CTV 1985/1986 Ferguson TX 20E2; TX90 Chassis

Got a Tea or Coffee? You're going to need one to get through this saga. One of the three TV's I saved from Stan's Impending trip to the dump. This one was not a well TV, the following saga is condenesed from a weeks worth of battling it into submission.

Introduction

This is a Ferguson 20E2, released I believe around 1985/86. It took the TX90 chassis which was mostly seen in portables and presented it in a 20" set. The TV on the whole inside and out is in remarkable condition. This version I have is the remote equivelant of the 20E1, as usual the remote was missing, but more on that later. A visual inspection of the four PCB modules and nothing looked untoward, notice the nice big fat transformer.

The First Problem

I powered up, and the TV came into standby mode with a dash “-” showing on the LED display, only problem, I could not 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!

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’d not come across that type before, and sods law I’ve none in stock. Before jumping to conclusions, I removed and tested. The set not coming out of standby was found to be nothing more than PL108 not seated due to a broken retaining clip.

The Second Problem

With the plug fully seated, I powered on again. Pressing set channel and the TV jumped into life, followed by vast plumes of smoke emanating from the rear, I quickly switched off.  I had not fully taken in where the smoke came from, so reluctantly had to try again, this time paying attention to where it was coming from.

It was R190(4.7K), burnt to a crisp and gone high at 144K. R201(82R) the resistor is in the bottom end of RV202, the height 100R preset, had also gone high. I replaced R190, R201 and RV202 which had also 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 yet again!

As this was my first foray into such a new (to me) TV and a TX90, it was at this point I contacted Mr Thorn for some of his usual sage advice as I don't think there is one Thorn TV he's not seen. John suggested checking C171, D106, C172, the scan coils etc. Thankfully 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.

Considering I had recently worked on a 22" GEC that had a faulty TDA3561, IC's were foremost in my mind. Therefore, the first thing I did was isolate pin 3 of IC102 (TDA5400), hey presto, the short has now gone, so IC102 has died, unbelievable!

Nagging question; why, what caused it?

The TV is a little tricky to work on and John gave me a word of warning, I should remove the CRT PCB. To not do so will lead to a Psssssssst, as the small CRT tube neck is easily broken, guess how he knows. I found the TV far easier to work on with all the circuits boards removed, and as there’s so much to remove and check, it makes sense.

I next checked the field output pair, a TIP29E and a Darlington TIP112H, both on the TX90 are OK. The newly fitted 100R RV202, had also survived. I then set about replacing the TDA4500, and replaced R190, R201, C171 & C172. To play safe and stop component getting fried if there is other problems, R190 was temporarily made a 4W. The field output pair were left out of circuit to first see if the line stage was functioning without faults. The line stage was fine, so time to put the field output transistors back in circuit. With the field output transistors installed, I powered on and had a partial field collapse. I tuned into a signal, test card F from my generator, I could see the test card, and I had colour.

Now to find what is causing the partial collapse; followed by A Major Screw-up!

This was traced to R203, a 33R fusible resistor that was open, I didn't have any in stock. This is where I made a fundamentally stupid mistake. I'd been lulled into thinking all my problems must have been due to the TDA4500 short, so as a temp solution I fitted a normal 33R 1/4W resistor. Powered on and FumpH!! Flames  😲 shooting out of it and up the heatsink of TR105, no time to switch off the TV did that itself a fuse blowing. Worst of all, the flames continued OMG!!!! Panic. I took in a massive intake of breath and blew, thankfully that took it out. The workshop by this time was a tad foggy and smelly.

The Damage

R203 well and truly 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”. I paid the price for my own stupidity! First time I’ve made such an idiotic mistake, resulting in damage. It burnt a hole right through, quite crusty and carbonised now.

Onwards

A few days later to chill down and then come back to it. 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 Thorn were cost-cutting, 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, to recap, prior to this disaster I had the TX90 up with a partial field collapse. Finding the 33R fusible 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 perhaps one or both of the output pair, although testing good on the DCA55 when under load, must have been faulty.

I couldn't for love or money find on the market a 33R fusible flameproof resistor. I did however find 47R fusible flameproof on RS, so ordered a batch. I set about installing 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 I pushed the channel to activate, the TV came up without any smoke or drama. Unfortunately, I’m back to where I was before, with the same partial field collapse.

A quick in circuit check of R203 (47R) and it was reading in the high megohms, I suspected R203 fusible had gone open again! After removing, indeed it was open, something was killing R203, it’s doing its job and why I have a partial field collapse. So, the assumption that one or both the original output pair being faulty under load, appeared to be incorrect.

The big question is, what is killing R203 as I have no idea, again I was flummoxed. Speaking with John, it was decided to check the drive out of pin 2 of the newly installed TDA4500 to see what was there. This is what I could see, certainly looked like nothing but to be sure instead of using my highly sensitive 200mHz Siglent, I checked with my 50mHz TEK, that indeed confirmed there really was no drive.

Finally Getting Somewhere

This was followed by some resistance checks on Pin2 and pin 3 of the TDA4500. These were 1.7K and 25+K respectively, so the TDA looks to be OK, and the no drive lies elswhere. 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 even 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 further remembered I’d not been able to locate R200 so had moved along, subsequently totally forgetting about 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 circuit states it is a 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 R203, the fusible resistor, which was now open, in circuit whilst I tried to diagnose the fault. I didn’t want another burn up of the field transistors.

Now as things had moved forward, I could replace it. Replacing the o/c R203 fusible resistor gives me a 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 the other day when I tried to test the CRT.

After All That; Wishy Washy Mullard Tube

So out came 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.

Backgrounds askew! I then had to adjust the background pots on the rear of the CRT PCB, I think as the CRT has aged someone just adjusted the pots to compensate. After doing this, the 20E2 is now giving a rather good picture.

I still have no idea why R200 being open would result in R203 going open or why it would be dumping so much current. If anyone can enlighten me, I would appreciate the education. Below, the parts it took to get it working.

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 T770 remote control on eBay. I swiftly purchased it and happy to say it functions.

Conclusion

This is the newest set I've worked on, (except for the 51A2 FST and another set, but that's a story for another day) but certainly my first foray into a Thorn TX90. The Ferguson 20E2 TX90 chassis will now join the rest of the Thorn collection, in fact it's sitting alongside the 1973 Ultra 6713 in the stock room for now.

I never worked in the trade, but over the past 12-years have worked on the various offerings from the Thorn stable. I have to say you can see and feel the decline, it's right there before your eyes. Unlike the marvellous 2K, 3K, 4K (in my opinion) 8K, 9k, TX9, TX10, TX100, then this! You can see how it was desgned down to a price and nothing more. I guess the writing was on the wall for the once great giant.

If you’ve got this far reading the repair log, many thanks for taking the time, and hope I've not bored you all too much. As ever, my thanks go to John @Jayceebee for lending me his Thorn brain, and putting up with my constant stupid questions. 👍