QRX-5500A FM audio problem

[crkeith65]

I am getting real close on this but I seem to be at a loss from this point on.
1. The Problem: Very low volume on FM and FM Stereo> In other words I have to turn the volume all the way up to even hear it. (AM AUX and Tape in work fine)

2. What I have noticed: Troubleshooting this problem I first found that I could inject an audio signal to the outputs Pins 14 and 15 of the F-1517 Tuner Circuit board and hear Audio which rules out dirty switches and pushbuttons, etc. Then I noticed that if I connected the wires that were connected to Pins 14 and 15 to JP6 and JP7 I could hear perfectly working FM Stereo which rules out any RF or IF problems. Next I noticed that if I reinstalled the wires to Pins 14 and 15 and then lifted R58 FM seemed to work fine. I could remove TR-11 with same results.

3. What I have done so far: I checked or replaced TR11, TR15, TR16, D05, C59, C47, and Checked the FM Muting switch which grounds Pin 9.
Someone good at analyzing schematics can probably pick this out in a matter of minutes. Not me I struggle at every level. I usually end up getting it but always takes too long. Please respond with any suggestions. Thanks.

Here is a link to my service manual:
http://akdatabase.org/AKview/albums/userpics/10007/Sansui QRX-5500A, QRX-7500A Service Manual.pdf

 

[Tom B]

With FM muting off, do you get reasonable sound including static in between stations?

 

[crkeith65]

With FM muting off, do you get reasonable sound including static in between stations?

Yes when I have the resistor lifted. Actually there is no change in sound whether the muting is on or off. With the resistor lifted the FM sounds perfect.

 

[crkeith65]

Has anybody had a chance to study my schematic to see what the effect is of lifting R58? With it lifted FM stereo works perfect. With it in very faint FM but still a good signal. Is it part of the FM muting circuit since there is no change with the FM mute button in or out? What would be the harm in leaving R58 out of the circuit? Not that I want to but this has become very time consuming and keeping me from going any further such as a noisy popping Quad board. ( Not related, the unit sounds wonderful in stereo with the F-2048 quad board removed. This will be a discussion for a later date.) Here is a page from the schematic: The full SM is linked above.

 

[rcs16]

R58 is the mute signal to Tr15,16.
Sounds like mute is always on. The switch S41 grounds this signal, which is the same as lifting R58!! Why is it not doing its job?
The level detector is not working or the schmitt trigger switch has bad and/or improper installment of components.
The IF signal is taken off at TP 2, amplified by Tr7,8, peaked using T5 (TP5 monitor point)and rectified by D03. The DC signal goes through 2 pots for adjusting level, one for the signal meter and the other for the input to the schmitt trigger mute switch.
Check C46,47 in the mute ckt!
Is the signal level meter working okay?

Good Luck

 

[crkeith65]

R58 is the mute signal to Tr15,16.
Sounds like mute is always on. The switch S41 grounds this signal, which is the same as lifting R58!! Why is it not doing its job?
The level detector is not working or the schmitt trigger switch has bad and/or improper installment of components.
The IF signal is taken off at TP 2, amplified by Tr7,8, peaked using T5 (TP5 monitor point)and rectified by D03. The DC signal goes through 2 pots for adjusting level, one for the signal meter and the other for the input to the schmitt trigger mute switch.
Check C46,47 in the mute ckt!
Is the signal level meter working okay?

Good Luck[/QUOTEThanks for the good ideas rcs16. Yes the signal meter works. The mute switch does ground so I will check the other suggestions.

Thanks for the good ideas rcs16. Yes the signal meter works. The mute switch does ground so I will check the other suggestions.

 

[crkeith65]

R58 is the mute signal to Tr15,16.
Sounds like mute is always on. The switch S41 grounds this signal, which is the same as lifting R58!! Why is it not doing its job?
The level detector is not working or the schmitt trigger switch has bad and/or improper installment of components.
The IF signal is taken off at TP 2, amplified by Tr7,8, peaked using T5 (TP5 monitor point)and rectified by D03. The DC signal goes through 2 pots for adjusting level, one for the signal meter and the other for the input to the schmitt trigger mute switch.
Check C46,47 in the mute ckt!
Is the signal level meter working okay?

Good Luck

Thanks for helping me understand the mute circuit. Help on this forum is more valuable than a hundred YouTube videos. I did get the circuit working thanks to your suggestions. Either c46 or c47 was the problem. They checked good but when I replaced them it worked. Thanks again.

 

[rcs16]

Great news ... I laugh at my friends asking to fix something and I say just have to find that $0.10 component, problem is finding it or just change them all in hope that you get the right one.
In your case it was only a few possibilities.
Enjoy