A640 chassis help needed.

[Tazman1966]

Another vote here for a gassy DY802.

There were numerous faults noted after replacing a section of the mains dropper with a "Polo mint" dropper section. These including: line hold drift, poor frame and line sync, odd frame linearity, hum on audio to name but a few. A fair amount of the timebase troubles and line hold drift were due to those carbon resistors with the circular clip ends being way out of value. Also a couple of the half blue/yellow Dubilier capacitors were leaky. The rubbish sync was traced to the video amplifier/sync separator valve's (PFL200) screen grid decoupler electrolytic which was virtually open. The hummy audio was due to a cooked cathode bias resistor and dodgy associated electrolytic. I popped a new PCL82 in to save future troubles and seeing as I was unlikely to use my stash of valves in my lifetime, I treated the set to a brand new set of valves throughout! It is now working absolutely tip top!

[HamishBoxer]

Great job,I think I meant blue/yellow caps not white/blue.

[linescan87]

I'm only after reviving one of these after a long sleep in storage. I bought it a while ago from a back lane antique dealer in Dublin city, complete with chrome stand, I did a write up on it at the time.

Anyway, when I woke it up, I too was surprised to see a bright blue glow in the rectifier. It had gone to air in the mean time. I did a short video on it, showing it working on both standards. It needs a bit of work but I'm inclined to leave it as it is as it's more or less original. I have two others in fully restored condition. I think these rectifier valves are prone to going soft as I've found a few that way over the years.

[dryad1313]

I did have a spare (used) DY802 but it behaved in exactly the same way. Hopefully the NOS one will arrive in the post soon. Updates to follow.

[Nuvistor]

Did you try it without the valve to check if the EHT and boost voltages etc came back to normal?

[dryad1313]

Frank - I've just tested the boost voltage with the valve removed, and it rose to about 124v until I could hear the line whistle starting, then it dropped right down. Interestingly, I just took a scope reading at point J (according to the service data I have) and it's wrong; although the voltage is approximately right at 17v it looks like a dirty sine wave. See photos of what it looks like and what it should look like:

[Nuvistor]

Part of the frame Oscillator circuit is fed from the boost HT so that waveform may take you away from the fault.
If the boost and other voltages haven’t returned to normal with the DY802 out of circuit you still have a fault to find including possibly defective DY802.

[dryad1313]

UPDATE: The new DY802 arrived, fitted it...exactly the same as before. But I've noticed something I didn't notice before; with the lights out I can see that the anode of PL504 is glowing slightly red (as Terry suggested previously). I've replaced 3R25 and 3R76, as well as 3C15, 3C13, 3R19, 3R20 and 3R22. I've also tried three different PL504 valves, and have changed all the others. I must also add that I can't see the heater of DY802 glowing at all. Any ideas?

[Nuvistor]

Check the line drive is correct, if it is then it’s a possibility that both your transformers are faulty. If you have a scope try ringing the windings.

[Heatercathodeshort]

Remove the top cap from the DY802 and see if you can see the heater glowing. If you can, replace the top cap and remove the CRT connector and try again. If the DY802 still glows purple you must have a short on the heater winding to the core. This can be rewound by hand. It's just a turn or two.
The PL504 will glow a dull red if there is a short on the EHT supply. The transformer must be supplying a good supply of AC for the DY802 to glow purple. John.

[PortugalTV]

the heater of the eht rectifier will not glow if the flyback is "not working"
check the line drive as said before. you should have a sawtooth waveform going from plate of the oscillator to grid of the horizontal output tube
also on the grid of the horizontal output tube a negative voltage should arise maybe -30 to -40vdc measured with a multimeter..that indicates that all is OK
also you should hear a 15khz whistle that changes as you turn the hold control
if the tests are positive there is something loading down the high voltage. sometimes a short to the core or between turns on the high voltage winding causes this... if the high voltage winding is separate from the primary winding you can disconnect it and see the result... if that is the fault you can generally replace it with another from a different set.

but:
if the tube doesnt have air in it the only way it can glow purple is the heater not working but eht is present....so it must be a bad heater winding like someone mentioned (easily replaceable) or a bad socket (happens often) or another bad tube (also happens)

if you didn't understand something feel free to ask, English is not my 1st language so sorry for any mistakes

[dryad1313]

Thanks for the useful tips eveyone. I'm planning to get working on it again tomorrow, and will update as soon as I make any progress (or not!).

