Marshall DSL50

[Ant]

Hi,

I recently bought a JCM2000 DSL 50 head which was only a few months old.
I've it hooked up to a 4x12 cab. Everything was fine for a while but over the last month or so when I switch the amp on from stand-by, after a few seconds i'm getting a type of hi pitched squealing that lasts for several seconds and then goes away. Sometimes it comes back again after another few seconds and cuts out again. On a few occassions when the amp was in "squeal mode" i removed everything except the speaker lead so it's not coming from the input or effects loop. After these initial few seconds/minute of problems, the amp works fine so it appears to be ok when it's warmed up a bit outside of standby mode. My first reaction is to open it up and check for dry joints, reseat tubes etc, but if anybody on here has seen this problem before and tracked it down to a specific cause then it would save me a lot of time. I've a gig tomorrow night and I'm starting to worry that the amp will fall over mid-set.

[Amp Kat]

Well the amp has so many connections that it would be a good idea to go over all of them and make sure they are pushed down good. For oscillations you almost have to use a scope and find out where it's coming from if it's not a preamp tube. One thing that is good is it's easy to take that board off. I mark all the connectors with a sharpie then pull them out. Turn it over and take out the outside screws to the pan and remove the 4 tube socket screws and the whole pan pulls out. Then the screws in the plastic holdown and the whole power board comes out. The pads are very weak so be careful with that iron but it is very possible that it could be a bad connection on that board. You could try swapping the wires on the secondary of the OT or check the scematic to see if they are going to the right connectors. I couldn't find the schematic so I used the 100 watt which is different component numbers but layed out the same.

[Ant]

Thanks for this, I'll get a look later tonight to see if I can find anything obvious. As I said in my original post, its a new amp so I can't see any reason for anything being mis wired. I'm hoping it's down to either a bad connection (which I would expect to cause a problem all the time and not just as start-up and therefore kinda rule out) or a dry joint that gets made properly after the current has been through it for a few seconds and warmed it up so it stays good (more likely?). Only other thing I can think of is a dodgy tube but I don't know alot about these things so can't comment apart from to say that this fault at present only occurs after power up for the first minute or so and then the amp is fine so again I would expect a dodgy tube to be dodgy all the time and not just on powering up.

[Amp Kat]

Thanks for this, I'll get a look later tonight to see if I can find anything obvious. As I said in my original post, its a new amp so I can't see any reason for anything being mis wired.

Well I can and have seen it several times in Marshalls and the same symtoms as yours. Had a JTM 60 sqealing and it turned out the wires from the OT going to the board were right but the ones going to the transformer were backwards or off color code when they went to connect them they used color code and it was backwards. I swapped them and it killed the sqeal. As long as it clears up I wouldn't worry about it to much.

[pdf64]

Ant, the first thing I'd suspect is a dodgy tube (probably a pre amp 12ax7 = ecc83, though I wouldn't rule out it being a power tube). At least try swapping them around, after testing which controls if any affect the squeal, then see if that changes following the swap, even better if you can try substituting in a known good spare tube.
Good luck - Peter

[Ant]

An Update:

I was checking out my amp last night and as usual when I powered it up it started the squealing noise for several seconds and then stopped again and appeared to be ok until it started again for another few seconds but when the squealing stopped the 2nd time I noticed that the volume was up by at least 10%. I then took the amp apart and I re-soldered anything that looked dodgy and put it all back together again. When I tried it again the problem was still there so I took it all apart again (good job I left the tags on the cables) and re-soldered anything and everything I could see this time and put it back together only outside of the wooden case. I powered up and the problem was still the same with intermittent squealing for the first 30 seconds or so and then normal followed by another short squeal and higher output volume then another half a second squeal and the volume was back down again by 10%. So it looks like I can have normal or slightly less power output depending on how the amp settles out!


Anyway, with the amp sitting out of the case, I gently tapped each of the pre-amp and power valves and this seemed to bring the squealing on an off pretty regularly. This appeared to be more so around the 4th preamp valve and the 1st power valve which are beside each other. So for the 3rd time I took the thing apart and concentrated around this area of the board but could find nothing worth talking about but I re-soldered again including the valve connectors which are pretty much welded onto the board anyway. Again when I put everything back together the problem was still there but I did give the power valve a good tap and couldn't get the problem to come back after that although at this stage it was nearly 1am and I was completely ticked off and went to bed.

Conclusion? I think it's one of 3 things. 1. It's a dodgy valve either pre-amp or power, and from what Ive been reading up on this site the stock valves supplied by Marshall are crap anyway and should be replaced as a matter of course. 2. It's a dodgy connection between the valve and the valve seat but there's no way to re-tension these pin sockets easily or 3. There is still a dry joint somewhere that I haven't found.

Any advice?

[Amp Kat]

In these situations mainly squeal's, the first step we normally take is to change the preamp valves because they are so sparatic these days it's not uncommon at all to get a Microphonic even a selected 12AX7 right out of the box. The JJ's are by far the most popular valves on the market right now and most players use them in their amps and I'd do the same unless you want to spring for some NOS Mullards which IMO would be awesome in that amp. The gain gets a little noisy after about 8 and more if the Reverb is up a tad but it sounds good there and a tad more noisy & brighter in Ultra. I was under the impression you had allready swapped valves but you did yourself a favor by going in and touching up and adding solder to the waved machine PCB connections. If I bought one of these amps that's the first thing I'd do to it. Once you get it straight if needed, your good to go for a while but let me stress DO NOT leave those power tubes in it to long as this amp doesn't fair well to tube shorts. The resistors standing up were hand soldered and I've pulled them straight out of the board so check those. If it's not a valve the only other thing I've seen is a microphonic coupling or tone capacitor between the first and second stage. Had the same problem with a JCM800 and that did the trick. Hopefully someone else may have some input on it.

