Fisher 200-T FM Signal Strength Meter Problem

[Pioneer Vir]

The FM signal strength meter stays pegged at maximum strength as soon as the receiver is turned on and the reading will not change until it is turned off. Stays pegged regardless of function selected - Aux, FM, FM Local, Phono. FM is working great in either FM or FM Local. Stereo Beacon is working correctly. Stereo / Mono switch is working correctly. I verified all of the connections and voltages on the Meter and Muting board and everything is SAT. I did the FM Tuning Meter Calibration from the service manual - no change. I have not changed any components on the Meter and Muting board yet. I'm hoping that won't be necessary. If anyone has any ideas or if I'm missing something simple here would appreciate your input on it.

Don

 

[Tom B]

With the unit unplugged, check the trimpot on the board and make sure it is working properly. Its actually used as a variable resistor, so it should range from 1k ohm to a short as you turn it. If the trimpot is ok then you hay have a bad UA703 IC or Q701 is bad.

 

[Pioneer Vir]

Trim pot is working correctly - 1K down to 0 ohms. Will look at the IC and/or transistor next.

 

[larryderouin]

On the IC use a sig-gen and a separate amp, with a probe on the end. The probe can be as simple as an RCA Cord plugged into the AUX terminal on the separate amp. Solder a .01uf 600v cap to the center pin on the RCA plug. Ground the chassis together. Probe Pins 3 & 5 INPUTS. The Probe Pins 1 & 7 OUTPUT's. They should be equal or a little louder on 1 & 7. NTE is the only supplier out there and they ain't cheap. North of $10.00 last I heard. (See pdf. ) Last time I got the 703 was 10 years ago or so and had to get it from NEWARK Electronics. They may or may not carry NTE Now. I know Mouser quit about 2 years ago. You can also check Digikey.

The BC147B is no longer being made. It's replacement is the BC547B. The 2 main differences are the pinout's are different (get a copy of the datasheet for the 547) and the 547 is in a TO-92 case. other than that they look to be viable replacment.
BDENT has them for $0.34each. I'd grab a handful as the early FISHER's used them a lot. I'd check TR701 1st along with 702 (both BC147b's) b4 messing with the IC.
http://www.bdent.com/bc547b-philips-general-purpose-transistor.html

Larry

 

[Pioneer Vir]

Thanks Larry. I'll definitely go the transistor route first.

 

[Tom B]

I used to troubleshoot those UA703 IC circuits in the 70's using a .01uf orange cap. As these IC's were usually used as the IF amps, and many times there were 2 or 3 on the IF board. I would use the .01uf cap and jumper pin 3 to pin 7 if I had a bad FM problem. If upon bridging those 2 connections with the cap the FM level increased, or I got some FM signal whereas before I had none, I would replace the IC as it was bad, and not passing the signal. If I jumped across those 2 pins and the FM went lower, it meant the IC was fine (it was amplifying). Remember though, that when you jumpt the input to the output to test if an amplifier is actually amplifying, alway use a cap like a 0.01uf @100v or more, never use a piece of wire or resistor. The capacitor blocks any DC from passing, only allowing the "Signal" to pass, while the wire or resistor will pass DC and may cause problems with the component or others.
But this meter circuit is different, and dont think the jumpering with a cap will be a worthwhile exercise. As you are getting good FM and Stereo light, if the problem is in the IC, its more related to a shorted junction internal, rather than an open junction internal.

 

[Pioneer Vir]

Fixed. Bad TR701. Found some BC147B transistors in the UK. Replaced TR702 while I had the board out. Signal strength meter working correctly now.

Don