Adcom GTP 500 Preamp Request for Help - diyAudio
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Old 23rd December 2008, 09:08 PM   #1
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Default Adcom GTP 500 Preamp Request for Help

I picked up this tuner \ preamp to use in a bedroom system with a small T-amp. I thought it would be nice to have the preamp and tuner in one box.

I tested the Adcom GTP 500 with an Adcom GFA 535 amplifier that I purchased from the same CL seller. The gain on the preamp is low compared to other preamps I own and the bass is very muddy unless I use the low filter. At first I didn't get equal output from both channels. I cleaned all the pots and switches with Deoxit and now sound levels are balanced. When the sound wasn't balanced I believe the higher output channel sounded better and louder than what I'm hearing now. I know the amp is okay because it sounds really good when driven with either my Hafler Series 915 or my Harman Kardon HK 725. The gain on the Adcom preamplifier appears to be less than half of the gain from the other preamps. The headphone output of the Adcom is also lower than it should be.

Some of the capacitors look suspicious to me. It looks like there has been leakage but I also notice the same stuff around a few other connections on the PCB. Could what I'm seeing be glue or solder flux? Attached are some pictures. Any advice on how to get this going would be greatly appreciated.

Thanks
Bruce
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Old 24th December 2008, 12:52 AM   #2
m2003br is offline m2003br  Brazil
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It's only glue (bad one).
best is to get it removed, because this kind of glue causes corrosion on components leads, in some specific climates.

Cheers,
Marcos
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Old 24th December 2008, 05:34 AM   #3
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Thanks Marcos. I was hoping that replacing a cap would fix this but I'm afraid it is not going to be that easy.

Bruce
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Old 24th December 2008, 01:53 PM   #4
m2003br is offline m2003br  Brazil
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Hi Bruce,

If the sound is the same from both channels, it's unlikely you have problems with elcos.
Is here the loss of gain very apparent, or with the volume control at 3:00 pm is enough for a very loud audition? case "yes", the "low" gain is only a design choice and not a fault. (and a good sign, for sure)

marcos
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Old 24th December 2008, 02:13 PM   #5
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ussually when i see capacitors leaking electrolyte that bad i say a prayer for the pcb underneath of it. i would get that leaking cap fixed in a hurry.
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Old 24th December 2008, 02:44 PM   #6
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I think Marcos is right about it being glue around the caps and other connectors. I tried to clean it with alcohol and a toothbrush and it doesn't come off. If the stuff was electrolyte would it come off with alcohol? The same stuff is also around wires that connect to the pcb.

The unit may have a lower gain by design but it appears to be lower than anyone would design. There is also the problem with the bass being to high and muddy. I have to use the low filter to make is sound tolerable.


Summary of symptoms:
Lower output, muddy base, highs somewhat rolled off.
Same both channels
Same all inputs
Headphone output is also the same
Same tone controls in or out


Done to date:
Cleaned volume, balance, and tone pots. (deoxit)
Cleaned switches but they look semi sealed.


Is the power supply the most logical place to look?

Thank you for the suggestions and I welcome all and any thoughts.

Bruce
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Old 25th December 2008, 11:53 PM   #7
m2003br is offline m2003br  Brazil
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Bruce,
is unlikely you have a problem in the ps.
Just in case, check voltages in IC554 (about -19V pin 4, +19V pin 8, 0V pins 1 and 7) and in IC551 or 552, you must have obtain about 15V on pin 7 and -15V on pin 4, and 0 volt (+- 50mV) on pin 6.
it's enough to cover all preamp voltages (tuner have separated ps)
If some electrolitic caps are defective on power supply, you get a "hum" in your speakers or at least a reduction in some voltage readings.
The best way to analize your GTP500 is by using a Audio generator and a oscilloscope.

regards,
Marcos
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Old 26th December 2008, 12:56 PM   #8
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I'll check the voltages but don't have or know how to use an oscilloscope. Thanks for all the help.

Bruce
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