Dr. Z maz 38 Senoir oscillates at low freq.

[Corben]

Ive been dealing with this amp for a long time now. I fixed most the problems except the last problem. No matter what i do their is a constant oscillation at some where from 2-3 cycles per second that really clashes with the lower freq. It almost sounds like a tremolo should.
I thought it was the fact that this amp was so picky about matched tubes so i bought a set of matched el84's from euro tubes and even bought a balanced phase inverter tube. I checked all the resistors and checked the power supply and filter caps but they are all fine. Im really out of ideas here. Ive even got to the point where im trying to move every wire encase its picking up some feedback or something. Any advice would really help.

[Corben]

if anyone has a schematic that could be helpfull to

[Gaz]

Sounds very strange indeed, and I don't have any advice, however I do have a schematic, so PM your email and I'll send it you way. I really do hope you get it fixed, that amp is one of my all time favs.

[Corben]

i know these amps can cause trouble with rf interference problems with a noisy reverb, it is real picky on the 12atz reverb tube v3 Ive gone through 3 tubes until i found one that didnt make the reverb hum.
There are also some very microphonic instrument cable used inside the amp, if you tap on the cable it makes loud pops esp comeing from v3. I can see precautions that were made esp. to keep this cable from vibrating much.
At first i even thought there was a weak short in the cable or bad joint but i replaced and retouched the joints.
I read on a few sites that the reason for the cut off control was to dial out some of the static and interference at times when it got bad.

[imaradiostar]

This sounds very wrong to me...Dr z stuff is usually neat as can be and all the ones I've played (quite a few, Nashville loves Dr Z amps) have been dead quiet and reliable.

So the question remains- what happened?

I'm sorry if it's a problem you inherited, understand that I'm not trying to pick on you!

Please show us some pictures- it sounds like motorboating from a wiring issue.

jamie

[Corben]

Dont know what happend. But that is the problem im experiencing. Taming the noise in the MAZ 38
Here is some of these problems direct from the drzamp boards or goggle "dr. z maz 38 noise"
Ive expeirenced problems with this model amp 2 times before
Not trying to knock it i mean they sound great when they work and their other models ive heard dont seam to have this problem as often obviously.
Ive seen online on a few places and from a few experiences with this model theres a problem that come up esp with the reverb and with picking up stray emf.

I hope i can find my camera if i do ill def attach some picks. But the only wire that is in any way out of place from any other 38 is the cable running from pin 2, i think, from v3 the reverb tube and ive tried replaceing and moveing it all around.

Most of the wires are really nice solid core and very hard to move and go perpendicular when they can of the other wires. And ive physically tried moveing each wire as far as it would budge but the osc didnt change in the least.

Maybe theres something that needs to be tweaked futher like replacing a short wire with a longer wire to give it more room to get around another or maybe some component value needs to be changed a bit and tuned specifically for this amp. It looks like the osc. is starting at v4 and then gets extreamly noticable after the phase inverter tube. Ill recheck this when i get back to the bench in a bit.

[Steve Conner]

This sort of problem is often caused by bad electrolytic caps. Not the main filter caps, but the smaller ones that decouple the preamp supplies. How did you check them?

[Corben]

well i checked all the filter caps for esr. The smaller on all the way to the very end of the turret board actually looked like it had bean replaced. I tried putting in a cap of a higher capacitance and same voltage in parallel with it and no change. then i removed the cap from the circuit and put my larger cap in place of it and i think it may have changed very slightly but i just atributed that to a little better filtering. Also i checked all the caps for ac leakage on my dmm. ANd i tried putting a cap in parellal with the two caps in the tube mounted to the chassis and still no change.

[imaradiostar]

Have you called Dr. Z about it? He might be able to recommend a fix or fix it for you.

jamie

[Corben]

emailed him and called the number on the website. The number told me to email the repair email. Hope to get some kind of help there soon maybe