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1974 22" Ultra 6714: Thorn 3500 chassis

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Jayceebee
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I can't see any harm in trying but if you do I would suggest shorting the sync input end of R503 to deck just in case of any stray pickup. Don't worry too much at this stage about the shape of the waveform, just see if by adjusting L501 you can get the frequency to 15.625KHz. 

John.

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Posted : 12/09/2016 10:49 pm
crustytv
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Well I tried getting the frequency but to no avail.

With R504 set at mid point I adjusted L501, it could not get anywhere near 15.625kHz. Then with R504 set to it lowest setting and adjusting L501, I got it to 15.9 but any further and it started to climb again. So this confirms why the line speed is never going to lock in its current state.

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Topic starter Posted : 12/09/2016 11:06 pm
Jayceebee
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Apart from the electroltyitcs causing line drift which manifested itself as loss of colour when the set warmed up this part of circuit didn't really cause much trouble, there was also a mod to improve temperature stability by adding a resistor in series with X501. It was either 120 or 220 ohm, can't remember which.

Might be worth trying a replacement reactance transistor VT501, also diodes W501,2,3. R527 might be worthwhile checking out of circuit, also that the -150v line pulse is present at the top end of R506. C509 and C510 are worth a look also, it appears from the pictures in the original thread that C512 has already been replaced?

John.

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Posted : 13/09/2016 12:12 am
crustytv
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Process of elimination

Removed and tested VT501 ..Tests OK
Removed W501, W502.. Tests OK

Next W503

This is when I find one side of R527 goes to ground the other side should be soldered to W503's cathode which in turn connects both to the oscillator ( VT502) base. However the tweezers reveals that end of R527 it is not at all its just resting ever so close but not connected. What's more when R527 is tested its also out of tolerance. It should be an 82K 10% so max being 90.2K it reads 97.9K but regardless of that fact, not being in circuit is the likely culprit, well fingers crossed.

I  tested W503 for good measure and it OK

Bed now and back to this in the morning with a replacement R527 and then scope the oscillator to see if I can get the frequency pulled in.

R527.jpg

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Topic starter Posted : 13/09/2016 2:01 am
crustytv
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Well I was convinced that was going to fix it, nope! With a new R527 installed, LTB back on the rig, monitoring the frequency on the base of VT503 the best I can get with R504 one ended is 16.1kHz. Fault must be somewhere else. 

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Topic starter Posted : 13/09/2016 1:50 pm
crustytv
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Checked VT501,W501,W502,W503, R527(replaced and soldered),C509 and C510 all OK

Now to the -150V line pulse at the top end of R506 (4.7K).......its not there!

Top end has .412V other side has .392V Removed R506, it was high at 8.3K even so that would not acount for the lack of the -150V. Replaced R506 as it clearly was out of tol. Rechecked voltage and we now have .400V and .405V

So I guess that's what I'm looking for now the lack of -150V. I checked "B" & "C"  wiring on the EHT/tx "C" was loose, resoldered, still no -150V

ccct3.jpg

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Topic starter Posted : 13/09/2016 2:38 pm
sideband
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Chris, I've never used a component tester/analyser so don't know how good they are. Personally I would still be tempted to try temporary subs for the reactance transistor and possibly the flywheel diodes. I have come across situations where (for instance) an oscillator doesn't work. Transistor voltages check fine and on a testmeter HFE socket, the gain measures as OK. Half an hour later having got nowhere, decide to sub the transistor anyway.....oscillator now working! Why? Who knows? Probably down to some obscure parameter, maybe internal capacitance, that doesn't show up with a simple gain test.  

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Posted : 13/09/2016 2:45 pm
crustytv
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With the -180V line pulse still missing I thought I would check the reference sawtooth that is fed from the B C winding of the EHT/tx  at the junction of R503/C501 as this is what feeds the flywheel discriminator with the negative going pulses. Not a nice saw tooth ( see below)

sawtooth.jpg

Hi Rich, To be honest I'm floundering a bit here, need to sit and have a think.......confused

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Topic starter Posted : 13/09/2016 5:11 pm
Cathovisor
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Well, the first thought that's crossing my mind is; are you looking at this in the jig and as a result, is your scope's earth connected to the earthy end of the pulse winding for the phase discriminator or somewhere else? Because I assume that winding is continuous and of course, that part of the circuit shows separate earths because of the beam limiter arrangement. A 180V pulse is a big thing to go walkies but, you should be able to get the LO down to 15625Hz anyway in order for the discriminator to 'steer' the oscillator...

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Posted : 13/09/2016 6:06 pm
crustytv
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Hi Mike yes all work is taking place on the jig. I picked the earth at point (R) the line timebase earth point. Not only used the scope but looked for the -180V line on the top end of R506, nothing! just .4V

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Topic starter Posted : 13/09/2016 6:20 pm
Cathovisor
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So, let me get this right in my head.

