DSL 50 Not Working

[aspguitars]

Hi! guys,
could do with a bit of help here. I've got an old JCM 2000 DSL 50 that doesn't work. I took all the Tubes out and had them tested by an old valve guy that I know, and both of the Power Tubes (EL34's) proved to be faulty, the other 4 pre amp ones were good. I have now replaced all 6 of the valves as I thought this to be good practice, but still no sound. The Power Valves all seem to be OK, the heaters are all lit up as they should be. The ECC83's although warm do not seem to be glowing as I would expect them to. I checked the Bias and the most that I can get is about 13.8 mV which I believe should read about 40-45mv.
I did suspect the Bridge Rectifier as apparently they do sometimes break down, so I've replaced it and it has made no difference. Not sure where to go now guys?
Any help would be gratefully accepted.
P.S. I did the output socket mod on the 16 ohm output as recommended, but still nothing. However if I plug the speaker into the 16 ohm socket I get a strange crackly echoey type of sound but still very low volume.
Ade

 

[Marshallmaniac]

Cool man. Provided you replaced the rectifier the right way around with right spec, then your next culptits are
1. diodes
2. filter caps
3. screen grids
4. transformer

In no particular order

 

[scat7s]

verify the following ohms readings at your speaker out jacks. powered OFF

16 ohm jack- approx 1.9 to 2 ohms on meter
8 ohm jack- approx 1 ohm

i forgot you r bias is way out...but do these checks anyway, to know if your output trans is healthy on the secondary...

next thing, if your secondary is ok, power it up, and take voltage readings at the plates of the power tubes, in DC and AC, note them, and post back. your amp needs to plugged into a speaker load when it is taken out of standby, or you can damage your OT.

 

[aspguitars]

Hi! and thanks very much for the suggestions. Could you just please clarify which pins you mean when you say the plates of the Power Tubes. As a domestic electrician, plates mean nothing to me!

 

[aspguitars]

Further to my last, I've just measured the 16 and 8 ohm jack outputs.
I only got 0.8 R on the 16 ohm and only 0.3 R on the 8 ohm jacks.
Does this mean that my Output Transformer is no good. I compared the readings last week with a brand new one on the input and output and they both gave the same readings

 

[aspguitars]

The latest edition of the DSL 50 Tests. I've checked for anything obvious, i.e. burned out Resistors etc and can't seem to find anything. I put a brand new Output Transformer in just in case, and now the Bias on one side is about 60mv and the other about 50mv.
The bias pots are as low as they can go, so something is not right there.
Also when I plug the speaker into the 16 ohm socket, I get sound, but really crackly. Nothing out of the two 4 & 8 ohms outputs.
Not sure where to go from here guys, need help.
One of the Power Valves went really bright before I had the chance to check the Bias, so I'm hoping I haven't blown that one. All the valves are brand new by the way. The preamp valves still don't seem to be lighting up like they should.

 

[scat7s]

Further to my last, I've just measured the 16 and 8 ohm jack outputs.
I only got 0.8 R on the 16 ohm and only 0.3 R on the 8 ohm jacks.
Does this mean that my Output Transformer is no good. I compared the readings last week with a brand new one on the input and output and they both gave the same readings

no, your readings sound good. thats a good sign.

the plates? look up the datasheet, i think it will be pin 3, but double check.

you should see high dc voltage, (im not sure what, but i'll guess 450 to 500vdc) and, virtually no AC, BUT, the reason ive asked you to check for AC is to see if weve got a blown diode, or leaky caps.

 

[aspguitars]

Hi! again. I've had a look at the Marshall Circuit Diagram, and it's not very clear. I can't see where the power valves get there DC supply from unless they are acting as rectifier valves. (not sure about this)
It looks to me like Pins 2 & 6 are the Heater Pins, 1(G3), 4(G2) & 5(G1) are Grids and pin 3 goes to W4 on the output transformer. Can I presume that this is where I get the plate voltage?
It's really hard unless you know where to test. There are no test points on these diagrams which is not much help.
Any help would be muchly appreciated.
Ade

 

[scat7s]

ok, you need to look up the data sheet for an el34. find the drawing that looks similar to the one on your schematic ( the tube pinout)

pin 3 is the plate, or anode 'a'

if you look very closely with a flashlight, there are usually numbers on the tube socket. but you can also locate it via the keyway on the tube socket. look at the drawing and compare it to your socket, work it out. you can do it.

2 and 7 are heater.

8 is the cathode, it is tied to ground, and is also connected to pin 1.

4 is grid 2, this is tied to your output trans primary. this is also very high voltage, check that one too.

pin 5 is grid 1, this should show a negative voltage, so watch for that one, it will also help you with your orientation.

tell you what, lets make this easier. i still think you should pull up the data sheet and try to make some sense of what ive said here b4 you begin this test, but.....
with your speaker plugged in, clip your black lead to chassis ground.
with your meter set to DC, start at pin one, (left side of the key)
ONE HAND BEHIND YOUR BACK, or in your pocket, you dont want to get zapped, and you especially dont want to get zapped and have the voltage go across your heart, so work with one hand whenever possible around the high voltage.

note the dc readings you get from each pin, write them down, they do not need to be precise, as in, i dont need decimal points or anything like that, round it off to the nearest whole number. .

go all the way around, every pin and write them down for DC, then set your meter to AC and do the same. then get back with your results.

some of the pins will give you readings testing in this manner, some will not, so dont worry about what it all means, just note them, and i'll try to help you from there.

please be careful!

