I found this on craiglist this morning. Not working. The grill was stapled in. Had some GE EL34's in it. Has a loud hum/buzz. Not quite a 60 cycle hum. Tried some 6L6's same noise. Transformers are not rusted badly.
Images:
Fri, 03/11/2016 - 07:59
#1
Joel DC
HOS entry
Looks like the poor thang
Looks like the poor thang suffered from emotional abuse. Good news is the fuse didn't blow when powered up. If you plan to repair it, start with the basics. Visually inspect for pimply electrolytic caps and burnt/cracked resistors and diodes. Replace any caps with pimples or gashes and components that have lost their smoke. Pull the IC's and tubes one at a time and clean the sockets with something like DeOxit. Clean the pots and jacks too. Once the internal visual cooties are taken care of, check to verify all power supply voltages are in spec, especially the low voltage supplies regulated by zener diodes. Good Luck. -mgriffin
Thanks for the info mgriffin!
Thanks for the info mgriffin! Yes, one of the big caps under pan looks very pimpled up almost ruptured looking. There is a place on the board where it has been very hot as it is darkened around the area two big white resistors and two small black cap looking thingys, right behind the middle pot. Been cleaning the cabinet all day. The chassis is next time. I'll be back with updates.
Which Model is it?
Hi Joel, RD65 or RD100 ? I have a 1981 Canadian Vintage RD65-112. Feel free to post close-up pics of the boards. -mgriffin
It is a RD100 that has 6CA7
It is a RD100 that has 6CA7 in it so I assume it is a earlier model. I am surprised at how light this amp is without the speaker. Took it apart, sprayed it down w/ waterhose...literally...and found some initials under where the reverb tank should be. My initials, JC, written in yellow chalk. I thought that was pretty cool. Next I gotta get the chassis cleaned up.
BTW, in case anyone hasn't mentioned this, Coke and aluminum foil really knocks the rust off of metal pieces. I put corners, chassis straps etc. in a bowl and poured Coke on it. It fizzed and took the rust off, then took some wadded up foil and knocked off what was left. Man, I love youtube!
It's clean now, doesn't look
It's clean now, doesn't look a lot better, but better. It is now my favorite 112 extension cab until I can get the amp going. I had a little G12L Celestion laying around so threw it in there for now. I'm thinking I'll prolly leave the old tolex on and find some aged grillcloth and go with the well used/beat-up look. It's been years since I had this much fun messing around with an amp.
Road Worn
Hi Joel. Clever use of Coke. One of the ingredients is phosphoric acid. The pause that refreshes. Same stuff is in oven cleaner and gasket remover solvent. Make you wonder what we're drinkin'. Thanks for sending the pics. All in all, the board components visually look pretty good. The 2 black things where the PC Board is discolored are the zener diode regulators for +/-16V (D15 and D16) and are connected to the 2 big white resistors R42 and R43 (390 Ω). Looks like excessive current has been flowing. With amp unplugged, use your DMM to see if the zeners are shorted in both directions. If they are, put them on your shopping list along with the pimpled caps under the HV supply can. The circuit is shown in the lower right part of the schematic. Just a safety note: be careful working in there with the amp powered up. The hi voltage supply under the metal cover to power the tubes has 700+VDC. Speaking from experience, it's rude if you get zapped. BTW: You mentioned a loud humm. If it was loud and not flaming, there's a good chance the tube section output amplifier is functioning. As you poke around in there, feel free to ask questions. Good Luck. -mgriffin
Thanks a lot mgriffin! I have
Thanks a lot mgriffin! I have recapped a coupla Fenders and Marshalls but I'm not confident enough to take on the mighty MM amps. I will take it to a tech when I can. Actually, I think I need caps in all these amps. Maybe I can refinance my house.
I finally got this amp going
I finally got this amp going and it sounds great...of course. I put a Eminence Lil Texas speaker in it so now it is decently light weight. I didn't weigh it but feels about like a Fender Deluxe.
Besides the caps being replaced, one of the bias transistors was bad. Now I just gotta move it down the hall and over to the bragging section.
Nice!
Joel,
glad to hear you got it up and running!
Cheers,
Lars