Vintage Fisher Receiver FM Tuner Repair. - Page 1
Forum section: electronics / repair

author: Arty30
author_rank: Regular Contributor
timestamp: August 12, 2024, 06:22:45 pm
content: Hello All, I'm working on an early 70s Fisher model 301 Receiver. I've had it for a while and did a re-cap on it probably a year ago. I haven't used it since and pulled it out to test for selling. The amp sounds great however the FM tuner is exhibiting an interesting fault that I have not been able to track down due to my lack of understanding of FM IF and RF circuits. I can pick up stations, however the sound is non-existent in the strongest part of the station, but if I move the dial to either edge of the signal I get sound, but extremely distorted. it almost seems like the signal is blown out in the middle and edges to the point of peaking/saturation. at the same time though I have to crank the volume to hear these noises clearly. hopefully this makes sense and someone might recognize the behavior and know what to look for. I've spent a while testing diodes and checking signals with my scope but am none the wiser, I cant find an audio signal that is obviously good/bad. Th fisher also has a signal strength and center tuning meter, the strength works, center does not. I got a schematic from HifiEngine.ThanksArty

author: Grandchuck
author_rank: Super Contributor
timestamp: August 12, 2024, 06:47:48 pm
content: I see you posted about this in the past and have the schematics. You could post the IF and FM detector sections here and perhaps get some clues from the gang.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 12, 2024, 06:49:25 pm
content: Post the schematic here.It sounds as if your receiver is demodulating by slope detection, if it was fine when you stored it, there's possibly a fault with the ratio detector.RegardsX

author: Arty30
author_rank: Regular Contributor
timestamp: August 13, 2024, 04:27:51 pm
content: Here is a schematic, not the best resolution, but the silkscreen resolution on the board is somehow worse.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 13, 2024, 07:44:15 pm
content: Check all components in the area shown in the attached.Start with C466 it is a common failure point in FM demodulators.

author: Arty30
author_rank: Regular Contributor
timestamp: August 13, 2024, 10:41:57 pm
content: C466 Has been replaced, other than that I'm not sure how to test those components. can someone explain how they work?

author: Arty30
author_rank: Regular Contributor
timestamp: August 13, 2024, 10:42:44 pm
content: both sections outlined with the dotted line are sealed units.

author: Xena E
author_rank: Frequent Contributor
timestamp: August 14, 2024, 05:38:48 am
content: Quote from: Arty30 on August 13, 2024, 10:41:57 pmHave youtestedit?QuoteIt's possible the inductor turns are open or one diode u/sRemove the modules from circuit to prevent spurious results.Do a diode check between 6 & 4 and then 6 & 3 on the ratio detector note their polarity is reversed relative to each other.On the second module, do resistance checks between the pins and refer to the values on the diagram.The small value capacitors in these units are quite reliable normally, and from your original description of the fault, alignment is not a problem.I have occasionally heard of one of the diodes going bad, this then effectively makes the receiver demodulate the signal as amplitude modulation, giving your symptoms.Further, try injecting a swept signal at the IF and check the discriminator slope with your scope. The full manual has details of this and is available on line, Electrotanya site has a free preliminary version of this.QuoteYes, they can, (Google is your friend).RegardsX

author: Xena E
author_rank: Frequent Contributor
timestamp: August 14, 2024, 05:43:45 am
content: Quote from: Arty30 on August 13, 2024, 10:42:44 pmVersions I've seen the cans will disassemble as normal, but you can still do basic tests.Being sealed doesn't mean they're not faulty.