Help with Marshall DSL100 head - Low output

[malrato]

Hi guys, I am new at the forum. I have been sneaking around, reading and getting great info from here for some time. Funny how you make a Google search about anything "amp repair" related, and most of the results are threads of this great forum.

I registered and contacted Enzo asking for help. He kindly instruct me to post a thread. This is a copy of the message.

I am looking for some input regarding a Marshall DSL 100.

I got this head from a friend, because it isnt working properly. It has low output. The head sounds great, all controlls work fine, no hum, no noises , nothing, sound really good, only with the volume on 6 you get "loud bedroom level" , I mean is loud but I can't see this competing with a drummer in a rehearsal situation.
I have a few other Marshalls and just retubed a TSL for a friend, which is really loud by comparison. So I know there is something going on.

The DSL was repaired a couple of years ago by the only authorized dealer here in Puerto Rico. It got the pcb of the power amp replaced, because of the runaway bias issue. So the board is new. Tubes are new, and testing OK. I suspected the OT, but I got another one to try it with the amp, and it sounds the same.

I have read about this happening to other people "after " the power amp PCB replacement. In fact I was reading in another forum, some guy had this same exact issue, with a DSL100 "after" replacing the power amp board> Funny thing is the guy said he was in Puerto Rico, So for a while I was thinking I got the same head, in fact I am not sure if it isn't.

I know it is almost impossible to give me a 100% answer without actually looking at the head but any help or info will be apreciated/ Today I replaced all the preamps tubes, but nothing. Also the jacks on the FX loop are in perfect order, and just in case I plugged a patch cable to test , but nothing.
I plug the guitar in the Loop's return to check but nothing, same output. I replaced the PI tube a couple of times and nothing.

I was looking at the controls board and I think one of the realys might have been replaced (the black one on the pics) as the other two are white. Althought all channels have this issue, the lead and ultra have a bit less output than the clean and crunch.

Last thing, I see on the replaced power amp board, conectors 15 , 16 and 12 are not used, they have nothing conected to them. Same with the terminal W8.
I talked to Marshall customer service and they told me conectors 16 and 12 are not used. They didn't say about the con 15 and W8.

The board is JCM2-60-00 ISS 20. (Just in case).

Again any help, will be appreciated.

[Jazz P Bass]

To narrow it down you need to measure the volts ac signal getting to the EL34 grid pins.
At full, flat out volume, you should see roughly double the bias dc voltage.
It may help to use a dummy load.
And while you are at it measure the full volume ac voltage on the dummy load.
Voltage squared divided by resistance will tell you the output wattage into the load.

[malrato]

Hi, thanks for the reply.
Yes, I already did that with a friend who is more knowledgeable and has the tools (i am not an amp tech). I don't remember the exact figures, but with the original tranny, it was puting out less than 30w and I think a bit less on the drive channel. That is what I suspected the OT, but I got another one and tried it out with the same results, the same output.
I need to check the info on the grid pins , I can find the paper I wrote all those figures (it was like 6 months ago).

Any input on those unused connectors on the power amp board? thanks.

[52 Bill]

That series of amps has been know to have a few odd problems, one being the speaker jacks. What speaker are you using with the amp? Did your friend check the speaker jacks for bad solder and a bad closed contact on the 16 ohm jack?

[malrato]

Its more complex than a problem with the jacks.
Anyway, the first thing I did when I got it was to check for cold solder joints and resolder the jacks conections to the board. I checked the jacks and just in case did the 16ohm jack mod ,but it was working Ok, and making good contact. So it isn't the jacks, or a loose fx loop jack conection or something like that.

[malrato]

any other ideas, anyone with the schematic for the JCM2-60-00 ISS20 board?

[Enzo]

What issue schematics do you already have? For that board I have issues 1 and 7, and I think even some others. I doubt I have issue 20. I am surprised they'd even go that high.

More important, what is different about your board from the schematics you have? The changes won;t be major.

[malrato]

I'll be honest, I checked a while ago, so I need to re-check. I know I have the schematic for issue 1 on my files. I have contacted marshall a few times and it seems incredible how dificult is to get an answer about those unused conectors from them and also about the ISS 20 info. They always keep not answering that one even when replying to my emails.

I need to sit down and look at the board and compare it again to the ISS 1 Schem I got.
I will let you guys know. In the meantime, any chance I can get that Issue 7 to compare.

[Enzo]

Oh let me see...

OK, the one called power amp is I think issue 5. Issue 7 is marked. The one called power and reverb covers the mains wiring, which includes some of your wire posts that are not on the power amp drawings.

[malrato]

Thanks Enzo, I will check these and compare to what I got on the board to see. I will let you guys know.

[malrato]

ISS 7 seems to be a match!!!!
So far my board (ISS20) its the same as ISS 7 (thanks for that)> The only discrepancies (and it is not really one) seems to be CON 8 on JCM2-60-00 mine goes to CON1 on reverb board, the schem says it goes to Con 4 , (my reverb board doesn't have a CON4) I believe it is a JCM2-63-02 ISS3 (I have the schem for the reverb board ISS6 and it seems to match) , on this one it says CON1 goes to CON8 on the power amp board (like on mine) so I guess it is alright. W8 on the power board is listed as going to chassis on the Issue 7 . Mine is not used, but when measuring continuity, it shows it is conected to chassis , so I gues it ok too.

On the ISS7 for the power amp board, conectors, 12, 15 and 16 are not listed (at least I couldn't found them). Am I supose to understand this are not used? left empty?

[Enzo]

It only matters if it is involved in your problem. If one channel works and not the other, for example, then we cannot suspect the mains wiring. If it used to work but now does not, unless we changed the mains wiring, it is not likely at fault. And so on.

[malrato]

Update/ So this past Friday I decided to take the power amp board out and look at the solder side... (remember , this board is a "New" replacement from Marshall) that is the reason it took me this long to check for "cold solder joints'. I assumed being a new board, it was flawless (hehe). Well, anyway, I retouched every single joint on the power amp board, and two days later tested the amp. There is a significant improvement in the overall power and output volume from the head.
I am not saying it is repaired , because I haven't been able to test it a lot and also I will want to check how many watts its putting out.
But , it does seem to have improved, to the point where it is actually loud. I will check and test it a bit more, I hope that is it.... I should have not assumed anything and checked, anyway....
Thanks for the help, I will check a bit more.