90R mystery, to me anyway

[junkjer]

I posted this on the tuner forum and someone suggested I try here.

Recently I bought a Fisher 90R and it's lovely but for one problem: After about an hour of use listening to 88.3mhz some nasty distortion takes over. Signal strength does not seem to be a problem, judging by the tuning indicator. But the distortion isn't evident on other stations as I go up the dial. Or maybe it is, just less so in a way that's tough for me to latch on to.

Does anyone have any idea how to solve this problem? I have no idea whether it might be out-of-spec parts (and which ones) or something serious with alignment. Since 88.3 is one of the two stations I listen to, this is really annoying.

Thanks.

 

[larryderouin]

1st try cleaning all of the tube socket's with a NON-RESIDUE Contact cleaner (Home Depot sells CRC QD Contact Cleaner). Also the tuner fins with a NON-RESIDUE Cleaner. DO not use DeOxit on the tuner fins. Then apply a little DeOxit (D-5) to the tube pins and rock them around in the sockets. Local Guitar Center has D-5 and possibly G-5.

Clean all the pots, switches with DeOxit (D-5) and follow up with Deoxit Gold (G-5).

Once you clean everything, fire it up and check it. I'd take a close look at the tubes in the tuner circuits (FM RF AMP(V50067/pcc88), OSC/MIX(6aq8)).EDITED to reflect correct info: The V50067/pcc88 is a 7DJ8(V1). As the heater is a 6.3V tap on the secondary, you could probably also use a 6DJ8 as V1. Fisher probably used the 7DJ8 to allow longer tube life with reduced heater voltage.

V2 is the 6AQ8. These are rode hard and put away wet. Replace any tubes that are questionable. Note that the 6AQ8 can be replaced with a 6DT8 for about 20% of the cost of a 6AQ8.

The FISHER 90R Manual is on the FISHERCONSOLES Website. The specific link is here.
http://www.fisherconsoles.com/non console manuals/fisher 90r 00000 19999 sm.pdf

There is a modification note on the schematic page in the lower right hand corner. It will cut down on ignition interference.

Replace all of the electrolytic caps, and any of the film caps that look like little brown "dog turds".

 

[dcgillespie]

If the signal strength is not varying from initial turn on to when the problem happens, there's no drifting, and the problem is not evident on AM operation (which eliminates the audio output section), then a good candidate would be C48 in the discriminator section. It may be heating up and giving out as it does.

Dave

 

[junkjer]

Thank you both. I've done the tube socket cleaning and I have non-residue QD on the way. A bit concerned about access to the fins - the gang is tucked away neatly in a box on the 90R. Will definitely look at C48 -- I think that corresponds to C3 on Sam's. A previous owner had all of the electrolytics replaced except the can caps, but I can do that. Will also hunt for some dog turds. New term to me, very handy. Great tip about 6DT8. Thanks!

 

[larryderouin]

C48 on the FISHER Manual corresponds to C5 on the SAMS. 8uf 50V. replace with 10uf 50V.

To gain access to the box, remove the cover on the underside. there will be 3 or 4 nuts to undo inside to remove the top box. Quite easy to do. Make absolutely sure before spraying the fins that the cleaner says NON RESIDUE on the can. Anything that leave a residue will affect the tuning as this is an air gap capacitor. Spray and let dry at least 24 hours. Close it up and play.

NOTE: I edited the Tube info above. V1 is actually a 7DJ8 (PCC88). Like I said above, they probably used the 7DJ8 to allow longer tube life due to the 6.3v Heater voltage. You could also probably use a 6DJ8 here (V1).

 

[junkjer]

A belated thank-you to all for the guidance. A year later -- after having to put this aside due to life annoyances -- I went back to it. Pointing me to the discriminator led me to R58 (Sam's #) which seemed to be out of spec. I replaced that and the FM seems to be working well now. (The caps had already been replaced.)