Adcom GFA-565 with very high DC offset - Page 2 - diyAudio
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Old 19th November 2012, 03:10 AM   #11
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Quote:
Originally Posted by Mayank View Post
Thanks for the suggestion Rob.

The resistor in question R154 (1.2K) is on the negative rail for the Bias section. The Input & Voltage Gain sections have a seperate (-) and (+) supply.

Studying the schematic, I thought there could only be two reasons for the high current across R154 (a) film capacitor C117 (1uF/100V) has a short to ground or (b) diode D115 and/or R150 are short and Q119 (2SA970) has a B-C short to ground.

Unfortunately all these 4 components test OK.

What have I overlooked ?

Rgds
Mayank
Well, something, but I don't know for sure what.

What you say is true, but the current through the long-tailed pair formed by Q101 and Q105 are sinked though R154 via Q116.

First, check R145 (499 ohms).
Next, follow the collector of Q116. It connects to the emitters of the differential input pair Q101 and Q105. The collectors of these connect to the emitters of Q103 and Q107. In addition, there is a DC servo IC, IC101, which connects to the bases of Q103 and Q107 and also to the collectors of Q101 and Q102. Theoretically, a DC servo should be maintaining the output to have negligible DC offset.

I suspect that one or more of these devices or the resistors feeding these transistors has a problem. A short in this area could result in a larger voltage drop across R154, hence the burn. There could be a problem with the opamp and/or C108, too. I think C107 and C108 are electrolytics. Replace with a plastic film or high quality ceramic cap.

The good news is that the circuit is symmetrical push-pull, so you should be able to compare voltages and currents of Q101, Q103, Q105, and Q107 with those of Q102, Q104, Q106, and Q108.

Good luck with the diagnostics.

Regards,

Rob
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Old 19th November 2012, 04:04 AM   #12
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Quote:
Originally Posted by Mayank View Post
Initial finding: There was capacitor leakage on the underside of the driver board.The 10R resistor R157 was bad. The servo IC 2A had a short on pins 4&6. Input stage transistors Q107 & Q108 (C3478/A1376) were bad. In the Voltage Gain section, transistor Q109 (A1015) had the B-E short.
Some more thoughts:

I see you already took a look at least some of the input stage transistors. Pin 6 of the IC on the schematic looks like the output pin to me. If you had a short on this pin, that chip is likely dead. Unfortunately, Adcom only shows a house part number.

Given that R157 was dead, did you check the zeners D116 - D119? Also check R155 and R156.

Also, there are 3 other fusible resistors: R121 (negative feedback to ground, 10 ohm), R135 and R136 in series with base of pre-driver transistors.

Regards,

Rob
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Old 3rd December 2012, 07:24 PM   #13
Mayank is offline Mayank  United States
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Default Calling GFA-565 owners

I am still working on this

Quick question: Can anyone confirm if the red wire from the 12A (+) fuse goes to pin 10 on the input board and the corresponding red wire from pin 5 to the +tive terminal on the main capacitor?

This is how it is on the schematic, but they are reversed in my unit. I am confident that no one has worked on this unit - wonder if these wires were accidently switched on the factory assembly line (no biggie as both are +86V).

Thanks
Mayank
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