ADCOM had sent me details on how to do the repair for the II (free of charge I might add)... Have you tried reaching out to them regarding your GTP-500?
Steve
Sure.. send me an e-mail at weiss.1966 at gmail.com and I'll get those docs to you... I don't want to post them here as that would be for ADCOM to do, not me.
[b]GTP-400 REPAIR AND ALIGNMENT[/b]
BACKGROUND
When Adcom tuners grow old, the FM reception becomes intermittent.
For me, stations dropped out one by one, climbing up the FM band, each replaced in turn by harsh white noise.
A trimmer capacitor inside needs to be replaced. The part # is CT-104, and it is located inside the metal box. Others have made this repair successfully multiple times using
Mouser P/N 659-GKG10015. ( 3-10 pf) Once replaced, you will need a DVM to bring the circuit back into alignment.
MY GTP-400 TRIMMER FIX
Dear ALL,
Here are my alignment numbers for replacing capacitor trimmer CT-104 in an Adcom GTP-400. Thank you everyone so much for this thread.
The old trimmer was set at 6 pf. The replacement was shipped at 13pf. I unscrewed it to lower the setting to 6 pf and installed it after opening the metal shielding box and removing the old trimmer.
I adjusted L107 with the set tuned to 88 MHz, and CT104 when tuned to 104MHz as described, but not to the voltages given for test points TP3 and TP4 at the rear of the printed circuit board.
L107 at 88MHz came up at 1.98 volts. Reception worked from 1.68 volts to 2.4V, otherwise noise. I left it at 1.98V.
CT104 at 104MHz was at 6.3V, and reception worked from 4.9 to 7.2 volts. I left it at 6.0V. Touchy to adjust, as you guys said.
I then put the set on a weak station at 91.5MHz. The test point voltage is of course lower at the low end of the dial; the TP reading was 2.44V. The station would only come in with CT104 from 2.42 to 2.46V -- 2.49V was dead reception. I left CT104 at 2.44V, confident I was at the golden middle, pressed the metal lid back onto the shielding box (damned if I would solder it again), and called it done.
While there, I adjusted the mute threshold pot (halfway back to front of circuit board) to less muting, so that I could hear weak stations when fussing with an antenna, for example.
I left the spare trimmers and instructions inside the set . . . my "Adcom Kit" if I ever get another one.
AND NOW FOR THE $64 QUESTION
What the blazes goes wrong with the damn trimmer?
My stations went off the dial one at a time from 88.5 on up -- horrible white noise.
Sometimes after a rest, the set worked again.
1. the old trimmer measured 6pf, that proved to be correct, you could adjust the old pot
2. the old trimmer had infinite DC resistance, as it should; no shorts.
3. the old trimmer had infinite ESR at 150KHz, as it should.
Thank you guys for this thread, there is nothing wrong with the trimmer that I would ever have found, it's Miracle Mystery Hour here for me.
--jerry
IEEE
BACKGROUND
When Adcom tuners grow old, the FM reception becomes intermittent.
For me, stations dropped out one by one, climbing up the FM band, each replaced in turn by harsh white noise.
A trimmer capacitor inside needs to be replaced. The part # is CT-104, and it is located inside the metal box. Others have made this repair successfully multiple times using
Mouser P/N 659-GKG10015. ( 3-10 pf) Once replaced, you will need a DVM to bring the circuit back into alignment.
MY GTP-400 TRIMMER FIX
Dear ALL,
Here are my alignment numbers for replacing capacitor trimmer CT-104 in an Adcom GTP-400. Thank you everyone so much for this thread.
The old trimmer was set at 6 pf. The replacement was shipped at 13pf. I unscrewed it to lower the setting to 6 pf and installed it after opening the metal shielding box and removing the old trimmer.
I adjusted L107 with the set tuned to 88 MHz, and CT104 when tuned to 104MHz as described, but not to the voltages given for test points TP3 and TP4 at the rear of the printed circuit board.
L107 at 88MHz came up at 1.98 volts. Reception worked from 1.68 volts to 2.4V, otherwise noise. I left it at 1.98V.
CT104 at 104MHz was at 6.3V, and reception worked from 4.9 to 7.2 volts. I left it at 6.0V. Touchy to adjust, as you guys said.
I then put the set on a weak station at 91.5MHz. The test point voltage is of course lower at the low end of the dial; the TP reading was 2.44V. The station would only come in with CT104 from 2.42 to 2.46V -- 2.49V was dead reception. I left CT104 at 2.44V, confident I was at the golden middle, pressed the metal lid back onto the shielding box (damned if I would solder it again), and called it done.
While there, I adjusted the mute threshold pot (halfway back to front of circuit board) to less muting, so that I could hear weak stations when fussing with an antenna, for example.
