2266 Hum on power

[sikter]

You can try to send signal from your VM to return of some other Amp ( which has loop)
It can confirm your theory of faulty power section.
I really can't come with any other suggeston right now. It seems like you've tried everything.

[jeffsmusic4919]

Hallo und guten Tag, Sie Meikel. You can start here:

http://www.marshallamps.com/support/faq/hums-hisses/

I have also heard that the transformers can become loose or have bad isolation washers causing hum or rattle. I didn't have time to read all five pages of information so hopefully this isn't just rehashing old information.

[thomaso24]

You can try to send signal from your VM to return of some other Amp ( which has loop)
It can confirm your theory of faulty power section.
I really can't come with any other suggeston right now. It seems like you've tried everything.

Slight OT:

I dont have hum on my 2466 but I do have it on my SJ 2550. The hum is loud enough to hear it when I stand next to it but goes completely away when I switch it on from the standby.

Anyway, this gave me a wild idea: take the loop send signal from jubilee and feed it into 2466 return. Or the other way around 2466 loop send to Jubilee return. I think I might get some interesting sounds?

Is this perfectly safe? I have both amps in the same wall outlet.

[sikter]

It's called slaving.
It's safe (I think, I did it many times but never used it as a setup) but you're not going to win much in your case.
I don't think you'll get any a-ha experience but let me know how it went.

Remember to have load (cabinet) connected to each amp all the time.

[Meikel]

Hi,

quick update (THXfor it!):

I am gone from the send of the 2266VM in the return of my Bugera V55, to test the basic behavior of the 2266VM preamps, as far as possible. The 2266VM I have not connected to ground to avoid a ground loop (which there would otherwise be).

Result: A quiet, totally buzz- and humfree signal from the 2266VM preamp into the power stage of the Bugera V55 (and the speakers connected to it).

So on my opinion it is only the area from the power stage input over the OT until to the Speaker Out of 2266VM.

Regards Michael

[Meikel]

Hi,

Update: The problem may be solved.

It was probably due to the rectifier diodes in the HV power supply. They spread their "switching frequency oscillation" in the HV which you see can very well on a high-resolution scope. An exchange of the diodes has corrected this problem. Well known under "Fast Recovery Diodes"...

I would like not too far out of the window, but I would almost say that here is a - let's say - weakness in the design: On the board and in the Schematic you can easily see the four rectifier diodes, but not them parallel connected capacitors (like "Snubber"). They are not stocked! This is actually an old and well-known problem when diodes has to switch inductive loads that RF interference can occur if you do not fast recovery diodes used or if you do not connect them capacitors parallel.

Marshall has saved here and I "may" now pay to repair it...

Regards Michael

[Meikel]

Hi,

Happy End.

State of affairs:

Yesterday I picked up the amp and now he appears to be running properly within its capability. "Within its capability" means that the rattle is not completely gone. But you have now already about 20 - 30 cm be directly to the ear at the level of the Cabinet / Speakers to at all to locate the rattle, it gets lost in the rest of the background noise of the amp.

But a Bugera V55 (250,- Euro?) runs silently here! More about this see below.

Praise and criticism:

I was good.

Because I could find confirmed by my research, my guess is that it was the final stage or its power supply. As far as I was able to locate and isolate the fault. I, however, was not consistent enough
to pinpoint the exact source of error. Here I was able to afford the targeted brain work, but not by measurement to prove the source of error. Therefore, the path led me now to Enzo Morgana and here I have to say that my decision was right to bring the amp to the Enzo. Because ...

Technical:

...Enzo has demonstrated not only the exact source of error measurement technology, but he has now built here "his" measured out HV diodes and completely, of course, equal to the missing, the diodes in parallel connected, installed capacitors. Here he has regarding the diodes built its know-how, which I would not, because I probably would have re-soldered the types specified in the schematic. My very best thanks again to take this opportunity to Enzo!

Marshall:

A clear statement: Since I exclude another error that has led to the misconduct of the rectifier diodes AND from the mp3, which I got thankfully from another 2266VM-user with the same problem (!), I conclude that Marshall concerning the interpretation and dimensioning of the power supply has not handled correctly here his homework. See Schematic regarding the marked fields. I 've attached the schematic of the power amplifier and the power supply twice. "Original" is more or less the factory wiring diagram, you can see here the circuit as for the 2266VM and 2466VM should be, including all comments. In "2266VM wiring" I have everything removed, which is not present in the amp on the boards and you can see now that is good, what is left - in particular in the power supply!

The HV rectifier diodes in the power supply are not of sufficient size, the series circuit is unfavorable for the occurrence of HF interference, and it is the appearance of this one scattering in the final stage RF interference favored also by the absence of their capacitors connected in parallel. For me, clearly a deficiency in the circuit design. What is even more annoying, since this issue since the mid-80s, bridge rectifier diodes well known in amplifiers (I actually could have come right on it ...).

So "keep your eyes on" for the 2266VM what if applicable terms of such disorders!

With best regards
Michael

[bz2022]

Just send PM to SteveD and wait for his detailed repair instructions. I simply attached some blu-tack on the chassis near the power transformer area, and it works for now. Before I did it, I touched the chassis which is a little bit hot, so I chose the relatively cooler outside area to attach the blu-tack. It absorbs the vibration caused by the power transformer and the chassis is no longer hum, but I will keep watching and see if it solves the problem consistently.

[HarryPotter69]

My VM has the same problem. So where to get this paper and can I do myself?