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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 14th Jun 2018, 9:01 pm   #1
Join Date: Jun 2018
Location: Egremont, Alberta, Canada
Posts: 1
Default Loewe Opta Domino 5824W dead oscillator

G'Day All

New to this forum...there seems to be a lot of knowledge here.
Just thought I would showcase this tough nut that may have been seen on ARF and see if anyone keen to diagnose possible cause of difficulty.
Loewe Opta 5824W (chassis type 5767W) has no FM oscillator.
Until I had the correct schematic I had placed some capacitors values to match another model. This had the oscillator able to start if tuned to past mid scale.
Now the correct schematic arrives and I put back original values. Some original caps were off value and I had to make values with series or parallel out of what I had in bins. Now it is dead ... careful exam showed break in foil coil on pc board ... thought I found it ... nope.
Took it all out again and measured all caps OK . Found R2 lead fell off when checking value with one end of winding of CH4 removed ... nope still dead .
Decided to order all the small value caps and shotgun this board. I had to make up 1 cap out of 12pf and 1pf (tolerance +- .5pf on both) and compromise a 2.3pf with a 2pf at C7 (did not think it would have much effect from pin 1 of ECC85).
During the installation of fresh caps the DVM found a 2.5 ohm short to ground on the coil L7 ... AH-HA got you now I thought. I could not for the life of me see anything even with jewelers loupe. In exasperation I washed the PC board with isopropyl and short GONE. Finished putting rest of caps in, trying trying to keep lead dress as I initially found it.
Aaaghh! still dead. The ECC85 seems to be biased correct, it has 85 volt on feed side of L8 from R5 39k ohm to plate pi6 changing upward to over 90 as tuning down the scale. If I reproduce the mistake I made of putting DVM probe on plate of mix/osc I get an oscillation around 8MHz ( unchanging of course ).
My tube theory is weak and under educated I suppose, but Armstrong oscillator was easy I thought.
Any one got any ideas how to test the triode amplification without a mutual conductance tester ( just have Conar emmision tester )? Only have three ECC85 one is weak testing.
Is there a passive ring test as we used to do on Flyback transformers?
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