bandit 112 problem

[JC@]

I have a problem with a peavey bandit 112, that sounds "too tubby", distorted even on clean.

Signal on power preamp out / power amp in is ok.
The problem begins around U2.

This is what I see on the scope
U2A
pin #3 sine wave is ok
pin#1 rounded at the bottom
pin#2 ok

U3A
#3 and #1 whips

U3B
#5 rounded at the bottom
#7 rounded at the bottom

U2B
#7 whips
#6 rounded on the top, whips
#5 very rounded on the top

Hi Enzo, in case you are reading this.

[Mars Amp Repair]

which bandit 112 is it? From around '89 or from 70's ( I guess they called that the Transtube Bandit...
Do the voltages on the opamps make sense...ie; Plus input & minus inputs equal ~0 volts & no offset on output pin & the like?
The schematic I find is for the Bandit 112 around '89..doesn't seem to match what you're saying about U-3...U-3 is in the reverb ckt on that amp.

glen

[Enzo]

Whips? What is that? The things awound you mowf?

I am looking at the Bandit 112 AND the Bandit 112 '89, and I agree, U3 is the reverb driver, and it is downhill from there.

Does it say Bandit 112 on the serial number plate? Or are you adding the "112" as a description?

In any case, I too would be looking for funny power rails. You want them even and at about 15 volts. Just which amp do you have?

[mykey]

a really simple way is to check the temperature of the IC's,
if one is getting hotter than the rest, it's usually bad.
you can feel it with your finger, but watch out- it can
get so hot when it shorts that it burns!
when an IC goes bad it usually drops the power supply
voltages, check the plus minus voltages at the IC supply.
I usually put in sockets when I change them, this makes
it easier to swap them out and poses less risk to the IC's
when soldering.
shorted IC's (op amps) are a very common problem in PV
amps which cause this type of distortion.

[JC@]

I am now back to this amp.
Yes it says Bandit 112 on the serial plate number, and is a Transtube(r).
I have partially rebuilt the low voltage power rails (replaced both 15v zeners and replaced C43 and C45 because they were leaky) and I get now +15.1v and -15.7v.

I havent tested C44 and C48, but I get +23v and -23v there so I believe that is ok. Am I?

But the amp keeps sounding distorted, except when I turn the Tube Dynamics knob all the way up. Then it cleans up perfectly. That is, at zero, very rounded wave form in it's upper side, then as I turn the knob up, it goes better, and at "10" it is ok.

I still need some help I'm afraid.

[Enzo]

Ah, Transtube is the key. Do you have the right schematic? Bandit/SUpreme Transtube? What drawing are you getting these part numbers from?

I might swap out U4 and U5 in the power amp. I would verify 0.33 ohm 5 watt R140 was OK and well soldered.

[teemuk]

Are you sure the T-Dynamics potentiometer is OK: a lifted wiper would impede the operation of the whole power amp/transtube/T-dynamics configuration - that includes reducing the range of the TD control. Other suspect might be a quirk in the feedback path between first stage of T-Dynamics and output. Operation of T-Dynamics is presented thoroughly in US Patent 5,675,656, Power Amplifier With Clipping Level Control, Sondermeyer et al., 7. October 1997. Note that patent shows incorrect pin order in the driver opamp of the power amp.

[JC@]

Ah, Transtube is the key. Do you have the right schematic? Bandit/SUpreme Transtube? What drawing are you getting these part numbers from?

I might swap out U4 and U5 in the power amp. I would verify 0.33 ohm 5 watt R140 was OK and well soldered.

The schem I have seems to be the right one for this model, two sheets, one for schematic diagram Bandit (r) 112 Pre AMP, and the other for the Power AMP.

There's no U4, U5 nor R140 on these drawings.

On the power amp diagram sheet's lower right corner it says: Front panel breakway board, and on the upper right side, (Bandit/Supreme only) and (Revolution only)

The Tube Dynamics pot is VR2 (10KACRA) and seems to be a double pot, but externally the pot on the amp looks like a single track pot.