I have a BIAS problem...

[Wickeviking]

Hello! I have tried to adjust the BIAS current on my QRX-3000 to specified 20mA. The problem is it will not stay there. It is drifting between 10mA and 35mA depending on temperature. It is so very temperature sensitive that it is enough to blow air on the board to put the current down to 10mA. All the boards are renewed with Elna silmic2 caps. An old repair, where the output transistors and BIAS diodes where changed, are changed back to original. I found new original transistors (old stock) and buyed the SV-02 diodes/varistors from Sanken. I suspect the data on the modern SV-02 are changed from the one made 1975 ??? Any one knows? Anyone who can sell me the Sanken SV-02 from around 1975? Or, what else can be wrong? Any ideas/advise will be very welcome!!!
/Goran

 

[Morden2004]

Try freezing the components

Hello! I have tried to adjust the BIAS current on my QRX-3000 to specified 20mA. The problem is it will not stay there. It is drifting between 10mA and 35mA depending on temperature. It is so very temperature sensitive that it is enough to blow air on the board to put the current down to 10mA. All the boards are renewed with Elna silmic2 caps. An old repair, where the output transistors and BIAS diodes where changed, are changed back to original. I found new original transistors (old stock) and buyed the SV-02 diodes/varistors from Sanken. I suspect the data on the modern SV-02 are changed from the one made 1975 ??? Any one knows? Anyone who can sell me the Sanken SV-02 from around 1975? Or, what else can be wrong? Any ideas/advise will be very welcome!!!
/Goran

This certainly sounds like a faulty semiconductor. I think your best approach is to get a can of 'freeze-it', like SUPER COLD by M.G. Chemicals.

If you're not familiar with this type of product, it is an aerosol can of gas that simply freezes the components you spray it on. Spray it on one component at a time with the unit powered up and at operating temperature. Wait a few seconds while you observe your meter or scope. If that device (it can be a semiconductor, a reststor, etc.) is heat sensitive, your circuit readings will jump erratically or change dramatically. When that happens, you have (usually) discovered the faulty component.

It's easy and quick to do. :yes:

Paul

 

[Wickeviking]

I have the same problem on all four channels :sigh: ,supersensitivity to temperature changes. I have allso discovered that the powersupply in my building are unstable. It varies between 210 and 240 Vac. I think that is not making it easier either...
Thank's for the advise :thmbsp: I will se what I can do with that freezing stuff.

 

[EchoWars]

Typical Sansui stuff...most seem to be undercompensated for temperature. The gain of transistors rises as they get warm/hot, so designers mount a thermal diode or bias servo transistor on the power transistor heatsink for compensation. Thing is, the amount of bias compensation on many Sansui amps is not really enough (and I shouldn't just pick on Sansui, as many amplifiers are the same way).

Make sure that the externally mounted component (whatever it might be on that amp) has good contact with the heatsink, and set the bias to the low end of the adjustment range...perhaps 10mA or something...and forget about it.

 

[Wickeviking]

Thank you! It is working fine now, sounding very good. But I had to adjust it to around 8mA .
I had a look at the datasheet for Sanken SV-02, and it seems to me that it is not starting to work before the heat sink reaches a temperature of 70*C ??. Take a look at the graph in the lower right corner. http://www.sanken-ele.co.jp/en/prod/semicon/pdf/sv02yse.pdf
Can this version of SV-02 really be the right thing to put on this board?
Anyway, it is working great now,,,adjusted out of spec...