Adcom gfa1a clipping issue

Mr. Sharpe

Hello all! My dads old gfa 1a has been hangin around his rta12 c's since they both came out new, I've taken this beast apart several times.

Here's the issue, one of the channels is working fine the other is not, symptom is simply that it sounds Like it's constantly clipping. I will see if I can post a video or sound clip of some sort so you can hear. It's weird too, one day it started doing this, the next I hooked it up at my house and theee were no issues at all, took it back to his house and the problem started back:( sad day.

I'm wondering what I can do about this to help him out. And by helping him out I mean me taking this unit apart and fixing it myself to save him the hassle and money sending it somewhere to fix if possible. I don't mind working with this unit even though it's numerous screws are annoying.

I even offered to let him take my parasound amp so he could keep the sda 1s he got so I could have his rtas and this amp once working but he won't do it.

[Deleted User]

What I would do is:
Swap the speaker cables or speakers themselves and see if the problem
follows it. (be careful) If it does, it is the speaker itself.

mhardy6647

^^^Good advice, that.

The input cable(s) to the amplifier and/or the physical connection (RCA plug on the cable and jack on the amp) are also always suspect. Swapping channels may help troubleshoot/identify the issue. It's a combinatorial exercise

A quick perusal of the web suggests that this amp has no attenuator pots (at least accessible on the outside of the amp). If it did, dirty pots (or switches) are always "one of the usual suspects" -- doesn't seem to be the case for this amp, though. There could be internal trim pots, but one would need a schematic to proceed prudently

Electronically, such a problem is likely either power supply, driver stage or output stage. That narrows it down a lot, eh? I assume it's a direct coupled amplifier. If you have a DMM (digital multimeter) it would be a good idea to check for a DC (direct current) "offset" on the "bad" channel. Significant levels of DC on the output is potentially lethal to a loudspeaker load

mhardy6647

The service manual (ostensibly covering the 1 and 1A) is available from hifiengine:
https://www.hifiengine.com/manual_library/adcom/gfa-1.shtml

Registration required to download (and well worth the effort, in case the OP isn't currently registered with them).

mhardy6647

Here's a primer of sorts on amplifiers and DC offset:
A little is OK (usually it's by design); a lot is not.

http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

littlewoodboats

What is your dad running for a pre amp? I would also suggest swapping the R&L from the pre out to the amplifier to see if the issue moves. It could be an issue with his pre amp considering the amplifier seemed to be fine at your house.

mhardy6647

littlewoodboats wrote: »

What is your dad running for a pre amp? I would also suggest swapping the R&L from the pre out to the amplifier to see if the issue moves. It could be an issue with his pre amp considering the amplifier seemed to be fine at your house.

Excellent point.
There is that, too!



So, it's still a combinatorial experiment (we call 'em "factorial" in my line of work, but you get the idea). Just a few more degrees of freedom.

... and I suppose dirty pots or switches in the preamp are suspect, too!
Especially if the components are older and haven't been maintained in some time (a few years, or more).

Cleaning pots and switches is relatively straightforward.
http://audiokarma.org/forums/index.php?threads/the-idiots-guide-to-using-deoxit-revisited.207005/

Mr. Sharpe

rednedtugent wrote: »

What I would do is:
Swap the speaker cables or speakers themselves and see if the problem
follows it. (be careful) If it does, it is the speaker itself.

I've tried the amp with several of his speakers, I first noticed with his infinity shelf speakers, then his big ones, and again with mine. I've at the same time used several different cables, monsters and 3 different audioquest models. Switch ok no them back and forth between outputs on twood different preamps.

His preamp is a adcom gfp 1 that he just got repaired locally.

I haven't done any serious multi meter yet but I've only new batteries for a week or so, just haven't started that.

I am curious how to check the voltage? Just turn it on and check voltage between the positive and negative right? Thank you so much for the replies, I was thinking of recapping this unit while I'm in there, even rebuilding it in another chassis hahaha might be fun/cool.

mhardy6647

Set the DMM to a fairly sensitive range for DC voltage and, yes, check between + and - with the amp at idle (no signal). There is, presumably, a specified, acceptable DC offset for the amplifier -- it's probably on the order of 10 or 20 mV (i.e., 0.01 or 0.02 V; positive or negative doesn't really matter). If you see 100s of mV, or, worse yet, an offset on the order of several Volts (i.e., 1000s of mV), somthing's awry -- and the loudspeaker's at risk.

Take a look at that primer on measuring DC offset at the AK link. It should be helpful.
Note that a DMM is a high impedance device. Connecting it across the two output terminals of an amplifier channel does not (ahem, should not) constitute a short circuit!

Transistors in a push-pull audio power amplifier "ride" on a "rail" of DC voltage that determines the power output capability of the amplifier. I think (!?!) that, for a complementary (i.e., output capacitor-less or "direct coupled") "200 watt" amplifier (into an 8 ohm load), this translates to (two) DC rails of + and - 50 VDC. You guys who know about solid state amplifiers can fact-check me on this.

A shorted driver or power output transistor might enable the entire DC voltage of one of those "rails" to appear at the speaker output. That could be a DC offset of up to 100 volts (or maybe just 50 V, if it's only one device shorted... I am not sure 'bout that, but you get the idea!). 50 or 100 VDC would cook the (woofer, at least) voice coil of a loudspeaker connected to it essentially instantly. Most ss amplifiers have some sort of protection built in to them ostensibly to prevent such immolation. However, as the Adcom amp's manual points out, no protection device is 100.0% effective (sort of like contraceptives!); it's not uncommon for an amplifier to cook a $1000 loudspeaker to protect a 50-cent fuse installed in the output circuit.

