Soundstream Class A 5.2 Won't Power Off

[madHD]

With C57 removed rail voltage is minimal ( mV range). This is the same with c57 installed.

 

[madHD]

C57 checked out ok.

 

[Perry Babin]

What is the DC voltage on all 3 legs of Q38 and Q51. Black probe on the amp's ground terminal.

 

[madHD]

Q51 (MPS8099) is 1-E, 2-B, 3-C and read 1 = 0V, 2 = 44mV, 3 = 105mV.
Q38 (J175) is 1-S, 2-G, 3-D and read 1 = 0, 2 = 105mV, 3 = 5.5mV.
I did my best to get the right pins but I am not sure I am indexed correctly on that.

 

[Perry Babin]

Confirm that you have 12v on one end of R125.

 

[madHD]

R125 measures 11.47V with a 12.2V B+. The other side measures 103mV.

 

[Perry Babin]

Remove Q51. Does the voltage on the collector pad for Q51 go to near 12v?

 

[madHD]

Yes. It went to 10.8V.

 

[Perry Babin]

Re-check it out of the circuit.

 

[madHD]

That is what the pad measures with the Q51 out of the circuit. The base and emitter pads read 0V

 

[madHD]

The rail voltage is reading + and -25V.

 

[Perry Babin]

Did you check Q51?

 

[madHD]

Unfortunately the lead for the base pulled out of the transistor when i was removing it. I'll have to replace it either way.

 

[madHD]

Does this narrow it done or do you see any other possobilities. If i have to order it i would like to order anything else that might be bad.

 

[Perry Babin]

Do you have any small transistors on hand?

 

[madHD]

None free. I might be able to find one at my work. Or I have a hifonics tourus amp with serious problems I could rob from. I dont see any 8099s but lots of other TO-92 transistors.

 

[Perry Babin]

These will work:
MPSA06
MPSA42

 

[madHD]

Yeah!!! Its working good. Q51 (MPS8099) was replaced and the amp is working. Thank you so much for all the help. This was an awesome experience and I couldn't have done it without you.

Is there anything else I should check at this time after making these replacements (bias or anything else)? It sounds good and seems to have full power. This is just awesome.

 

[azvrt]

That's good news ! I am happy for you.
Bias for this amplifier is supposed to be 25 mvdc per channel.
I am currently running 3 subs in series for a total impedance of 12 ohms mono on a 5.2 and it's doing well. I also have the Airbass accessory for it.

 

[madHD]

azrt - Thank you for the schematics as well. Couldn't have done it without those either. There is a toggle for the airbass on the board, not on the front panel - is that what you have "on". Mine also has the "Hawkins" bass control. Do you use that? I have 2 - 4ohm subs in parallel. I will play around with some of this when I tune the system. Thanks.