I left the spare trimmers and instructions inside the set . . . my "Adcom Kit" if I ever get another one.
AND NOW FOR THE $64 QUESTION
What the blazes goes wrong with the damn trimmer?
My stations went off the dial one at a time from 88.5 on up -- horrible white noise.
Sometimes after a rest, the set worked again.
1. the old trimmer measured 6pf, that proved to be correct, you could adjust the old pot
2. the old trimmer had infinite DC resistance, as it should; no shorts.
3. the old trimmer had infinite ESR at 150KHz, as it should.
Thank you guys for this thread, there is nothing wrong with the trimmer that I would ever have found, it's Miracle Mystery Hour here for me.
--jerry
IEEE
BTW, I'm working on repairing my second one of these today and on each one that I have looked at, I find lots of electrolytic capacitors that have high ESR including all of the 220uF caps on both units. Some of the access to the capacitors is very difficult in these things. Not at all an easy unit to work on.
I did re trim according to instruction without part replacement. I was able to set bot the L107 within specs as well as the C104 within specs . I went back and forth several times to verify that nothing had changed and if necessary tweeked each to get the voltage values. No change to the low reception . What I did find in my situation was bunch of melted wax like substance around L107 I cleaned it up but I did not see where it came from appeared all around L107. My problem is pretty much Isolated to the lower band. I listen to 88.9 only. Most of the other higher station top out the reception range and stereo indicator. My station some times gets 3 bars and no stereo and I have use to use high blend to even receive the station. I have GTP 500 as well no problems with that unit with the same dipole I get max bars and stereo indicator. In my situation since my issue is with the freq that is in the range of L107 also the waxy residue I am wondering if just changing C104 is going to fix my problem. Any ideas suggestions welcome.
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I tried a desperate measure. Since the trim adj to specs didn't work out. I said what the hey lets try adjusting while on line for my station. Worked wonders. The adj on L107 is at 3.3 v but the station has multi bars and the stereo indicator come online. I checked and the C104 is at 29.7 v so I just left it alone.
So the net is you can realign and adjust for component ageing. When it goes out all together then I can worry. EBC the ADCOM service facility charges 26 s + 146 to fix anything wrong. You also have to pay shipping there.
So the net is you can realign and adjust for component ageing. When it goes out all together then I can worry. EBC the ADCOM service facility charges 26 s + 146 to fix anything wrong. You also have to pay shipping there.
The wax is from the factory, I guess to minimize fluctuations due to either physical movement or temperature change. That trim cap is extremely sensitive.
Thank you for the suggestion, but only one of those mounts on through holes, but looks to be a different form factor and the rest are surface mount.
Anyone found a suitable direct replacement?
David.
Re-resurrecting this topic.... sort of. I’m interested in disabling/disconnecting and shutting down altogether the tuner section in the GTP 500ii. Removing all power and as much interconnections to the preamp section.
Does someone have any ideas? I’m pretty sure it’s a matter of pulling out some ribbon cables and removing a couple of caps/resistors from the main board.
I’d like to not only shut down the fluorescent display, but the whole tuning circuit, theoretically removing noise sources from the preamp signal.
Any ideas are welcome.
Cheers,
Luis
Does someone have any ideas? I’m pretty sure it’s a matter of pulling out some ribbon cables and removing a couple of caps/resistors from the main board.
I’d like to not only shut down the fluorescent display, but the whole tuning circuit, theoretically removing noise sources from the preamp signal.
Any ideas are welcome.
Cheers,
Luis
Probably a LOT simpler than all that.
I’d start with the schematic and look for the positive power feeds into the radio section. Shouldn’t involve more than cutting or disconnecting a few wires.
Shouldn’t.
Let us know if this helps.
You could always sell it on one of the auction or audio equipment sites and buy a 555 or 565 preamp.
Norm
These things are notoriously difficult to work on but it certainly can be done. You'll need the schematic which can be found on line. The inter board wiring and the component layout are not provided though and it is not very intuitive comparing everything to the schematic. Access to areas between boards is nuts too. I got one for free that had a bad tuner which I fixed by replacing the trimmer capacitor that is a regular problem with them. After replacing most of the electrolytic capacitors (ESR test fail), I sold it for a good price. I thought I'd repeat the magic with one I bought on ebay (broken tuner) but could never get the tuner working.
Power to the display should be easy to find as it will be an odd voltage compared to the others. Removing power to the tuner section will be more difficult. Cut power circuit traces to the shielded RF can area. There may be some PLL circuits that you can disable too but the analog part of the tuner audio path and filter will be OK to keep powered.
