NO more trouble with the 350B ....

[Oli]

Hi,

I need some advices as I could'nt manage to fix my 350B/2000S yet. I had some problems with a nasty LFO which came with an unusual humm. I realised that the tonecontol (Bass und Treble) had an effect on it, so I checked and changed all potis and the caps of the preamp section (see pic001). The amp did run fine for app. 20 min, the it stated to humm again.
What I did yesterday was to pull out the 12AX7 and the 6AN8 and checked everything with a scope. The hum seem to come from the Poweramp  section.... with the two tubes in it as possible to change to hum with the preampsection....anyway: the two 20YF Caps seemed to be down, so we changed them, that reduced the hum but it was still there.

This is where I would like to get some help:

1.) The peak of the sinewave on Pos. C is 3Volts, is this too much for the "DC" at this very point (see pic002)? What is exeptable?

2.) As I`m into the repairing I would like to change the two chokes (called 3 100C) aswell. I found these in another discussion: http://store.triodestore.com/c354.html - do they fit? I`ll try the cancab from Jan for this repair.

3.) ...and most important: Did anyone have or had the same problem and how did you slove it?

Any help is welcome,

Thanks & Greetings

Oli

[Oli]

Hi,

so, solved the problem after all:

The LFO came from the two sep. 20yF caps AND the two chokes - changed them. The main hum came from the old multi can cap., I use a new one from AskJanFirst, they fit 100%. Though I connected the spear 20yF from the can cab parallel to the other from the can and - wow, man the Amp is dead quiet now, no hum or fizzle when it is turned up to action. You should really think about this modification.

Greetings

Oli - the man who is rocking with his Sunn tonight !



  

[noel]

Oli,
I had encountered this and never knew the answer.  Kudos on the find!  Did you change the chokes too or just the 20uF caps?  Where did you find a can cap replacement?

[Oli]

Hi noel,

I changed "all powercaps" (so the two 20yF and the can multicaps) and the chokes. With the new chokes the LFO was gone, the cancap was responsible for the hum. I (well, lets say we) did it in a very save way:

1.) pulled out the 12AX7 preamp , still got the problem
2.) pulled out the 6AN8 phaseinverter, still got the problem, so we knew it as coming from the DC line. It`s hard to tell this from the scope picture but it was the best possibility where the LFO could come from. So we changed the chokes and the two separated 20yF cabs. The LFO was gone, the hum was much louder.
3.) disconnected the pink cable from pos. B (2000S schematics), so it was gone. So we knew that the cab was bad. I took one of these:

http://www.die-wuestens.de/eindex.htm, it`s the KTL13.

There were easy to install, took about half an hour.

Greetings

Oli

 

[noel]

Oli,
Thanks for the clarification and the link.  That looks like a great resource.  What did you replace the chokes with?  Where did you find matching chokes?

[Oli]

Hi noel,

I used two of these (see above):

http://store.triodestore.com/c354.html

They do fit 100%, no problem. The only difference is the colour.   The shop does have a good e-mail support aswell.

Greetings

Oli