Marshall JMP MkII 50 - acting funny

Repairs, trouble shooting, modding and so on.

Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Thu Mar 20, 2014 8:52 pm

I have a 1976 Marshall JMP MkII #1987 50watt head, with master volume & effects loop mods. I've used the amp for 30 years without any major issues. I moved to a different amp 5-10 years and stored the JMP. I recently pulled it out & fired it up, and its working'ish. It powers up and plays. But it has some weird things happening, and I have to shutdown the amp with 30sec-2min:

1) One of the two 6550 is glowing really bright. Like the entire metal innards are glowing red. The other 6550 is glowing as expected. The bright glowing tube is really giving off a lot of heat. After warming the amp up, the mains transformer is very warm to the touch.

2) A huge buzz starts to get louder & louder over time. Coming off Standby, the buzz is minor & somewhat typical, then within 30 seconds to 2 minutes, the buzz starts to creep up, getting louder & louder. As the buzz gets louder & louder, the output tone starts to degrade & get badly distorted (not the good kind of distortion...!). It keep getting louder to the point I have to shut off the amp. This is not normal for this amp.

3) I measured plate voltage (pin 3) on both 6550's, and initially after coming off Standby, both are 420VDC for a brief moment, then begins dropping & dropping, as if it won't stop, till it settles around 325VDC. The rate plate voltage drops seems to be directly related to the increasing buzz. As plate voltage drops, correspondingly, the amp buzz increases. This does not seem normal.

4) Heater voltage is 3.1-3.3VAC for all tubes, for both Standby & On. Seems low...

5) Plate voltage is around 6VDC on both 6550's when in Standby.

4) Output transformer DC resistance is 40.8 & 42.6 [from CT (brown) to each leg (white & red)]. This seems pretty normal.

5) Controls all work (Bass, Mid, Treble, Master, Volume), except the Presence doesn't seem to do anything, though with 30 sec - 2 min before shutting down, I didn't get a lot of time to look at it in depth.

Other than the above, I don't think there are other issues, though its not usable as-is. I tried swapping out all the tubes with another set (6550's & 12AX7's), and though the replacements are used, I think it proves its not tubed related, because after swapping all the tubes out, the exactly same scenario occurred. So the problem seems to be with the amp itself.

Initial tube set in the amp (used):
-6550's: matched set of Groove Tubes (H6)
-12AX7's: Mesa Boogie

Replacement tube set (used):
-6550's: matched set of Groove Tubes (H5)
-12AX7's: Ruby's

For the hell of it, I swung the bias adjustment high & low, or hot & cold, to see of there was a tipping point on this issue, that either brought the amp back in line, or made it fall completely over. But it had little affect on anything I describe above. I checked the main 4-5 silicon diodes in the output section, and they all are good.

So this leads me to think I have some components failing, probably a leaky or failed cap(s), but I don't know. Resistors don't usually go bad unless they burn up. The big inductor (choke) and transformers (mains & output) are pretty robust & would have, I think, more catastrophic issues.

This leads me to caps... Based on the above, does my analysis seem on track? Is there something besides caps that could cause those symptoms? I believe I've eliminated the tubes & bias adjust as a possibility. If not caps, what could it be? If caps, which ones do you think would be the likely culprit? How can I test them?

Any other ideas or insights on what could be wrong?

Just looking for a sanity check, because I can't seem to get anywhere on this.

Thanks!!!
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Re: Marshall JMP MkII 50 - acting funny

Postby dai h. » Fri Mar 21, 2014 6:22 am

hi welcome,

if an amp hasn't been powered up for a significant amount of time, generally it should be safer to apply voltage gradually with the current limited. Specifically, (something like) pulling all the tubes to take the heavy loads out of the power supply and apply voltage through a 100k resistor while monitoring the voltage drop across the R (and power up incrementally using a Variac). This way, you give the aluminum electrolytics (which will unform if left idle and no voltage applied) a chance to re-form and function normally again. I'd probably also check the contacts (jacks, switches, socket pins) and clean as necessary. If you simply power up the amp, (I suppose you could be lucky, but) there could be things that go wrong such as the bias filters not working and control grids not making good contact which leads to excessive current draw and an "out of control" condition for a power tube (which probably won't help filters to reform), excess hum and buzz (from filters not able to filter out the ripple from the power supply) and so on.
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Fri Mar 21, 2014 7:22 am

I've already powered the amp up several times. Not with a huge load though. Only with the tubes in place, an 8 ohm cab, and at very low volume (barely on, master way below 1), and for 30sec-2min at a time. So it's too late to do a gradual power up w/o the tubes in place. So whatever cap damage that could have been done, has probably already occurred.