 

[aspguitars]

Hi! again, just got your reply, but I'm at work at the moment. Will check out the voltages on both power valves as soon as I get in from work tonight and get back to you.
Ade

 

[aspguitars]

Hi! again Scat7S. I really should have mentioned that I teach Electrical Principles and how to use Test Equipment, so testing is no problem. I am however a Domestic Electrician by trade, and therefore not really into Electronics. I understand the principles of Transistors, Diodes, Resistors, Transformers etc, but Tube Valves are beyond me.
I've managed to carry out the tests that you mentioned both on DC and then on AC. The Readings don't seem very promising. I managed to see the pin numbers on the tops of the Tube sockets and then just reversed them to work out the pins from underneath, I just hope I've done it correctly.
These power valves are numbered V6 and V7 so.
V6 V7

Pin 5 0v DC 3v AC Pin 5 0v DC 3v AC
Pin 4 .001v DC 1.9v AC Pin 4 0v DC 0.002v AC
Pin 3 0v DC .002v AC Pin 3 0v DC 2.1v AC
Pin 2 0v DC 3v AC Pin 2 0v DC 2.2v AC
Pin 1 1.8v DC 0.019v AC Pin 1 0.002 Dc 2.38v AC
Pin 7 0v DC 0.002v AC Pin7 0.941v DC 0.002v AC
Pin 8 -33.4v DC 0.059v AC Pin 8 -33.0v DC 0.50v Ac

Please let me know the bad news
Ade

 

[aspguitars]

I hope you can read this.
It did'nt look anything like this when I typed it out
Hopefully you will see that the first readings on the left are for
V6 DC and AC
and the same on the right for V7
Ade

 

[scat7s]

please double check both your pin number orientation, and that you have a good connection to gnd with your meter.

based on what you have above noted as pin 8, should be pin 5.

everything else, looks to have no voltage whatsoever. which either means you did not have a good connection to gnd while testing those pins, or your power trans HT is not working.

check your HT fuse please, as well as any fuses on the circuit board, with continuity check.

 

[aspguitars]

Hi! again, as I said, I'm not too sure with tubes. The numbers that I've used are actually on the top of the tube sockets. I'm checking them from the solder joints on the circuit board from underneath though, so it's possible that I've got them around the wrong way. I'll have another go tomorrow as it's getting late now. What sort of voltages should I be getting from the Heater and HT terminals on the Power Transformer?
I did check the fuses. The ones on the circuit board are good, but I'll check the mains fuses again.
Thanks
Ade

 

[scat7s]

not the mains, the HT, its on the back panel, check it with a meter, not just visually.

you should see something like 450 to 500vdc on pins 3 and 4.

to measure the heater, use AC, pins 2 and 7, one with each probe. all other measurments are done to chassis ground. you should see 6.3 vac or something close to it.

 

[aspguitars]

Hi! again. We seem to be having a bit of a language barrier problem here lol!
When I say mains fuse, I mean where the mains lead plugs in to the back of the amp.
The power valves are actually powering up ( I presume these are the heaters) so the fuses must be OK. I think perhaps I'll check again just to make sure.
Will get back to you.
Thanks again for your patience.
Ade

 

[scat7s]

your mains fuse is obviously ok, as it still powers up. but it would appear you have no high voltage secondary from the power trans. this should also be fused.

look over the schematic, and locate this fuse.

 

[aspguitars]

Hi! again,
Just done some of the recommended testing that you suggested. I'm getting nothing on the HT at pins 3&4 of both the power valves (all less than 1 volt. I then checked the heater pins on pins 2&7 and again reading almost nothing, all under 1 volt. I checked both fuses and they are OK (obviously but double checked) I checked the input on the mains transformer. I am getting about 238volts going in, but pretty much nothing coming out. It looks to me like the mains transformer could be blown. Again on the Marshall Schematic Diagram there are no given voltages for the various connection, but I definately don't have anywhere near the desired 450 to 500 volts AC.
Do you have any recommedations for testing the transformer, or like me do you suspect that this is the problem?
The thing that has confused me is how the tubes are getting powered up at all! with little to no voltage. Even on the heater pins
Again, over to you lol!
and thanks for all your time and effort.
Ade

 

[scat7s]

it seems unusual to me that if your power trans secondary is bad, that it wouldnt continually pop fuses. usually when they go, they short out and take out the fuse.

can you read schematics?

i wouldnt run out and buy a PT just yet....im still not convinced your pinout is correct to be honest...if your tubes are lit, then your heater circuit is still working.

so, you did find the HT fuse then? what is its value? (voltage and amp/current rating)

heres a link to the PSU schematic. F2 is the HT fuse. please verify you have found and have tested this fuse...thanks.

http://www.drtube.com/schematics/marshall/dl50-63-02-iss3.pdf

 

[aspguitars]

Hi! again,
the fuses are situated either side of the mains plug socket so very easy to find and check. I checked these, but as I said nothing would power up if the fuses were blown. I have the schematic that you put the link to, the fuse holder says T500MA on it. The mains fuse is rated at 230v 1.6 Amps. (UK rating).There are also fuses F1,F2 and F3 on the main board which again are OK.