Fisher fm-90 no output.

[Carbsandcaps]

Recently acquired a fisher fm-90x that’s got no output. It turns on, the tubes light but I get no movement of the center meter or signal strength meter and no audio output. The guy clamed it worked but now I’m wondering if he just meant the light came on. I haven’t done anything other than a visual look over and detoxing the mute control. Is there anything I should be looking for on this that’s a common issue?

 

[gadget73]

Clean the pins on all tubes and sockets. I've had a lot of "dead" tuners from crusty tube issues.

If thats good make sure its got something near reasonable plate and screen voltages

 

[Carbsandcaps]

Clean the pins on all tubes and sockets. I've had a lot of "dead" tuners from crusty tube issues.

If thats good make sure its got something near reasonable plate and screen voltages

I got into it today. The golden cascade tube tests good. I cleaned all the sockets. Replaced all the capacitors. All the voltages were low. I ended up putting diodes in place of the tube rectifier just to see what happened and the voltages came up so I kept the diodes and put a resistor in to bring the voltages back down. So all the voltages are dead on now. It just doesn’t get any reception unless I put an fm broadcaster next to it. What I did noticed is it has a 6bq7b where the 6bq7a should be. I don’t know if there’s a difference because I can’t find any info on the 6bq7b.

 

[gadget73]

Offhand not sure either but i have my doubts thats the issue.

If it will receive something at broadcast frequency my guess is there is a fault in the RF amp section, though an extremely weak oscillator might do it. Usually those work or not though.

I had a broken solder joint on a coil in the RF front end of a Sherwood that acted mostly the same. Found that with a visual inspection underneath

 

[fred soop]

If it picks up the nearby broadcaster, then either the oscillator is working or the signal is large enough to overload it. Agreed with Gadget that there is likely a problem in or around the RF amplifier.

 

[Carbsandcaps]

I certainly agree since the signal strength gauge works off the IF amplifier tubes and the gauge barely moves even with the broadcaster directly next to it. I’m going to double check the tube sockets and all the solders joints tomorrow. I did pickup a normal station for a few minutes until I shut it off then I couldn’t get it back. The guy I got it from said he listened to it no problem for half hour. It was not packaged securely at all so I’m wondering if something came loose.

 

[Jalousie]

That looks like a nice tuner from the pictures. It's been a while, but I have owned two FM-90x tuners and they were pretty reliable and bulletproof. Unless the guy you got this from was lying about it working or it was shipped in a 13 gallon trash bag with forever stamps on it, I'm not sure about solder joints or tube sockets or shipping being the problem. I know this is too obvious, but is the antenna ok? Yeah, that unique 4-pin connector, etc.
"no movement of the center meter or signal strength meter and no audio output" sounds like the what happens if the antenna isn't plugged in or a bad wire there.

 

[DOBIEGUY]

Your tuner is circa 1957 and worth the effort to get it working. If you don’t already have the SAMS photofact for it there are a few on EBay.
With seven pages of info including alignment it’s a must have.
Happy listening!
James

 

[Carbsandcaps]

Oh I’m getting this thing working. It’s going to be my main tuner with a fisher MPX 100 adapter. The antenna plug is there and when I got it there was a cheap antenna soldered to the plug it seems to be wired correctly. I’m wondering about these small white capacitors that I haven’t replaced yet.

 

[Carbsandcaps]

Ok so it looks like it is something with the antenna circuit. When I bypass the whole 300/75ohm plug and inductors and got straight to to first fm gang with a wire on an alligator clip I pick up the strongest station in the area pretty well. I’m going to look into the antenna plug.

 

[dcgillespie]

Do NOT change out any caps within the front-end RF box. They are specifically chosen for their thermal characteristics so as to minimize drift during operation. They rarely if ever go bad anyway.

Dave

 

[Carbsandcaps]

Do NOT change out any caps within the front-end RF box. They are specifically chosen for their thermal characteristics so as to minimize drift during operation. They rarely if ever go bad anyway.

Dave

That’s why I didn’t want to plus I saw the oddball rating like 120pf and 300pf I figure they’re very specific

 

[gadget73]

What Dave said

Check the solder around each of those coils. On my Sherwood it was the chassis grounded end of what would be L3 I think that had broken loose.


someone has clearly been in here, I see newer caps and a modern power resistor so its highly possible its just something has a solder blob where it doesn't belong, or a broken connection.