dead FM

[Alfonsol]

Hi
I have a muskat mpx-20 stereo receiver with a dead FM section.

i tried to find the schematic, but i found very few references to the brand on google.

The AM section the phono input and the 8-track player are working, but the FM is almost dead.

when i select FM i hear only hum, but if i put full volume and use headphones i can listen to FM, but very very quiet.

i changed some caps, because the main filter cap was dented. and the other caps looked very ugly. i didn't changed all the electrolitic caps yet because is a very difficult board to work on. After changing some caps the AM reception improved a lot, but FM is still dead.

 

[Punker X]

Might be hard finding a scat for this, but I'd check you FM power first. Most likely 12 volts.

 

[Alfonsol]

today i've been changing some caps and the results are encouraging.
FM is a lot stronger,but i can listen only the left channel and it's still a lot quieter than AM.

While changing caps i've been figuring out the circuit to know where to measure voltages.

 

[amptramp]

The nice thing about having a fault in one stereo channel is you can measure both channels and see where the differences are. This will usually point to the fault.

 

[Alfonsol]

I found something.
the stereo decoder is an LM1307. the stereo output of this IC is connected to the input selector, after the input selector the audio path is common to all the other inputs and i know this is working. So i think i have a faulty LM1307. Voltages look good.

 

[Punker X]

If the MPX chip goes, you usually receive, but just not in stereo. What are your tuning meters doing? Do they swing when trying to tune.

 

[Alfonsol]

It doesn't have a tuning meter.
After changing all the electrolityc caps in the tuner i actually receive in FM but at low volume and only one channel. Every cap i changed made an improvement. it looks like the right channel of the mpx chip is dead. Also there is a burnt resistor on the pin that drives the stereo light.

Also after changing caps the AM section improved a lot.

Also i chaqnged the caps in the power supply, but i didn't changed yet the caps on the amp section.

Also listening carefully on FM, altough the volume is low, i noticed the sensitivity is good, but the selectivity is bad.

The AM section sounds loud, and the sensituvity is good, the selectivity is bad too.

 

[Alfonsol]

I confirmed the LM1307 is bad. i injected a sigtal just at the output of this IC and i have audio on both channels.

 

[Alfonsol]

I replaced the LM1307 and now FM sounds loud on both channels and in stereo. also the stereo light now works. It gets lots of stations.

but.. when i tune stations it's very scratchy and very touchy. some stations sounds clear, some other distorted. also some stations are not where they are suposed to be, it looks like the tuning cap is dirty, but i don't know how to clean it.

 

[Punker X]

Check out the stick thread at the top of the forum.

 

[Alfonsol]

thanks, i just posted there.

 

[Alfonsol]

I found some nos tuning caps with the same footprint and pinout. They say are suitable for AM/FM
i don't know the value of the caps inside the original one. I only know it have 2 caps for FM and 2 caps for AM. Should the replacement work? or there are too many values between differents caps to know?

I don't have the schematic, but i know where on the tuner cap are connected the oscillator and antenna, on both AM and FM and wich trimmers i should adjust.

For the IF cans will be more tricky, but i think i can figure it out.

 

[Alfonsol]

i put contact cleaner on the shaft and rotated it several times. it improved a lot.

still i have an odd behaviour. when tuned in a place where there are no stations. i can hear a whistle . the pith of the whistle is related to the position on the dial. the pitch at 108MHZ is higer than the pitch at 88MHZ