6101 MIDI Repair

[Mike_LA]

Ahhh,m Nope, it's the transistor that I just installed.

If I wiggle it, the midi board goes on then off.
Think I will hard wire the missing copper point on the board.

[Anitoli]

Ahhh,m Nope, it's the transistor that I just installed.

If I wiggle it, the midi board goes on then off.
Think I will hard wire the missing copper point on the board.

Good catch Mike.

[Mike_LA]

Watch Rocky as I pull a rabbit from my hat . . . . . . .

Ok, I wasn't aggressive enough scrapping the covering from the copper.

Resoldered and it seems ok, further testing in the coming days.

On the plus side, the old midi chips may still be ok.

Think I should switch them now or I'll never do it . . .

Thanks again Pete

[Anitoli]

Give it a good burn in before packing it up.

[Mike_LA]

Ok, I took out the (2) large new chips and replaced them with the old orginal marshall chips and I got all 3 LEDs lit at the same time.

Took the old ones out and put the new ones back in and it all works again.
Guess the one of the larger midi chips is fried.

The switching is a lot quieter with the damping set to low, thnx.

Will leave the amp on all day and will report back . . .

[Anitoli]

The 27C64 is old school memory, these chips were rated at 10 years for data retention- many go longer but bathtub effect comes into play here. The 80C31's onboard memory ( which loads the 27C64's data at every startup) is CMOS. any type of static spike can knock those out as well as simply burning out over time. Have seen boatloads of the latches and flip flops short out as well.

[Mike_LA]

Ok, up and running, both 6101s side by side.

The 6101 running 6L6GCs is dead quiet
The 6101 running EL-34s has a nasty humm.

If I put my decibel meter behind the speaker I get 63.5dB enough to be annoying at low volume.
It is quieter if I run, low power, pentode mode, and is still there when I remove all preamp tubes.
Got to be in the power circuit. Maybe another cap on the power board or the power tubes?
I moved wires around iN the chassis and had no result. I think my next choice is to get some compressed air,
Start spraying components and listen for change when the parts cool.
Any other trouble shooting tips that you can pass on?

Thnx M

[Anitoli]

Noise can be many things, old worn tubes whose current draw is to far apart, bridge rectifiers getting worn out, bad caps in the power supply.
do you have a scope?

[Mike_LA]

Noise can be many things, old worn tubes whose current draw is to far apart, bridge rectifiers getting worn out, bad caps in the power supply.
do you have a scope?

Well, it has new mullard power tubes so I don't think it's them but you never know.
Don't have a scope, haven't used one in years, of course this might be a good excuse to get one . . .
Let you know if I do . . . .
Still might try the compressed air route.
M

[Anitoli]

Also check the loops, try bypassing with jumpers. Do the chopstick test for bad solder joints. Did you remove the PI?

[Mike_LA]

Also check the loops, try bypassing with jumpers. Do the chopstick test for bad solder joints. Did you remove the PI?

I found the bad solder joint on the transistor with the chopstick, maybe I will try a bit more.

Thought I removed the phase inverter when I removed the preamp tubes but didn't

Will try bypassing the loops, and the PI Thnx

What do you think about this scope if I can get if for around $100?
It's local

https://www.ebay.com/itm/BK-Precision-2 ... Sw6rFefm6p

[Anitoli]

As long as it works properly that will more than do the job. I have a B&K 2160 and it has been a very good unit.

[Mike_LA]

Great, I zeroed in on a couple of scopes but I have 1 question.

BK PRECISION 2120 2-CHANNEL Oscilloscope 20 MHz
BK Precision 2190B Dual Trace Oscilloscope 100MHz.

Is a 20mzH scope or a 100mzH better for this type of stuff?
Thnx M

[Anitoli]

Both would work considering where audio frequencies reside but usually the faster ones tend to have slightly sharper trace lines. But then that is also dependent on the units usage and age. I can assure you of one thing, the 2120 is of the same generation as my 2160 and that has been an excellent piece of gear. I bought that new in 91 and have had no issues at all.

[Mike_LA]

Hey Pete,

Pulled the PI, no difference.

Got a BK 2120B coming, let you know when it gets here.

Got the manual and will check i t out before it arrives.

M