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ADCOM GTP-500 Help (please?)
ADCOM GTP-500 Help (please?)
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Old 15th May 2012, 07:19 PM   #1
immersifi is offline immersifi  United States
diyAudio Member
Join Date: May 2012
Location: Ann Arbor, MI
ADCOM GTP-500 Help (please?)
Default ADCOM GTP-500 Help (please?)

OK, so here's the situation... I have a GTP-500 preamp and have been quite happy with it. However, not long ago the right channel stopped working (line outs and headphone) independent of the source selected. If you cylce the power and leave it off for a while, the right channel will come back, but only for a little while, and then it drops out.

NOTE: I have yet to check the TAPE OUT section to see where the issue may be. My *guess* is that if the TAPE OUT continues to have the signals present (after the right channel goes mute), then it's probably not power supply-related (as all chips draw from one central supply, yes?) but instead is related to the chips that drive the output. Also, that the op-amps appear to be one per channel also suggests that may be the fault, because again, if it were one of the rails' supplies, then when this happens, the left channel would be distorted, but this isn't what is happening; if I can get my hands on a DMM shortly, I can check the supply voltages.

Anyway, from what I can tell, it looks like there are two chips (one for each channel, which from a twice-scanned, several-times-folded-spindled-and-mutilated Adobe doc appear to be ADCOM chip 6A) which seem to have the same pin-outs as a TI TL-082 (which is a J-FET op-amp) and some other op-amps out there than are the main chips fro driving the outputs. ASSUMING that my *guess* that the chip(s) 6A are the culprits, then as a fix, there appear to be a few J-FET op-amps out there that could work in their stead, so I'm definitely open to suggestions.

So, here are my questions:

1) Are there any GTP-500 owners out there who have had this issue and know the fix? I can always root around with a DMM and such, but as always, if there's an "E.W.O." (easy way out), I'll take the guidance, advice, wisdom, and insight of others who have seen this problem.

2) Assuming some pin-compatible and roughly similar replacements can be used off the shelf, does anyone have any recommendations as to which surrogate would be best to use?

Again, while I'm very comfortable with this subject matter, time is one thing that I find to be in ever decreasing quantities in my life, and particularly as a newb to this forum, any guidance that could be given would be most sincerely appreciated.

Many Thanks... Mark

[UPDATE] So I did the pre-out check...sho 'nuff, the signal is present at the TAPE OUT points. It appears that the issue is on one of the ADCOM "5F" chips - more specifically IC552...at least, this is what makes the greatest sense. I have been perusing the web and these forums, and it seems that Linear technology is the 'preferred' vendor for ADCOM. Nevertheless, I'm thinking that as long as the open loop gain parametrers and supply parameters are met, I should be OK. I did find a 'substitute' on the web (IC00001530 ADCOM 5F (SUB FOR ADCOM 5E) GTP-500) on adcomparts.com).

Can't be the +/- DC as the tape out functions perfectly...



[UPDATE 2] Well...this doesn't seem to be an economical fix. As it happens, it's not the op-amps. It appears to be related to the LISTEN TC9152P chip (24-pin DIP). I say this because I did indeed try a chip swap for the ones that I thought were bad, but the ame thing happened - after about 3-4 minutes of operation, the right channel mutes. Based on what I can see in the schematic, this makes the most sense, because the RECORD chip (i.e. signal to tape out) always operates, but it is on a separate chip (same variety as the 'listen' chip). I did manage to find the data sheet for the chip, and the chip does indeed have a MUTE function, it operates both channels...so it seems wrong to think the MUTE logic signal is being sent. That is, if that were the issue, both channels would mute and not just the right.

Unfortunately...the TC9152P chips are not in sockets like the 8-pin DIPs, so some careful desoldering would be required, and then a socket and replacement chip.

I'll look again at the schematic, but again, this seems the most logical diagnosis.

Last edited by immersifi; 27th May 2012 at 02:34 PM. Reason: more exploration
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Old 15th May 2012, 10:31 PM   #2
cihtog is offline cihtog  United States
diyAudio Member
Join Date: Nov 2006
Have you tried tracing the signal through the pre-amp, to see where it drops out at?

Adcoms are generally quite colorful and easy to trace, even without a schematic - not to mention that you have a working channel, that you can compare readings with.

My guess is a loose connector or solder joint - chop sticks make great wiggle, poke, and prod tools "and" powered PC speakers make excellent audio probes ;-)

Just plug an audio source into the input, and follow the sound through the circuit, until you can't hear it anymore.
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Old 16th May 2012, 01:13 PM   #3
immersifi is offline immersifi  United States
diyAudio Member
Join Date: May 2012
Location: Ann Arbor, MI
ADCOM GTP-500 Help (please?)
I thought about that, and to that end, I tried looking for the obvious - loose connectors (i.e. the ribbon cables etc) and even re-seated some of them. I kind of thought that it was a long shot, simply because if you leave it off for a while, turn it back on, and listen, then the right channel works - for a while. So, it seems that were the problem connector-related it would tend to be more constant. Also, that the channel drops out after a fashion seems to indicate something related either to drift, or to a thermal issue (even though this is a preamp and as such, doesn't really reject heat like a power amplifier would).

As far as actually tracing the signal, that occurred to me, but I don't have an operable 'scope that could be used to walk through the signal path.

I'm tempted to simply get two J-FET op-amps having the same pin-out as those of the ADCOM 6A chip, swap them out, and see what happens. I thought I had seen somewhere that the LT series of J-FET input Op-amps were a good choice for substitution.

Anyway, thanks for your input. If I have time tonight, I will indeed try the 'rough' approach to tracing it - like I said, if the signal disappears from the tape outs then that would seem to indicate a different issue than the probem being in the chips that drive the line outputs; it can't be the chip that drives the headphone output because when the channel loss occurs, it happens in the main outs (whether "LAB" or "Normal" outputs) as well as the headphones.
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