Based on the symptoms & voltages I read, what do you think the possible causes are? Based I on everything I reported, what could be wrong with the amp?
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Re: Marshall JMP MkII 50 - acting funny

Postby dai h. » Fri Mar 21, 2014 7:57 am

hi,

to be clear when I mention 'taking the heavy loads out of the power supply", I'm not referring to the speaker output but the loads on the power transformer windings.

As far as what is wrong with the amp, that is kind of hard to say (FWIW while I do know a bit about amps, I'm not a tech). The one power tube (metal innards cherry red?) could be toast (that is damaged to the point where it won't operate stably). Could be one or caps that aren't working. By the way, you might want to check the fuses just to be sure they are the correct value(s). Be safe.
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Sat Mar 22, 2014 5:25 am

Fuses are correct at 3A mains, .5A HT. It doesn't matter what tubes I put in, the symptoms are the same & the one tube starts glowing really bright red. I pulled the tubes and measured some things & use a DVM with a capacitance meter to check the caps. Here's what I found:

Cap values read with DVM

50uf+50uf on mains: OL + OL
50uf+50uf on power amp HT: 63uf + OL
50uf+50uf on preamp HT: 47uf + 66uf
(2) 10uf blue: 15uf + 22uf
(2) .022uf brown: .02uf + .02uf
(1) .22uf master volume: .24uf
(multiple) 22nf green: .02uf
(1) 330uf blue: OL
(1) 680nf yellow: 1uf
(1) .022uf brown: 0uf (no value read)
(1) 680nf yellow Presence cap: .84uf
(1) .005uf Treble film cap: 0uf (no value read, I think out of range of my meter)
(2) 250/500pf very small caps: 0uf (no value read, I think out of range of my meter)
(1) 470pf brown small cap: 0uf (no value read, I think out of range of my meter)

Voltages measured

(no tubes) Heater voltage, was at first 3.3VAC like before, but shortly after power up, popped up to 6.5VAC, which is more what I would expect
(tubes back in) Plate voltage on 6550 power tubes: Starts off at 410VDC, but immediately starts dropping and was at 320VDC when I shutdown.
Wasn't able to read bias voltage, was always around 0-1 VDC or VAC. No matter if bias was at min or max. Not sure where to read this value properly..?
Wasn't able to read screen voltage, was always around 0-1 VDC or VAC. Not sure where to read this value properly..?


Output Transformer DC resistance:

1 side 40.8 ohms
1 side 42.6 ohms

Inductor/choke: 106 ohms DC resistance

Something is causing the plate voltage to continuously drop, and it looks like some of the caps are bad...?

Any ideas? Do these cap values read look right, or am I right that (2) of the big 50uf+50uf caps, 330uf, and .022uf caps appear to be bad?

What about the bias & screen voltages? What should these be & where should I read them?

Thanks.
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Re: Marshall JMP MkII 50 - acting funny

Postby neikeel » Sat Mar 22, 2014 5:52 am

From your description your filter caps (all the 50.50 cans should be removed, disposed of and replaced with ARS/Ampmedic/F&T or similar.

Output tubes too

One output redplating like that also suggests a problem with your bias caps (also electrolytic) and we know your V1 cap is toast.

Ty that and report back.
Neil
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Sat Mar 22, 2014 6:48 am

Much appreciated neekeel!

Which ones are the Bias caps? And is the V1 cap the 330uf that shows OL?

What do the can replacements usually run?

No matter what output tubes I put it, the tube closest to the primary xfmr always glows too bright. So I don't think it's the tubes.
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Sat Mar 22, 2014 11:57 am

UPDATE:

I know the V1 330uf/10V Electrolytic is bad for sure. The 10uf/160V bias caps are reading 15uf & 22uf, so it looks like one is going bad for sure, I'll replace both. Since I have one tube location, regardless of which tube I put in it, that always glow bright red when in that one socket next to the PT, and when swapped to the other location doesn't glow red, could it be doing that because the one bias cap is at 22uf & failing?

I'm gonna replace all 3 of those electrolytics.

Regarding the big cans. I feel like a bonehead. I measured those value while they were wired into the circuit. They were reacting to the circuit impedance. I de-soldered all the Daly's & remeasured them. They all read pretty consistently about 60uf-63uf. Nothing is OL. I measured them on resistance, and they all have very high resistance in the M-ohm ranges. They look pristine and perfect. No bulging, discoloring, heat marks or anything. In fact they look brand new, but I guess that's just how Daly's look. But to me at all the cap taps on each can all read around 60uf & have high resistance. To me that seems like they're fine. What do you think?

