DSL 50 Not Working

[aspguitars]

Hi! just measured between the jack sleeve and ground (chassis) 0.2Ω


and yes I have some switch spray.

 

[scat7s]

ok, good.

first thing, STOP trying to run out of the 4/8's until we know they are electrically sound. you will damage your amplifier!

now, with the cable still plugged in, measure the tip to each of the 4 and 8 ohm taps on the output transformer, as they are plugged into the appropriate jack. refer to the schematic as necessary to ID the 4 and 8 ohm taps on the OT

let me know what you get.


if you find that you do not have a very low resistence, spray out the jacks and run the cable in and out several times, like 20 times, each jack. and remeasure....let me know what you find.

(also, do this same task on the loop send and return while your at it)

 

[aspguitars]

Hi! I think we may have found at least one problem. I checked for continuity as you suggested from the output tags on both the 4 and 8 ohm connections on the output transformer. First I got nothing and then when I put the jack in another socket, I got a really strange fluctuating reading on both tags. That lead me to the 4/8 Ohm changeover switch. I noticed that I had some really bad dry joints on all the connections. I've now soldered them and am now getting the exact same low crackly sound from all three sockets. Also when I turned the amp off, I got a crackling sort of hissy noise that got quieter after just a few seconds. That sounds like a Capacitor discharging to me. Do you think although having solved one problem that I could have some blown caps somewhere. Not sure how to test them, as I don't have that sort of test gear.
At least all the output sockets are the same now!

 

[aspguitars]

What time is it where you are? 2123hrs here

 

[scat7s]

its 4:30 in the afternoon.

ok, so we have all of our jacks working now, it seems anyway.

what do you get for continuity on the tip of the cable to all three taps when plugged into the appropriate jacks??

and did you clean all the jacks with contact cleaner as i described above? measure and note your continuity numbers b4 and after cleaning the jacks please. im not convinced this is your issue yet, but we need to rule it out 100%

also, please clean your loop jacks as i described and verify that it does not improve your crackling output. EDIT: reflow the solder joints on these jacks as well, lets be sure we have good uninterrupted signal thru the loop jacks.

please do all of these things as described so we can move forward KNOWING that we have eliminated these possible issues. thank you.

i dont know about blown caps yet, its possible, but i dont want to jump around all over the place. one thing at a time, and hopefully, in a logical order. i am suspecting dirty connections and/or bad solder joints at the moment, but we need to work methodically.

 

[scat7s]

after you have done all of the things mentioned above, i will need you to check your bias again like i asked a while back. and please post that up as well.

 

[aspguitars]

I had already sprayed and cleaned all the jacks before I fixed the 4/8 ohm switch, so sorry, can't give you a before and after reading. When I took a reading from each output to the transformer to each output respectively when switched to both 4Ω and 8Ω, I'm only getting about 0.2Ω on each one. This must be good surely?.
I still can't get the bias down though. 59.0mV on one side and 63.0mV on the other. Thats as low as I can get them.
I've resoldered all the sockets on the back panel now including the loop and return and the footswitch and reverb. Hopefully that should be OK now. I've got a feeling that there may still be some problems elsewhere on the board.
What do you think?

 

[scat7s]

ok, no big deal. so you cleaned the loop already too then, right?

 

[aspguitars]

Yes all cleaned and resoldered just to make sure

 

[scat7s]

ok, your OT to speaker jacks look good.

so now you have hot bias? this is why i keep asking you to repeat tests as we move along...this is what i was going by:

I checked the Bias and the most that I can get is about 13.8 mV which I believe should read about 40-45mv.

so now, we are at the other end of the spectrum. you need to adjust R68, which according to the schematic is 22k. but you need to verify this, as we have already discovered, the schematic is not without errors...or intentional misleading. and we dont know if this value has been modified by someone along the way.
verify it is 22k, and if it is, then piggy back another 22k on top of it. do you know what i mean by that?


that will bring our range resistor value to 11K, which should do the trick, but we may have to expriment with different values to get into a comfortable range. start with what ive suggested.

if it is NOT 22k, check back and let us know what value it really is. ok? (before making any changes)

also, now that we know (or know a little better, maybe) that our connections are solid and clean, do you still have crackle from the guitar into the loop return? im assuming you still do, but lets check and be sure. we still need to correct the bias either way...

 

[scat7s]

wait: b4 you change any resistors in the bias network, lets make sure ALL of our resistors in this part of the circuit are healthy and of the correct values.

the resistors for the Bias circuit: R67, R68, R69, R77. find these on the schematic and check them all. lets make sure we are in spec...

