Adcom GTP-500ii popping while changing inputs

luicas

Member
2013-08-05 7:48 pm
Hi, I just acquired a used and “working well” GTP 500ii tuner/preamp off from the bay. Truth is it’s not working well...

Problem is that it pops (considerably loud) when changing input sources, or when pressing the tone in button.

Volume knob has a scratchy-like behavior but more a breathing sound than a scratch.

Cleaned and lubed all front panel pots and buttons to no avail.

I’m inclined to thinking there’s an issue with a shunt capacitor somewhere. Sort of like having a DC current surge not going straight to ground but leaking into the signal path. But that’s just an idea.

Hopefully someone with experience in these units can chime in with informed ideas.

Thanks,
Luis
 

luicas

Member
2013-08-05 7:48 pm
Thanks Rayma.

I've had feedback from another source, indicating that this problem is opamp related. Related to the electronic input switching circuit.

Hopefully someone else can chime in with a bit more detailed info.

Forgot to say in my initial post, that you can actually listen to music. It's just that a loud pop is heard whenever switching inputs or pressing tone in/out.

Cheers,
Luis
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
This beast is a maze. Both the circuit and the schematic I found.

Check the lower-voltage power supplies. If +/-12V is really +12V/-2V it could be real out of balance and leaky. Not that I expect power trouble; but it comes to mind when two functions both channels are ill, and it is a lot easier than tracing the signal path/maze.
 

luicas

Member
2013-08-05 7:48 pm
Ok. So far I’ve found 12 volts dc using lab output, both channels, max volume, tone controls out.

0 volts with tone controls in, even at maximum volume.

Please chime in with further clues. It seems like the DC leak is happening in the first stage of the signal path.

Cheers,
Luis
 

luicas

Member
2013-08-05 7:48 pm
Following up on this post. The problem was in two buffer op amps, IC601 and IC602, IIRC.

Replaced them with OP275 op amps (originals are Adcom 7A - LT1057). Installed dip sockets for opamp rolling, and a 0.1uF ceramic decoupling cap between pins 4 and 8 for added oscillation stability. I’m currently testing with LME49720’s but might not be the best match. Too forward, metallic treble. Probably oscillating?

OP275’s sound better, more tonally balanced.

Also, followed a service bulletin requiring 11.5kohm MF resistors at R951/952, to slightly lower the rail voltage to all buffer and gain op amps, adding a bit of safety margin on over voltage conditions. Also changed the tone control opamp just for the sake of OCD sanity, with a new LT1057 (same part as original op amp). All is good now. Might probably also renew the Toshiba analog switch IC’s as there is a slight audio bleed through from phono to CD.

This preamp is GOOD sounding.

If someone knows how to lower the thermal noise (hiss), please chime in... My guess the issue is in the gain stage, after the volume control. I’m considering replacing all caps and resistors on the final gain stage, and possibly replacing the gain op amps too, with new generation low noise and higher spec components.

My other guess is that the final gain stage is wired for a very extreme gain ratio. The volume knob at 9 o’clock is concert house SPL level.... and according to the preamps specs, it has a rated 10V output..... who needs 10V rms line level output?? 2 Volts output would be more than enough for my GFA5300 amp (or any amp, for the matter).

If someone knows how to change the gain ratio, please feel free to add your insights.

Cheers,
Luis