marshall jmd1 r48 resistor blowing problem

[mark allen]

I am trying to fix this marshall jmd 1 amp and have found both r48 resistor and r56 resistor legs burned off. They are beside each other in the circuit board but look downstream in the schematic from each other. I tested the tubes and they appear to be good with no shorts on my dads vintage tube tester.
I replaced both resistors and turned the amp on . Then turned the standby on and it took out r48 1 ohm 1w metal oxide resistor again.
Any idea on why it is blowing this resistor.

[g1]

Please post or link to the schematic.

[mark allen]

schematic link.
http://forum.metroamp.com/download/f...e724f8453a1b01

[g1]

A power tube fault that will burn R48 will not necessarily show up on a tube tester. The tube tester is probably not using as high a voltage for it's test.
I don't see any reason R56 would burn. Are you sure what is called R56 on the board matches the schematic?

[Jazz P Bass]

R48 is the sense resistor for the bias setting.
1 ohm.

R56 is part of the bias adjust circuit but for the other tube pair.
220K ohms.

You got me as they are unrelated.

[Enzo]

Pull all power tubes. Power up the amp, and also turn on standby switch. Anything burn now?

Still with no power tubes, measure voltage at pins 3,4,and 5 of each socket. Do you have good B+ at pins 3 AND 4 of each? Is there good bias voltage at pin 5 of each, like -45 or -50 volts or so?

And power off, measure resistance to ground from pin 8 of each and pin 1 of each. All should measure 1 ohm.

[mark allen]

i could not see any reason why r56 resistor would go out. My first thought was a dead stinkbug found in the amp caused a short between the resistors. The resistor is correct . r56 is a 220k.
Any other reasons beside a bad tube that could do this? r56 did not reblow

[mark allen]

I will try to do what Enzo is speaking of.
This will take some time to do and figure out.
Thanks.

[mark allen]

marshall jmd pulled tubes tested voltage

Pull all power tubes. Power up the amp, and also turn on standby switch. Anything burn now?

Still with no power tubes, measure voltage at pins 3,4,and 5 of each socket. Do you have good B+ at pins 3 AND 4 of each? Is there good bias voltage at pin 5 of each, like -45 or -50 volts or so?

And power off, measure resistance to ground from pin 8 of each and pin 1 of each. All should measure 1 ohm
.

I pulled the tubes . it did not blow the new resistor. pin 3 all read about 492v on 3 tube sockets and a strange reading of .027 on the other tube socket ( Problem )
pin 4 all read about the same 491
pin 5 all read about -40.8

2 of the tubes on the same end one of which was the one that read low @ .027 on pin 3 read a resistance of 100.8 on my meter on pin 1 resistance to ground
2 of the tubes on the other end had no resistance to ground . no reading at all.

I do not have a 8th pin on the tube socket to check resistance. Any ideas?

[Enzo]

No resistance to ground means zero ohms. May I assume you mean no continuity to ground? Same as infinite resistance, an open?

You better have 8 pins per socket, EL34s have 8-pin bases. Pins 1 and 8 need to be wired together. Pin 8 is the cathode, pin 1 is the suppressor grid, and is tied to the cathode or in some amps the negative supply.

If the two tubes on one side have an open to ground from pin 8/1, then your 1 ohm resistor is open OR the circuit path through it is open. That is R48 or 49. The other side reads 100 ohms? Well that is the other resistor of that pair, and it SHOULD be 1 ohm, not 100. So make sure you do not have a 100 ohm resistor installed for R48 or R49.

The two pins 3 on one side should be wired together, and the two on the other two tubes should be wired between them selves. So if you get 490v on one pin3, but not on its buddy next door, look at the connections between the two sockets. And the sockets themselves. Look at the pin 3 that DOES have the 490v, there will be a trace on the board coming in from the transformer wire, and also a trace over to the other socket pin 3. The transformer wire trace might go to one then on to the other, or it may come along with a branch to each. In any case, follow it between the tubes looking for:
cracked/broken trace or even board under trace
cracked solder on one of the socket pins
cracked solder on any jumper wires on the board, if any
Copper trace burnt open like a fuse at some point, usually the narrowest point of a trace.

[mark allen]

amp is working

I will try to do what Enzo is speaking of.
This will take some time to do and figure out.
Thanks.

I was getting a reading from the wrong place about the 100 ohm thing. The resistors are the correct value. It indeed does have 8 pins on the tube .On the other side of the circuit only showed 7 contact points. I found the r48 resistor blown also.
I replaced the blown resistor , then put tubes back in, flipped the standby switch and now blew out the fuse. Took out tubes again and replaced fuse and installed tubes from my other amp, Turned on again and bingo! Working perfect!
My tester did not show the short on the tube that was bad like mentioned on one of these replies. Found fuse was bigger than what is noted on circuit board. To sum up what probably happened is the tube shorted and caused the resistor blowouts because of the wrong value fuse found .

Thank you for all your help once again.

[GBS76]

Hi everybody

After 1 year I decided also to have a look at my JMD not working anymore and found this thread...
Note: Preamp (output) has always worked perfectly since no more sound on the speakers as I used my JMD connected to another power amp.
So I opened , checked and :
I have quite the same issue .... R48 burnt and Fuse F1 also ... R56 is ok...

I am about to buy a new 1 ohm resistor and a 1 amp fuse but does anybody have any idea about the root cause ?
Will it work again of I have to trouble shoot somewhere else before to repair ? ( even if those 2 components are really not expensive ... )

Thanks for your support and sorry for my poor English (I m froggy ^^)

Thanks you all by advance

[52 Bill]

Welcome to the place!

I am about to buy a new 1 ohm resistor and a 1 amp fuse but does anybody have any idea about the root cause ?

The root cause would be a shorted power tube that caused the resistor to burn and the fuse to blow.

If you re-read the above thread, the original poster had to change the power tubes along with the otner parts.

[GBS76]

Thx Bill;

I have replaced 48 and discovered also 49 was burnt. I removed V1 and V4 power tubes and 49 burnt again after around 30 sec.
Question now is ; As I left V2 and V3 connected, shall I have to replace 49 again and make a try without any tubes ? ( 4 removed ?)

Is there any link between R48/49 and somewhere else ?
Remark: F1 fuse is still OK ( why was it burnt when I discovered during the first time check ?)

Thanks for your support. I m a bit lost.

Eric
F

[Jazz P Bass]

Have you replaced the power tubes?
(all four)

The only 'link' between R48 & R49 to ground is the output tubes.

They are configured in two parallel pairs.
Pair 1: V1 & V2
Pair 2: V3 & V4

It appears that you have more than one bad tube.

I would replace the quad.