Fisher TFM 1000 problems

[speaker dave]

I'm trying to sort out a Fisher TFM 1000 that I've owned for a number of years now. It has low sensitivity but does receive.

Has anybody worked on one out there?

I have a Soundtech generator and can do a full alignment, but it is distinctly low in sensitivity. It doesn't start to limit until about 200 uv. The signal strength meter never gets off the peg although I know the meter circuit works by injecting a little current into it.

This means that either the front end or the IF are weak somewhere, not the limiter or pulse count detector that come after metering.

Voltages throughout look plausible. Good through the IF transistors. The manual gives front end current numbers through the first three FETs and the oscillator. First FET current is high at 7.1 vs. 4.5 ma spec. 2nd is close to spec and the third is low at 0.5 vs. 0.9 spec. Does either direction off mean anything?

The front end and the IF have individual agc loops but they don't seem to be causing the problems. All of the RF and IF transformers peak to some degree.

Any Ideas?
David

 

[Punker X]

Try to inject 10.7M at the first IF stage. If signal passes and everything peaks well, then you have trouble in the frontend/local osc. If not trace the IF path to the failure.

Check all your grounds and all connections, look for bad solder joints etc.

 

[speaker dave]

Try to inject 10.7M at the first IF stage. If signal passes and everything peaks well, then you have trouble in the frontend/local osc. If not trace the IF path to the failure.

Check all your grounds and all connections, look for bad solder joints etc.

Hey Punker, thanks for the suggestions. I was thinking similarly about a capacitor bypass shunting each IF stage in turn and then pulling the IF signal out of a working tuner. Most front ends have a coax out and you should be able to patch a working front end into a suspect IF (since I don't have a 10.7MHz generator).

But... I think I found the primary problem. Being an early transistor tuner, Fisher upped their overload margin with an attenuating diode just before the front end. For signal inputs above 100mV it was supposed to attenuate the input and turn on a front panel light. Of course the whole circuit is driven via the light and a burned out light sets the whole circuit off and attenuates the input. Bypass the light and the quieting commences around 10uV (75 ohm). A lot better than 200uv that it was.

So I will replace the light (pea light or LED?). There are some secondary issues because the sensitivity is still a bit off and the metering circuit still shows nothing, indicating low IF gain.

Still, one step at a time.

David