I have a 1980 Mark III 400 BH, bought new in the box as NOS in 1982. I had owned it for about 6 months when it fried, mid-gig. Just went completely dead. I had it repaired under warranty, but it was never really right again, intermittent problems where one channel would cut out, or it would distort and crackle, etc. It's posible I damaged it myself using an instrument cable between head and speaker, but who knows now. I eventually got another amp, and this one has been in a closet, moved with me over the years. I haven't found the footswitch yet, so that may be long gone. Every 10 years or so I'd stumble on it and say "gotta get that fixed". Now getting rid of it is on the Honeydo list. I dragged it out of storage (dry and warm) yesterday, with the following result.
The good news:
It fires up and produces sound through both inputs in both channels and the Automix input. The last I played it (probably 5 years ago) one of the channels didn't work at all, think it was B; which has somehow miraculously fixed itself. Problems with this amp were always intermittent, which was why I kind of gave up on it. You never knew if it was going to work, so I couldn't trust it to play out.
EQ knobs and graphic EQ function, didn't play loud enough for the compressor/limiter to kick in, but I assume it works.
An inspection of interior shows no obvious popped caps, burned or cracked boards, loose wires or connectors, bad fuses, etc. No dried up beer, food, or other debris, and nearly pristine except a little dust. I was VERY careful to let it sit unplugged for a good long while before opening, and did not touch any of the caps.
The bad news:
1. With a brand new speaker cord connecting head to my 1x15 cab, no inputs into channel A, B, or Automix, and pre/post gain at 0, I get a constant buzz/hum, like really bad 60 cycle noise. The noise increases with volume turned up. I can eq some of it out by cutting highs, but it's pretty loud. It's the same wth on switch in either position.
2. With bass plugged into either channel A, B, or Automix, I get undistorted bass sound, but the buzz/hum is constant and increases with volume adjustment. It's nearly as loud as the bass sound, and unbearable at any decent volume.
3. Tried switching the on/off/on switch to opposite "on" and got a LOUD pop, like an amplified bass drum kick. This happens every time you shut the amp off, regardless of channel or which of two speaker outlets is used. After hearing it a couple times, I'm afraid to try it again for fear of damaging my speaker cab.
4. The channel B pots are scratchy, and the Paramid knob makes a strange "whoop" (best description I can come up with) noise around mid-range of adjustment. All the other tone knobs and EQ sliders seem to work fine. I'm hoping this is just dirty old pots.
5. Both speaker out jacks crackle and cut out if you work the speaker cable jack around a little. Im hoping this is just old cracked solder joints at the output jacks that can be re-flowed.
I tried two different basses (one single coils, one humbuckers), two instrument cords, and two different speakers with same results, so it's definitely the head that's buzzing. I am hopeful this is something minor, since the amp functions, albeit noisily. Any rough ideas what these symptoms indicate? Thanks for any advice on this, and sorry to be long-winded. It's an old amp, but has a lot of nostalgic appeal to me from my college bar-band days, so I'd like to fix it if the cost doesn't exceed the used value, which seems to about $150-200.