Could it be that all my trouble & symptoms could be caused by one bad V1 cap & one bias cap?
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Re: Marshall JMP MkII 50 - acting funny

Postby dai h. » Sat Mar 22, 2014 5:22 pm

aluminum electrolytics have loose tolerances, so a 10uF reading 22uF is nothing extraordinary. Have you checked that the socket (in which the power tube always goes red) has bias voltage going to the pin (number 5)? (This will be a negative DC voltage in the tens of volts, around -35V or thereabouts.) Also, is that pin making good contact? Additionally, what about the master volume? Is everything along that path connected and functioning as it should be? Lastly, I'm not sure that a simple capacitance reading is a good indicator of whether an alu electro is good or not. You probably want to know if it's leaky and perhaps an ESR (Equivalent Series Resistance) might be useful to suss out a bad cap as well.
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Re: Marshall JMP MkII 50 - acting funny

Postby neikeel » Sun Mar 23, 2014 8:13 am

dai h. wrote:aluminum electrolytics have loose tolerances, so a 10uF reading 22uF is nothing extraordinary. Lastly, I'm not sure that a simple capacitance reading is a good indicator of whether an alu electro is good or not. You probably want to know if it's leaky and perhaps an ESR (Equivalent Series Resistance) might be useful to suss out a bad cap as well.


Of course Dai is right about checking the whole amp out.
Simply checking a cap with a meter is not sufficient, you need the correct gear Dai mentions (ESR and leakage).

In the absence of a tester, the age of the amp, the the amp has stood not being used and the symptoms you describe replacement is a pragmatic approach
Neil
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Sun Mar 23, 2014 4:39 pm

Dai/Neikeel: Thanks so much for both your feedback & advice.

Dai,
I'll check pin 5 on both tubes & the sockets, they do look pretty old & dirty. If the bias voltage is not as expected (around -35VDC), and remains consistent regardless of tubes are installed, what could or would likely be the cause?

The master volume appears to be fine. If functions properly & everything is connected. It is a mod though & not stock 1987 Super Lead. On 0, the amp has no sound. As I raise the volume level, the output correspondingly goes up as expected. Nothing appears out of whack with the master volume. What should I be looking for? Since its a mod, I'm not 100% sure if the circuit & component values are correct. I drew up the circuit to show you, but I don't see a way to attach a file on this msg board.

Will an ESR meter show if the cans are leaky or not? What' a good, yet inexpensive, ESR meter that can meter those 50uf+50uf cans? What range window would be considered 'good'? Say 40-60uf?
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Re: Marshall JMP MkII 50 - acting funny

Postby neikeel » Sun Mar 23, 2014 5:17 pm

kdude12345 wrote: I drew up the circuit to show you, but I don't see a way to attach a file on this msg board.

Will an ESR meter show if the cans are leaky or not? What' a good, yet inexpensive, ESR meter that can meter those 50uf+50uf cans? What range window would be considered 'good'? Say 40-60uf?


[img]your%20link%20to%20web%20host%20here[/img]

Most decent cap testers are expensive (why I have not got one!) eg:

http://www.ebay.co.uk/sch/i.html?_odkw=sencore+cap+tester&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xsencore+capacitor+tester&_nkw=sencore+capacitor+tester&_sacat=0

http://www.teknetelectronics.com/DataSheet/SENCORE/SENCOLC103.pdf
Neil
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Sun Mar 23, 2014 6:36 pm

Here's the Master Volume Mod drawing:

https://www.facebook.com/photo.php?fbid ... =3&theater

Image
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Re: Marshall JMP MkII 50 - acting funny

Postby dai h. » Mon Mar 24, 2014 6:21 am

kdude12345 wrote:Dai/Neikeel: Thanks so much for both your feedback & advice.

Dai,
I'll check pin 5 on both tubes & the sockets, they do look pretty old & dirty. If the bias voltage is not as expected (around -35VDC), and remains consistent regardless of tubes are installed, what could or would likely be the cause?


pin 5 is the control grid. This is where the signal comes in from the phase inverter. The negative voltage here sets the operating point of the tube. Pin 5 goes to what looks like a spiral coil inside the tube which surrounds the cathode. The negative voltage applied to the grid (from the bias circuit) acts as sort of a force field and allows you to control the flow of current from the cathode to the plate, sort of like a faucet handle mechanism allows you to control the flow of water coming out of a faucet. If you have an open connection at the control grid, that means there is no control of the current flow, so current will flow uncontrolled until something in that path gives out (fuse, power tube, OT winding, etc.). That is why I'm suggesting to check along that path (the pin, solder joints, back to the source of the negative voltage(bias circuit). (The "tube turning red" suggests it is drawing way too much current.) Something easy to do to try to ensure a decent connection is to gently move the tube in and out of the socket so the pin rubs the socket contact (my understanding is that oxidation can build up and impede the connection). A loose socket pin is also a possibility but if the amp was working, then it seems less likely. Also, note that some tubes have oversized pins (such as Tesla EL34), so they could conceivably "stretch out" the contacts and make things a bit loose for another tube (with the correct size pins).

