Need help with GTP500 repair. Tone and Phono sections.

First post on this site. I'm working on a GTP500 Preamp for a friend and I dont have a lot of experience with the IC based circuits but fully capable of swapping components and troubleshooting. Looking for some confirmation I am heading in the right direction.

Currently unit performs great with CD/Tape/FM/AM inputs but the following problems occur.

#1. Tone In selection causes both channels to cut out with a POP and very low sound on one side, none on other. Schematic looks like IC553 (20421DD) is only thing in this section. Looks like Mouser: 513-NJM2068DD is a good drop in replacement.

#2. When testing Phono Input I get nothing with a TT and if try iPhone at low volume it starts popping/distorting with very low sound in background as start to increase input level from phone. There are two IC's in the Phono Amp section IC501 & IC502 for both sides. This is listed as an IC Adcom5E. I am hoping someone has seen this and knows what IC's replace this component.

Attaching couple snips of the schematic here.

Any help will be greatly appreciated;

JD
 

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PRR

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Joined 2003
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It is unlikely a chip has failed. Caps and solder-joints FAR more likely. I don't see all the switches and jacks, but they are popular problems also.

Before ripping-out, _I_ would get DC voltages all around. A dead chip will be revealed by not doing what its pins say to do. Sick caps and joints give different numbers.
 
First post on this site. I'm working on a GTP500 Preamp for a friend and I dont have a lot of experience with the IC based circuits but fully capable of swapping components and troubleshooting. Looking for some confirmation I am heading in the right direction.
Swapping parts as a trouble shooting technique is not good form esp. when used in-lieu of proper test gear. IMO usually only a method of last resort, even then it's a bad idea. why? It's a good way of destroying perfectly good parts when swapped into failed circuit.
Currently unit performs great with CD/Tape/FM/AM inputs but the following problems occur.

#1. Tone In selection causes both channels to cut out with a POP and very low sound on one side, none on other. Schematic looks like IC553 (20421DD) is only thing in this section. Looks like Mouser: 513-NJM2068DD is a good drop in replacement.
1st look for DC on signal lines from leaky caps, poor or dirty switch/ pot contacts, cracked solder joints. Compare the bad channel to the good side for clues.IMO A good cleaning, re-seating connectors, and inspection will probably bring joy. Seeing a fault on both channels is a very good clue. It might even be a design flaw Unlikely for Adcom, .
#2. When testing Phono Input I get nothing with a TT and if try iPhone at low volume it starts popping/distorting with very low sound in background as start to increase input level from phone. There are two IC's in the Phono Amp section IC501 & IC502 for both sides. This is listed as an IC Adcom5E. I am hoping someone has seen this and knows what IC's replace this component
. see PRRs post above. Don't go replacing parts until you've located the problem with high confidence, E.g you really need more troubleshooting to reveal symptoms or clues. TT preamp need levels in the tens of mV. A phone output will be ~ 100's of mV so at really low volume it might be an unstable signal source. Build an add-on attenuator between the phone to preAmps TT input. Do use recordings of single tones at 400Hz so you can use a DMM on lower level AC ranges
Attaching couple snips of the schematic here.
Post a link to a clearer / higher resolution schematic.
 
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Thanks PRR and Infinia;

The owner has expressed desire to cash out on this preamp and look for something newer to use with his GFA-454 Amp. For my own learning I will continue with this unit and get to the bottom of this. Starting with the DC voltages and connections. I did a cleaning of the controls as best I could without pulling the main board but may have to do that to get better access. Not easy to access component side with power on but the board is well marked and can follow traces easily on the soldered side.

Sorry for poor quality images that's from HE copy which was already in poor resolution and almost impossible to read. Here is link though: Adcom GTP-500 - Manual - Tuner / Pre-Amplifier - HiFi Engine
 
i'm trying to fix my GTP-500ii which i picked up a couple of years ago from someone in houston, nonetheless! (drove down from dallas for the buy - i'm back in atlanta now myself) it seems to be having the same issue as yours with the tone controls (loud pop+no sound when switching them in with "tone in" button when the amp is wired to the 'lab' output, still a pop but slight output when wired to the 'norm'. did you ever have any luck with this? i went against the good advice of others and basically pulled out about half the electrolytics (any that looked like they were leaking or covered in glue that has since turned brown/black) and just got my replacement nichicons; i'm eying the styrols as well, since those seem to be quite heat sensitive.
 
bumping an old thread, again... just wanted to share what worked for me - thanks to ebc electronics for the repair tip, i replaced IC553 (marked adcom 7A) with this opamp, the LT1057 from mouser. that was the fix! i'd like to think i gave it a nice refresh as well by replacing (most of) the teapos with nichicon audio audio grade electrolytics, and all the styrols (with much higher voltage rated replacements from justradios.com) but it sounds fantastic now to my ears.


xfCd56Bl.jpg


Adcom GTP-500II repair - Album on Imgur
 
Phono input chips are clearly single opamps in standard pinout, but which sort? NE5534A is a clear possibility, assuming the circuit has enough gain for use without compensation (which is likely given its a phono section).


BTW Its really handy to have a reverse-RIAA converter/attentuator for testing phono inputs, since any signal source can then be used. A few resistors/caps in a little box with phono in/out is all you need - high accuracy RIAA curve isn't needed for troubleshooting, and no cartridge is put at risk.
 
Yet another bump. I replaced IC553 with an AD712JN which fixed the "TONE IN" pop on my GTP-500 II. I have a couple of spare AD712JNs if anyone might be interested - send me a PM.

More detail, as I understand it: The pop is caused by the preamp sending the failed tone control IC V+ or V- voltage to the preamp outputs when the TONE IN button is depressed and the tone control section is inserted into the signal path. The DC offset is maintained on the LAB output as it is DC-coupled (I measured 17.6 VDC). The NORM output is series capacitor coupled, so the DC voltage is not maintained on that output. Needless to say this DC transient (present on both outputs) and the sustained offset on the LAB output are Definitely Not Good for downstream devices.

Failure of IC553 also blocks the output signal, leading to silence on one or both channels.

In hindsight I would suggest a couple of things:
  1. Don't use the LAB output unless you are characterizing the amp
  2. If you bi-amp, connect the preamp to the amps with 1=>2 Y-cables connected to the NORM output.
  3. Replace R951 and R952 with lower values, e.g., 11.7K, to lower the supply voltages of IC553 (and others) and provide some margin away from the absolute maximum ratings (typically +/-18 V). I understand this is called for in the service bulletin, but I was not able to obtain that from Adcom.
  4. Document (with a camera) every single 'ribbon cable' termination BEFORE removing anything.
  5. Remove all of the ribbon cables to move the daughter board out of the way, don't try to force it out of the way.
  6. Use copper solder wick to remove parts if you lack a vacuum pump solder sucker.
  7. Clean the pads with alcohol or other suitable solvent prior to re-soldering.
  8. Use a flux pen to prepare the pads for re-soldering.
Last but not least a huge shout-out to Jim Fuller of Fuller Sound, Goodwin's High End, and EBC Electronics for their suggestions, as well as to the folks on this forum and others for reporting their experiences.

Tim
 
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