GFA-555 question

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gemmellsc

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Joined 2012
#1
Hi All. I have a GFA-555 (I believe a MK1 from my research) that is still working but has some issues. I'm a Controls and Automation Engineer, but I have zero experience working with amplifiers. This amp has tremendous sentimental value to me (my deceased Brother and I used to listen together with this amp) , and I'm not sure I trust myself to try and repair on my own. The amp is making the 'mouse fart noise' when it powers down, and I've lost a tweeter in one of my mains (Polk rti12). I'm not sure if the two are related, but I've pulled the amp from service until I can restore it.

I have decent Electrical knowledge, but it's mostly around Industrial Controls (480 3PH and 24V DC). My soldering is decent as I'm big into building and flying large RC Helicopters. None of this is related to amp repair, but I wanted to give an idea of my background and basic skills. I found the Hoppe's Brain site, but I'm not sure if repairing the amp is something that is easy or difficult? Am I just swapping out boards and replacing caps, or is a full restoration super complicated? If the amp didn't have so much sentimental value, I would probably dive right in. I would feel horrible if I damaged it beyond repair.

Is this something I may have the skills to tackle, or should I play it safe and leave it to the professionals? I live in Greenville, SC. Not sure what to do here. What are your thoughts about the best way to tackle this?

Thanks in advance,
Bill
 

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#4
Mr. Pass, what an honor! I do have access to all three items and basic, board-level experience. I've never worked on anything amplifier related. Most of my experience is in the Industrial Controls realm.

Thank you and best regards,
Bill
 
#5
Well the "Mouse Fart" 😸 is normal in any GFA-535, 545 or 555, just because there is no relay to disconnect the speakers as you power down, so you hear the noise of the amp getting unstable and oscillating a bit once the gain collapses at low voltage.

But it's unusual for it to develop a fart noise when it didn't have one before... that would be a little concerning.
 
#7
I didn't know how else to accurately describe the sound, but it does make me smile every time I shut the amp down. 🙂 I think it has always been there, but it seems to have gotten a lot worse (2-3 times as loud) and maybe even unbalanced left/right (louder on one side). I'll find some time this week and power it back up for the requested measurements. I'm assuming you want me to short each unbalanced input at the back of the amp (short between + and - of each unbalanced input) and then measure the VDC at the corresponding speaker output?

Thanks for the help!
 
#9
gemmellsc said:
I didn't know how else to accurately describe the sound, but it does make me smile every time I shut the amp down. 🙂 I think it has always been there, but it seems to have gotten a lot worse (2-3 times as loud) and maybe even unbalanced left/right (louder on one side). I'll find some time this week and power it back up for the requested measurements. I'm assuming you want me to short each unbalanced input at the back of the amp (short between + and - of each unbalanced input) and then measure the VDC at the corresponding speaker output?

Thanks for the help!
Click to expand...
Correct, you should see less than 50mV at the speaker output, depending how close the input transistors are matched, among some other factors.

The fact that the noise is changing is concerning.
 
#11
Hi all and thanks so much for everyone weighing in. I fired the amp up and took the requested measurements after about a 30 minute warm up period. No loads were connected on the output channels.

Below are the results:

Left channel:
-23.1mV with input not shorted
+4.8mV with input shorted

Right channel:
-30.4mV with input not shorted
+9.4mV with input shorted

I left the meter connected during power down just out of curiosity, and the voltage increased up to about -188mV as the caps drained. This looked somewhat linear at first and then looked more exponential around the last 25% of the shutdown period. I didn't measure the shutdown period, but I'm guesstimating it was around 5 seconds or so total. This measurement was with the inputs not shorted.

I'll wait to hear back on any suggestions. I did bring the amp to work just in case further measurements are requested. 👍

Thanks so much for the help!
Bill
 
#12
Thanks again for everyone's help. Is this an acceptable difference between the L and R channels, or do adjustments or component replacements need to be made? I was concerned when I lost a tweeter, but it could be totally non-related. The speakers are over 20yrs old, and I do like my Floyd and Dire Straits a little loud sometimes... 🙂
 
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