[terrykc]

Catching up after a week, I'm amazed to see how far down he DY802 diversionary route this thread has blundered! The next diversion, it seems, is to be the field sync.

Yet nothing has been done to address the low HT situation or to check for the reason behind the low line drive. Is the line drive problem the cause of the low HT or is it the other way round?

You say that the smoothing can runs cool, so we know it isn't leaking but an open circuit capacitor doesn't leak, either! It can also be low capacity and cause low HT: hence my original suggestion to temporarily connect a know good capacitor with a suitable voltage rating and reasonable high capacity across each cap in turn and monitor the results. (I used to keep a suitable capacitor handy with two wires connected ans a crocodile clip on the negative and a probe on the positive. Then it was a simple matter to put the clip on the earth rail and check out each of the electrolytics.)

Remember that whatever you do, the test cap will still be fully charged when you finish!

As a matter of interest, can you measure the voltage across 3C43 while you are at it?

The slightly red hot PL504 anode is to be expected because of the low drive.

As PortugalTV says, don't expect to see the DY802 heater glow - the heater voltage is derived from the LOPT and, as we already know, it will be low!

[terrykc]

Originally Posted by Chris55000

I don't have handy access to the circuit without rooting thro' all the American Radio History's "Practical Television" archives ...

Check out the April & May 1970 issues.

Alternatively, try this link - starts page 300.

[dryad1313]

Thanks Terry, I tried temporarily connecting a suitable cap across all the smoothing caps in turn, but there was no change. I didn't need to worry about any of the caps holding a charge afterwards because they dropped down to a few volts within about six seconds, which is the complete opposite of when I switch the set on with the CRT disconnected - it takes a good ten minutes to get to a safe voltage. I measured across 3C43 and it was 268v.

Thanks for the link - I'll study those now.

[terrykc]

Originally Posted by dryad1313

I didn't need to worry about any of the caps holding a charge afterwards because they dropped down to a few volts within about six seconds ...

Err, it was the charge remaining in the test capacitor I was warning against! I would expect that to retain its charge for quite some time!

268V across 3C43 is low, as was to be expected, but the voltage at that point should be 280V/290V, dependent on system selected, for an input of 240V AC.

Now I must hold my hand up here because there are really two other things I should have asked - and didn't. The actual input voltage and which system was selected at the time of the measurement because, taking 285V as the midpoint of what the voltage should be, the drop is 17V.

An additional 17V drop across the 14Ω part if 3R57 is a whopping 824mA increase on what it should normally be and I find that very hard to believe!

The original voltages you quoted in post #1 were taken in the 625 line position and I don't think you've mentioned any difference between 405 and 625, as far as the fault is concerned.

Skimming back through the the thread, I can't spot any reference to 3C24.the S correction cap in series with the scan coils. There are another two caps associated with the S correction but these are only brought into circuit on 405 lines. If the set is permanently switched to 625, 3C24 could well be the problem but I don't think it would have the same effect on 405.

A bit of a long shot but something which briefly crossed my mind, so I thought I'd mention it.

There was a brief reference to the boost cap, 3C18, very early on in the thread but I can't see that this has been eliminated while skimming through again.

[dryad1313]

Originally Posted by terrykc

Err, it was the charge remaining in the test capacitor I was warning against! I would expect that to retain its charge for quite some time!

I know, but I had left the test cap attached when I switched it off, so it discharged along with the ones in the set! I've had too many belts off electrolytics over the decades to be very casual about these things!

The actual voltage applied to the set is your typical UK mains voltage (if that's what you mean) and I had it set in 625 mode as I have no way of using 405 anyway.

I replaced 3C24 and 3C18, and also 3C31, 3C22, 3C27, 3C76, 3C15 and 3C58. I've also replaced most of the resistors in that area. Additionally, I have replaced 3C13, 3C28, 3C33, 3C31, 2C48, 2C44, 3C37, 3C36, 3C34, 3C32, 3C35 and most of the resistors in that area too.

As you can imagine, it's driving me mad - especially as someone who is unfamiliar with working on televisions.

[Hybrid tellies]

I think at this stage I would disconnect the EHT cap to the CRT, then making sure the EHT cap is secured is secured safely run the set. If the voltages return to normal and your getting a nice healthy EHT hiss from the CRT EHT final anode cap you could well be looking at a CRT fault, a short on the CRT base or a PFL200 video stage fault which is robbing the CRT cathode of its voltage turning the CRT hard on and severely loading the line output stage. I have seen similar faults causing the same symptoms on colour and black and white tv's of this era.