loewe-opta: FM front end problem or other issue?

I am restoring an Olympic 5711W and could use some help troubleshooting the FM.

I have replaced all the paper and electrolytic caps and replaced the selenium rectifier with diodes and a dropping resistor.  B+ voltage is on spec at 250VDC.  Resistance readings are all within spec.

Voltages are in spec except for pin 1 of the ECH81 (184 actual vs 70 spec), pin 8 of the EF89 (110 vs 65 spec) and pins 1, 2, 3, and 6 of the EABC80 (all are substantially more negative than spec, like -21.8 vs -.7 spec on pin 2).

I opened up the front end can and all coils have continuity.  I have not opened up the IF cans.

Functionally AM and an outside source into the phono jack work well.  FM however is completely silent.  No sound at all, no static when turning the dial.

I have swapped all tubes for known good tubes but there was no change in performance or voltages.

I fed a 10.7mhz signal into pin 2 of the ECH81 and I get an audible tone.   The same is true if I feed the signal into pin 2 of the EF89.  If I understand correctly, that tells me that IF can pass correctly from the ECH81 to the EL84.  I then created a little wire loop and ran the 10.7 signal through it and held the loop near the L8 and L9 and again, I get an audible tone.  If I place the loop around the ECC85 or near the antenna, I do not get any audible tone. 

I am still a bit of a novice so could use suggestions for next steps from the experts here.  Much appreciated,

 

Mark

 

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I guess the voltage readings in the schematic are valid for zero signal conditions, i. e. when there the AGC controlled tubes are working at full gain and full power consumption. When the AGC voltage starts going negative, the anode and screen currents drawn by these tubes like ECH81 and EF89 will fall and so the voltage drops across the screen resistors also fall and the screen voltages rise.

In this case, i would guess that something is oscillating at the IF frequency  causing a high negative AGC voltage from the ratio detector.  Try removing one tube after another - first ECC85, then ECH81 and last EF89 - and see if and when the negative voltages at EABC80 disappear.

Possible reasons could be some loose shield plate or ground connection, some open-circuited decoupling capacitor, or maybe an alignment error. If this set has polystyrene (styrol, styroflex and other names) tuning capacitors for the IF coils, beware that some brands of such capacitors can get open circuit causing de-tuning.

 

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Torbjorn:

 

Thank you so much for your help,  I think you are on to something.  When I pull the ECC85, the voltages for the EABC80 revert back to spec.  I've recleaned the switches, checked continuity and poked around with a chopstick but didn't find anything amiss.  I did manage, however, when poking with the chopstick between the 2 FM switchbanks to create a short and burned up R41.  I'm putting it aside for the moment but if removing the ECC85 with success gives you any other ideas, I'd love to hear them.

Regards

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