Marshall JCM 2000 DSL 401 problem

[DrGonz78]

Hello, this amp has an apparent issue with the power tubes as they wont power up. THe power switch light comes on and there is power running to the board (as measured from ground) but they wont heat up. THe pre-amp tubes all have power as well but they don't light up either. I suspect it is a bad ground, or some component blown between the ground side of the circuit and ground itself. Has anybody here ever seen this type of problem before? I do have the schematic and it looks like R12, and R14 are on the ground side of T2, but I cant tell which tube is T2. Any advice would be most appreciated. THanks in advance!

[oc disorder]

Hi DrGonz78 and welcome to the forum.

I'm a little confused to your reference re "T2 , R12 & R14 " as T2 is a transistor MPSA13 and it and the resistors
are part of the switching circuit for channel change etc notice the stereo (Tip ,Ring and Sleeve) jack to the left
where the footswitch plugs in.
However on that page to the right is F4 the heater or filament fuse which is a slow blow 6.3 amp fuse (T6.3 ).

This is probably blown which is why the tubes (or valves) are not lighting up.
In the States they are referred to as tubes 'spose cause they look tubular in shape wheras in the UK they are called
valves. So V1 V2 refers to tubes and T1 T2 refers to transistors. Straightforward isn't it ????

In my experience the fuse can go for two reasons :-
1. There is a internal short in one of the output tubes which popped it
2. The fuse was fatigued and blew.

So if the output tubes have had a considerable amount of use and or haven't been changed for a while
perhaps now is the time to do it and then fit a replacement fuse.

Otherwise replace the fuse and keep your hand on the power switch in case there is any crackling/hum
suggesting an internal fault which will blow the fuse again and possibly stress some other components in the process.

On this forum there is almost a novel written about problems with the bridge rectifier BR1 that supplies the
first pre-amp tubes with DC for the heaters (page 1 ,right - centre)but as all the heaters are not lighting
in your case I think it's just the fuse.

[DrGonz78]

To be more specific the term "T2" comes from the actual schematics of this particular amp. T2 (or we could identify as V2) is the first in the series of power tubes section, so the amp schematic says... "R12" & "R14" are resistors that are in parallel circuit to ground from the T2(tube or V2). Amp schematic>> Marshall Diagrams, Schematics and Service Manuals for FREE updated: Ok ok I just Finally!! found this other document that show way more important info to the amp schematic.>>> http://www.drtube.com/schematics/mar...60-02-iss9.pdf This will hopefully set me on a better track to diagnose this problem, and I am writing this after completing all my other parts of this post. Now it makes way more sense looking at this other diagram. Note: Please help still and continue reading, while ignoring references to T2, lol. Will update more as I study the correct diagram and am still wondering which the first valve that takes power first. Thanks... Sorry so confusing to read.

While diagnosing this amp I have taken it apart and put it back together, thoroughly inspecting everything for something that could be a problem. I bought this amp in this condition with the intention to troubleshoot and repair the amp to full working condition. It's not any of the three fuses located on the amp(HT, Main, or the one on the board.)

Yes, reading about the main troubles with this amp(heating of power tubes)>>> I have already spent time fixing any cold solder joints in the power tube sections on the main board. The BR1 may have actually been the trouble as well but looking at the schematic for the amp there is nothing on there that shows where BR1 is located. (Update: BR1 feeds preamp tube V3? Or is it Fil1 & Fil2 which leads to the power tubes?). So that leads me to know help till I figure which is the first power tube in the series "T2" on the schematic. Now I would gladly replace the EL-84's with new ones and assume that would fix the problem... However, the preamp tubes are also not lighting up as well. All of the (preamp & power) tubes are getting 120V current (according to the multimeter check) and should be turning on... So that is why I believe there might be some kind of short to ground occurring that is not connecting the circuit between Tube2 "T2" and "R12/R14" to ground. The biggest trouble is that "T2" is not labeled on the main board where the power tubes are located, the tubes are not labeled w/ as anything that makes any sense. In fact, I am first looking to find out simply if anyone knows which one of the power tubes is "T2" or "V2." That way I can check for grounding/resistor issues from the first in the series of power tubes. Doing this without a real service manual is hard since they don't tell you this stuff from the amp schematic.

