6101 No treble output on the clean channel 6100

[Mike_LA]

I started this over in the The 6100 Darkside lounge as a rant but since I'm going to try and fix it I didn't want the info to get lost in the mondo thread.

The story so far . . .

F@ck'in Hell, this amp is driving me crazy.
I've noticed a lack of high end on channel 1, so i turned a few knobs and found out there is no output from the treble pot.
Mids and low are working fine, guess it's time to take another look inside.
nothing looks bad and the pot seems to work.
gonna reflow the connections and put it back together.

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V1 goin south?

Alas, not something that simple.
Took it all apart didn't see anything, put it back together, same problem.
No treble output on clean channel

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Sounds like a bad cap in the preamp section ...

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Are you getting a sweep reading from the treble pot?
Should be 220k.

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Well I put it back together swapped tubes, let it warm up for a long while.
Same issue.
I will recheck the pot again. I seemed to work but I will recheck.
I will also see which caps are closest in the treble circuit and attempt to test them.
Do you test a cap for continuity? Or do you have to power up and see if it holds a charge?
Attached is a pic of my meter

Thanks for the help guys.

[Anitoli]

Looking at the schematic C3 seems to be the only one that feeds the treble pot.
http://home.myfairpoint.net/vze1u14s/si ... preamp.pdf
The rest of the signal feeds through the treble pot wiper, out to the MV and C2. Check that cap or that mounting side of the pot.

[Mike_LA]

Thanks Pete, I think you are right if our friend from the great white north is also correct.
C3 looks like the best place to start,cuz the bright switch on the other side works.
Would you say it's a 220uf, a little fuzzy on the pdf.
I should have time to put it back on the bench tonight and I will have a look.

[Mike_LA]

Ok, took the beast apart again.

The treble pot works fine, from 0 to 215K so it's not that.

I think cap #3 is good as well.

When I put the tester on ohms the reading goes up until OL (Charging the cap), then if I switch to DC the readings go down as it discharges.

C6 and C5 perform the same way

Now C2 goes to 2.5 meg ohms immediately and holds it, then discharges really quickly.
But this is a tiny cap so maybe that's how it functions, fast loading ,fast dump?

Poked around a bit more but didn't find anything yet.

[ONFLOOR AUDIO]

Mike , i will ask my amp tech. friend tomorrow and see what he thinks , then report back asap !!
I hate to see a 6100 down ...

[Anitoli]

Mike look over the circuit trace REAL good. Are there any broken trace lines?

[Mike_LA]

yah, i've been doing continutiy checks .....
Let you know when i finish

[mickeydg5]

Hi Mike
Was C3 in the circuit when tested? Usually a multimeter cannot pickup the ohmmeter/DC test on a 220pF capacitor.
You should try the capacitor reading measurement with your meter. It still has to be out of circuit for measuring, lifting at least one lead/leg.

If you jump the C3 capacitor with a wire or alligator test lead does the high frequencies pass? That would indicate a problem with C3.
Likewise if you jump from VR2 TREBLE potentiometer side of C3 to the wiper lug of the TREBLE pot and get high frequencies then the problem would be in the potentiometer.
If a jumper is placed from that C3 point to the treble side VR2 lug and get high frequencies then that would indicate a bad connection or trace between the two components.

[Mike_LA]

HUmmmmmmm, well it works.

Took it apart 3 times, swapped tubes around in the preamp section and now it's good to go.

Must have been a dirty tube socket or if it was a weak valve and I haven't noticed where it ended up in the circuit.

Let you know if it fails again, Thanks for everyone's help.

Now on to fixing the footswitch . . . .

[Mike_LA]

Ok, same problem again.
No treble on channel 1.
intermittent problem are the worst . . . .

[mickeydg5]

Try the jumper.

[Mike_LA]

Try the jumper.

Will check that as soon as I get it taken apart again . . .

Thnx

[Mike_LA]

I've said it before and I'll say it again, "YOU GUYS ROCK ! ! ! ! ! "

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Without even taking the board out I'm pretty sure there is a problem with the C3 Cap since it looks like it has burst open on the top and 1 side. It also is the only cap which is not soldered close to the board but has long legs. Hummmm. Wonder if this has failed before?

220pF capacitor Humm, have an electrolytic guess a quick jaunt to radio Shack is in order, 35% chance that they will have one . . .
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Crushing defeat, nothing at RS.
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More as progress warrants.

[Mike_LA]

Ok, once again, full blown gonzo confused.

Went to mouser and since the cap next to it looks like it was rated a 600volts I searched mouser and got a way huge capacitor..

What voltage am I looking for?

thnx

[Mike_LA]

Still planing on checking a local shop or 2 but since they seem a bit special I grabbed a couple of these.

NPA220M25 220uF 25V Axial Capacitor #:NPA220M25
The NPR and NPA Series subminiature aluminum electrolytic capacitors are especially designed for use in circuits whose polarity is reversed or unknown.