Servicing a MkII Graphic EQ. Channel switching relay buzz.

[acorkos]

I have a 1982 MkII with a dead 750Hz EQ slider. Anyone repair one of these before? I'm experienced with working on tube amps, never one of these graphics, though. It looks like all the jacks, knobs, pot nuts, switches and faceplate need to come off before i can pull the grahic EQ board off.

If anyone has dealt with this problem before, what was your experience....what part(s) was bad? inductor? resistor? capacitor? slide pot?

Also, when I use the footswitch to change channels, I can hear the relay buzzing when in clean mode with the LED on. No noise if i disconnect the footswitch and use the front panel pull-switch. Also, no noise when i removed the LED from the footswitch...something about the LED being on is causing the relay to make noise. Any ideas what could be going on?

Thanks for your help!

 

[Restless Rocks]

Likely the 750 slider pot has been crunched if one of the transistors went out the entire EQ would be out.

If you can determine wheather or not the ft sw has been modified (a smaller value resistor to make the LED brighter would lower the voltage just enough to make the relay buzz.) Of course you should check the power supply to the switching circuit to make sure it is correct. And then the relay it'self could need replacement. There should be a 680ohm resistor in series with the LED. Also you should see about 8vdc at the ft sw jack when in clean ch.[/b]

 

[acorkos]

thanks for the reply

the FS has an 820ohm resistor. do you think the higher value is messing with the relay?

 

[acorkos]

Turns out the channel switching relay was faulty and the .22H inductor for the 750Hz EQ band was open. I got the replacement parts from Mesa last week. While I had the amp opened up, I did a complete cap job, replaced the preamp plate resistors, bias adjustment, and removed/replaced a dozen or so extra components....looks like someone was maybe trying to make this more of a IIC+? Installed an Eminence Delta Pro 12....this thing wails. I'm diggin' it stock as it was designed.

IIb's rule. This was the amp I wanted in 1981. As a college student, I couldn't justify the $1500 price tag and 6 month waiting list. Instead I ended up taking home a new $500 Fender 75...Fender's take on a hot-rodded channel switcher. I kept that amp for 20 years. But i don't miss it now.

 

[Restless Rocks]

Glad you were sucessful with your restoration. I've yet to see an inductor fail in a Mesa EQ. When I was in school in 81 I too couldn't live without a Boogie amp. Like you funds were a major obstacle so I got hold of a IIb schematic and converted a Fender into a point to point Clone. (don't tell Randall) I played that amp in full time bands untill 85 when I got hold of the new MkIII schematic and rebuilt it. Then with that amp I earned the doe to buy my first real purple stripe MkIII in 86. Both of them are still in my collection and now I wish I hadn't converted the IIb coz it rocked.