GF-555 Preamp gain problem

[Robertsozo]

Hello!
I am restoring a Sharp GF-555 that I got for a very reasonable price. So far I have fixed some usual suspects of this machine (bad belts, broken wiper in volume pot, e.t.c.)
Now however I have encountered a problem that I cant find the solution to. Speaker output of the machine is incredibly loud. It is excessively loud even when volume pot is at less than 1/4 of its travel. Also when radio is tuned to a station, the VU meters are in the red almost all the time. This leads me to believe that there is a problem in one of its amplifier IC`s and somewhere the signal level is too high.
I started to investigate this by inputting a 0,3V 1kHz sine wave signal into ''Line in'' of the unit. When probing ''Line out'' I got a 1,4V output signal. I feel this is strange, because I think that Line in and Line out output levels should be almost the same. Please correct me if I am wrong.

After that I moved my suspicion to ''IC 104'' which is a Rec & Line amp according to service manual. When probing its output pin I got an even higher 2.6 V signal level.
After this I tried to find a datasheet for this IC to maybe understand how the input gain is controlled. No luck here, there is no datasheet for this IC. All I could find was a block diagram of it. I am not an electronics expert at all, and this tells me very little about how the gain is controlled.

Can someone please advise if I am looking in the right direction, maybe these signals are as they should be. Also maybe someone has knowledge on how input gain is controlled for this preamp IC.

Any help is appreciated, Thanks!

Probing ''Line out''


The results: CH1 is what I input to ''Line In'' and CH2 is output of ''Line Out''



Probing output pin of IC104



Results:



The IC in question in the schematic diagram:



IC104 block diagram:

 

[caution]

Going to be honest with you, there aren't many folks on this site capable of helping you with an issue this complex.
You might want to try Audiokarma or Tapeheads.
Good luck.

 

[Superduper]

It is not unusual to for an amplifier to "amplify", it is what they do. Nor is it unusual for the signal level to be amplified prior to the tone controls, as those can have the effect of attenuating a signal during the signal shaping process. Keep in mind that the tone controls section is a passive section, and not active. If you think about this for a second, tone controls at neutral position needs to be able to increase the signal level when the control is turned up to max. Since these are passive circuits, it can't literally "turn up" a signal, all it can do is attentuate less at the high position as opposed to the middle or low position. Also it is unclear if the purpose of that line amp is to actually amplify the voltage, or simply act as a buffer amp so that the signal can be split without being dragged down. I think you are on the right track though, the signal does sound like it's being over-amplified.

You can try the following:

(1) Replace that IC
(2) As you have the schematic diagram with voltage specs, test (all of) the pins of the IC and compare the actual voltages you observe with the specs as per the schematic. If you find anything significant, that could indicate an issue with an appurtenant component, or the IC itself. However, first things first, check the voltages and see how close you get. Especially pins 2/15/16 as those are related to the gain control.
(3) Check your PCB carefully. The top edge appears to have significant corrosion issues which could have compromised the continuity of the traces and solder joints. Any alterations would change the dynamics of the circuitry.