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Adcom GFA-5400 / Help please...
Adcom GFA-5400 / Help please...
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Old 23rd April 2015, 04:13 PM   #1
luicas is offline luicas
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Join Date: Aug 2013
Default Adcom GFA-5400 / Help please...

Hello everyone. Great forum. Have been reading lots of good stuff and it's time to post my first call for help.

A few days ago I bought this pristine looking GFA 5400. Inside and out.

As always, upon receiving a used amp bought off eBay, I fire it up with no speakers connected, let it simmer for a few minutes, and take DC offset readings off the speaker posts.


First thing I noticed is that upon turning the unit on, Both distortion alert leds lit up, right channel dimmed quickly, but left channel took aprox 30 seconds to completely fade away. Both ended up unlit after this.

The amp simmered for a few moments, checked DC offset, 3mV on both channels. Left channel jumped occasionally to 20mV (or thereabout), and then went back to 3mV. Right channel stayed put at 3mV.


Turned the unit off. Waited a few minutes. Turned it on again, and no distortion alert lights went off. Everything was now normal. ??

Hooked it up to my system. Worked as expected. When turning off, no whistling, pops, thuds or anything. When powering on, just the common thump, nothing bad.


Then....THE NEXT MORNING.... turned the thing on and Boom! distortion lights on, and a burst of crackle scared the hell out of me (and my daughter...) on the left channel, then silence, then another burst on the right channel.... Immediately turned the thing off. Disconnected speakers. Turned it on again, and as expected, the lights faded within 30 seconds and the amp worked fine after that.


So, I opened it up. Changed the big caps (I have all new electro's at hand since I bought this amp to do a cap-restoration). One main cap seemed to have a cold solder (discolored at least). Put in the new caps (15k uF SMH Chemicons). Fired it up, and no distortion lights. Bias was a bit low (20-23mV), so rebiased to 36-39mV, recalibrated offset to within 5mV. All was good. (or so it seemed). Left channel was sensibly warmer than right, at same bias current. That was yesterday.

This morning, fired it up, and boom, distortion lights....and the same 30 seconds before they faded out completely. Mainly left channel as I said before. Right channel light barely lights up and fades quickly.

I'm thinking of replacing the rest of the PS caps. But if someone has some light to shed here, I will be very interested in knowing.

It seems that this failure is only present after a prolonged (several hours) standby time.


Any hints? Comments?

Cheers,
Luis
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Old 25th April 2015, 12:50 PM   #2
luicas is offline luicas
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Join Date: Aug 2013
Replaced all the PS electrolytics, Chemi-con SMH on the big caps, the rest are Nichicon FG. But problem persists. My 5400 is the Version 2, with the additional output protection board.

Next idea is to replace Left channel electrolytics.

Any suggestions on where the problem lies?
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Old 29th April 2015, 01:07 PM   #3
luicas is offline luicas
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Changed three low voltage electrolytics on the left channel board and that seems to have fixed it.

They (old ones) all tested fine with an eBay ESR meter.
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Old 29th April 2015, 06:34 PM   #4
AMV8 is offline AMV8  United Kingdom
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Location: near london
Hi
I have no idea what you have checked on the amplifier so I may mention seemingly simple things that you have already looked at.
However if it was my amp I would start by examining all the connection on the mains power supply to the amp and then on the power supply wiring within the amp to the transformer, including any relays if fitted.
Next I would check all connections within the transformer secondry wiring and after that the connection to the diode bridge and the diode bridge itself.
Poor mains connections can cause crackle and bangs as the connections make and break and affect both channels.
Look at earth connection especially from the power supply.
If the problem still persists I would use a multimeter to check wiring integrity and transformer integrity.
After that you know the power suply is OK.
Don
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Old 5th May 2015, 09:50 PM   #5
luicas is offline luicas
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Join Date: Aug 2013
Thanks Don. The amp had been working OK after my last post (#3) until today, when at power up, the Left channel distortion light barely lit up. No crackle this time. I assume there is a relation between these factors:

1. turn on thump (DC surge)
2. Left channel is always warmer than right channel at same bias current
3. when turning off the amp (listening to music at low volume), the left channel always runs out of juice much quickly than the Right channel.
4. distortion light lighting up due to a current surge/oscillation (I really don't know how the circuit works)

Bad resistor ??

Aside from this, the amp works very well (to my best critical listening), plenty of clarity, power and no distortion at all. It can play really loud and clean and have yet to see the distortion lights activate while playing music.

No hum, pops, clicks or anything while idling. Just the typical hiss when you go near the tweeter.
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Old 6th May 2015, 06:25 PM   #6
luicas is offline luicas
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Join Date: Aug 2013
Update from today... Power on, left distortion led lit, brief squeal from left channel followed by another brief burst of lower freq noise (800-1200 hz estimated). Turned the thing off immediately.

I'm at a loss.

As always, if you leave it on for 30 seconds (no speakers connected)(until distortion led fades away), you can plug the speakers and listen for hours with none or at least indiscernible ill effects.

I don't want to take it to an electronic repair shop because it is my will to learn how to fix this thing.

Regards,
Luis
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Old 5th December 2017, 10:40 PM   #7
Andersonix is offline Andersonix  Sweden
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Adcom GFA-5400 / Help please...
Luis, do you have an update?
Could it be a bad electrolytic cap on the L channel amp board (feedback cap C605 in particular)?
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Old 7th December 2017, 07:06 AM   #8
Trollet is offline Trollet  Sweden
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Adcom GFA-5400 / Help please...
I've got a Phonic PWA2400 that behaves similar.
Sometimes protection on ch2 goes crazy and the relay starts "bouncing" in random frequency.

In your case- may it have something to do with protection circuit?

Figge
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