JCM 2000 bias pins maxing at 15mV

[Leo_Gnardo]

If you guys have any other suggestions, I'm open minded to new solutions. But I have a feeling my only option is to try and do the drilling pin 5 out routine, and I'm not up to the task. Thanks guys.

So you don't have another annoying too high / too low test run again, check the bias voltage on pin 5, all output tubes before commencing. I'd recommend, dial the adjustment pots so that you read say -43.0V if you can get there, or as close as you can. Same bias V for each pair of tubes. Warm up your prize, read the bias on the test points, it will start out low then increase as you adjust the bias pots closer to -40V.

[Djizix]

So you don't have another annoying too high / too low test run again, check the bias voltage on pin 5, all output tubes before commencing. I'd recommend, dial the adjustment pots so that you read say -43.0V if you can get there, or as close as you can. Same bias V for each pair of tubes. Warm up your prize, read the bias on the test points, it will start out low then increase as you adjust the bias pots closer to -40V.

I've tried swapping tubes around and have gotten the same result every time I fire it up. And I'm not 100% how to test the bias voltage on pin 5 either. I'm confident that a new PCB would fix the problem and I know I have the ability to install it. But for the sake of knowledge, I'd love to know how to test the bias voltage in case I run into similar problems in the future.

[Leo_Gnardo]

I've tried swapping tubes around and have gotten the same result every time I fire it up. And I'm not 100% how to test the bias voltage on pin 5 either.

Clip your meter common probe on chassis ground. Put your meter measuring probe on pin 5. Turn on the amp. Read the meter. Now you are reading the voltage on pin 5.

I'm asking for readings on ALL the output tube sockets in case say you have the bias voltage at -45V for one pair and -35V for the other pair, that sort of thing would explain why you're getting widely different bias readings from the test points. It would make sense to start with the bias voltage matched and on the high side, that is further away from zero, then adjust to the appropriate level.

I fear that if you cannot make these kind of measurements you're over your head with or without a new board, and perhaps this task is best left to someone with more expertise. Where are you located, perhaps some kindly MEFster is within easy driving distance, you could take it there and get help.

[Djizix]

I fear that if you cannot make these kind of measurements you're over your head with or without a new board, and perhaps this task is best left to someone with more expertise. Where are you located, perhaps some kindly MEFster is within easy driving distance, you could take it there and get help.

I'm located in southern Illinois, at least 2 hours from any big towns with any known techs that I am aware of. Which is why I've tried to tackle this myself. I'm at technician full time and work with electricity regularly. I'm not flying completely blind, I'm just not as experienced with circuitry. Trouble shooting amplifiers is something I'm trying to learn simply because I'm a lifelong musician. Not to say I wouldn't take help, that's the reason for this thread.

That being said, I suppose when I said I wasn't 100% sure how to do it, I didn't mean I didn't know how to test voltage. That's pretty cut and dry. Perhaps though, I was measuring this bias voltage wrong because my reading wasn't a negative voltage when I attempted this.

So again, if I'm understanding this correctly, I'm to take one tube out at a time, ground to black, and red to the pin 5 insert of the extracted tube? And when and if I test it correctly, the bias pots will adjust this at an appropriate level of negative voltage?

[Leo_Gnardo]

So again, if I'm understanding this correctly, I'm to take one tube out at a time, ground to black, and red to the pin 5 insert of the extracted tube? And when and if I test it correctly, the bias pots will adjust this at an appropriate level of negative voltage?

OK then looks like we have enough confidence to press on, good . Also, you never know where us MEFsters are, scattered around the globe, places populated and others not so much (hear me you guys on Prince Edward Island?)

You could do it that way, or if chassis out of box, you could approach pins 5 from the back of the circuit board, with tubes in sockets, no need for removal. Of course being careful as I'm sure you know how, one hand in your pocket so you can't get a shock. Another of this amp's unexpected quirks, the bias trim pots adjust the pair of tubes - and test point - opposite the trim pot location. Another Marshall circuit trick to drive us crazy.

Perhaps though, I was measuring this bias voltage wrong because my reading wasn't a negative voltage when I attempted this.

Possible we're not square on the bias voltage = the voltage you measure at the control grids of the output tubes, pins 5, should be somewhere between say -37 and -42V once we're done and everything's working right. Not to be confused with the bias test point voltage, what you measure on those handy pins sticking thru the pan covering the bottom of the circuit board, there you'll see millivolts on the two outer pins, with the center pin common. Each millivolt corresponds to a milliamp of bias current. What you're measuring here is the voltage across a pair of 1 ohm current sensing resistors placed between ground and output tube cathodes, in pairs. When things are running right, I like to see about 70 mV on each test point though Marshall specs 90. My opinion, others may differ if they wish, 35 milliamps per EL34 is enough.

I hope this helps!

[DrGonz78]

So I tested the 22k variable resistors on the bias board to the best of my ability. I'm having trouble identifying which is positive and which is negative, but I'm fairly sure I have the neutral nailed down. Either way, I alternated the black lead accordingly and my readings were on one side, fluctuating around 11-22k and the other side was 14 ohms turned counter clockwise, and 25 ohms clockwise. Doesn't seem right, but I don't really know.

Something about this part of your observations here needs more understanding. If one side is 11-22k and the other bias trim pot adjustment is between 14-25ohms then something there needs to be looked at better. I guess this part left me confused a bit as to whether I understood it correctly. Then I read that one pair of tube is pulling a butt load of current and the other is barely pulling any. Of course measure your pin 5 bias supply voltages like Leo mentioned first. Still perhaps there is something off about that that Bias Board adjustment readings.

