Need Bias Current Help with Sansui AU-20000 (0 mv)

[Utk]

Hey everyone,

I made some custom wires to bring out both driver boards so I could test them on my desk. After swapping the VD1212 with the original diode on the board and some Transistors, everything seems to have started working. However, I noticed that the Bias setting for 35mV is at the very edge of the 1K VR02 in the clockwise direction. It ramps up very quickly, and I only get about half a turn on my 25 or 22-turn pot to reach the correct value and it's at like at the 3rd turn from the end. It seems like the scaling might have drifted over time. Not sure if this is because the transistors are old because all the Resistors are within 1% of the spech.

I also changed some transistors. Here is the new SUB_LIST: The last CHANGED_TO Column is what I have now:





One question:

I set the bias and bias current after the amp was warm, but when it cold starts, there is a significant DC offset that dissipates in about 4-5 seconds. However, the speaker relay usually clicks in within 2 seconds. If the speaker selector is on System A, for example, I hear a loud pop from the speakers when RL01 clicks. If I have the selector set to 'off' on power up and then switch to Speaker A, the pop isn't there.

Additionally, I noticed that the first cold start pop is much louder than subsequent ones, which get fainter as I keep turning the amp off and on, but it doesn't completely go away. I've swapped everything but the main filter caps and the cap on the power switch. The DC offset at the speaker terminal poses a problem, as it takes some time to settle, and I suspect RL01 clicks too quickly. I've made sure to replace the relays and caps on the F-2511 board with the exact values that were there before.

Lastly, I want to thank everyone for all the help they have provided. I couldn't have done it without your Help!

 

[Utk]

Looks, like the Timing is controlled by TR05, R23 & C905 (RC Circuit determining when TR05 turns on/off ?). Or is it controlled on the lower side? I can try to increase the R23 from 1.2K to 2.4K or C905 from 10uF to 20uF might double the timing for this relay. Or is this a hack and usually not recommended?

 

[richardpp]

The delay for speaker relay RL01 is mostly due to the amount of time it takes for C10 to discharge. The schematic shows a value 10µF but there is usually 4.7µF in parallel on the conductor side of the F-2511 board which increases the capacitance to around 15µF for a delay time around 7 seconds.

R23 limits current to the base of TR05 which controls the LED switching. C905, while it would also contribute to the delay, is there to ensure a positive turn on/ turn off the relay, i.e. preventing minor voltage fluctuations from effecting the relay switching. I would try experimenting first with C10 first.