STA-2100 (Left Channel AMP) Problem

[Super Noob]

Currently rebuilding my STA-2100, and have run into a problem. It revolves around the main amp board in the left channel. In my infinite wisdom, I thought I would change a few transistors on the AMP boards. The SA872s (TR501 & TR502) which I used 992s HFE matched to less than 1%. As well, I changed the TR513 & TR514 (2SC1175 / 2SA659). I incorrectly replaced the 2SA659 with KSC2383... when I tested with DBT the 60w light was bright and would not power on (1st relay would not engage). I pulled the AMP board and found the error. Corrected the replacement of 2SA659 with KSA1013. I then re-checked all the transistors & diodes on the board and everything checked out including the outputs.

Reassembled and tested, and the initial relay would not engage and the DBT glows bright. Since then I reinstalled the original transistors and still cannot get the main relay to engage to power the unit. Here is the interesting part. If I unplug the left channel amp board, the unit powers on (main relay engages) and then the protection engages and the bulb goes dim. Since then, I pulled very cap and resistor out of circuit and they test good / within tolerance on the problem AMP board. Am really perplexed.

Before I rebuilt the board, the receiver powered on with good offset + biasing and was working fine. To say I am embarrassed is an understatement. This is a self inflicted problem to a working set. I am looking for guidance to what might be causing the main power relay to not engage. Been grinding looking for problem for a good while now.

https://drive.google.com/open?id=1EiEV2EeO8CQOM3d50LmQmBbXPs845wO-

 

[cademan]

This may not be the problem, but make sure the new transistors are oriented the correct way. Compare the diagram on the packaging to the print on the circuit board.

 

[Super Noob]

This may not be the problem, but make sure the new transistors are oriented the correct way. Compare the diagram on the packaging to the print on the circuit board.

I have spent hours on this board... checking all the components. Everything checks out.

Something on AMP board pin 13 (L) has an effect on the voltage (energizing) of the main relay coil on the main relay PCB. I follow the 13L over to protection board pin 2. The signal sets ready to be passed through once the protection relay makes up (if it is within tolerance); however, I don't understand how it is impacting main relay coil / phase 1 turn on? Wish my troubleshooting skills / schematic tracing was better.

 

[Tom B]

Pin 13 is the output it appears. What is the voltage on it? You may want to do the DC balance (DC offset adjustment) to bring the voltage to 0 volts if you can. Anything above 1 volt DC will usually keep the unit in protect without energizing the relay. If it is high Dc and wont adjust, you may have bad components or broken foil connection on the board. Bias adjustment should also be at minimum initially.

 

[Super Noob]

Pin 13 is the output it appears. What is the voltage on it? You may want to do the DC balance (DC offset adjustment) to bring the voltage to 0 volts if you can. Anything above 1 volt DC will usually keep the unit in protect without energizing the relay. If it is high Dc and wont adjust, you may have bad components or broken foil connection on the board. Bias adjustment should also be at minimum initially.

Thanks Tom... the unit doesn’t turn on. Main relay not protect. Going to hook everything back up... and upsize the DBT wattage from 60w. Maybe unhook right and see what happens. I set the new offset trimmer to same resistance as the old one before installing.

*EDIT *
100W light in DBT = same result. Will not pull in power relay with left channel amp attached?

Unplug and the unit powers on (main relay) pull in / then protection relay pulls in. Something is not right = maybe it is me.

 

[petehall347]

I have spent hours on this board... checking all the components. Everything checks out.

Something on AMP board pin 13 (L) has an effect on the voltage (energizing) of the main relay coil on the main relay PCB. I follow the 13L over to protection board pin 2. The signal sets ready to be passed through once the protection relay makes up (if it is within tolerance); however, I don't understand how it is impacting main relay coil / phase 1 turn on? Wish my troubleshooting skills / schematic tracing was better.

whats going on with r501a ?

 

[Super Noob]

whats going on with r501a ?

Hey Petey... measured every resistor out of circuit. R501 measured out good. I will get the value vs. reading in a few minutes.

 

[petehall347]

vr501A i meant .. might be the trick of the picture but looks very odd to me . as if a large is missing .

 

[Super Noob]

vr501A i meant .. might be the trick of the picture but looks very odd to me . as if a large is missing .

VR501 =
PV36P502C01B00 (mouser)
Bourns 5K / 1/2w

 

[petehall347]

i was looking how it is installed and the shape of it . somehow doesn't look right to me .

 

[Super Noob]

i was looking how it is installed and the shape of it . somehow doesn't look right to me .

 

[petehall347]

 

[Super Noob]

View attachment 1256632

Petey... it is the angle. You are seeing the heat sink of one the transistors. It is sitting on the other side. It is cocked ever so slightly to get on the 25 turn screw once it is installed in the receiver.

 

[petehall347]

rules that out then ..

 

[Tom B]

Thanks Tom... the unit doesn’t turn on. Main relay not protect. Going to hook everything back up... and upsize the DBT wattage from 60w. Maybe unhook right and see what happens. I set the new offset trimmer to same resistance as the old one before installing.

*EDIT *
100W light in DBT = same result. Will not pull in power relay with left channel amp attached?

Unplug and the unit powers on (main relay) pull in / then protection relay pulls in. Something is not right = maybe it is me.

Did you measure the voltage on pin 13 even though unit stays in protect?? Pin 13 is the output of the power amp, prior to going to the protect relay, so you will be able to see voltage. If your DC balance (offset) is badly mis adjusted, the unit wont come out of protect.

 

[Super Noob]

Tom... I got it going. An output wasn't seated properly (say hello to stupid). Today I made another mistake - I crossed the power supply links on the amp board when I trying to get a better clip on the resistor for biasing (= stupid). Thinking the main rectifier may be toast now - just walked away for a day or so. I have rebuild SX1250, SX850, SX750, STA-2000, STA-2100 (previously), Technics SA-800, Sansui 800, Marantz 2325, but I am just struggling with my STA-2100. Just frustrated with my dumb errors. They have said , if your going to be stupid = you better be tough.