[Djizix]

Something about this part of your observations here needs more understanding. If one side is 11-22k and the other bias trim pot adjustment is between 14-25ohms then something there needs to be looked at better. I guess this part left me confused a bit as to whether I understood it correctly. Then I read that one pair of tube is pulling a butt load of current and the other is barely pulling any. Of course measure your pin 5 bias supply voltages like Leo mentioned first. Still perhaps there is something off about that that Bias Board adjustment readings.

Hmm, how to explain it...both pots measured the same. What I meant was when I'm on the positive side (I think) if fluctuates from 11-22k. When I'm on the negative side it measures the 14-25 ohms (14 when the pot it set to minimum and 25 when maxed)...I dont know what those resistances should measure at because I dont fully understand this variable resistor...it has a 22k max I'm assuming from the letters on it. But what Im calling negative shouldnt test at low resistance like 14-25 ohms should it.

Also tested pin 5. On the left pair I was able to get my bias voltage at -35v. This is the side that is biasing at 130mV. Yes I know the pins are backwards. The right side pair was at -48v. This side would only bias at 8mV. So I have no idea where tis leads me.

[Leo_Gnardo]

Also tested pin 5. On the left pair I was able to get my bias voltage at -35v. This is the side that is biasing at 130mV. Yes I know the pins are backwards. The right side pair was at -48v. This side would only bias at 8mV. So I have no idea where tis leads me.

That set of figures, makes sense. Given those bias voltages we'd expect the tubes to act that way. It looks like there's something amiss with the bias control card, maybe one or both of the pots are kaput. "Fluctuates" doesn't sound healthy, and a pot that ranges from 14 to 25 ohms only doesn't sound good either.

[Jazz P Bass]

Here is a schematic of the 'bias' board.

jcm2-64-02.pdf

If you want to test the controls, unhook the connector at the main board.
At the bias board header, measure the resistance from pin 1 to pin 2 & from pin 1 to pin 6.
When you rotate the adjustments, the value should change from zero ohms at one end of the adjustment to 22K at the other.
Both controls should read the same.
Reading across pin 1 and pin 7 should be double the value of each 22K control. (they are in parallel).
The adjustment screw has no affect on this reading.

[Djizix]

I will tear her open and test as soon as time allows. In the meantime though, if I do have shot variable resistors, would anyone have any idea where to get a matching set? Id prefer to not have to mod this tiny little chip. Also, does anyone know if a new pcb would come with a new bias chip?

[Leo_Gnardo]

I will tear her open and test as soon as time allows. In the meantime though, if I do have shot variable resistors, would anyone have any idea where to get a matching set?

Mouser's my go to parts place. I narrowed the selection somewhat but you'll have to pick the trimpot that has the approprate pin locations for the Marshall board. Here's a selection from 20K to 25K.

I'm sure Digikey, maybe Newark or Allied will also have these or others like them.

http://www.mouser.com/Bourns/Passive...yzbppyZ1z0z54u

[Djizix]

Here is a schematic of the 'bias' board.

[ATTACH]42619[/ATTACH]

If you want to test the controls, unhook the connector at the main board.
At the bias board header, measure the resistance from pin 1 to pin 2 & from pin 1 to pin 6.
When you rotate the adjustments, the value should change from zero ohms at one end of the adjustment to 22K at the other.
Both controls should read the same.
Reading across pin 1 and pin 7 should be double the value of each 22K control. (they are in parallel).
The adjustment screw has no affect on this reading.

They tested fine, so I suppose its time to get the new pcb on order. Has anyone had any trouble with the reissue that theyre selling now?

[DrGonz78]

https://www.amplifiedparts.com/produ...acement-dsl100

That is the source that I know of to replace the main board on the DSL100. Pretty expensive but it does help rebuild your confidence in the reliability of the amp again. Did you every measure bias voltage at pin 5? Also, it is wise to check all the output speaker jacks to see if any are worn out and open. The jacks are supposed to be closed circuit when no plug is inserted. I have one here that one of the 4 ohm jacks tip is open and will not mechanically close any more. That part needs to be investigated either way if you buy a new board or not since your still going to be using the output board in the amp.

[Leo_Gnardo]

https://www.amplifiedparts.com/produ...acement-dsl100

That is the source that I know of to replace the main board on the DSL100. Pretty expensive but it does help rebuild your confidence in the reliability of the amp again.

Ambiguous answers as of 8 Nov 2016, apparently out of stock, they're expecting to have them by the end of May. Woops check that, another answer says they expect to have them in 2 or 3 weeks, that was 1 Dec 2016. Better call to find out for sure.

Otherwise, excellent find DrGonz! And in your neighborhood too. If they're in stock, sure beats sending away to England, in hassle, time waiting for delivery and $$$.

Antique and CE customers take note, same part #. What a coincidence!

Also, it is wise to check all the output speaker jacks to see if any are worn out and open. The jacks are supposed to be closed circuit when no plug is inserted. I have one here that one of the 4 ohm jacks tip is open and will not mechanically close any more. That part needs to be investigated either way if you buy a new board or not since your still going to be using the output board in the amp.

??? Closed circuit, as in shorted like Fenders? I know there's a feature in the output jack field that DISconnects the 4/8 ohm jacks if there's a plug inserted in the 16 ohm jack, intended to prevent users from driving speakers from both taps at once. And how many times I've found the switch built into the common electrodes of the 16 ohm jack with its contacts roasted, no longer making a good connection to the 4/8 jack pair, one more quirk to beware of in this series of amps.

It's been said before, so I'll say it again. If this series of JCM2000 amps were cars, Marshall would have to conduct a recall program and fix their problems at no cost to the customer. Add that to your aggravation list folks. Grrrrr......

[DrGonz78]

??? Closed circuit, as in shorted like Fenders?

Sorry my wording was confusing there, I really meant Normally Closed (NC). Just that the normally closed latching switch part of the jack gets worn to point that it does not latch closed any more.