Kmd gv-100s intermittent problems

[DPM309]

I have a KMD GV-100S head on my bench that is experiencing a couple of intermittent problems. The first is after each note there is a compressed/distorted/clipped sound. I read on this forum that this could be a bias problem and I will check and re-adjust the bias if necessary. The second is that when you turn on the amp, sometimes there is no sound coming out on either of the standby settings (high or low). When I touched the 1st filter cap with my multi-meter probe, there was a loud noise and then the amp worked perfectly. Here are my voltages with the standby switch on the high setting:

1 2 3 4 5 6 7 8
V1 6L6 - - 772 386 22 - 6.8 AC 68
V2 6L6 22* - 772 386 22 - 6.85 AC 68
* - Pins 1 and 5 on V2 connected together.
1st filter cap - 772 2nd filter cap - 386

On the low standby setting (I assume some kind of output attention) I get 514 V on the first filter cap and 256 on the 2nd. Pin 3 is 514V and pin 4 is 256V. I was able to re-create this several times and it came back each time I touched my probe to the first cap. What could be causing this? I have attached a copy of the schematic from Kaman. They were very helpful and apologized for the poor quality of the schematic as it has been scanned an re-printed several times. Would anyone have a clearer copy.

Thanks,

Dan
KMD GV100S schematic.pdf

[Jazz P Bass]

The very first thing that I would do is resolder that capacitor.
And while you are in there, hit all the big ticket items.

[teemuk]

I can't unfortunately help with a clearer schematic but that design is largely identical to Music Man GP3, right down to circuit architectures and component values.

[DPM309]

teemuk - Thanks for the suggestion for the MM schematic. It is almost identical to the KMD. JPB - I resoldered the filter caps and as of now, the amp seems to be working fine. I let the filter caps drain completely for about 10 minutes, and turned it back on with no problem. I will let it sit for a few more hours and try it again.

As far as biasing this amp, there is a 100 ohm trim pot on the power amp board and I don't know if this is the bias pot, balance pot or hum pot. I am having a hard time finding it on either schematic. I e-mailed the tech at Kaman to see if he has any more information on this. Will report back later today to see if the amp is still working.

Dan

[DPM309]

Well I waited about one hour and the amp is still not producing any sound, except for a loud hum, through the speakers until I touch the MM probe to the filter caps or plate pins on the power tubes or when I touch the tip end of a guitar cord plugged into the amp. When I do this, I get a loud pop and the hum disappears and I get sound. The amp sounds good when it is working. Since the filter caps have to be at least 30 years old, could this be the cause of the problem?

Dan

[DPM309]

Replaced the caps and everything was working fine when the owner picked it up. Got a call a couple of days later and it is still cutting out intermittently. Opened it up again and as soon as I pull out the 1/4" stereo jack out of the footswitch, the amp cut out until I touched or shook the end of the cord. Removed a few inches of the cord and re-soldered the jack and also re-soldered the contacts on the board where the 3-conductor footswitch wire is attached. I tested the cable and jack and there are no shorts or intermittent connections. When the amp was in cutout mode, all of the B+, plate, and cathode voltages were unchanged and I get no sound or signal out of line out. The problem seems to lie somewhere in the pre-amp or switching circuits which are all solid state.

Have been running the amp for a while after re-soldering the footswitch connections with no cutting out. Now when I pull out the footswitch cable, the amp cuts out sometimes when I touch the plug with my fingers and then comes back in when I touch it again. Other times, it doesn't do it. Could a small static electricity charge be causing something in the switching circuit to cut out?

Re-soldering the footswitch connections seems to have improved the problem a bit but I it is still cutting out. Any other ideas?

Thanks,

Dan

[Jazz P Bass]

. Now when I pull out the footswitch cable, the amp cuts out sometimes when I touch the plug with my fingers and then comes back in when I touch it again. Other times, it doesn't do it. Could a small static electricity charge be causing something in the switching circuit to cut out?

Seriously, why would you do this?
Is the customer going to do it?
At times, it is best to limit your troubleshooting.

[Gtr_tech]

Cracked solder somewhere. Look at the solder connections for the FS jack. Also if there's a loop, clean the shorting contacts and check the solder.

[DPM309]

Seriously, why would you do this?
Is the customer going to do it?
At times, it is best to limit your troubleshooting

This is the first amp that has come back in over 10 years and I am resolved to get to the bottom of this problem. Was just trying to re-create the problem so I could check voltages and rule out some of the causes. I watched the owner plug the cord into the footswitch when the amp was off standby, giving a loud pop and then the amp cutout. Have been running the amp for some time now and have not been able to re-create the problem so hopefully the re-soldering of the footswitch leads did the trick. Have also cleaned and re-soldered the effects loop jacks.

Thanks,

Dan

[J M Fahey]

Maybe thereŽs no problem with the jack itself, but plugging something there slightly flexes the chassis or board and some cracked solder or bad contact ,makes contact again .... temporarily.
IŽd probe everywhere with a plastic pen or similar, causing some telltale thump or click.
In a nutshell: sounds "mechanical" to me.

[DPM309]

I went ahead and replaced the stereo jack that plugs into the footswitch and removed another 6" of cable and the amp has been running without any problems for a couple days. I measured the voltages on the footswitch leads and was getting about 40VDC on the channel switching lead and virtually nothing on the reverb lead. Before I changed the plug, I was able to get the amp to cut out and measured 40VDC on the reverb lead which led me to believe that I had a bad plug which was shorting out between the reverb and channel switching lead. Will ship back out to the owner and keep my fingers crossed that I solved the issue.