• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Fisher Allegro FM-19 tuner question

Ziradog

Ziradog

AK Subscriber
Subscriber
I have been working on the FM-19 tuner from an Allegro console, with an installed MX-78 multiplex. I have it working, and it sounds good in mono. In stereo, it sounds good when tuned but between stations it roars. I know there is no muting circuit on this tuner, but in mono I get normal-sounding hiss between stations, in stereo it is much more of a roar. Is this normal? If not, where do I start? The unit has been recapped (by me), and the power supply on the amp has been redone (this tuner pulls power from the amp). Any thoughts?

Thanks!
 
Register to hide this ad
In mono mode, the audio signal is taken directly from the ratio detector circuit, and therefore does not pass through the MPX sub-chassis.

You mention that the unit has been recapped -- did this include the multiplex sub-chassis as well? If not, check the 1 uF electrolytic cap on it for failure. This is a common part failure after 50 or so years if it has not already been replaced. Leakage of this cap can upset the balance of the ring diode detectors and allow for elevated noise. Either that, or check the diodes themselves.

Good luck with it!

Dave
 
Dave, I did replace the electrolytic in the MPX, but did not replace (or test) the diodes. I will try that.

Yes Larry, this is the same unit. I posted this question here as it seemed more Fisher-specific.
 
Wonder if the "separation" pot on top of the MPX has been "adjusted". It should be about the middle of it's travel, not up against the stops. If you are looking at the unit top from the side, the pot is sticking up between V101 and V102. "Hitting the stops" The marker on the knob(looks like a small wedge on the side of the knob on the base of the knob) is either at about 10:30 or 1:30. Middle of travel the marker is between 5:30 and 6:00 .

If it's already at mid point DON'T MESS WITH IT.
 
Last edited:
I will check. Interestingly, once I put ~20 minutes on the tuner, the "roaring" between stations went away, and has stayed away ever since (even when starting up from cold).
 
You must log in or register to reply here.
Back
Top Bottom