The Fine Things are Always Hand Made

800 B FM Eye Tube Issue (Weird one)

HI everyone,

I've been working on Bruce's 800-B and have everything working great (including Norman's Bass mod) but I can't figure out what is wrong with the FM eye tube--I can't get any action on it (nor any negative voltage to the grid.)

I've recapped everything on both chasses, verified good connections between the chasses via Jack 4, and all resistors in the FM eye tube circuit are now in spec.

I did find the 2.2 meg and 27K resistors connected directly to the grid (pin 4) of the FM second limiter V17 (rather than as shown on the schematic) and so since the eye tube wasn't working, I rewired these resistors ahead of the 27 ohm resistor that is connected to the IF transformer (T6) (per the schematic) but still no voltage at the eye tube grid . Also swapped the 2 6SJ7 tubes but no affect.

Anyone had this problem before? Other than this the set is working great.

Thanks,

Joe

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Permalink Reply by Norman S Braithwaite on October 8, 2013 at 3:51pm

Have you checked the FM IF and limiter alignment? FM radios have a characteristic of being able to pick up stations even if the alignment is way out of whack. And if so, the eye tube will not respond.

Norman

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Permalink Reply by Joe Miller on October 8, 2013 at 4:12pm

Interesting,

No I haven't checked alignment. I assumed it was fine since the FM came in so boldly. I've never done an FM IF alignment. Not sure what the IF frequency is on this set either.

Any tips on how to do it or any reference I can go look at?

Thanks Norman

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Permalink Reply by Norman S Braithwaite on October 9, 2013 at 12:36pm

I believe both Riders and Sams have FM alignment data provided by Scott Radio Laboratories. Included is a method by which the IF can be aligned using a standard signal generator and VTVM rather than a sweep signal generator and oscilloscope. I have found the alignment procedure using the standard signal generator to be fully adequate for this receiver. All but one of the adjustments consists of adjusting for a peak at 10.7-mhz. Adjustment of the discriminator is a little more complicated and requires some iteration but it works.

Norman