Marshall DSL 100, loudspeaker out, 16 ohms ok but parallel 4,8 ohm no sound

[rrrdddrrrddd]

I need help. My parallel loudspeaker output does not work but the 16 ohm output works fine. Checked all internal and external fuses. Where does the problem lie. Is it internal switching between the 16 ohm out and the the parallel out?

[acorkos]

check the 16 ohm jack itself....there is a switching mechanism built into the jack. with no plug inserted, the moveable metal elements on the top of the jack should be making contact with the fixed metal parts on the jack....otherwise you have an open circuit and no connection to the 8/4 ohm jacks

[rrrdddrrrddd]

I'll tell you what I did. I did remove the amplifier from its housing to get direct access to the three loudspeaker outputs (red) and circuit board from underneath. The 16ohm 1/4" output was tested with my multimeter for continuity, the contacts are clean and are in perfect shape. The same was done with the two parallel 1/4" outputs and the 4/8 ohm switch.

This amp is like new. This switching must be done within the circuit board as all the 1/4" outputs are identical and don't seem to have any switching mechanism on them.

The only theory of operation I know about this amp is that if the 16ohm output circuit is closed (plugged to a speaker 16ohm cabinet) then the other two parallel outputs are opened (won't work).

Also nothing looked "fried" on the circuit board. There is one transducer thingy (black chip) on this board and I wander if this is responsible for switching and if so can it be tested. This is new "territory" for me.

Anyways, thanks for the reply. If you know anything else please let me know.

[oc disorder]

Hi rrrdddrrrddd and welcome to the forum.
The usual problem with these with the exact same symptoms you describe
is caused by the sleeve connection (the one closest to the back panel)not going back and touching on the contact underneath in the 16 ohm jack.
This is the main ground for the 4 or 8 ohm sockets.
The switch (8 or 4) moves the "live" or hot output wire to either the 4 or 8 ohm tappings from the transformer.
If there is no ground it doesnt matter what the tip of the output jack is connected to it just won't work .
See this post http://music-electronics-forum.com/t12900/
and if you scroll down to post 27 I attached a picture of someones faulty jack
and a small section of the schematic that shows this section.
Either you will have to clean it and or retension it (not often successful)
or replace it with a new one.
Another alternative is to bridge out the two front solder connections underneath on the 16 ohm jack ie the sleeve connection and the contact that sits directly beneath it (the ones that join or are supposed to when you pull out the 16 ohm jack plug) .
That will mean you will loose that safety feature if someone plugs into all three !
Hope that helps illuminate the issue.

[rrrdddrrrddd]

Thanks for the great instructions. I took apart my amp again flip it upside downs to get underneath it and tested the 16ohm input. I tested for continuity from the JS2 through the JS1, JS3 and all the way to W5. There are no issues of grounding. The contacts are imaculate, lots of tension, can't make em fail if I tried to. The diagram was very helpful.

The only strange thing I found was that there was continuity from point W6 (red wire) which on the diagram is the ground to the W5 (black wire). Same between the point "t" and "s" on the JS3. Is this normal?

[rrrdddrrrddd]

Problem solved!!

I went back again to test the continuity of the grounds and I finally made it fail. Yes, the JS3 steel contact stuck up and sure enough was not make any contact therefore not providing a ground for the parallel outputs.

I just plugged in my 1960A and 1960B cabinets for the first time with the DSL 100. Full stack finally!

Thanks for helping me out. I'm going to bed.

Richard D
New Brunswick, Canada

[rrrdddrrrddd]

You are right!

[oc disorder]

Glad you found it even though it eluded you at first.
Although acorkos nailed it first time - these simple things can sometimes
appear to work when you are not expecting it and vice versa.
"The only strange thing I found was that there was continuity from point W6 (red wire) which on the diagram is the ground to the W5 (black wire). Same between the point "t" and "s" on the JS3. Is this normal?"

Yes the output secondary windings (the ones the speakers connect to)
have very low resistance ..hardly any turns compared to the primary which
connects to the o/p tubes.
What's not shown is the ground connection on the opposite end of the transformer to the 16 ohm tapping.
So the output can appear like a shorted connection.
A low ohms meter that is accurate + calibrating it first can help explain away
some of these "appears shorted" measurements.