Another DSL Marshall problem..

[Valvehead]

This one is weird..
I have strange heater voltage at V1-4 , and also V5 . The last 3 el34's are fine. The fuses are good . Good bias and plate voltage , I have no voltage at either side of R69 , (100ohm) and i CANT find R73 (the other 100ohm) . These are in the heater circuit .
Can anyone tell me where on the board r73 is? ive looked for a half hour now!

are these 100ohm a common problem on these amps? I also read about the heater bridge rect being a common problem(or the joints to it) , but why would 3 out of the 8 tubes read ok ?


Heres what im getting at the sockets -no tubes in=

V1 4.7vdc to ground (pins 4/5/9) (no ac)

V2 4.7vdc as above but 9.4dc across 9 and 4/5 (no ac)

V3 2VAC across 9 and 4/5 with 0vdc

V4 2VAC across 9 and 4/5 , also pin 9 shows 3.6VAC to ground


power tubes:


V5 1.8VAC across heaters


V6-V8 -look halfway normal with 7.2 vac across the heaters.

[Enzo]

Geez o pete I wish folks would use model numbers. WHICH DSL? I see DSL201, DSL401, DSL50, DSL100, and who knows, maybe there are others. They don't use the same boards or schematics.


SOunds like open trace or cracked solder, most likely on a jumper wire between traces. Follow continuity down the heater feed traces socket to socket. Leave the tube sout of course, and disconnect the heater voltage wires from the transformer where they meet the board. That will prevent the low, low resistance of the winding from confusing things. If the heater current gets to the first three powr tubes and not the fourth, I'd be looking between the third and fourth sockets first. Also, if there are individual heqater fuses at each socket, check the fuse - pull it out and use a meter. Can;t check fuses visually reliably.

If this is a DSL 100, then the power tube heaters are fused individually.

I see R67 and R73, both 100 ohms, as the typical virtual center tap for the heaters, just as half the Fender amps ever made are wired. I don;t know where they are on the board. Of course if it is one of the other DSLs I am not going to get the rest of them out to check numbers.

Those resistors typically blow when power tubes short out, and more than that, they REALLY often blow when someone breaks off the plastic center post on a power tube, then sticks the tub back in the socket in the wrong orientation.

The heater current comes on the board at posts W7, W8. R73 runs from the circuit of W8 to ground. It will be on the same trace as W8 between that post and the row of heater fuses at the power tubes.

It is possible someone has put wires in place of the four heater fuses, but even so, check the wires.


But try this. The tubes are out. Measure resistance to ground from pin 2 and from pin 7 of one of the working power tube sockets. if the transformer wires are still on the board, you should see about 50 ohms to ground from either side. With those wires disconected, each side will show 100 ohms to ground. If the hidden resistor is OK, the readings will be right. If one is burnt open, you will measure 100 ohms with the wires on and with the wires off, one side will be open to ground.

If the readings are OK, then who cares wher the resistor is?

If your heater supply current is not getting as far as V5, then it ain't going to get to V4 or any other place useful. SOlve the V5 heater issue, and the rest of them will probably fall into place on their own.

[Valvehead]

cheers enzo , its a dsl100 head.

will get to it soon...

[Valvehead]

But try this. The tubes are out. Measure resistance to ground from pin 2 and from pin 7 of one of the working power tube sockets. if the transformer wires are still on the board, you should see about 50 ohms to ground from either side. With those wires disconected, each side will show 100 ohms to ground. If the hidden resistor is OK, the readings will be right. If one is burnt open, you will measure 100 ohms with the wires on and with the wires off, one side will be open to ground."



i tried this before i pull the board since i have a few minutes right now,

V6 /7/8 are testing good

the rest are NOT passing this test. some read open , some read 3- 9Meg .


still cant find R73 ................