Your scope is connected between the emitter of the line transistor, and you are measuring at the top of R506 or connections 20/1 and 20/4, and you have just a few volts of pulse? And you've checked C503?

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Posted : 13/09/2016 6:31 pm
crustytv
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No, let me try and clear it up.

I've been looking for the -180V line and started with the dmm looking at the top end of R506 its is not there. I then just for the hell of it looked at the junction of R503/C501 for the sawtooth.

I went back to the EHT/tx winding B/C and found C loose resoldered, rechecked no change all as above.

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Topic starter Posted : 13/09/2016 6:42 pm
Cathovisor
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You say '-180V line', but the diagram says adjacent to pins 20/1 and 20/4, "-180V pulse". By its location, it will be a short pulse because it's derived from flyback and given the discussion we've just been having about how waveform shape affects meter readings...

For what it's worth, I actually think you've taken a walk up that metaphorical garden path - I think the waveform at the top of the discriminator diodes is right; if you think about it, that pulse (once integrated and rectified) will have to produce a nominal DC that balances the DC potential from the horizontal hold control for the discriminator to balance.

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Posted : 13/09/2016 6:58 pm
crustytv
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Cathovisor said
You say '-180V line', but the diagram says adjacent to pins 20/1 and 20/4, "-180V pulse".

I did but if you look back over the other posts I clearly say "Line pulse" it was an omission due to hastily typing as I was off on my evening jog.

OK so I've gone up the garden path, I was just following what John said, check that the -180V line pulse is present at the top of R506. Danmed if I can get a reading to confirm yay or nay.

So if we ignore that for the moment I'm still left with this osc unable to get down to 15.625kHz.

I'll go back to the drawing board on that one for now

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Topic starter Posted : 13/09/2016 7:33 pm
crustytv
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Right just to prove I'm not mad and the jig is not fibbing.I prefer working at the jig as it in the workshop, on the bench, mag lamps, soldering kit and parts to hand. The TV is in the display room, poor lighting and PSU/LTB access once in the chassis is restrictive. Besides I should be able to sort this on the jig using the scope to get the the oscillator working correctly.

Anyway lets try and do this by the book or in the case the service manual. I reassembled the TV, reinstalling the PSU and LTB. Advanced the brightness to maximum. Shorted the flywheel TP to chassis. Using an AVO 9 ( Ruggerdised 8) I connected to the slider of R504.

Objective being to set the voltage via rotating R504 until it reads 6.2V. Then adjust L501 (osc coil) for a floating picture, remove flywheel short and there you have a lock.

What happens? The voltage at one end of R504 is 17V, at the other end 12V, so unable to get down to 6.2V

Back to the jig, bench and scope to try and find what must be blindly obvious but is eluding me thus far

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Topic starter Posted : 13/09/2016 10:04 pm
Red_to_Black
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Check the components at the bottom end of R504, and R504 itself, after all the bottom end is connected to chassis via those components, also something Cathy mentioned, make sure you are using the correct chassis point for your 0V line reference when measuring as there are two.

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Posted : 13/09/2016 10:15 pm
Cathovisor
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chris said

What happens? The voltage at one end of R504 is 17V, at the other end 12V, so unable to get down to 6.2V

Back to the jig, bench and scope to try and find what must be blindly obvious but is eluding me thus far  

Right - now we're getting somewhere! thumb_gif As R2B has said, you now need to look at the components at the bottom of the chain. My money is actually on X501 being o/c - a very simple calculation of potential division should give you an idea why you can't get the voltage down. Actually, try shorting out X501/R524 and see what voltages/frequency you get.

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Posted : 13/09/2016 10:25 pm
crustytv
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Hi Baz,

I listed all the components that have been checked and replaced in post #26 R504 was removed, checked and found to be OK.

As for the ground for the AVO there is no special mention of where this should be in the service manual. I would have thought if there was a potential to choose the wrong one they would state which to use. Just for clarity I clipped to the main chassis frame, same place the flywheel is shorted.

The only other ground point would be post (R) on the LTB which is the Line Timebase earth point, this is where the beam limiter board connects itself. Again if the procedure to set up R504 needed to use this groun, I'm surprised it does not say so.

I wonder if we're all going of at tangents. I will continue checking components that might be causing R504 to be upset.

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Topic starter Posted : 13/09/2016 10:27 pm
crustytv
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Cathovisor said 
 My money is actually on X501 being o/c - a very simple calculation of potential division should give you an idea why you can't get the voltage down. Actually, try shorting out X501/R524 and see what voltages/frequency you get.  

I started to wonder about R524, X501 and R505 myself 

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Topic starter Posted : 13/09/2016 10:34 pm
crustytv
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nuts_gif X501 tests fine as does R524 steamy_gif

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Topic starter Posted : 13/09/2016 10:48 pm
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