Thorn TX90 - line timebase problem and field collapse
 

I think this set just about counts as vintage as it's about 25 years old.

Line scan is a bit irregular - verticals are slightly ragged every 10 lines or so. The irregularity is itself not stable and the ragging varies randomly. I've got the full service manual so probing around isn't hard. The line drive has the same irregular timing as the linescan. The feedback to IC102 (TDA4500) has same irregularities too. So it's chicken and egg - is it the line output stage with its somewhat complex boost regulator or the line oscillator deep within the TD4500. The manual gives a procedure for setting line hold, RV129 by connecting IC102 pin 28 to ground and connecting 2K7 between pins 7 and 26 of IC102. This appears to disable the flywheel loop. Did this and the irregularity seems still to be present on the line drive and scan. Also I cannot get the picture to float through nicely as I adjust RV129, it's always very restless but perhaps that's normal on this chassis. Before I condemn the TDA4500 is there anything I
ought to check? If it's the TDA4500 has anyone got a lurking spare that they could sell me?

I did remember to do a couple of "silly" checks. I've been caught more than once, mainly on professional kit, where the fault has simply been due to the main power rails being adrift. The main 12V supply is accurate and clean. The 95V boosted HT is within 1V of nominal and looks pretty clean on the scope.

Re: Thorn TX90 - line timebase problem
 

Hi Jeffrey, I had a quick look at my components dealer. They have one TDA4500 in stock. 10 Euros.

Do you want it?

Kind regards,
Darius

Re: Thorn TX90 - line timebase problem
 

Darius, thanks for the offer. Grandata stock the TDA4500 for £2.30+VAT so it looks easy enough to buy in the UK.

I would like to prove that the TDA4500 is faulty before ordering a replacement. Does anyone know if it is well known for causing the fault I described?

Re: Thorn TX90 - line timebase problem
 

I realised I had another set with TX90 chassis so I borrowed the TDA4505 from it. The problem is not the TDA4505. A lot of unsoldering just to prove the point.

I suppose I'm now going to change all the odd R and C around the TDA4505 that relate to the line scan and hope this cures it. Unless I've made a mistaken diagnosis and the problem is really in the line output.

Re: Thorn TX90 - line timebase problem
 

These sort of ripple effects are ALWAYS due to ht smoothing and caused by duff electrolytics. I know you have said the HT looks fine on the scope, but look again. I think you will find there is a second filter cap on the line by the time it reaches the line transformer.

Dave

Re: Thorn TX90 - line timebase problem
 

ISTR dry joints, et al - especially around the LOPTx, etc.

Al / G8DLH

Re: Thorn TX90 - line timebase problem
 

Hello, the fault that you have described is almost certain to be c189, c191 both 22mfd.
Also make sure that the HT volts can be varied with the HT pot RV224.
If it has little effect, replace tr107 bd839. C125 68n, c130 2.2mfd, c197 100nf.
C124 2n7 also gives problems with this fault.
I have serviced hundreds of these sets over the years and many still continue to give good service.

Re: Thorn TX90 - line timebase problem
 

Great little sets these, well worth the effort and you should not be disapointed with the results as the tubes did last well.
I seem to remember that caps did cause a few problems around the power supply as Malc has mentioned.
BTW the power supply was rather unusual in that it used a sliding tap principle. A very clever bit of design from the backroom boys of Thorn Consumer Electronics, another big name now sadly consigned to the scrapheap.

Re: Thorn TX90 - line timebase problem
 

Fixed! I finally got back to this set just now and picked the right capacitor first time. It was C189 which was hardly a capacitor at all. Thanks for the suggestions.

The set is also reported as having occasional field collapse which I've not seen. I suspect this is a dry joint so I'll resolder everything in the field scan area (and around the LOPT since this is reputed to suffer dry joints).

Re: Thorn TX90 - line timebase problem
 

Well done, Jeffrey, nice to hear that another little TX90's been saved. I've seen far too many of these dumped recently:clap: .

And congratulations to Malc Scott (& Dave etc.) for being spot-on with the diagnosis...

Re: Thorn TX90 - line timebase problem
 

The set actually belongs to Jackie's mum who uses it as her kitchen set. It wasn't as greasy and horrible as I was expecting:)

Nick, if you come across the odd dumped but potentially salvageable TX90 I'd quite appreciate having it.

My own TX90 is actually the MCO1 monitor version. Handy little set for all sorts of odd jobs.

Re: Thorn TX90 - line timebase problem
 

Hi
In case you get a similar fault, especially after doing a repair, it can be caused by the ferrite slug falling out of the line driver transformer!

Glyn

Re: Thorn TX90 - line timebase problem
 

The set has come back to me with field collapse. This time it looks permanent which at least saves me the hassle of finding an intermittent fault. Before I point at the TDA4500 (again!) may I ask the experienced TV menders for an opinion please. Field swtooth at IC102 pin 1 is correct and locked at about 1.3Vp-p. Nothing except DC at pin 2 (drive to output darlington) and nothing except DC at pin 3 (field feedback). Power is present at pins 7 and 22 though a fraction low at 11.3V as the manual says 12V. I've tried tapping gently around IC102 and the field output. Also tried freezer on U102. No effect in either case.

Anything else I should check before I condemn the TDA4500? Could a failure in the field output stage completely foul up the field drive?

Re: Thorn TX90 - line timebase problem
 

Well I would check the PSU for dried up caps, etc. as something killed it!

Cheers,

Steve P

Re: Thorn TX90 - line timebase problem
 

Hi
Unsolder the field transistors and resolder them - check the print around them with a magnifing glass, then suspect the darlington transistor.

Check the supply to the output stage - these sets all had dries on the frame outputs.

The 4500 and 4505s were actually reliable devices.

Danny

Re: Thorn TX90 - line timebase problem
 

I would confirm. Intermittent field collapse on TX90s are ALWAYS dries on the output transistors.

Re: Thorn TX90 - line timebase problem
 

Thanks for the advice.

The field collapse seems pretty solid now, despite how it was described to me. I resoldered the transistors (and much else) last time the set was on the bench. I've just pulled out the darlington TR105 and it's showing reverse conduction from base to collector (not a short but it shouldn't conduct at all) so I reckon it's had it. I don't see how an output stage fault could stop field drive being present on pin 2 of the TD4500 unless an output stage fault damaged the TDA4500. Unless some odd effect in the feedback loop can stop the field drive.

I think I'll order a TDA4500, TIP112H and TIP29E from Grandata which will only come to about £5 the lot. I'll also recheck the soldering in the output stage and change C172 and C171 since either could have dried out.

Re: Thorn TX90 - line timebase problem
 

Grandata are out of stock on TIP112H and TIP29E. They have TIP112 and TIP29A. Are these OK as substitutes? I can find data on the TIP29A and TIP112 but not on the original parts.

Re: Thorn TX90 - line timebase problem
 

Fitted a new TIP112 (instead of the TIP112H) field output darlington and the set's working. It's on soak at the moment. Making this post is meant to provoke it into going wrong:)

Thanks again to all who offered advice.