Adcom GTP-500ii FM Problem

gregbed

Member
2008-06-27 1:07 pm
Terry C
I am trying to do this repair for the first time. I have a solid metal cage around FM section. It looks like the top is soldered on. How do open this? Do you just desolder the corners and inside side and pry it open? Any tips or tricks would be appreciated.
Greg
 
gregbed said:
Terry C
I am trying to do this repair for the first time. I have a solid metal cage around FM section. It looks like the top is soldered on. How do open this? Do you just desolder the corners and inside side and pry it open? Any tips or tricks would be appreciated.
Greg

Yes, one corner at a time. Any cheap soldering iron with a large tip will work. I used some wick to get the majority of the solder off, then heat it up again and pry it open.
 
The part # is CT-104, and it is located inside the metal box. I've made this repair successfully multiple times using Mouser P/N 659-GKG10015. Once replaced, you will need a DVM to bring the circuit back into alignment. I can provide instructions for performing the alignment.

Old thread bump alert !

Intermittent FM on a GTP-600, no schematic. It also has a CT104 that I'd like to swap out, to see if it is the cause. Can anyone confirm it is the same part as on this model? Also nearby are trim caps CT102, CT103, but apparently these are a different value (blue case as opposed to red).
 
Old thread bump alert !

Intermittent FM on a GTP-600, no schematic. It also has a CT104 that I'd like to swap out, to see if it is the cause. Can anyone confirm it is the same part as on this model? Also nearby are trim caps CT102, CT103, but apparently these are a different value (blue case as opposed to red).

I don't have the manual for that model, so I can't confirm it is the same. However, Adcom used this tuner circuit design in many of their tuner/preamp models, so it's likely the same.
 
I don't have the manual for that model, so I can't confirm it is the same. However, Adcom used this tuner circuit design in many of their tuner/preamp models, so it's likely the same.

Assuming that's true: can you supply the values for CT101-103? I think the three of them are identical to each other, but different from CT104. This is just based on the color of the casings, blue vs. red.

CT104 (3-10 pf based on the Mouser part number given in an earlier post) is probably the prime suspect but I don't know why other components of the same type on the same board wouldn't be likely to fail around the same time. I'll probably order a handful of these trim caps, since they cost roughly nothing.

edit: I see you were the one who provided that part number :)
 
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gregbed

Member
2008-06-27 1:07 pm
I had this project sidelined for over 3 years. I originally couldn't get the lid off the metal tuner box section and was a little afraid of the soldering. I had given it to an acquaintance who was a pro. But he had a lot of personal problems and never got to it and I just took it back when I got the chance.
Anyway, a new year and I decided it was time to take this on or kill it trying. I have very little experience working with desoldering and soldering pc boards. The key to the metal lid is to pry the side of it outward from the box while holding the soldering iron on the box close to the lid. (make sure you have a tiny screwdriver and that you really are between the lid and the box and not behind the box as well.) Desoldering took some time ( I don't have this braid thing down.) I used a little piece (1" cube) of diced foam wedged in the metal box to hold the CT104 in place while I soldered it. (The first time I tried soldering it I pushed it out from the bottom). Once soldered in, I did the alignment (step 1 & 2 only). The L107 is adjusted with a small allen wrench in the center of it. (i think it was 2mm, but not 100% sure). I had to turn it pretty far clockwise (inside part moves downward) to get down to 3V. The CT104 is pretty touchy and it was hard to get right on 23V, I ended up settling for 22.7.
I hooked up my rooftop antenna to the 75 Ohm antenna tap, plugged in the headphones and MUSIC. It worked. Very cool.
 
I purchased one of these preamps off of ebay after reading about the fix. Since some of the information is incomplete/out of date I thought I'd add my two cents--for the benefit of future web searchers.

1) I ordered the cap TerryC mentioned from Mouser, P/N 659-GKG10015. They still have them, and I suppose I could've gotten a replacement from Adcom as well. I was fixing several other items as well and decided to make a bulk order of caps from one source.

While waiting for the cap I looked up the web links given earlier. The link Gregbed listed is missing in action, but it can still be read (minus the pictures) by searching for the page on archive org: http://www.vgernet.net/bks0731/comms/adcom.html.

The calibration info given in the second link by TerryC (http://i41.photobucket.com/albums/e2...500IIAlign.jpg) is a photograph and it's hard to read. I'll save you the trouble below by listing the two pieces of info you'll need.

2) One of the unit's previous owners had already nearly desoldered the metal shield over the tuner. Once that was completely removed I identified the position of CT-104. With the unit's front panel facing me and tuner in the upper right of the case, I saw two sections inside the tuner. CT-104 (as well as the tuning coil that requires adjusting) were next to each other in the section of the tuner closest to the front panel. The two test points (TP3 and TP4) required for adjusting the coil and cap are just outside the tuner box, on the side closest to the front panel.

3) I flipped the unit over and removed the bottom panel screws before lifting the panel off. Thoughtful design by Adcom--otherwise the entire unit would've had to be disassembled to replace the cap. Desoldering the cap and replacing it was easy. I noticed there was a 'flat' side of the cap on the PC board's silkscreen, so I inserted the replacement with it's flat side oriented the same way.

4) After replacing the bottom cover I dug out my DVM and performed the adjustments mentioned using TP3 and TP4. I placed my DVM's positive lead on TP3 and it's negative lead on TP4, selected DC Volts, and chose it's 20-volt range. (If you're reading negative voltages you have your positive and negative leads reversed.) After turning on the preamp and setting the tuner to 87.5 MHz, I used a non-conductive hex adjustment tool to turn the coil's center until the DVM read 3 volts, +/- 0.1 volt.

Next I adjusted my DVM to the 200 volt range and set the tuner to 108 MHz. Then I adjusted CT-104 with a non-conductive adjustment tool until the DVM read 23 Volts, +/- 0.2 volts.

NOTE: This is an RF circuit and adjusting it accurately can be difficult if you use a metal Allen wrench or screwdriver (as one poster did). These plastic tools can be ordered from many sources including MCM Electronics (http://http://www.mcmelectronics.com/product/JONARD-TK-AT5-/22-3915).

I was successful in getting my unit adjusted after cap replacement to the exact proper voltages with no problem. The FM tuner works great now. If you follow my steps you'll do fine with repairing your GTP-500ii.
 
Hi,

I have a GTP-500 (not the 500 II) with the tuner out as well.


I was going to try swapping out CT104 as well but I have not been able to find TP3 or TP4 either! Did anyone ever get back to you?


Does anyone have a readable schematic and/or service manual for this unit? All I have found are blurry PCB figures and unreadable schematics. I am going blind trying to figure out my tuner problem!!!

Can anyone help?

Any help will be much appreciated!

Mitch