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Adcom GFA-535L power amp distortion clipping warning LED

strat68

strat68

Member
Hi all, Adcom question, but some background first, if too long-winded skip the next 3 paragraphs of how I got the amp and speakers.

I recently picked up some gear from Craigslist, I was after decent floor standers with WAF as we got new furniture just before the holidays and the double stacked (and battered) Advents had to go to the testing laboratory, aka basment :)

So I picked up a pair of Linn Keilidh in good working and cosmetic condition for $450. They were also selling an "adcom amp $50" so after asking the model number (GFA-535L), I asked them if they were selling the pre-amp as well, and they didn't quite understand, so I had to explain there is probably a similar sized unit with more knobs, likely an adcom, but could be another brand... Then they came back saying, oh that- yea my son has it let me see if he's willing to part... It was a GTP-450, and another $50. I had to bring a CD source and wire to test everything and it sounded great. I bought it with no haggle as I thought the Linns were fair market value, and the Adcom stuff a little low in price.

So I get it home and enjoy for several days with the Linns speaker terminals strapped with one 8 ga. cable run. Later I take out the jumper and run the same exact wire and length for bi-wiring, one amp. Since I have to use bare wire at the moment on the Adcom, I have them into the A and B jacks, tweets on A, woofs on B.

I added a turntable preamp as the adcom has no phono stage, and I noticed some clipping. I thought the gain may have been to high so I lowered it. Then problem became intermittant. Seems to happen early on, even with no source playing, then go away after 10 minutes or so of playing.

I pullled the Adcom from the system and put back my previous integrated amp, a Denon PMA-860 with no problems. Bi-wired using A+B speakers. I opened the Adcom, and I don't see anything fried or blown fuses. Just a little dust, I will try some compressed air then deoxit on the bias adjustment pots (unless someone here advises not to).

I read in another forum, posted by an authorized tech that he's only seen one power unit fail and that-
"Most failures were 10 and 100 ohm 1/4 watt fuse resistors that would open causing grounding issues on the front end thus causing DC offset putting the amp into protection mode."

If I take a picture of the board would someone be able to help identify the resistors he's talking about?

Thanks,
Steve
 
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mars_volta said:
First, measure the output with a dc volt meter. No signal.
Click to expand...

Thanks, only basic electronic skills changing pickups and pots in guitars, never messed with amps... so outputs as in the speaker terminals, so Left A, Right A, Left B etc.? Amp running no source or preamp plugged in?
 
Strat68,
Welcome to AK.
Use care when measuring voltages. There are certain areas of an amplifier that contain dangerous voltages. Be careful around power supply capacitors. They retain voltages for some time even with the amp unplugged.
 
jblmar said:
Strat68,
Welcome to AK.
Use care when measuring voltages. There are certain areas of an amplifier that contain dangerous voltages. Be careful around power supply capacitors. They retain voltages for some time even with the amp unplugged.
Click to expand...

I measured with the cover back on. Connected a short wire to the (annoying) Adcom speaker connectors and put the probes of the meter on that. Thanks for the warning, it looks like 4 caps at 63 volts each. I haven't had to learn to discharge caps on a solid state audio amp yet, but have done it for safety on my guitar amps (tube), usually to retention power tube sockets.
 
Steve[/QUOTE]
strat68 said:
Thanks for your replies guys. On the right which is the channel acting up it's floats between 1.5 and 2.0 mV. On the left channel which hasn't misbehaved it's at 4.9 mV.

Steve
Click to expand...
This is acceptable. Be certain the fuses are good and making good contact in the clips, you can turn them to scrub contacts.
 
strat68 said:
I measured with the cover back on. Connected a short wire to the (annoying) Adcom speaker connectors and put the probes of the meter on that. Thanks for the warning, it looks like 4 caps at 63 volts each. I haven't had to learn to discharge caps on a solid state audio amp yet, but have done it for safety on my guitar amps (tube), usually to retention power tube sockets.
Click to expand...
The power supply filter caps are the large caps that store power so the PS doesn't 'run out of gas'. They also serve to convert ac to dc voltages for the amps circuits to use.
They retain this voltage over time. Forced discharge isn't the best way to go.
 
Follow-up to this. I did the de-oxit and CRC compressed air cleanup, and the problem has not returned as of yet. The GFA-535L has left and right output levels, not in the traditional volume knob way, but a flat head screw-type adjustment on the front panel by the A/B speaker. I put the deoxit D cleaner in the pots and let it sit for a few, worked back and forth many times, blew some air in the vent, then used the deoxit F lubricant and turned it some more. Also removed the fuses and cleaned the fuse clips and ends of the fuses themseleves with deoxit.

