Gibson GPA HD100 PA mixer- 4x2 channel-reverb..1972

[jozluck]

Ok, been reworked once...sprague caps etc..but out of nowhere started feedback-tap on faceplate by on/off 3 way 6 pole switch...and hum would stop- then intervals became closer.. checked everything, switch OK, breaker OK, caps- one seems to register higher than the other on the duel 1013-725 50vcds... but also now_ transformer hums, with or without speaker... ideas?  also:  How to test transformer?  And the gp100 schematic does not show the same switch...

[jozluck]

...and note:  can hear mic...but its a buzz-not a feedback hum...which increase's with turning ANY volume knob, from master to what-ever channel -including not plugged into...

[J M Fahey]

Start by posting the schematic.
And please tell your Tech to register and ask himself, otherwise communication becomes a torture.
As of those Gibsons, they usually were quite conventional and classic, which is good , it simplifies repair or getting parts.

[Roly]

tap on faceplate ...and hum would stop

Physically sensitive almost always means a dirty contact or failed solder joint, and is almost certainly nothing to do with the power transformer itself.

[jozluck]

Thanks, YOUR RIGHT so far- went back through starting with Transformer leads...so far found 2 GREENed corroded wires under the solder at connection bar from transformer..stripped back, cleaned, sanded bar..fluxed/rewired..found one Ground terminal poorly soldered/dirty-wouldn't respond to re-heating-cleaned etc......going through rest...1972 and probably stored for a while at some point in time...Thanks again, we'll see where it ends up... that's 3 I missed as I presumed they seemed tight and had continuity..novice mistake...

[J M Fahey]

Remember to post the schematic.
Thanks.

[jozluck]

http://www.gibson.com/Files/schematics/G100A.pdf     This is as 'close' as they have on it..and not the same switch..have a 6 pole three way...on/off/on
here is another guy with problems, but: mine is cherry inside and out...5 pic down the two large caps-..I same # but get two different readings, in fact the one where the orange/yellow lead goes has twice the resistance for some reason....that is the only anomaly I can find...
AFTER CLEANING-RE-SOLDERING  CHECKING GROUNDS - HAVE CONSTANT, BUT CLEAR, HUM WHICH RELATES DIRECTLY TO THE VOLUME-MASTER AND CHANNEL-TONE/REVERB HAVE NO AFFECT AND CAN HEAR MIC, BUT HUM IS TOO LOUD FOR RECORDING OR PERFORMANCE

1.While there are six leads coming into the on/off switch ( which I don't understand the significance of , can I replace it with a two way?  While it checks out, this is where the problem seemed to start.   Tapped it and hum would leave...then simply continued after intervals getting closer and closer.
2. checked breaker/3.4 plunger type which does engage shutdown when pushed down- red plunger is in the out position and active.  It does not hold on a 'down' position and do not know if this is normal.
Im in TN and have breakers and power strips on everything...we are by a power station and have spikes all the time...fries cable box's (they are used to giving them out like m&ms some days), one computer.. had to get an ATP 200 for the computer as conventional's won't handle the load.  I don't leave the PA plugged in, but a spike could have occurred while playing.
Thank you for your time..

[Enzo]

While anything is possible, it seems unlikly the power switch would make hum.  Taooinbg it may make the hum stop, but when you tap on it, you are also tapping on the chassis, which vibrates a number of things.

Try this.  Unplug the amp, take the nut off the power switch and pull it out of the hole on the panel.  Suspend it in mid air by its wires.  Now power up and see if the hum comes by.  If tapping on the panel where the switch used to be still makes the hum stop, then it isn;t the switch's fault.   But if tapping on the switch itself hanging there, then you are right, it is the switch.

The switch in the schematic you posted is wired to reverse the polarity of the incoming AC mains voltage.   Usually it doesn;t matter, but in some casesd it will reduce hum.  If yours is different, it swhould be easy to follow the six wires and see wher they go.  That would tell us their purpose.

[Roly]

Eu não entendo nada - alguém pode traduzir?

Você é um homem corajoso Enzo.   8|

[Enzo]

Nós fazemos o que podemos, outros estão lendo sempre sabendo que podem se beneficiar



Português? Sério?

[J M Fahey]

Eu não entendo nada - alguém pode traduzir?

Você é um homem corajoso Enzo.   8|

Hey Roly, I'd thought I had recognized you, but the makeup made me doubt


EDIT:
dear Enzo, maybe you should update your Sumo fighter picture:

[Roly]

Bah, my doctor tells me I can lose weight, but I'll never get rid of the pot belly at my age.   

Enzo: Nah, just confused.