1972 Ferguson Colourstar 3713 - Thorn 8500 Chassis

Whilst waiting for the frame output transistors to arrive and the fuses, today I might try and figure why the brightness circuit seems dead. I cannot vary the frame collapse intensity.

Started with the brightness control transistor V121, looks like the issues are present here, voltages here are roughly half what the should be

Next check was W110, all seems OK at the anode, not sure what voltage I expected to see at the cathode

Checked the voltages at the RGB output transistors, these are OK

Wondering if the Tant C190 could be the culprit, the quest continues to find what's bring the voltages down on VT121

Perhaps it is time to read the text description of the frame circuit.

You should be looking for a tube protection section that kills the brightness to prevent a fault printing a line across the tube. I have no idea if the 8500 chassis has such a thing but it was the era when they started to appear in sets.

The symptom looks like a partial frame collapse leading to the variations in brightness.
Perhaps a capacitor or two.

I think the D.C. coupling might be off due to VT116, the voltages being wrong.

This likely accounts for the skewed brightness control transistor voltages.

I've read brightness faults affecting all guns ( this is my scenario) usually have their root cause in either the feedback loop or the fly-back blanking circuit. As I have skewed forward bias from VT121, this results in cut-off of all guns, no raster ( again my scenario, although after about 15 mins a barely visible frame collapse is evident). Its suggests C190 might be the cause, I suspected this in a previous post above( Tantalum) or it could be VT121/VT116 might be faulty. W110 going O/C would produce the same effects also, a few components to remove and test then. I will start with the easy Tant ( C190) replacement.

The hunt continues......

When you adjust the brightness preset and user controls, what does TP18 do?
Does the base of VT121 track 0.6 volts below this voltage?
The brightness circuit does not look like it includes tube protection but that does not mean that it could not be happening by another means.

It may be worth disconnecting the tube heater just in case the brightness control bursts back into life again while the frame is down.

Well it clearly looks like VT116 is s/c base- collector. Have you taken it out and measured it?

Hi Chris.

I've twice had the frame driver transistor (BC142) O/C causing frame collapse.

Tas

Well it clearly looks like VT116 is s/c base- collector. Have you taken it out and measured it?

Yes, belt and brace tested it, there's nothing wrong with it. I did simple OHMS test for an NPN using my meter, which checked out fine. I then tested it on my B&K transistor tester, it passed, finally checked on a Hansen which showed no leakage.

Hi Chris,
I would be inclined to fix the field timebase first because of the missing vertical flyback pulse fed into pin 6 of SN76227N as many of these types of device require all signals to be present before the correct output levels can be established.

Regarding field timebase transistors, Thorn had a habbit of producing different Schedule boards using different components. I do recall an issue with a particular board using American 2Nxxxx (RCA) field output transistors having slighly different pin outs with tracks routed to suit! While considering this point I think base and collector were swapped. Worth checking before fitting replacements. Also check diodes W411 and W412.

Looking back in an old issue of Scope - A Technical Bulletin for Service Engineers, published by TCE dated November 1975, it mentions fuse rating was increased from 2A to 3.15A antisurge. 4A sounds a bit high and probably a bodge to fix an underlying nuisance fuse blowing problem. Intermittent thermal cut-out contacts or arcing mains switch have been known to cause fuse failure, also the snubber choke can be implicated. Thermistor (TH7) wired in series with the choke can spark spectacularly!

Cheers
Rich

Hi Chris,
I would be inclined to fix the field timebase first because of the missing vertical flyback pulse fed into pin 6 of SN76227N as many of these types of device require all signals to be present before the correct output levels can be established.

Cheers
Rich

Agreed.

I replaced C190 ( 1uF Tant) and after putting VT116 back in circuit tested the voltages. I get E=9.7V, B=10.4V, C=10V so still out of whack.

Will pick the story back up later this week when all the parts required have been delivered as typically I don't have them. Over the years I've stocked up on valves but since switching to SS, the transistor stocks are not as well covered yet.

Still waiting on the 3-legged fuses (transistors) so I can address the frame collapse, however the 3.15A fuses have turned up. Removed the wire wrap that was used as an HT fuse ( F302), happy to report the new fuse does not blow.

Hi Chris
My recently aqquired 8500 is suffering from frame collapse too. When I tuned in a test card the frame opened up! Obviously there is a fault there but I won't hijack this thread with my woes.

Finally the transistors have arrived, BC142 ( Driver) & BD139 ( Frame output pair)

Now to the task of removing each to see which component has failed. Frame driver? (VT409) or One or both of the frame output pair VT410, VT411? or perhaps all of them are faulty, we shall see. Will report back my findings.

Finally the transistors have arrived, BC142 ( Driver) & BD139 ( Frame output pair)

Careful Chris, only one of the outputs is BD139 (npn). The other one should be BD140 (pnp).

Pete

Yes indeed VT411 is the PNP and VT410 the NPN

Caught some bench-time this even and set about first removing the frame driver VT409 ( BC142) despite looking a bit rough it tested perfectly fine.

Next out was the first frame output, VT410 which was a BD222, that tested fine to. Last was to remove VT411 which was a BD225, that tested OK as well.

So with the transistors looking Ok I will check the diodes and caps associated with the stage.

Edit: Found R458 ( 10R 1W) O/C, replaced it, no change.

Hi Chris,

Use of the 'scope might give you a better idea what's going on. Check waveforms 26 and 27 to see if the frame oscillator is running, see what's on the base of VT409. Three lockfits here

John

Hi John,

Spooky I've just been looking through the manual at the waveforms, I think that's the plan of action for tomorrow morning. Hooked up the PM5515T, tuned the TV and have a nice strong test tone.

Update

Found three resistors O/C in the frame circuit

R403 10R 1W
R418 10K 7W
R458 10R 1W

These had all suffered rust through on one leg, all were replaced. Powered on, optimism that R458 was the prime culprit. Nope all hopes were dashed with no improvement from the previous fault condition

So out with all the transistors, sods law they all tested OK. I took voltage readings to baseline where I am at present

Finally scoped TP26/27 , nothing at TP27, should be a 2V sawtooth. As for TP26 all I could only get was a raggedy trace at .1V @20uS when it should be around 10V. I think its safe to say the Oscillator is not running. As to what the cause is I've yet to find the culprit/s

A quote from Television 1976

Apart from output transistor failure the frame timebase circuit gives rise to few servicing problems. This is perhaps just as well since fault finding is not easy in this sort of circuit

You can say that again! add 45 years into the mix and its a whole new ball game.

Looking at your rather nice schematic if the wiper on the height control was oxidised or not touching it's carbon track then C433 & C436 would be just twiddling their thumbs instead of charging.

Eddie

Hi Chris,
Looking at your voltage table VT406 and 407 are not conducting. Higher than normal voltages at various points indicate low or no current being drawn. For VT406 to turn on, current must flow through W408 (1S44) and R438 (120k) and R439 (50k Vertical Hold). One of those has either gone high resistance or open circuit. Possibly also C430 might be short.

Cheers
Rich

Ah yes correct Rich, I was reading nothing on TP27 as 0V didn't spot there was 38V on the collector of Vt407.

Eddie