 

[aspguitars]

Hi! again,
I've just been doing some more checking as recommended, and all seems OK with the exception of R77. According to the Schematic that should be a 3K9 Ohm resistor, when in the board it is actually a 10K one. But it's OK so maybe it won't be too much of a problem. It looks to be original, unless someone is very good at soldering. I can't imagine why anyone would put the wrong resistor in if they had replaced it, so I think it's a factory mistake, or a mistake on the schematic, which seems to be common.
However! on the Output Board, I've just found a burnt out resistor R21 which is a 100 Ohm 1w resistor. Once I've replaced that, I'll get back to you. I don't have that value of resistor in my box, so I'll have to go get some.
I'll get back to you.
Ade

 

[aspguitars]

Hi! again, I've just put the new R21 100R 1W resistor in the output board and everything is working. However the Classic Gain Channel is really crackly. The Ultra Gain seems OK now.
Any advice would be much appreciated. Bias still very high. 56mV on one side and 59mV on the other at the lowest setting.
Ade

 

[scat7s]

see post 50 about the bias. what value do you have in R68?

RE: crackling

have you tried swapping some preamps in and out? you can discount the PI tube as the issue, as your high gain is working.
swap and /or reseat v1 thru v3 see if that helps.

if not, then i would be looking for another bad resistor or a cold solder joint...

 

[aspguitars]

HI! R68 22K which is OK.
Have changed all the valves for known good ones and exactly the same.
This time I got 79mV on each side of the bias setting. Much too high. Starting to lose interest now!

 

[scat7s]

i hear you. take a break for a few days, a week, whatever. come back to it.

 

[aspguitars]

I've read that the Electrolytic Capacitors dry out and should be changed after a few years. I'm not sure of the exact year of manufacture, but this DSL 50 has got to be fairly old. Do you think that changing say the C38 and C39 large Caps would get rid of the crackling?
And what would cause the bias readings to be so high?

 

[Lane Sparber]

I'll throw my hat into this ring.

First, there should be a white sticker on the back of the chassis with the serial number. If that sticker is still there, post the number, and we can decode the date of manufacture from there.

Next, we need to know what revisions of the circuit boards we're dealing with. Go to this page on JCMJMP's site and scroll to the bottom. He'll tell you how to determine the board revision numbers in any DSL amp.

Marshall DSL

As to the bias issue, it might be the tubes (yes, even if they're new). Try this test. Remove all power tubes from the amp, and with the power and standby switches ON, clip your black meter lead to the chassis, and probe pin 5 of each power tube socket for DC voltage. Do this once with each bias pot in the full CCW position, and once with each bias pot in the full CW position. Please post the numbers here. To clarify, you should be posting TWO numbers for each power tube socket's pin 5. That will tell us what your bias circuit is doing.

All component numbers I list below are based on Rev 7 (or later) boards:

As to the crackle, if it's only on on the green channel, then the good news is that there are only a limited number of components specific to that channel. For starters, check for bad solder joints or crackly resistors at R2 through R13 and check solder joints on capacitors C2, and C4-C7. All of these components are on the POT board where the knobs are mounted. You might want to re-flow these joints anyway just to be sure. Also make sure that CON 3 on that board is seated cleanly and firmly, and check to make sure that the other end of this cable is making good contact at CON 15 on the main tube board. This is going to be tricky without a scope or test device of some sort, so good luck!

I hope this helps.

-Lane

 

[scat7s]

thanks lane....was hoping youd come along.

 

[scat7s]

I've read that the Electrolytic Capacitors dry out and should be changed after a few years. I'm not sure of the exact year of manufacture, but this DSL 50 has got to be fairly old. Do you think that changing say the C38 and C39 large Caps would get rid of the crackling?
And what would cause the bias readings to be so high?

i dont think the filter caps your talking about will help you, since you have a good clean signal thru your high gain channel.

it sounds to me, where your bias has gone from 13mV, to 59mV, to 70 something mV....that you have a componant failure, i couldnt say which one for sure, but R68 is what dictates your bias range. i guess if it were me, i would start with R68 and the trim pots, making sure that they are clean and have smooth transition from one end of its range to the other, (the trims i mean).

i dont know if tired filter caps in the bias circuit could cause erratic/high bias or not, seems like maybe the opposite would be true (low bias supply, under load).... maybe lane can fill us both in on that.

anyway, i'll step aside, follow along quietly, and let lane take care of you from here, your in good hands with him.