Help with a 9090 right distorted channel

[Caughtthebug]

Let me start at the beginning. My father in law bought this new. He put it away years ago when the pots became full of static. He ask me about it and I said let's Deoxit all the switches and pots. After a period of dry time we power it up. Everything was fine. And then nothing. Did a little research and found out about the fuse resistors. Tested and found two opened up. So I ordered every thing to rebuild the driver board and recap and replacement transistors for the other boards except the tuner section. Re did the driver board and everything was fine. Adjusted the dc offset and bias. Then I moved on to f2546, f2541, f2431, f2547 ,f2543 ,f2544, f2542, f2550 and f2597. After done found right channel some what muted and distorted. I've been over all the transistors placement as far as e c b. Pulled all of them and metered them all tested good. Check all caps and placement. Even know I checked the value of them when ordered and as I replaced. I rechecked them again. I've been checking the silk screen on the board and board layout to the schematic. And even my notes. I've been at this for a few weeks now. I know it's something stupid but where. Any help or guidance would be much appreciated. Also I ran this out of preamp in to another amp so it's not in the amp. And I have it in the aux. As well.

 

[avionic]

Check your fuses with and ohmmeter. The fuses that you had to remove to adjust bias.

 

[Tom B]

Also with the same eraser pencil, tap on the speaker protect relay to see if you have any change. I usually use headphones to be able to hear for changes (popping as you tap, level go up and down as you tap, distortion comes and goes as you tap). I the relay is real dirty, it can cause this type of issue as well as what the fellows above have mentioned.

 

[Caughtthebug]

Thanks guys for the help. I have all ready check the fuses. But I do as I'm told. I pulled and tested the fuses. Took an eraser to fuse holders and a moment of joy. The one for the right was loose. re flowed all of the joints. Check with dim bulb. Powered off and re hooked up. But it's still there. This is what I noticed. with using rca's and going from left to right. That it is in the pre amp side. The the right watt meter shows less than the left. The stereo light was working. But not now(could just be the bulb) But just seems strange its not. I'll test that. I have thought of the relay. But it was fine before. But I will remove and clean contacts and inspect. And post back.

 

[Caughtthebug]

Ok I did get the relay off and cleaned the contacts. One of the for fingers had a heat mark just above the contact. Twisted every so slightly. I have good continuity. Part of me doesn't like the heat mark. It doesn't show up on a pic for me to post. Part of me thinks just order a new one. Are they troublesome.

 

[Caughtthebug]

Would anyone now what a good reading be for the coil side of the relay.

 

[The Fuxtor]

You can buy a perfect replacement from Mouser ( i think?) or from where i bought mine, Digikey. I dont have the part number handy tho.....

 

[Hipocrates]

9090------->Omron MY4-02-DC24
9090db---->Omron MY3-02-DC24

 

[Caughtthebug]

Well today I decided to work on unfinished projects. I put the new relay in and it didn't change anything. I'm going to look at what was replaced in the right signal path and re check my work for the millionth time. Any thoughts or suggestions welcome.

 

[LBPete]

To start narrowing it down, pull the preout jumpers and use an rca cable to cross connect the outputs so the left pre is driving the right main and vise versa. If the noise swiches sides, the trouble is in the pre amp. If it stays on the same side, it's in the amp. If it's in the preamp, "exercise" ALL the controls to see if any of them change the noise. If so, clean that control again. If it's in the amp, concentrate on the circuit after the driver board. This includes the protection board, protection relay and speaker switch.

Can you post the serial number for the database? Click on the link in my signature.

- Pete

 

[Caughtthebug]

Thanks Pete I have done that with the RCA's. And I ran the amp section with an other pre amp. And pre through another amp. And it seems to be isolated to the right pre amp. I have not found any difference working the switches. But did clean the switches for the second time and blown them out with air. Just in case any foreign objects fell in.

 

[LBPete]

Then it's time to trace the signal with an oscilloscope. It's the only good way to find something like this. On second thought, it's a little work but may be worth a try. Sansui was very consistent using blue and gray wire for the audio signal. If you look at the boards in the preamp, you will see blue and gray wires running from board to board. Follow the signal from the input on the back. Where it transitions from one board to the next, unsolder and swap the wires left to right. When the noise transfers to the other channel, you found the board with the noise.

- Pete

 

[Caughtthebug]

Pete I do not have a oscilloscope. And to be honest I'm afraid of messing it up past repair at this point. I may find a good shop to scope it. But switching the wire is a good Idea. And would help pinpoint the area. Thanks Pete I will get back to you soon.

 

[Caughtthebug]

Pete I I did what you suggested. I started at the equalizer board and moved forward. I found no difference until f2543 master vr and balance vr board. Switch leads 3and4 going to the tone control board. Every thing work fine. Turned balance control left and right. Both channels clear and equal. Replaced leads back to the correct location and it went back to right being muffled and lacking power. The leads ohm out good. I reversed leads again to get the result. Both clear and equal.

 

[Caughtthebug]

Pete I've been thinking about this. It makes no sense. How does switching channels fix it. But when there back to normal I have distortion in the right channel. I'm going to pick up wire to make new leads tomorrow. Correct me if I'm wrong. But just reversing left with right leads fix it. But putting them back cause the distortion. And I reversed them again and it fix it. In my mind then I don't have a component failure or have any thing in wrong.

 

[LBPete]

Switching the leads left to right Won't fix it. What it will do is identify the board where the channel drop is occurring. Once you identify the board, you need to look at that board to find the source of the channel drop.

- Pete