Adcom GFA-565 with very high DC offset - diyAudio
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Old 28th October 2012, 07:49 PM   #1
Mayank is offline Mayank  United States
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Default Adcom GFA-565 with very high DC offset

I had picked up a pair of original, unmolested, unopened units a couple of months ago. *The former owner had kept them in storage for a few years but listed them as "in perfect condition".

The first test I did on reaching home was to check for DC offset. One had 63V and the other 40mV. I am working on the very high offset unit first.

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.

I washed the board in Simple Green solution (4 times with careful scrubbing). Replaced the five electrolytics, servo IC, 10R resistor, Q107, Q108, Q109 transistors. Checked all the rest of transistors, diodes and resistors. Everything looked good and there was no bad smell from the leaked electrolyte.

On powering up, the 1.2K resistor (R154) on the negative rail on the board smoked instantaneously. I know, I should have powered up with the variac

What am I missing? Could this be due to a short on one of the output transistors?*

Any suggestions/guidance appreciated.

Thanks
Mayank
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Old 28th October 2012, 09:48 PM   #2
AMV8 is offline AMV8  United Kingdom
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Check the input ac voltage to the bridge rectifier and the dc output voltage of the bridge rectifier. It could be a rectifier fault or a connection to the rectifier that is faulty.
Then start up with a variac if possible.
Don
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Old 28th October 2012, 11:28 PM   #3
Mayank is offline Mayank  United States
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Thanks for responding Don.

Measurements with everything disconnected except the soft start circuit:

AC input to bridge rectifier: 62.1V and 62.1V
DC output from bridge rectifier: +57.1V and -57.0V

I had disconnected the two large main psu capacitors.

Shouldn't the DC voltage be 62.1 x 1.414=87.8V?

Rgds
Mayank
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Old 28th October 2012, 11:53 PM   #4
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Quote:
Originally Posted by Mayank View Post
Thanks for responding Don.

Measurements with everything disconnected except the soft start circuit:

AC input to bridge rectifier: 62.1V and 62.1V
DC output from bridge rectifier: +57.1V and -57.0V

I had disconnected the two large main psu capacitors.

Shouldn't the DC voltage be 62.1 x 1.414=87.8V?

Rgds
Mayank
Yes it should be!!! But you "have to" put back those PSP caps and measure voltage again! Check for shorted output transsistors, disconnect them and measure them.
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Old 29th October 2012, 03:15 AM   #5
Mayank is offline Mayank  United States
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With the two PSU caps connected, the voltages look OK at +85.2V and -85.6V. So I am ruling out bad power supply or rectifier bridge.
I am removing the two huge heat sinks to check the 10 pairs of output transistors for a short.

Mayank
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Old 30th October 2012, 04:29 AM   #6
Mayank is offline Mayank  United States
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I think I found the issue (was home all day due to Hurricane Sandy).

4 of the 10 output transistors D424 had a dead short on the B-C and also had their 10R base resistors open. On the other side, all 10 of the B554 were found to be OK.

Before I go ahead and replace the bad outputs and base resistors, I would like to know why this happened and what was the reason just 4 of these were affected and not all. I think I should replace all 10 just in case.

Mayank
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Old 30th October 2012, 06:20 AM   #7
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Originally Posted by Mayank View Post
I think I found the issue (was home all day due to Hurricane Sandy).

4 of the 10 output transistors D424 had a dead short on the B-C and also had their 10R base resistors open. On the other side, all 10 of the B554 were found to be OK.

Before I go ahead and replace the bad outputs and base resistors, I would like to know why this happened and what was the reason just 4 of these were affected and not all. I think I should replace all 10 just in case.

Mayank
it is not easy to say, but it can be many reasons why! if it was not pushed to the limits, shorted it out by accident,it could be just stress. even if amp has fuses as protection, that will not guarantee 100% protection for output transistors. But I suggest you to change "all" 10 output transistors. make sure you test them before and after soldering them. also after taking out all output transistors "measure" all components near by like: resistors, caps,diodes. and then put the output transistors. use the "lamp" method to plug it...just in case if there is a short somewhere.also do not connect speakers until you are sure you do not have high voltages to the speakers. Good luck!
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Old 15th November 2012, 02:42 AM   #8
Mayank is offline Mayank  United States
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Update:

I replaced the D424 output transistors and all the corresponding 10R base resistors. Also, 4 of the 10 0.33R/5W ceramic emitter resistors had also opened which were replaced. I checked all the other resistors, diodes and capacitors on the output boards which were found to be OK.

On powering on with a 100W bulb in my newly made Light Bulb Limiter - I got an initial bright glow which quickly "simmered" to a faint red. Hmm, I thought this looks nice - no shorts, the output stage is good and the amp should now be OK.

However, after about 10 seconds with the lamp continuing to be faint red: FZZZ - the same 1.2K 1/4W resistor (R154) on the negative rail on the input board, smoked happily.

Before I start the test with a variac, is there any obvious component/fault that I am overlooking? Also, if I were to measure the current on the negative rail, shouldn't it be less than square root [1/4W / 1.2K] = 14.4 mA?

Thanks
Mayank

Last edited by Mayank; 15th November 2012 at 02:47 AM.
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Old 16th November 2012, 10:46 PM   #9
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Quote:
Originally Posted by Mayank View Post
Update:

I replaced the D424 output transistors and all the corresponding 10R base resistors. Also, 4 of the 10 0.33R/5W ceramic emitter resistors had also opened which were replaced. I checked all the other resistors, diodes and capacitors on the output boards which were found to be OK.

On powering on with a 100W bulb in my newly made Light Bulb Limiter - I got an initial bright glow which quickly "simmered" to a faint red. Hmm, I thought this looks nice - no shorts, the output stage is good and the amp should now be OK.

However, after about 10 seconds with the lamp continuing to be faint red: FZZZ - the same 1.2K 1/4W resistor (R154) on the negative rail on the input board, smoked happily.

Before I start the test with a variac, is there any obvious component/fault that I am overlooking? Also, if I were to measure the current on the negative rail, shouldn't it be less than square root [1/4W / 1.2K] = 14.4 mA?

Thanks
Mayank
Maybe take a look at the long-tailed pair that is ultimately sinking current through that resistor.

Regards,

Rob
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Old 18th November 2012, 05:24 AM   #10
Mayank is offline Mayank  United States
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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
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