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Adcom GFA-555 Help Needed
Adcom GFA-555 Help Needed
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Old 7th January 2019, 12:21 AM   #1
eliasb1979 is offline eliasb1979  United States
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Join Date: Jun 2018
Default Adcom GFA-555 Help Needed

Hello,

I picked up an Adcom GFA-555 with a blown left channel. As you can see, I've circled in red the burnt resistors (R1 and R7) and transistor (Q8). I'm assuming the problem is a blown output transistor, but I wanted to see if anyone has experience with this particular problem. If you agree that this is the most likely cause, I plan to update the left channel with the following parts (and later upgrade the right if it works as planned):

Q1-Q3, Q10: 2SC2240 (need 4 per channel) - Replace with KSC1845-FTA
Q4: 2SC2912 - Replace with KSC3503 DSTU
Q5: 2SA1016 - Replace with KSA992-FATA
Q6: 2SA1207 - Replace with KSA992-FATA
Q7: 2SA1210 - Replace with KSA1381 ESTU
Q8,Q9: 2SA970 - Replace with KSA992-FATA
Output Transistors
Q11: 2SB595 - Replace with MJE15031
Q12: 2SD525 - Replace with MJE15030
Q13-Q16: 2SB554 (PNP) - Replace with MJ21193G
Q17-Q20: 2SD424 (NPN) - Replace with MJ21194G

Does this seem right? Does anyone see any problem with these choices? I will do the caps as well.

Your help and advice is much appreciated.
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Old 7th January 2019, 01:31 AM   #2
cogeniac is offline cogeniac  United States
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Join Date: Feb 2015
Location: Petaluma, CA
Have you examined the schematic to determine what those resistors do? I would recommend against wholesale changing out of parts until you really understand what's wrong with the amp.

R1 is the INPUT series resistor. For that to blow means that someone must have either applied some sort of high voltage to the input, or when Q1/Q8 shorted (presumably), the short applied rail voltage to the input (presumably frying the guy's source/preamp output stage). Q8 is a bias transistor that is in parallel with R7, so whatever killed Q8, also probably killed R7. I'd also check Q7 and Q3 before lighting up the amp, since they are in the current path for the parts that got fried.

At any rate, if these are only parts that are fried, I'd start with replacing Q1 and Q2 and the resistors. I'd also read the posts on the GFA 565, which is the big brother of the GFA 555. The circuits are similar. See my thread on the Adcom 565. Yet Another Adcom GFA-565 Thread

In that thread there is a matcher circuit used for finding matched pairs of differential transistors. You will need to do that.

I'd carefully check the Q1 transistor that you remove. I'll bet it is shorted collector to base, and that's the problem.

Unless you are certain there is a problem in the output stage, I'd leave that alone. It is expensive and technically difficult to rebuild (AMHIK).

Here is the schematic:

Click the image to open in full size.

And here is the manual that sort of explains the circuit.

https://www.vintageshifi.com/reperto...ice-Manual.pdf

The 555 is a great amp! So, good luck getting yours going!

Scott

Last edited by cogeniac; 7th January 2019 at 01:53 AM.
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Old 8th January 2019, 02:40 AM   #3
eliasb1979 is offline eliasb1979  United States
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Join Date: Jun 2018
Default Q1 Measurement

Using a multimeter on the Q1 transistor, appears to reveal the problem. Connecting the negative lead to the base and the positive lead to the collector, it measures .061V nstead of OL. Reversing the leads reveals the same measurement.

I'll report back after testing Q2, Q7, and Q8.

Your help is MUCH appreciated.
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Old 8th January 2019, 03:16 AM   #4
eliasb1979 is offline eliasb1979  United States
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Default Adcom

Q2 and Q7 tested fine. Q8 was obviously fried. So I am going to replace Q1, Q8 and the two burned resistors.

Would you feel good firing it up for a test after that or should I test anything else first?
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Old Today, 06:08 AM   #5
cogeniac is offline cogeniac  United States
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Join Date: Feb 2015
Location: Petaluma, CA
I would check R17 and Q4 to assure that whatever happened didn't propagate into the output stage. It is probably OK, but out of an abundance of caution...

I'd go ahead and try it with replacement transistors. You will eventually want to replace Q1 and Q2 with a matched pair. See the thread I suggested above for the matcher circuit. You should be able to breadboard that easily, and then test a handful of transistors to find two with similar characteristics.

I am curious. The description says: "Input amplification and feedback occur in Ql and Q2, a standard differential pair input stage, with R3 and R4 providing local feedback for open loop gain control."

However, in the diagram, I can't find R4. One would expect that it would be in the emitter leg of Q2, since differential pairs are usually symmetrical. Wondering if there is a typo in the schematic.
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Old Today, 07:38 AM   #6
Halauhula is offline Halauhula  United States
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Join Date: Feb 2014
Location: Bethesda MD
I also note that on the PCB stuffing diagram, there are two R3. I was not able to find R4 on the stuffing diagram.
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