Will post more later, sorry the problem has suddenly shifted gears... All the above may just be me going in circles while searching, and Finally finding the full amp diagram schematic.
Appreciate the feedback and will followup with anyone who might help fix this problem, thanks.

[Mark Black]

In fact, I am first looking to find out simply if anyone knows which one of the power tubes is "T2" or "V2."

None of the power tubes are "T2" or "V2". They are V5, V6, V7, and V8. As "oc disorder" pointed out (very eloquently and in-depth I think) there are in fact no tubes at all labeled "T" anything on these Marshall schematics or on the PCB designators.

[DrGonz78]

Please read my last post Mark Black, because I found the correct documentation now. As you were typing your reply I also started to realize my error. I see the light now!! So, is it V5 that takes the DC power in first?

Sorry for my ignorance but it took a long time to find the right schematic. When I searched for the schematic most of the web sites only give you the page 2 of 2 document. When I first printed it out the other page was Setting Bias Document. Thanks for getting me to look closer and finally have it now yay! Will post more as I go thanks so much.

[oc disorder]

Hello again DrGonz78

Glad you have the full schematic now.
On page 2 (ref:E 2) are connectors W20 & W19.
They are connectors from the 6.3 volt heater winding on the power transformer.
W 19 goes to the T6.3 amp fuse and half of the "hum reduction circuit" R34 a 1 watt 100 ohm resistor to ground.
It is then labeled Fil 1 and is the distribution point from which the 4 output valves (or tubes V5,6,7 & V8) the phase
inverter (or splitter V4) are connected for their filament or heater voltage F1 connection.
Also Fil 1 is connected to BR1 on page 1 the bridge rectifier that supplies the "low noise" DC filaments for the first
3 pre-amp tubes V1,V2 & V3.
Likewise W20 is connected to R35 the other half of the "hum reduction circuit" also a 1 watt 100ohm resistor to ground.
It is then labeled Fil 2 and is the distribution point from which the 4 output valves (or tubes V5,6,7 & V8)the phase
inverter (or splitter V4) are connected for their filament or heater voltage F2 connection.
Also Fil 2 is connected to BR1 on page 1 .
You should be able to measure approx. 6.3 volts A.C. between W20 and W19 and also on the non ground side
of R34 and R35.

All of the (preamp & power) tubes are getting 120V current

?

I would leave the standby switch in standby position and possibly remove the HT fuse F1 a slow blow 300mA
while investigating the heater supply.

In this amp all the A.C. heaters are in parallel so its not really considered that a particular tube gets it first !
The 3 pre-amp tubes are also paralleled to their D.C. supply.

I haven't the amp in front of me only the schematic so you will have to visually follow the circuit board track
from the connectors W19 & W20 to see if there is some break in continuity .

[DrGonz78]

Hello OC,

Thanks so much for providing so much detailed and useful information! I will study this schematic the next few days and keep studying, in general, how to read diagrams better. I want to learn more about fixing these types of problems. Unfortunately, my multimeter is not working that great and I really need to just go buy a better one. I am gonna get one that won't crap out on me while trying to search for continuity breaks. Also, I have somewhat of a mentor that is guiding me through this process and will hopefully meet up with him again next weekend. So, I will keep you posted as to what problems I fix or encounter.

Now I can see how Fil1 & Fil2 connect to the power tubes and the BR1. I am starting to see how each component relates to the other and it's starting to make more sense. Also, will look at the circuit board to get familiar to where all the components that need testing are located. When I start testing for continuity then I will finally know where to be looking. Will follow up with the community as soon as I get a new multimeter. Thanks so much again for setting me straight on this issue.

[DrGonz78]

One more quick question about your post OC...

When you say "follow the circuit board track from the connectors W19 & W20..." >>> Should I also find the traces side of the boards connections and test those as well? Hypothetically, if there was a bad trace line, can things like that be fixed?

Sorry just had to ask, since it is something I have wondered about.
Thanks again man!

[Enzo]

If a copper trace on a circuit board is cracked or burnt away even, we can repair it with a piece of wire.

Most boards cover the copper tracks - traces - with a green or blue lacquer called solder mask. It is not present in the spots where solder would want to go - component holes for example. If a trace is cracked across, we would abradde away that coating on eithe side of the break, then take a piece of bare wire, lay it lengthwise along the trace and across the break, and solder it to the newly bared copper.

