Adcom GTP500 II no sound right channel

[old_marantz]

Yo
I'm pretty much a newbie when it comes to diagnosis of issues. But, if someone can tell me which part might be defective I can easily replace since I have the tools. Example, on this same preamp, there was a thread about what needed to be replaced to fix the FM - worked like a champ when I replaced what the thread recommended.

My current issue is no right channel sound. It occasionally will work but most of the time will not. Left works fine. Switch cables definitely isolates the issue to the right channel. Headphones confirm right channel doesn't work. The Tape outs for right and left work just fine.

Anybody have any suggest on what I need to replace?

 

[jupitersspot]

I don't know of a common failure of this sort on the GTP500 II, so I don't have a simple "oh, just replace X and Y, and you'll be good" sort of answer. If it were me I'd get a copy of the service manual (try HiFiEngine) and have a look to see what I could rule out. For example, do you get different results if you twiddle the balance knob, or the bass/treble mix? If you do, then it's likely the tone controls are the issue and your amplification stages are OK. Does it matter which input you're selecting -- does it work fine on tuner, but not on CD? If so, the issues are in the CD input circuitry. If it gets to the point that all inputs are affected and nothing you can twiddle makes a difference, then you're likely looking at failed components in the amplification stages (yes, it's a pre-amp, but it still does some amplification based on the position of the Volume knob). Look for bad capacitors shorted to ground, dead transistors, open resistors. Since one channel works, compare voltages from the working Right channel with the non-working Left channel counterparts -- find the differences, and you're getting close to identifying the part(s) that need work.

Good luck.

 

[LWB]

There is a variable capacitor in the Tuner section that takes down these ADCOM GTP500II tuner/pre-amps. It's located under the faraday-box shield covering the Tuner board at the right rear corner. It's part CT104 and you can order a replacement from Mouser, 659-GKG10015, about $2. I just replaced one in a GTP500II given to me that would power up but soundwise was DOA, and it made the unit sing again. Remove the top and bottom covers of the GTP500II to access the boards. If I recall correctly there is a chassis rail that can also be dismantled to further improve access to remove the solder that holds the right side of the shield in place. Replace CT104. The capacitor has a flat side on the PC board's silkscreen, install the new cap with it's flat side oriented the same way. Don't mess with anything else unless you wish to replace the old electrolytic caps under the shield. Given their age and the PIA of removing the shield, this is probably a good thing to do.

To calibrate the variable cap, use the test points TP3 and TP4, outside the tuner box, on the side closest to the front panel. Use a DMM to make the adjustments described at the top of page 1 in the service manual. Page 2 of the SM shows the board layout. If you don't have the service manual, here is what you do:

Step one: Put DMM's positive lead on TP3 and the negative lead on TP4. Turn on the preamp, set the tuner to 87.5 MHz, then use a non-conductive hex adjustment tool to turn L107 (beside CT104) until the DMM reads 3 volts DC, +/- 0.1 volt.

Step 2: Set the tuner to 108 MHz. Then adjust CT-104 with the non-conductive hex adjustment tool until the DMM reads 23 Volts, +/- 0.2 volts. This is an RF circuit, so do not use a metal adjusting tool.

Now you can replace the shield.

That should do it, unless something else is wonky.

 

[kychucky]

Dude - you just handed us a silver bullet? I Have one of these also but working fine. Love the inter-webs and AUdioKarma!

OP Old Marantz - let us know if this works !

 

[old_marantz]

Sorry forgot to reply. Channel still drops out. When I giggle balance it comes back for a while. Drenched it with Dexoit FROM FRONT....still drops.

 

[old_marantz]

Just doused it again...working. Something wrong with balance switch or in that area. I guess I really need to open top and make sure dexoit is getting inside. Thought I had done that when I replaced FM stuff...maybe I forgot. Otherwise it’s some other issue

 

[northpaw]

Intermittent sound is usually a dirty contact or bad solder issue.

If your application of deoxit "from front" means behind the knob on the front panel without removing the top cover, yes, you need to open it up an apply directly to the pot, preferably to an opening in it if it is not a sealed pot. Just a little deoxit so applied should be needed, and some exercising of the control. It would be good to hit the other pots and switches also at the internal location of the actual component, not the controller on the exterior panel.

I don't know your unit, but some have shafts or little tracks or the like leading from the front panel to the component mounted somewhere mid board. In those cases, go for the component itself, not the controller. Good luck.