Fisher 400 - Weak FM

Trying to get the FM sorted on this rebuilt 400. The FM on all selections is just audible at full volume. It's the late model with three 6AU6s, 6HA5 & 6AB4 - all test good. The voltages on the plates of the 6AU6s seem OK compared to the schematic. The screens are a bit high - V5 reads 84v s/b 61v and V6 - 98v s/b 76. The weak sound is pretty clear doesn't seem distorted. Running out of ideas and could use some help.

If the unit will quiet (no background RF noise) on the normal stations the unit should pick up, then everything up to the limiter stage is likely OK. Also, if the same problem exists in FM Mono, then the stereo multiplex unit is also eliminated. This would imply that the problem is either around the audio selector switch somehow, or in the limiter stage itself.

Try seeing if the normal audio level is present at the test jack on the multiplex chassis when FM stereo is selected. If so, then the problem is likely associated with the selector switch. If no audio is present at the test jack, then start working backwards from the limiter stage until the problem shows itself.

Good luck with it!

Dave

Aux in is the proper volume?
Those two little tubes next to the tuner wheel Ok?
FM work Ok in mono?
Have you traced the signal on the signal pins on those tubes you mentioned?
Tuning eye work normally for signal?

Call me crazy but I recently fixed volume and balance problems on a Yamaha, a Marantz, and a Pioneer with liberal doses of deoxit. It's my new friend :thmbsp:

I'd hit that selector switch again.

Oh yeah, check those "on off" switches up front. Riding slightly in between drove me nuts once....

Have you given the FM sockets a good cleaning and retensioning? It might be something simple like that.

I've had fair results "fixing" tuners by pulling each tube and cleaning the sockets and pins with contact cleaner. It works at least as often as it does not, and its by far the easiest thing you can do with it.

Thanks everyone! I cleaned those sockets and pins with IPA but haven't retensioned them - I'll try that today. I gave the selector switch a real good going over with deoxit but will do again, and the slider switches.

AUX is good and loud. FM at full volume is about what AUX is at 9:00, maybe lower. Tuning eye works. FM mono works but low volume as well.

dcgillespie said:

If the unit will quiet (no background RF noise) on the normal stations the unit should pick up, then everything up to the limiter stage is likely OK. Also, if the same problem exists in FM Mono, then the stereo multiplex unit is also eliminated. This would imply that the problem is either around the audio selector switch somehow, or in the limiter stage itself.

Try seeing if the normal audio level is present at the test jack on the multiplex chassis when FM stereo is selected. If so, then the problem is likely associated with the selector switch. If no audio is present at the test jack, then start working backwards from the limiter stage until the problem shows itself.

Good luck with it!

Dave

Click to expand...

Dave, I'm pretty much a newbie on the radio section - not sure where the test jack is or how to detect audio but I'll try to figure it out. Thanks for your help.

Well first of all, phooey on me! It is (some of) the Fisher tuners that include the test jack, not the receivers. See? That's what I get for responding from a remote location and relying on memory only as to which units included the jack! I apologize for the confusion. So, now that I'm back at home, then:

Second, I'd like to confirm exactly which type of unit you are working on. The later unit does use three 6AU6s, but those models ALSO include a 6BA6 tube in the IF strip as well -- except that you didn't mention that tube. On the other hand, the earlier units do not include the 6BA6 stage, and use only two 6AU6s and one 6HS6 in the limiter stage. If this is the unit you are working on, and you are using a 6AU6 in place of the 6HS6 (in essence, three 6AU6s again), then performance will be compromised. The 6HS6 has basically twice the Gm and amplification factor as does that of a 6AU6, and for those models without the extra IF stage, using the proper tube is a must.

OK. So now I'm back to my original questions. Does the unit quiet properly so that there is no static or rushing sound behind the audio signal? In other words, is the signal otherwise normal other than having to turn the volume up much higher than normal to produce otherwise normal sound? And is this the case when FM Mono is selected?

If so, then the problem almost certainly will be located around the final IF strip (ratio detector) transformer (Z4 or Z5 depending on which model you are working on). It is in these transformers where the audio component is actually recovered from the FM signal.

In the case of a normally captured FM signal, the audio output level is not dependent on the strength of signal received, but on the frequency deviation as created at the transmitter. Since we can assume that the station is not the problem, then the problem must be associated with the ratio detector transformer that recovers the audio.

Fisher included the detector diodes within these transformers, so it may be than disassembly is required to test those.

Let us know what you find!

Dave

Also, has anyone by chance been into the tuner and "tightened the screws" or something else that might have knocked the alignment off? If the detector is off kilter, it might produce weak sound, though I would expect it to be distorted.

Well, it's gone from bad to worse. Retensioned all the sockets and cleaned pins & sockets with deoxit, put it all back and no sound - just hiss but maybe the faintest hint of a station at full volume. Gadget, not sure if someone's turned any screws.

Dave, sorry forgot to include the 6BA6 which it has, no 6HS6. When I had an audio signal there was no static or rushing sound. I thought to myself that it sounded good but very weak. Mono was the same.

I located these test points on the schematic but couldn't find any write-up on how to test.

Bruce -- Is the Stereo Beacon operating properly in stereo mode? If so, that -- once again -- helps to narrow things down to V6 and Z5. Since moving things around -- even ever so slightly by retensioning the socket pins -- has made things go from bad to worse, I think it it time for the chop stick procedure to slightly push at different terminals while it is operating to see what impact that has on the audio output.

Dave

Bruce; The Test points are listed in the FM Alignment. Connect the DC VTVM during various evolutions.



Dip a pipe cleaner in DeOxit and clean the pins THEN re-tension them. Also DeOxit on the Tube pins (use a toothbrush.) I find IPA doesn't do that much to clean them, unless it's De-Natured Alcohol. Last thing to check is poor or cold solder joints on the sockets themselves.

Larry

solder joints are worth checking since it got worse after cleaning and moving the socket terminals a bit. I had a broken solder joint on one of the resistors in my Sherwood that caused the FM RF amp to not operate. It sort-of received but only one station and just barely at that. Moving the tube around would sometimes make it connect and then it would blow your eardrums out.

Woo-hoo, dancing banana time

Just finished moon-walking across the floor. Found that rascal! I was doing the voltages at the test points per Larry's chart and couldn't get a reading at Test Point 4 - see schematic below. There's a black wire that comes off L7 that goes to the MPX section and hooks up with a 47k resistor. Tested connectivity and nothing. Wiggled until I got the buzzer on my DMM. Fired her up and lovely FM sound. I'm a happy camper!

Many thanks you guys for all your help :thmbsp:


Here's a shot where that black wire comes in and the cold joint - you were right, gadget.

Glad you found it! L7 is in fact the output of the Ratio Detector circuit, which is right at the point in the circuit that your collective symptoms indicated the problem should be. Persistence pays off every time!

Dave

Thanks, Dave. I always learn something when you contribute to the thread. Your observations are educational as well as instructive. You helped me understand more about the 400 radio circuit but I still have a long way to go

Cool. Finding loose connections like that can be amazingly time consuming for what ends up being a very simple fix.

Can I have my Ball of String and tin can's back now?

larryderouin said:

can i have my ball of string and tin can's back now? :d

Click to expand...

:d :d