The job of any trace is to connect point A to point B. SO if the above method is not convenient, it is just as valid to find out where the trace came FROM and where it was going TO, then run a piece of wire between those two points.

[DrGonz78]

Thanks again to Enzo this time! I figured it was like jumping the circuit or something like that... So glad I found you guys here, because your answers are really well put. You guys rock!

[DrGonz78]

Okay, it has been a bit of time since I have worked on this amp and had put it on the back burner. As we know from prior posts I was to test the amp to make sure it was reading 6.3volts on the heater winding from the power transformer. To answer that part of a prior posting>>> yes it is. I have tested everything for continuity on the back of the circuit board and soldered some cold joints. Still nothing fixed the problem and as I am about to tell you will see why.

Now I have gone through every resistor on the preamp circuit. This is where I am finding all sorts of resistor problems. Here's the list...

R117 (1M r) Not cigar blown but totally dead and it will work on a continuity test. Continuity test should not work on my multimeter w/ 1M resistance.
R120 (1M r) Getting reading of 55r on the multimeter so it's failing bad.
R137 (1M r) Getting reading of .8M on the setting of 2M of the multimeter.
R128 (150k r) Getting reading of 136k which is lower than what it should be.
R98 (1m r) Getting reading or .323M on meter.
R99 (478k r) Getting reading of .320M on meter.
R100 (47k r) Getting reading of 0.00 @ 200k and 0.04 @ 20k setting on meter.
R126 (22k r) Getting reading of .003 @ 20k setting and nothing at 200k setting.
>> This last resistor below is after the bridge rectifier and is the only affected resistor in the output circuit.
R131 (2M2 r) Reads at 136k on the the 200k setting and will not read on 2M or 20M. I know it should be measured on the 20M setting so it must be bad too.

Basically, I can replace all the resistors and hope the problem does not reoccur. I am hesitant to do that but think it might be a way to get the amp working again. I bought this amp in this condition and have no idea what occurred to cause so many resistor failures. So, does anyone have any idea what could cause this problem and what steps are best to proceed on a fix? Note that these are not burnt resistors and when the amp is on there is not smell of smoke or overheating. Thanks for any advice.

[Prairie Dawg]

Have you checked the diode bridge?

[DrGonz78]

The bridge rectifier could have problems as well but have not done any tests on it yet. I have soldered the joints on it since they looked a bit cold. I guess I need to check it out but not sure what readings to look for... You have any ideas?

[Enzo]

First, please don;t change all of your resistors. They don;'t measure "right" because they are wired to other parts in the circuit. This causes parallel paths for your meter to measure. It cannot ignore them. Also, sometimes tiny bits of voltage remain in capacitors, and this interferes with the tiny voltage from your meter when it measures ohms.

When we measure resistors "in circuit" we have to keep this in mind. We may find a resistor open, but they almost never go lower in resistance, and a parallel circuit path will MAKE a resistor read low. Aging resistors usually measure higher. When we have reason to suspect a p[articular resistor, and want a true reading, we unsolder one end of it and lift it away from the board. Now, with one end free, it is no longer in a circuit, and the meter will measure its true resistance. AT least within the abilities of the meter.

If you pl;an to do any amount of this sort of work, you really need to have a decent meter. It doesn;t have to be A $200 meter, just one that works reasonably. And make sure the batteries in it are fresh enough. A 9v battery in a meter can be as low as 5 volts and the meter still functions, but I wouldn;t trust the readings then. I have a nice FLuke hand meter, and some HP bench meters. Good stuff. But Harbor Freight sells a cheap little meter for $10, and often have a coupon offering it for $2, and often enough it is even their free item. They always have something for free to get you into the store. I have gathered several of these things. While they are not the smoothest meters, they do seem to function and read close enough for basic work. I leave a couple out in the warehouse for continuity and resistance readings on speakers, and basic voltage measurements. SO FREE is pretty cheap. Point is that you can get a meter that works for not a lot.

[Prairie Dawg]

If I had to take a bet I'd probably change that bad boy. You can get them at Radio Shack. They benefit from installing a heat sink on them as well.