Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on May 14, 2011, 01:26:29 pm

Title: 1982 Marshall Anniversary Series 4210 / Input Noise Problem
Post by: plexi50 on May 14, 2011, 01:26:29 pm

1982 Marshall AS 4210 amplifier. I have a small amount of voltage at the input jack. When i turn my volume pot on the guitar it wisps back and forth. It does this using 5 different guitars so a guitar isnt the problem. If i run a screwdriver on the chassis you get that voltage crackle.  This is suppose to be the same as the 2205 but i see differences so i am not going by the 2205 schematic.  All i can think of is i have a bad coupling cap on V1 but i dont see one on the 4210 schematic until you get to V2A. This amp is weird. It is loud but not as loud as i think it should be with two 6550 power tubes. The Clean channel breaks up early and has no clean headroom . There is no clean to it period.  :help:

The clean channels EQ is wired way differently than what i am used to seeing in most all amp

All tubes are brand new as of yesterday

Title: Re: 1982 Marshall Anniversary Series 4210 / Input Noise Problem
Post by: plexi50 on May 15, 2011, 07:27:58 pm

Found my problem. Some one has did some hacking and removed the .047 cap that was a part of the input grid on this amp

Also they changed many cathode resistors out for 820's on V1 & V2 that made the amp overly gainy and dark

Changed the 220K plate resistors out for 100K values. Changed tonestack values for clean channel to 250pf / .1 Bass and .047 Mid
Slope was 33K but is now 100K for the clean channel

Slope for Dirty channel on schematic says 68K but is a stock 33K. Treble cap is 220pf and have changed Bass cap to .1 & Mid to .047

Will fire up tommorrow

Title: Re: 1982 Marshall Anniversary Series 4210 / Input Noise Problem
Post by: LooseChange on May 16, 2011, 04:35:02 am

Not that I though the cap was missing, but that would have been my first place to look.

Title: Re: 1982 Marshall Anniversary Series 4210 / Input Noise Problem
Post by: plexi50 on May 16, 2011, 08:40:00 am

A first for me. This amp is using 9 Tropical fish caps. The solder on them looks to be original. How can that be? I thought those caps were a 60's-70's thing. Just seems to me like some one made a lot of changes. Though i have not seen this amp before

It is finished. It sounds great. It almost seems to me that this is the worst Marshall i have ever seen as far as the clean channels EQ goes. More along the lines of a Peavey Triumph in the way it is layed out and put together

At least i now know what to do when the next one comes along