P2100 bias issue

[two.dogs]

The service manual says to set idle current to get 13mV just after turn on or 22mV stabilized, One channel works great, but the other wont go below 26mV hot or cold. The weird thing is that the bias circuit is working on both channels. On the good channel I can adjust the b1 and b2 voltages to +.6v and -.6v and the idle current is bang on. I can turn down the adjusting pot and the bias voltages decrease down to about +/-.2v and the idle current pretty much shuts off. But I can do the same on the bad channel. Even when I turn down to +/-.2v the idle current is too high at 26mV. DC offset is less then .05v so that isn't a problem. I pulled the output transistors and they tested good on my Atlas. The .47ohm resistors also measure OK. With a bias of .2v the output transistors should be in cut off. I'm confused.

John

 

[avionic]

On the bad channel. What is the +B and -B voltage on the output collectors?

 

[two.dogs]

Measured directly at the collector (after the molex connector)
+51.35
-51.45

 

[avionic]

Check all fusible resistors in the bias circuit.Transistors as well.

 

[two.dogs]

OK. Looking at the schematic I see 8 of these per channel - all small value 330ohm or less. Resistor symbol with small open circles on either side. Must I pull them out or can I measure in circuit comparing one side to the other? Also count 8 transistors in the bias circuit - or do you mean the phase splitter as well?

 

[avionic]

Nice avatar btw.

 

[two.dogs]

Transistors TR8-15 check good on the Atlas. Fuse resistors also check good.
223 (220),
224 (220),
475 (470),
68 (68),
22 (22),
331, (330),
335 (330),
22 (22),
122 (120)

 

[two.dogs]

Measuring from PE to E, I can adjust the bias voltage up but then as I turn the pot ccw it loses its effect on the bias voltage at about the 11 o'clock position. However the pot still continues to lower the base voltage (b1 and b2) well into what should be the cutoff range for the outputs. Perhaps P1 is being held high. Maybe remove the output transistors and test voltages?

 

[two.dogs]

Got it mostly reassembled and removed the output transistors. This shows voltage between PE and E on both sides. Hmmm.

 

[avionic]

Is there a test point for O (output). I'm not seeing how test point E can be used for bias adjustment. PE and O would put you across one of the emitter resistors. Ohms law --- 21.1 mv across an .47Ω resistor would put your bias at 45ma.

I think there is an error in the published adjustment procedure.

Test point E is an electrical ground.

 

[two.dogs]

Yep, there is a mistake in the service manual. Idle current should be set by measuring between PE and CT, not PE and E. If you do what the service manual says, then you'll get idle current plus DC offset. My DC offset is 26mv. Point "O" and "CT" are the same potential. My amp is fine.

 

[avionic]

. My amp is fine.

 

[drumbum]

Got it mostly reassembled and removed the output transistors. This shows voltage between PE and E on both sides. Hmmm.


View attachment 1383398 View attachment 1383399

ALERT! Don't attempt to use cottage cheese as a thermal paste. Use the proper product!

BTDT.

 

[two.dogs]

I find that cottage cheese adds a tube-like warmness to the sound.

 

[dino74]

I find that cottage cheese adds a tube-like warmness to the sound.

Is it dependent on fat percentage? I would think a non-fat cottage cheese would give a drier sound.

 

[tonefoto]

Yep, there is a mistake in the service manual. Idle current should be set by measuring between PE and CT, not PE and E. If you do what the service manual says, then you'll get idle current plus DC offset. My DC offset is 26mv. Point "O" and "CT" are the same potential. My amp is fine.

Glad I found this in time. Was having the exact issue, now its fine, thanks for this.