns, and they all came up to about the 16.5 mark on the respective meters, great improvement.
I then had to adjust the background pots on the rear of the CRT PCB, I think as the CRT aged someone just adjusted the pots to compensate. After doing this, the 20E2 is now giving a rather good picture.
Oh, before I forget. The 20E2 is the remote control version of the 20E1, sadly the remote as usual was missing. However, after a tip off from John @jayceebee there was a T77o remote control on eBay. I swiftly purchased it and happy to say it functions.
Now that ends this repair blog for the 1985/86 Ferguson 20E2 TX90 chassis. The TV will join the rest of the Thorn collection. If you’ve got this far reading the blog, many thanks for taking the time, and look out for more repairs blogs in the collection page here.
As ever, my thanks to John (member Jayceebee) for lending me his Thorn brain, and wise counsel.