[Enzo]

ok, so at V6-8 you DO get the 100 ohms to ground from either side of the heaters, OR you get the 50 (with transformer connected)? In that case, your resistor is OK, wherever it might be. Move on.

since V5,4 are wired in parallel with V6-8, the readings should all be the same. They are not. Obviouosly there is a lack of continuity between V6-8 and the rest. it only takes one trace open to kill the string downstream.

Isolate the problem.

Pins 2 of all the power tubes are wired together to one side of the 6v. Pins 7 of all the power tubes are wired together to the other side of the 6v. Stick your ohm meter probe into pin 2 of V6, and the other probe into pin 2 of V5. is there continuity? if not, there is the problem. If there is, then plug your ohm meter into pin 7 of V6 and the other probe into pin 7 of V5. Is there continuity there?

One side or the other is bound to be open. (or both) When you determine which, you follow the path from V6 to V5 inch by inch. Check the solder at V6 socket pin. Follow the copper trace along its path. Look closely for any tiny cracks or even burnt open spots. Is there a wire jumper across some space? Check the solder at both ends of that. Eventually you find yourself at V5. Check the solder at its socket pin.

There is no magic here. Imagine you had a row of power tube sockets connected by wires as in any old Fender Twin or whatever. You got 6v across three of them, but the fourth has no 6v. If the voltage was fed into the end of the row and went tube to tube, you would have to find where the break in the circuit was. Obviously the wires worked on the first three tubes, but between the third and fourth, something was open. You would check the solder on both wires at each end, you would check to see if either wire was broken off the solder connection, you would check to see if either wire was broken or cut in half. Not much else to it. This isn;t different in the slightest just because it is on a circuit board. Instead of wires, there are flat ribbons of copper on the board. But circuits are circuits.

[Valvehead]

i pulled the board tonight. what a pain with the 4,000 connectors....

There was a bunch of "goo" spilled into the amp--right around V5... looked like Jager ...or cough syrup.

I cleaned all that out and touched up all the heater joints.

The amp works great now.

By the way--the schematic is way off as far as part # / values / connector numbers. It matches my main board as far as the amp / part number , jcm-60-00 or whatever it is..but when you start looking at connectors and whats labeled what and so on.....not even close. Hence my search for r73 , etc....

Thanks Marshall!

[Enzo]

If you have Adobe Reader 5.0, the instructions for Adobe Reader 7.0 won;'t be very helpful. The owners manual for my 2002 truck won't be very useful for someone with a 2008 truck. And someone with an issue 1 DSL100 won't find the schematic for an issue 5 DSL100 all the useful either.

What revision is your board?

[Enzo]

And a shop tip: I never take those boards out. it would take an hour to chart and label all the cables. ANy more than I would take the board out of a Hot Rod DeVille.

If I have to work on the main board, I pull the tubes and take out the socket screws. Flip it over and dismount the rear jack board. A few regular insulated wires come over from the output transformer and a couple from the main board, and they all go to push-on connectors, mostly near the impedance switch. I disconect those and one small 6 or 7 pin small connector. Make a chart what color goes where. I usually clip the two tie wraps along the rear chassis holding them.

Now the rear board is free to move, though some wires are still attached. Move it aside to get at the main board mounting screws below. You can already get at the top side - the component side - of the main board just by swinging the jack board aside. To get at the solder side of the main board, I disconnect the heavy wires from the power transformer. Seems to me just the heater ones have to come off, the others reach... I think.

ANyway, removing those wires allows enough slack everywhere else. SO with the amp upside down front towards me, I flip the jack board over the front, unplug the little bias board, and flip the main board up and towards me. Bottom exposed. It will face away from you a little. If you don;t like that, tilt the chassis towards you a bit to lower the front edge. I like to work on them facing the front. But you could just as easily turn it around, working from the rear side, and then the main board is facing you.

I think it took longer to tell than it does to do. I can have the main board over and ready to solder in a very few minutes

[Valvehead]

ok, ill try that next time .

i dont know what version the board was--the amp has been picked up.