PENDD61 instability?

[HamishBoxer]

It sounds very similar to those faults that are cured by turning on a light switch.They are Not easy to trace but I am sure you Will crack it and we all look forward to the cure.Just a point what voltages change when in fault mode as against ok?Start with the PENDD61 area.

Good Luck

PS I get my fair share of grief jobs too.

[oldgeezer]

Its now working perfectly!

Totally unsatisfactory!

I changed the mains capacitor - (should have done it first really I know). Squashed every valve holder pin a little. Made sure every earth connection was resoldered.

Used a can of freeze spray on every component I could whilst it was on to make it jump. (as it had stopped!)

Its now been going a couple of hours perfectly. I have no idea, but am quietly confident the fault will return.

Oh - almost every voltage changed when it jumped. HT by just 1v, Oscillator anode by 6 volts, AGC by a few volts (even with aerial disconnected), etc.
Something was drawing more current I think, and that was going to be the next test to measure each valve. But I'll leave that till it goes wrong again......

Not a satisfactory conclusion I fear, as I dont think I fixed anything.
Not going mad as fault confirmed by independent observer! Thanks Richard!

Thanks for all help - if / when it starts jumping again I'll report back
regards
Keith

[Biggles]

Just a thought...if the AGC voltage changes this could indicate instability in the IF stages?

[HamishBoxer]

Guess the screen de coupling cap(s) has been changed on the IF,s Also are there any caps across the IF transformers?

[oldgeezer]

Decoupling caps changed, and it is possible that IF stage could be unstable.

I realigned the set to the right IF freq, and tried detuning to see if it made any difference. I could induce oscillation in the IF stage but could not affect the jumping.

I may have another go at it over the weekend, but its still working OK at the moment.

It's really weird!
I have a friend going to Harpenden - hopefully he'll bring back a 6A7 (frequency changer) valve - clutching at straws time!
regards
Keith

[G4XWDJim]

I think you could do now with localising the faulty stage either with the use of a scope, a signal tracer or even by connecting the output of the FC stage to the IF input on another set and wait for the fault to come back. At least then you'll then have divided the set in half.

Jim

[Dickie]

You would also need to disable the AGC and go "open loop", which wold be reasonably straightforward.

[oldgeezer]

I've tried disconnecting the AGC by removing R8. Still jumps!

I've got visitors this weekend so will have to have a go Monday.

Thanks again for loan of valve and confirmation its not my imagination
cheers
Keith

[Marconi_MPT4]

Interesting problem, but would not be surprised with this set due to the use of unscreened aerial, oscillator and last IF transformer coils. Component placements may also be critical if any parts are replaced.

Instability may occur if the IF trap circuit C17 and L1 is not working with the capacitor adjusted to give minimum output at the IF test frequency. In practice exactly the same signal generator setting should be used to adjust C28, C27 and C26 for maximum output and C17 at minimum for optimum rejection of the IF signal.

Problems with frequency shifting might be due to R3 or leakage through C4.

Under no signal conditions the normal cathode voltage of V1 is estimated to be roughly 6.4V, does it go up or down when fault occurs?

Cheers
Rich