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Old 27th February 2010, 01:47 AM   #1
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Default Will Donate $30 to Forum for Help Repairing GAS Gandson Amp

$30 to the forum when amp is fixed. It's a GAS Grandson amp. It has 6 volts offset on each channel. I replaced ALL the electro's including the 10,000 power supply caps. 1 channel has Toshiba outputs 2sd426 & 2sb556. The other has MJ2955 & 10261. It had incorrect fuses... Nothing is burnt, all the resistors are within specs. Each channel has a 14 pin dip socket with 4 transistors plugged in for matching?? Everything looks original except for the 1 set of outputs. I'm not sure where to measure the bias current also? .. Your expertise is absolutely appreciated!
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Old 27th February 2010, 02:26 AM   #2
llwhtt is offline llwhtt  United States
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Do you need a schematic?

Craig
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Old 27th February 2010, 02:40 AM   #3
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Thx, The schematic I found is at .. ABELTRONICS.CO.UK under GAS. It doesn't have much for voltage test points. If you have a more detailed repair manual, I surely could use it....
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Old 27th February 2010, 02:59 AM   #4
llwhtt is offline llwhtt  United States
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That's the schematic that I have and there were no service manuals that I'm aware of. Anyway IC301 (and associated parts) is the servo that is responsible for keeping the offset at 0VDC or close to it. Check for the proper DC voltages, pins 4 and 8 if they are OK swap out the IC. If they're wrong check the zeners, and there are two more caps(C303/4) that could use replacing. Since both channels are bad the servo is very suspect. Also I would consider replacing all of the small electrolyic capacitors, there are several in the audio path and in the servo itself. I'm doing the same on a Son. Digikey has Panasonics that are popular on this forum and Mouser has Nichicon Muse capacitors.

Craig
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Old 27th February 2010, 03:05 AM   #5
llwhtt is offline llwhtt  United States
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Oops, missed the part you replaced the electrolytics, nevermind.

Craig
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Old 27th February 2010, 12:55 PM   #6
sregor is offline sregor  United States
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Craig seems to have covered most of the possibilities. From the schematic, it looks like the 14 pin sockets with the transistors in them are part of the Biasing IC detail. (SAE used CA3086 or LM3086 transistor arrays, and they are no longer available. I'm guessing that GAS used similar chips. Are they mounted so the transistors are on the heat sinks near the outputs or drivers? Good luck.
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Old 27th February 2010, 01:11 PM   #7
sangram is offline sangram  India
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A few potshots:

That servo is limited to a max offset range of +/- 700mV because of the 1N4148 diodes clamping the output of the servo to ground. At 6V, it is not able to affect the offset at all. Removing the clamping diodes may help with the offset, but may introduce other issues.

You can measure voltage across R14/15 0.2 ohm resistors to figure out the bias at idle. Maybe a more experienced member will comment on what it should be at, about 60mA should be a fair starting point.

Are the power meters (if that's what they are) at rest position when the amp is turned on and at 6V output offset, or are they about halfway?
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Old 27th February 2010, 01:57 PM   #8
sregor is offline sregor  United States
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Actually, 600 mv at the output of the Servo IC would produce several volts out at the output. The servo goes through two 150k resistors, as a voltage divider, to the + input of the amp, which gives about 300 mv (minus the input current difference between the PNP and NPN input transistors), which is amplified by the amp (gain is 25) to 7.5 volts on the output which is pretty close to what is occuring. But, as it is on both channels, I would still go for the 15 volt zeners or the resistors (r302 and r303) supplying them power.
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Last edited by sregor; 27th February 2010 at 02:04 PM. Reason: math error
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Old 27th February 2010, 02:18 PM   #9
sangram is offline sangram  India
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^^Right. I neglected to see the feedback loop was DC-coupled.
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Old 27th February 2010, 03:48 PM   #10
llwhtt is offline llwhtt  United States
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The 14 pin IC socket IS for the input diff. transistors, all GAS products used that technique, I think my Thalia and Thaedra preamps have it also. The bias IC SHOULD be on the heatsink somewhere with thermal goop on it's top, though I haven't seen a Grandson yet.

Craig
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