The Adcom system has been used in my basement system since the first post almost daily with no issues. I am running both A/B speakers but in this case it is stacked "New Large Advent" and Dynaco A-25s. Sounds very good, but I will bring the Adcom upstairs eventually to see if the problem returns bi-wiring the Linn speakers.
 
strat68 said:
So I get it home and enjoy for several days with the Linns speaker terminals strapped with one 8 ga. cable run. Later I take out the jumper and run the same exact wire and length for bi-wiring, one amp. Since I have to use bare wire at the moment on the Adcom, I have them into the A and B jacks, tweets on A, woofs on B.
Click to expand...
There's really no reason to bi-wire the speakers this way with this amp,it's does'nt really acheive anything meaningful by doing such.

strat68 said:
I added a turntable preamp as the adcom has no phono stage, and I noticed some clipping. I thought the gain may have been to high so I lowered it. Then problem became intermittant. Seems to happen early on, even with no source playing, then go away after 10 minutes or so of playing.
Click to expand...
Odds are there's still too much gain,though it would help to know more about the phono preamp to say anything for sure one way or the other.

strat68 said:
I read in another forum, posted by an authorized tech that he's only seen one power unit fail and that-
"Most failures were 10 and 100 ohm 1/4 watt fuse resistors that would open causing grounding issues on the front end thus causing DC offset putting the amp into protection mode."
Click to expand...
This amp has no "protection mode" that I'm aware of,so I'd be extremelty hesitant to heed any advice from that source.

That's one of the biggest complaints of those that dont care for Adcom amps (no protection).

If this amp goes into DC offset,you're gonna know pretty quickly as the smoke coming from the voice coils will let you know that's the case.

Hope you get this all figured out.

FWIW

Bret P.
 
xero-D-hero said:
There's really no reason to bi-wire the speakers this way with this amp,it's does'nt really acheive anything meaningful by doing such.
Click to expand...

According to the Linn Keilidh manual, they say it will yield better results. But not as much as bi-wiring with 2 amps. It was actually a big difference removing the jumpers.

xero-D-hero said:
Odds are there's still too much gain,though it would help to know more about the phono preamp to say anything for sure one way or the other.
Click to expand...

I was going to say the phono preamp was a red herring, as it was the only change prior to writing the first post and I happened to notice it while using the turntable. As this thread progressed, I noticed it happening with the CD player and ruled out the phono preamp. It's a Rega, btw, the kind with a usb- forget the model number. Now thinking about it I wonder if the Adcom preamp was the culprit? On the Adcom GTP-450, it has several buttons for the input selection. Sometimes it shows all input lights ON- CD, Aux, tuner etc. I can power it off/on and then the input buttons will work normally. It could be when all lights were on it was giving input overload (the phono preamp is powered on with TT off). But then odd that it only occurred on one channel.

xero-D-hero said:
This amp has no "protection mode" that I'm aware of,so I'd be extremelty hesitant to heed any advice from that source. That's one of the biggest complaints of those that dont care for Adcom amps (no protection). If this amp goes into DC offset,you're gonna know pretty quickly as the smoke coming from the voice coils will let you know that's the case.

Hope you get this all figured out.
Bret P.
Click to expand...

I was always confused by the lack of protection discussion regarding Adcoms, It must be something other than the 4 fuses inside, or the single external fuse. I guess I need to google it :)
 
strat68 said:
According to the Linn Keilidh manual, they say it will yield better results. But not as much as bi-wiring with 2 amps. It was actually a big difference removing the jumpers.



I was going to say the phono preamp was a red herring, as it was the only change prior to writing the first post and I happened to notice it while using the turntable. As this thread progressed, I noticed it happening with the CD player and ruled out the phono preamp. It's a Rega, btw, the kind with a usb- forget the model number. Now thinking about it I wonder if the Adcom preamp was the culprit? On the Adcom GTP-450, it has several buttons for the input selection. Sometimes it shows all input lights ON- CD, Aux, tuner etc. I can power it off/on and then the input buttons will work normally. It could be when all lights were on it was giving input overload (the phono preamp is powered on with TT off). But then odd that it only occurred on one channel.



I was always confused by the lack of protection discussion regarding Adcoms, It must be something other than the 4 fuses inside, or the single external fuse. I guess I need to google it :)
Click to expand...
The so-called "lack of protection" refers to there being no dc offset triggered speaker relay circuit. Many amps dont have this feature, including many examples of high end audiophile gear.
 
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