Adcom GFA-555 repair

[Stormrider]

Yes, another thread on one of these amps. Sorry, but I need a place to jot down my ideas/plans, and maybe get some input from the more experienced.

I received two of these amps from a board member, knowing they needed work. I've started working on the first; the following is what I know:

No blown fuses, and power supply is good. +/-78Vdc.

This amp (and the other one, but I'm going one at a time) were victims of a shoddy tech at one time or another. Not knowing what the original failure was I'm not willing to power up the signal boards without some investigation. R3 (33.2R, Emitter degeneration for Q1) was blown, got hot enough to char the PCB. Was replaced with two resistors soldered on top of each other, and the previous tech managed to lift tracks in that area.

Q1, Q2 test good on the IT-18. They look like orginal 2SC2240. Going from the solder joints alone I don't think they've been replaced. Q3 also tested good. VAS transistors (Q4, Q7, Q8) test good. Because R3 blew I'm planning on replacing Q1, Q2 and Q3 regardless. They must have been stressed for enough current to flow through R3 to burn the PCB. I'll be using KSC1845.

Q5 in both channels test leaky, in the 100's of uA's. One fails the beta test, pegs the needle to right of scale. I'm not sure what the failure mechanism for Q5 would be; seems strange that it would cause R3 to burn if they were the problem, but I'm sticking to what I know.

I have not pulled and tested Q6, Q9, Q10 or anything in the output stage.

The 47uF caps looked slightly bulged, but they didn't leak. They're getting replaced, of course.

I will be posting pics; comments/criticism welcome.

[tloftis]

where do you get the parts to repair the adcom??? I want to replace mine.

[tloftis]

how do you test the capacitors?

[PreCD]

Stormrider:

Sounds like you have your hands full. Please keep us updated on the project. I purchased a GFA-555 in October of last year and have did the following:

• Replaced all electrolytics on the input PCB
• Replaced the filter caps (lowered my offset by 15mv and cured hum in left channel)
• Replaced the power switch (it fizzled when switched on or off)
• Adjusted bias current ( I have the 555 with Test Points on the output PCBs so maybe a little different procedure for yours)
• Replaced the common bus with a larger awg wire (originally a 12-14 awg wire)
• Replaced ceramic capacitors with silver mica on output PCBs.

I have been kicking around replacing the diff pair transistors but the amp is sounding very good at this time and the DC offset of both channels are identical. At this time I am leaving it alone.



Keep us updated!

[PreCD]

Stormrider:

Sounds like you have your hands full. Please keep us updated on the project. I purchased a GFA-555 in October of last year and have did the following:

• Replaced all electrolytics on the input PCB
• Replaced the filter caps (lowered my offset by 15mv and cured hum in left channel)
• Replaced the power switch (it fizzled when switched on or off)
• Adjusted bias current ( I have the 555 with Test Points on the output PCBs so maybe a little different procedure for yours)
• Replaced the common bus with a larger awg wire (originally a 12-14 awg wire)
• Replaced ceramic capacitors with silver mica on output PCBs.

I have been kicking around replacing the diff pair transistors but the amp is sounding very good at this time and the DC offset of both channels are identical. At this time I am leaving it alone.



Keep us updated!

[Stormrider]

Originally Posted by tloftis

where do you get the parts to repair the adcom??? I want to replace mine.

I order most of my parts for any project from Mouser or Digikey. I prefer Fairchild and On-Semi semiconductors to avoid fakes. For this reason I'll be using KSA992/KSC1845 (available at Mouser) for the small signal parts, KSA1381/KSC3503 for the VAS, MJE15034/35 for the drivers.

Originally Posted by tloftis

how do you test the capacitors?

I have a B&K LCR meter but for an amp this old I replace all electrolytic's regardless of measured value. I didn't even bother measuring them this time as the 47uF's looked somewhat bloated.

Originally Posted by PreCD

Stormrider:

Sounds like you have your hands full. Please keep us updated on the project. I purchased a GFA-555 in October of last year and have did the following:

• Replaced all electrolytics on the input PCB
• Replaced the filter caps (lowered my offset by 15mv and cured hum in left channel)
• Replaced the power switch (it fizzled when switched on or off)
• Adjusted bias current ( I have the 555 with Test Points on the output PCBs so maybe a little different procedure for yours)
• Replaced the common bus with a larger awg wire (originally a 12-14 awg wire)
• Replaced ceramic capacitors with silver mica on output PCBs.

I have been kicking around replacing the diff pair transistors but the amp is sounding very good at this time and the DC offset of both channels are identical. At this time I am leaving it alone.



Keep us updated!

The support/comments are much appreciated, thanks.

[embrown057]

What are you plaing to use the KSA992/C1845's for. Not the 2sa940/c2240 are you? If so why not just use the orginal parts that you can readly order. You can order most everything from B&D enterprises and get orginal parts, no fakes from them ever.

[Stormrider]

B&D Enterprises may be legitimate, but I still have reservations buying parts that have been out of production for years. Call me paranoid.

The KSA/KSC parts should drop right in. If there are compatibility problems please let me know.

[PreCD]

Compare spec sheets and then there is not a question.

Originally Posted by Stormrider

B&D Enterprises may be legitimate, but I still have reservations buying parts that have been out of production for years. Call me paranoid.

The KSA/KSC parts should drop right in. If there are compatibility problems please let me know.

[Stormrider]

Sorry, what spec is un-sat for KSC/KSA parts? I must be missing something.