I've already done some work on this pre-tuner. So far I'm liking what I'm hearing. Seeing I'll never use the tuner part, I'll try to see if I can disable it. As far as removing it's analog path to the rest of the unit, it's fairly easy, just following the traces from the Toshiba analog switch IC's. and severing those traces both at the base of the Toshiba chips and at the very border of the tuner section. next come power... and disabling the display and control button circuitry. I'll study the schematic to see how to do it. Will post any updates, and in the meantime, if someone has done this before, please chime in.
Adding to the general GTP-500ii topic, I had to replace a couple of damaged op amps in the Listen / Record section. line buffers I asume. IC601 and IC602, originally labeled as Adcom 7A. These parts are LT1057 op amps. Replaced them with OP275. Very nice sound. Also changed a couple of resistors, as specified in an Adcom Service Bulletin for this unit. lowering rail voltage to the buffer and gain op amps and protecting them from future damage.
I'm considering refurbishing the gain stage with better components and if possible, lowering the gain ratio. In order to diminish quiescent hiss. I have very sensitive speakers (Klipsch Heresy II) and a rather highish gain power amp (GFA5300) 29 db. For this, I'm thinking newer generation MF resistors, OPA134 op amps (replacing older LT1056), poly caps, and will have to study how to lower the gain via the feedback resistors, reducing gain by 30-40%, even 50% could do. Right now, you can't go any higher than 9 o'clock on the volume knob before going deaf and blind and cuckoo....
Just a project to keep me entertained during the lazy hours....
Adding to the general GTP-500ii topic, I had to replace a couple of damaged op amps in the Listen / Record section. line buffers I asume. IC601 and IC602, originally labeled as Adcom 7A. These parts are LT1057 op amps. Replaced them with OP275. Very nice sound. Also changed a couple of resistors, as specified in an Adcom Service Bulletin for this unit. lowering rail voltage to the buffer and gain op amps and protecting them from future damage.
I'm considering refurbishing the gain stage with better components and if possible, lowering the gain ratio. In order to diminish quiescent hiss. I have very sensitive speakers (Klipsch Heresy II) and a rather highish gain power amp (GFA5300) 29 db. For this, I'm thinking newer generation MF resistors, OPA134 op amps (replacing older LT1056), poly caps, and will have to study how to lower the gain via the feedback resistors, reducing gain by 30-40%, even 50% could do. Right now, you can't go any higher than 9 o'clock on the volume knob before going deaf and blind and cuckoo....
Just a project to keep me entertained during the lazy hours....
Sounds good. I like the Heresys too and love the Le Scallas although I've never owned either one. I've got 96db sensitive speakers so I can relate. I like the OPA134 series of op amps too.
It was a couple of years ago that I did my work and then sold them both. I just looked at the service manual again this afternoon on Hifiengine and remember now how terrible it was to work with as the manual is like a third generation copy that was scanned and everything you see just melts together. Also how the inter-board wires would pull out of their poor saddles without much effort. Getting between the upper and main circuit board to replace capacitors was frustrating too.
I've got a good capacitor ESR meter which tests in circuit and most of the caps on my two were out of range so I replaced. New caps in the signal path reduces distortion and any other caps replaced would reduce noise. Plenty of room to upsize the power supply capacitors or the decoupling capacitors in this one too. Tack on some small film caps across them on the bottom of the board and that helps too.
Replacing ceramic capacitors with film caps in the feedback loops will help and also do that with the power supply decoupling caps near the op amps too.
Good luck with your project!
It was a couple of years ago that I did my work and then sold them both. I just looked at the service manual again this afternoon on Hifiengine and remember now how terrible it was to work with as the manual is like a third generation copy that was scanned and everything you see just melts together. Also how the inter-board wires would pull out of their poor saddles without much effort. Getting between the upper and main circuit board to replace capacitors was frustrating too.
I've got a good capacitor ESR meter which tests in circuit and most of the caps on my two were out of range so I replaced. New caps in the signal path reduces distortion and any other caps replaced would reduce noise. Plenty of room to upsize the power supply capacitors or the decoupling capacitors in this one too. Tack on some small film caps across them on the bottom of the board and that helps too.
Replacing ceramic capacitors with film caps in the feedback loops will help and also do that with the power supply decoupling caps near the op amps too.
Good luck with your project!
FYI...this is an easy fix if your Adcom is no FM or fades out.
I've had 2 Adcoms and replacing the CT102 has fixed it each time. Fixed a GTP500II tonight.
Even if you have no electronic skills but are good at DIY this is really simple home fix and you can learn to solder. Hardest part is getting the tin cover off the FM section to replace the CT102
I've had 2 Adcoms and replacing the CT102 has fixed it each time. Fixed a GTP500II tonight.
Even if you have no electronic skills but are good at DIY this is really simple home fix and you can learn to solder. Hardest part is getting the tin cover off the FM section to replace the CT102