Here's a nice little primer I found in a quick googling on this topic. Again, I ain't a solid state guy, so I can't vouch for all of it in terms of accuracy.
http://education.lenardaudio.com/en/12_amps_4.html

mhardy6647

Oh, I should also mention: remember that the "motor" in a so-called "dynamic" (voice-coil & magnet type) loudspeaker driver (e.g., a woofer) is an electromagnet. Since the electrical analog of a music signal (waveform) is AC, it causes the electromagnet to "repolarize" rapidly as the signal passes through the voice coil. This makes the voice coil move in the fixed field provided by the driver's magnet. The voice coil's attached to the cone (or dome or membrane), which "wiggles" as the voice coil moves. That wiggling makes sound -- thus is the electrical (AC) signal fed to the speaker by the amplifier "transduced" to audio.



If you instead "feed" DC to the voice coil, you make a regular-old electromagnet (like the kind they use to lift scrap iron and steel at a junkyard. A speaker fed DC will exhibit an instant displacement ("in" or "out") of the cone as that electromagnet is either attracted or repelled by the fixed magnet mounted on the speaker driver. The problem is that, since the DC resistance of the voice coil is low (a few ohms), a tremendous amount of current will be flowing through the voice coil. The voice coil is made of fine wire (to help keep the mass low), and the heat generated by that current will literally cook it (burn it "open"). End of voice coil.

Mr. Sharpe

I checked voltages on both speaker terminals and neither read anything, my meter only has one v setting though so I'm sure that's why it's not picking it up.




I'm guessing these are the output transistors, that article you gave was insightful and since this unit four 50w amps I'm guessing each is for 25 watts. This thing is ridiculous, I don't even know how to remove the motherboard at all. Maybe take all transistors out? Should I check all of them out?

Mr. Sharpe

New development. I pulled all of the transistors out and sprayed deoxit in the plugs. Now I'm reading .667v oUT of each speaker terminal... checking voltage on each transistor is going from 5. Something to 1. Something....

What's bothering me is the side that the transistors are reading voltage is blowing cool.... the other side of the amp is not reading any volts at all and is blowing warm:(

I am also now getting transformer hum, and before the unit relays would "click" on, now they arent. probably destroyed it.

mhardy6647

Sounds like perhaps one transistor was inserted 'backwards', perhaps.
Not sure why you were pulling out transistors...

Mr. Sharpe

mhardy6647 wrote: »

Sounds like perhaps one transistor was inserted 'backwards', perhaps.
Not sure why you were pulling out transistors...

Is .667 a normal read for the outpputs then? I did clean all the transistor contacts and before I was getting a reading of 0.0000 volts. Maybe this is beyond me, if you believe it is let me know please because I think it is haha

mhardy6647

Given that you went from troubleshooting to intervention, yes, you're probably in way over your head. Sorry, I guess I should have been more explicit about not doing anything until you read through that AK thread and felt confident enough to proceed. My suggestion was strictly for troubleshooting, not intervention. If you have DC offset, the troubleshooting part should have led to the conclusion "find a competent repair person and get an estimate" - as opposed to "start pulling out components".

If you got an offset of 0.667 DC volts, yes that is too much (even without knowing the offset spec on that amplifier). If those are not the units, I can't even guess.
How does your DMM select to measure AC vs. DC voltage? Do you have a manual for it? If not, what is the make & model?

Mr. Sharpe

I read that article, maybe it didn't sink in enough lol. I also read the service manual and probably should've left it alone. But I'm not understanding why I didn't read volts before and now I am. Oh well. My DMM didn't come with a manual, it's more of a straight forward anyone can use meter.

heiney9

You were in over your head before you started.

I'd say you mucked it up a bit worse. I am almost 100% sure those exact transistors are no longer available. So that means if one is bad you should replace them all with whatever the current replacement is (pun intended ).

Don't buy them on Ebay or anywhere else unless like that unless you can confirm they are NOT counterfeits.

Good luck. Might be cheaper to find another GFA-1 and either use it or cannibalize it.

H9

pitdogg2

I'd just go find a nice used Adcom GFA 555.

VSAT88

Mr. Sharpe wrote: »

mhardy6647 wrote: »

Sounds like perhaps one transistor was inserted 'backwards', perhaps.
Not sure why you were pulling out transistors...

Is .667 a normal read for the outpputs then? I did clean all the transistor contacts and before I was getting a reading of 0.0000 volts. Maybe this is beyond me, if you believe it is let me know please because I think it is haha

Hey man, look up how to test a transistor on the web. Those do not put out volts when they are pulled out like that. You could have had the meter in "diode check" position with a reading like 667, that would be a correct reading (one way) with the meter in diode check position. Oh brother be real careful of those big huge beer can lookin main filter capacitors. They can give you one hell of a shocking experience. This is if you decide to go any farther, no disrespect intended but in my humble opinion you should not.

https://www.elexp.com/electrical_measurements_part_3.aspx

Mr. Sharpe

I can take criticism fiNE thanks for all the replies. I know they won't put out volts unplugged though lol and I am very familiar with capacitors and their electrifying personalities;)

tonyb

Take it to a shop that knows what they are doing and have the right gear to test it out on. Your just shooting in the dark here, spending your time and not making headway. Sometimes you just have to bite that bullet.

Mr. Sharpe

Yea you right. I am going to go in and make sure those transistors are in the right way bc I did take a picture of them then leave it be.