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Old 31st December 2009, 09:44 PM   #1
fredlf is offline fredlf  United States
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Default Another high DC Adcom GFA-555

Hello all,
I'm an earnest noob trying to learn theory and practice while keeping my aging system running. I have good mechanical skills, but only rudimentary knowledge of testing and troubleshooting. I've been reading and studying theory, but have a long way to go.

My faithful GFA-555 couldn't wait for me to get studied up. It's blowing the fuse on the left Output Board II and that channel has 82vdc at the speaker out! (The good channel was a respectable 41mvdc, btw).

After reading many threads here, there and everywhere, I pulled that board and tested Q11 (the driver transistor, right?). It passes the diode test. My DMM can measure HFE, but I'm not sure what I'd be looking for. (I said I was a noob.) If I replace it, am I correct that it does not need to be matched to Q12?

A more experienced acquaintance recommended I look first for blown output transistors, and the ones on this board do look a little suspect with all the drippy thermal grease running down (see attached). There was also a little greasy, sticky spot at the bottom of the case below this heatsink. The other heatsinks are much cleaner looking with no grease spots.

There are no other obvious burnt, bulging or cracked parts on this board or the front end board.

I reckon my next step is to remove and check these transistors (Q13-16)? If one or more is bad, it's my understanding I would have to replace all four with matched pairs to Q17-20.

Any and all help is greatly appreciated.

I know I'm asking a lot here. I know it's a questionable undertaking to try and repair something you don't really understand. As a bike mechanic (back in college) it always bothered me when people assumed it was simple to perform a given procedure. But I don't have many options for getting this puppy fixed, and I miss my music, so I throw myself on the mercy of the Internet.
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Old 31st December 2009, 11:03 PM   #2
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If the transistors are blown you need to take care that nothing else is blown that would blow up a new set of transistors.
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Old 31st December 2009, 11:47 PM   #3
SQLGuy is offline SQLGuy  United States
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First question I have is, is this really a GFA-555, or is it a 555-II?

Reason I ask is that I have a 555-II that came to me with 80V of DC at the output, and it's common on those amps, and has to do with bad pre-driver transistors on the input board.

However, my amp wasn't blowing fuses. If you're blowing fuses without a load hooked up to the amp, then you probably have blown output transistors on one rail. The quick check would be, on the output transistors of the bad channel (with the amp off and fully discharged), check resistance from collector (case) to emitter on the two polarities of output transistors. This should be very high (many K Ohms). You can compare to the working channel for a reference of what a good reading should be. One polarity of transistors is NPN and will have numbers starting with 2SC or 2SD or C or D. The other ones will be PNP and will have numbers starting with 2SA or 2SB or A or B. All the output transistors of the same polarity are connected together in parallel, so you only need to check one of each polarity on the bad channel to see whether the resistance is reasonable or not.

(Edit: Just looked at your picture and saw the 2SB554 designations. These would be the PNP output transistors. The NPN ones will be marked 2SD424. This also answers my question about whether it's a 555 or 555-II... it's a 555.)

I expect you'll find one or both polarities where the resistance is < 1 Ohm. That will indicate that one, or more likely all, of the transistors for that polarity are shorted. If that's the case, they will need to be replaced, and further testing will be needed to see whether there are other devices further up the chain that are also damaged.

Let us know what you find in your tests.

BTW, under what circumstances did the amp fail? Was its output shorted accidentally or hooked up to a bad speaker?

Cheers,
Paul

Last edited by SQLGuy; 1st January 2010 at 12:05 AM.
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Old 1st January 2010, 12:23 AM   #4
SQLGuy is offline SQLGuy  United States
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Oh, in more direct response to your other questions:

1. No, you don't need to remove the output transistors to check them, unless you find that one or more are shorted... then you'll at least need to isolate them enough to see which ones are shorted.

2. No, matched devices aren't required, as the emitter degeneration resistors will force current sharing for the output transistors. Matching won't hurt, but it's not critical. Suitable substitute output devices would be On Semi MJ15022G and MJ15023G. If you buy them new, from On or Mouser, etc, they should come from the same lot number and be reasonably similar in behavoir even unmatched. Companies like Tech-DIY sell matched sets of transistors, but I don't see that they carry the 15022's and 23's.

Cheers,
Paul
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Old 1st January 2010, 01:14 AM   #5
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You don't need to remove the transistors to find the shorted device but you need a VOM that reads reliably to less than an ohm. Measure the E-C resistance on each of the transistors. The good ones will read a short (the bad one) plus the 2 emitter resistors. According to my GFA-555 book that would be 0.82 x 2 while the shorted one will read 0. Remove the bad one and check the other group of 4 outs. This simultaneously verifies the emitter resistors as being good. If any transistor reads more than 2 ohms you better check the emitter resistor on that device. I've never seen 2 devices in parallel fail but almost always 1 in each group in series between the power rails. When you've removed the bad transistor on the other leg (positive and negative supply rails, it is safe to power it up with the fuses in. It should not be blowing fuses and the DC output should be nominal +/- 50mV. You can run signals into it with no or light load and check the output with a scope. Shut it down and replace the missing transistors. I agree with SQLGuy's OnSemi suggestion but I would not mix part numbers in a channel. IOW, all Toshibas or all OnSemis in a channel.

Happy New Year

G
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Old 2nd January 2010, 03:29 AM   #6
fredlf is offline fredlf  United States
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Hi all,
Many, many thanks for replies.

So if I'm understanding correctly, in order to do the tests suggested by Paul and G2, I need to re-mount Q11 and then re-mount/re-connect the board back in place in the case. Unless there's another way to test the transistors with the board un-mounted/disconnected?

FWIW, with the board removed, measuring across E-C of Q13-16 read 8.9 MOhm (and 180 MOhm with the test leads reversed, C-E).

Thanks again for the help, guys.
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Old 2nd January 2010, 04:12 AM   #7
SQLGuy is offline SQLGuy  United States
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Those readings sound fine for that set of output transistors. What are you finding for the set on the other polarity (the 2SD424's)?
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Old 2nd January 2010, 06:23 AM   #8
fredlf is offline fredlf  United States
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The other set, the 424's (Q17-20), read 60 and 103 kOhm. Much lower...

That output board still has its driver transistor attached and is still connected to the input board and PS cap.

cheers,
Fred
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Old 2nd January 2010, 02:14 PM   #9
SQLGuy is offline SQLGuy  United States
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Well, so far, I don't see anything that should cause you to blow fuses without a load connected to the amp.

Before we go any further, please confirm that that is the behavior, and give us the details of the circumstances surrounding the failure of the amp.
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Old 2nd January 2010, 05:21 PM   #10
fredlf is offline fredlf  United States
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Oh yeah, to answer the question, I'm not sure what caused the channel to fail. I just noticed the channel out. There is some possibility a speaker wire got shorted when I was moving some stuff around, but I don't know for sure.
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