The master volume appears to be fine. If functions properly & everything is connected. It is a mod though & not stock 1987 Super Lead. On 0, the amp has no sound. As I raise the volume level, the output correspondingly goes up as expected. Nothing appears out of whack with the master volume. What should I be looking for? Since its a mod, I'm not 100% sure if the circuit & component values are correct. I drew up the circuit to show you, but I don't see a way to attach a file on this msg board.


okay, that looks like a "Jose Master" ("Jose" as in Jose Arredondo). I was thinking you might have had a post-phase inverter type of master which is later on in the bias circuit and possibly something going on there but you have a different type of master.

Will an ESR meter show if the cans are leaky or not? What' a good, yet inexpensive, ESR meter that can meter those 50uf+50uf cans? What range window would be considered 'good'? Say 40-60uf?


you can buy one if you want, but it will probably be cheaper to use a series resistor, apply a voltage to a cap, and measure the voltage to figure out the leakage. There was also a cheap test I vaguely remember from Tom Mitchell's amp servicing book (IIRC) where a 9V battery was applied to a cap (that has been removed from circuit), and the DC voltage was observed (you hook up the DVM to the cap to measure DCV, then hook up the 9V battery, then disconnect it then observe if the cap holds the charge or not. (According to that test) if it won't hold the charge (that is, if the voltage drops very drastically and quickly), the cap is bad.) Note that caps can hold a charge even if you turn the amp off, so beware of this. (I've found that for 50W without "bleeder resistors", if I turn off the standby and keep the heaters on for a while (maybe a minute) the voltage bleeds off. You still want to measure and check though since it isn't fun getting shocked. And also, a power tube needs to be in for the bleeding off to take place, *I think*. Also REMEMBER TO TURN OFF AND UNPLUG THE AMP before working on it.)
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Re: Marshall JMP MkII 50 - acting funny

Postby kdude12345 » Mon Mar 24, 2014 10:46 pm

Thanks Dai. You have a discerning eye! That IS in fact a "Jose Master" ("Jose" as in Jose Arredondo) master volume mod & is (pre) phase-inverter, and Lee Jackson from Metaltronix did the 12AX7 driven effects loop.

I checked the bias path, the master volume path, and everything around the phase inverter. The other things I can see that are possible questions marls are:

1) The bias electrolytic caps

2) The 100K ohm pull up resistor coming off the resistor bridge at the phase inverter input, and on the other side of the presence pot, is measuring only 4K ohms. Seems very low for a 100K resistor. The only reason it could be that low is I'm measuring it in circuit, and there could be a very low impedance circuit load in parallel with the 100k resistor. It's labeled 47x or 47k with (100K) in parenthesis just under the 47x. Its is in fact a 100K resistor that's installed there.

3) I can't read DC resistance through the two .022uf caps feeding the phase inverter output from V3 into the power amp stage input (V4 & V5). This could be fine & normal, because they are caps.

4) The total resistance read from pin 5 of the output tubes to the connection on V3 phase inverter output those two caps connect to is around 1M-1.5M for both legs. This could also be fine & normal.

5) A side thing I noticed is the secondary side of the output xmfr, which connects to the speaker jacks via the impedance selector read nearly a short to each other and also to ground. Does this sound normal for the speaker side of the xfmr? Seems like awfully low resistance.

> Orange to Violet = .4 ohms
> Orange to Grey - .8 ohms
> Orange to Green = .6 ohms
> Orange to Yellow = .4 ohms
> Orange to Ground = 0.0 ohms

Could it be the bias caps causing the current to keep increasing & the voltage to keep dropping from 410VDC all the way down to 320VDC?

What else could be causing the current to keep increasing, besides the bias caps ?

How do I soecifically do this to assess the 50+50uf cans? ---> "it will probably be cheaper to use a series resistor, apply a voltage to a cap, and measure the voltage to figure out the leakage"

On the 9VDC battery test, do I just jump (+) & (-) of the battery directly to the 50+50uf cap can (+)n & (-) cap inputs? How long do I apply the battery to the caps? If the caps are good, should I just see 9VDC (or closer to that) across the caps? How long should the caps hold that 9VDC charge before they dissipate back down to nothing? I drained all the can caps using a resistor on added to the jumper clip.
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