[Ant]

Thanks for the info on this problem Amp Kat. Really appreciate it.

Right, next stage I need to change my valves so this is unchartered territory to me (I'm an ex-tv repairman).

At the minute my amp has 4 of ecc83's and 2 of EL34's. Should I change everything? if so, there seems to be a hord of alternative valves (6CA7's) and a range of EL34's to choose from so which are best - on the pre-amp side what is best the ecc83's or the 12ax7's? are these all fully interchangable without too much set-up?

What should I totally avoid? I don't mind spending a couple of quid extra in order to get a quality replacement that will sound good

[Ptron]

I fixed a JCM2000 DSL 50 head a year or so ago with the same problem. It was a lead dress problem. I believe it was the OT wires running too close to something. What exactly I don't remember.

[Amp Kat]

Thanks for the info on this problem Amp Kat. Really appreciate it.

Right, next stage I need to change my valves so this is unchartered territory to me (I'm an ex-tv repairman).

At the minute my amp has 4 of ecc83's and 2 of EL34's. Should I change everything? if so, there seems to be a hord of alternative valves (6CA7's) and a range of EL34's to choose from so which are best - on the pre-amp side what is best the ecc83's or the 12ax7's? are these all fully interchangable without too much set-up?

What should I totally avoid? I don't mind spending a couple of quid extra in order to get a quality replacement that will sound good

I would start with the preamp tubes and one by one to see if you can isolate the bad one. The ecc83 and the 12AX7 are basically the same tube. The mu of them is @ around 100 mho's but some may have more noise and some may have more hum. Most of the distributors of tubes don't test them because theres so many and in a lot there may be 1 out of a 1000 that is a special tube. Bob at Eurotubes test his and is pretty cool to talk to over the phone so if you call him he can send you in the right direction. I like the Sovtek LPS and the JJ ecc83 or 803s. Of course getting into NOS is a whole different ball game and you can get microphonic ones there too but most are very good especially Mullards and Telefunkens but they have a big pricetag. The power tubes could cause it also but but I can't say I've seen it very much. Those usually pop, growl or cause eratic noises and hum. The 6CA7 and El-34 are also interchangeble and the 6CA7 NOS are very good and very reliable. The current production EL-34's are pretty close with maybe an edge to the Svetlana and JJ's. I think there is a EL-34S that is a higher wattage tube than the EL-34 so you might have to do a bias change on that one which I don't recall seeing a bias pot on that amp so it would have to be a resistor change. Valves are very subjective and you'll hear many different views on them but IMHO the amp won't change the tone drastically with a tube change like many seem to believe happens. You can also try what Ptron posted as it could be anything but try the valves then repoty back.

[Ant]

Hi All
I bought a new pre-amp valve (ecc83) and rotated through each position but the problem is still there! I had the amp on tonight upside down mounted on a couple of blocks under the tranmsformers to keep the tubes of the bench and I switched it on. The problem was there again (intermittent squealing followed by reduced volume) and i did my old tv repair trick of tapping the circuit board with the handle of an artists paintbrush to try and find the component that was causing the problem. It seems that no matter where I touch the circuit board I can pretty much induce the fault. This is very frustrating and I am running out of ideas accept to say that if the volume is reduced in the fault state, then I should be able to try and trace the fault through voltage checks. However, I can't see any reference voltages stated on the schemaic diagram I have so I'm kinda stumped.

Have any of you guru's got any voltage reference points I should check for starters? I've noticed when I bring the amp on from cold i get about 30 seconds or so of normal high volume output before the sqealing starts and the volumes drops. I could use this window to check before and after voltages on the relevant reference points.

Could it be one of the power tubes?

[Ptron]

Try pushing some wires around and see if the squeeling changes as you move any of them. esp the OT wires.

[Ant]

I've just about given up on repairing this amp myself guys

I have re-soldered absolutely everthing, tightened valve seats, tightened all spade connections, pulled and shook every cable, swapped around the pre-amp valves with a new one - and the problem is still there!!!! There has been a slight change thou in that the amp no longer squeals but instead powers up ok at normal volume for about a minute and then the volume dops about 10% and stays like that for the duration. In absence of any tech info on reference voltages to check I sort of stuck

[Amp Kat]

I wouldnt' give up on it yet and I know it's frustrating but you have to isolate the problem and narrow it down to a certain point. I know I stress this a bunch but it's because it works and that's how I fix amps. Use the send out to another amps input or return. Does it still do it ? If so it's in the preamp from the send back. Now take the other amps preamp out send or whatever you have that produces a signal and bring that to the return of the DSL. Does it still do it ? If so it's in the power section from the return to the OT. Then we just keep narrowing it down until we find the source. Could be an open resistor across a cap or the cap itself. Note that I said before I didn't see bias pots. Well I found them and they are on the back as PR1 AND PR2. Theres three pins sticking out and if you put your black meter lead on the middle pin and red to the left it will read the mv's across the 1 ohm Cathode resistor as a 1:1 ratio of ma's so -45 mv's is 45 ma's which is where it is supposed to be so if it's not tweek PR1 to -45mv's. Now move the red lead to the other pin on the right side and leave the black lead where it is. Now check the mv's again and tweek PR2 to -45 if it's not there. Monitor this voltage and make sure it's not slipping after a period.