Audio Research Stereo Amplifier HELP

[valco]

Perhaps I'm measuring the wrong component? Its a 40311 on a terminal strip down below, in between the can caps. Yes?

 

[valco]

Okay, something is screwy. The amp was turned off for a while now. I turned it back on and remeasured.

C 7.18vdc
B 52.7vdc
E Was 20vdc when I first turned it on. Check one last time before posting this and it says 11.6vdc.

 

[valco]

Brown paper cap is now reading different voltages than I posted earlier as well.

 

[valco]

Seems the 40311 has dropped down to less than 1v again.

 

[llwhtt]

I don't have one to look at so I don't know the color of anything or it's location. I checked the Service Bulletin and the 40311 transistor is the later upgrade. Do you have a schematic to look at? Can you find D16? measure both sides.

 

[valco]

Very sorry, I just realized a typo. The 52.7 off the regulator is mV not V!

 

[llwhtt]

That's a bit different! Between the base and collector should be an 820 Ohm resistor, check that. Between the base and ground should a capacitor, C51-B, 1500Uf and 20V Zener, check those.

 

[valco]

D16: 14.9mV (anode) and 18V (cathode)

 

[llwhtt]

That makes sense since the regulator output, on the anode, is kaput.

 

[valco]

B to ground cap is getting 52mV. Thats the triangle lug of the brown triple-can.

Don't see the zener. I'll keep looking. Its really cramped!

And I apologize for the mV vs V readings. My meter has a small indicator for that and I don't always notice when it changes from one to the other. I'm making sure of it now.

 

[valco]

So perhaps a new regulator? Seems a NTE128 is a usable replacement.

 

[llwhtt]

The 20V Zener, D9, on the base of Q28 sets the output of Q28. If you don't have a voltage on the base there is no output from the regulator. Are you able to find R114, 820 Ohm resistor, that biases the Zener? So we need R114 to bias the Zener which in turn turns on Q28, the 40311, series pass transistor, we need ALL of this to work. So you need to find these parts and test them. Also make sure the triangle section of C51 is not shorted to ground, if shorted this will make the Zener look shorted and vice versa since they in parallel with each other.

 

[valco]

This is all great info. Thanks. My kids just got home. I’ll check all those components tomorrow and post them here.

 

[valco]

Okay:

Triangle of C51 is connected to Base of Q28. I confirmed it is not connected to ground. D9 is connected from Base of Q28 to a ground lug of C51.
I found R114. I did not remove it while measuring. I found zero resistance while connected.

 

[valco]

Current readings on the regulator:
C 7.2vdc
B 52mvdc
E 10.5mvdc

 

[valco]

D9, in circuit, is reading 0.051v on diode setting on multimeter.

 

[llwhtt]

Either D9 is shorted or the triangle section of the capacitor is shorted. Remove one end of the D9 and test again. Zeners are easy to get, not so much that style of capacitor. If one is shorted that would explain the low voltage on the collector and 0VDC on the base of Q28.

 

[valco]

removed one end of D9 and checked it again. Now reading 0v. So a fried diode it seems.

 

[valco]

1n968B a 20v 0.5w zener.

I found for much cheaper the 1n4747A, a 20v 1w zener. that should work, yes?

 

[llwhtt]

How's about a 1N5250B? They're only pennies to begin with. You might as well test the transistor while you're at it and make sure that R114 is still good, it was passing more current than normal.