Slightly OT - Marshall JMP-1 blows fuses

[shredgd]

Hi guys,

my Marshall JMP-1 is driving me crazy, because it has blown its fuse three times now, even though it does not do that immediately after I change it but after several hours (and meanwhile the preamp works perfectly, by the way).
I definitely want to fix it myself: considering my stompbox-builder skills I can't spend money to get the job done by someone else!

I already excluded a tubes problem, because the DMM reads about 150mA across the fuse slot (the fuse is a 80mA one) with another pair of tubes as well.
Voltages are also ok everywhere I could test, i.e. at the +15v, -15v and +5v after the voltage regulators, and voltage drops are ok across plate and cathode resistors.

Schematic is here:

http://www.drtube.com/schematics/marshall/jmp1-61-04.pdf

I planned to start by replacing the 6 main electrolytic caps in the power section (C14, C15, C16, C17, C28, C57), do you think it is a good idea, too?
I just wanted some opinions.
Unfortunately it is not that easy to work on it, because everything is board mounted, even the power transformer, so I can't do the typical "disconnect everything from the secondaries and connect one at a time to see where the problem is" troubleshooting. That's why I thouht about a "trial and error" method, starting with the power electros, which are often the culprits based on what I read around the web.
Thank you for your suggestions.

Giulio

[WelshWonder]

I would like to know this as well. Got a problem with a solid state marshall. Maybe the same kind of problem.

[Cardboard Tube Samurai]

I had this happen with my old JMP100. Turned out it was the output transistor... it had died. Unfortunately, I just replaced the fuse a couple of times, but this just allowed it to completely fry the whole thing, inlcuding the input transistor. Needless to say, it cost me a LOT to get fixed, but I got all the big caps and valves replaced at the same time and it is now the nicest sounding amp I have ever heard... unfortunately it is too loud!

[MKB]

Hi guys,

my Marshall JMP-1 is driving me crazy, because it has blown its fuse three times now, even though it does not do that immediately after I change it but after several hours (and meanwhile the preamp works perfectly, by the way).
I definitely want to fix it myself: considering my stompbox-builder skills I can't spend money to get the job done by someone else!

I already excluded a tubes problem, because the DMM reads about 150mA across the fuse slot (the fuse is a 80mA one) with another pair of tubes as well.
Voltages are also ok everywhere I could test, i.e. at the +15v, -15v and +5v after the voltage regulators, and voltage drops are ok across plate and cathode resistors.

Schematic is here:

http://www.drtube.com/schematics/marshall/jmp1-61-04.pdf

I planned to start by replacing the 6 main electrolytic caps in the power section (C14, C15, C16, C17, C28, C57), do you think it is a good idea, too?
I just wanted some opinions.
Unfortunately it is not that easy to work on it, because everything is board mounted, even the power transformer, so I can't do the typical "disconnect everything from the secondaries and connect one at a time to see where the problem is" troubleshooting. That's why I thouht about a "trial and error" method, starting with the power electros, which are often the culprits based on what I read around the web.
Thank you for your suggestions.

Giulio

What is your line voltage?  The fuse is rated at 80mA for 230VAC, and 160mA for 115VAC.  If your line voltage is 115VAC, you should use a 160mA fuse.  That might be your problem, especially as you read 150mA current draw when it is operating.

[shredgd]

What is your line voltage?  The fuse is rated at 80mA for 230VAC, and 160mA for 115VAC.  If your line voltage is 115VAC, you should use a 160mA fuse.  That might be your problem, especially as you read 150mA current draw when it is operating.

I'm in Italy, so 230VAC, 80mA fuse.

[MarcoMike]

Hi, I don't really have an answer to your problem, I just wanted to wish you a patriotic "good luck"   
Then... IMHO those capacitors are not involved in your trouble... I mean... it's kind of a "new" amp, isn't it?
Why don't you measure the current absorption in the "post-transformer" fuses? if they are all ok, then it may be the transformer leaking somewhere... if not (if the current passing through one of them is much higher than expected) you may at least understand which part of the circuitry is sucking current... step ahead. (just a guess)
And, as I've been told just a couple of days ago.... be careful when working with HT caps!

ciao!

[shredgd]

Then... IMHO those capacitors are not involved in your trouble... I mean... it's kind of a "new" amp, isn't it?
Why don't you measure the current absorption in the "post-transformer" fuses? if they are all ok, then it may be the transformer leaking somewhere... if not (if the current passing through one of them is much higher than expected) you may at least understand which part of the circuitry is sucking current... step ahead. (just a guess)
And, as I've been told just a couple of days ago.... be careful when working with HT caps!

ciao!

I have just replaced those capacitors and, as Marco predicted, nothing has changed, still 160mA of current draw (and another blown fuse, by the way...)

Unfortunately, that one is the only fuse in the preamp, there are no post-transformer fuses. To be exact, if you look at the schematic there are two resistors (R85 and R86) which have "fusible" written underneath; however I measure a correct voltage drop across them, so I have excluded them from the potential culprits, am I correct?
Should I simply unsolder one end of them, instead, and measure the current draw?

Any suggestion? I don't think replacing all those 22u/25v caps after the voltage regulators is worth the hassle, is it?

So even the transformer may be leaky? I really wish it doesn't turn out to be my case!!!

Thank you,

Giulio

[MKB]

Unfortunately I'm afraid you have a bit of a tough repair there.  Here's a few suggestions:

First, check and see if anything is getting hot, i.e. unusually hot, especially the regulators (IC5, IC6, IC7, IC8).  If one is getting really warm maybe something it is driving is breaking down.

Another check would be current drops in the various power supply sections; you can measure voltage drops across R85 or R86 for that section,  across R37 for the HT.  The digital section would be a bit harder I'm afraid.  These should be measured when the preamp is cold and is operating correctly.  Then when it is hot you can see if any of the current draws are increasing.

Check connector CN4 for any brown discoloration, if you have a dirty connector it could get hot and cause problems.  You can even get a type of thermal runaway in some power supplies (resistance in contact causes a voltage drop, power supply increases current to compensate, higher current increases voltage drop across contact, higher heat increases contact resistance, and on and on), but this is usually with switching supplies.  You can disassemble and clean most connectors if you're careful.  Also check all the solder connections in the power supply, especially on the power transformer and line voltage components.  Any brown discoloration on the PCB in the power supply section could indicate a cold solder joint there as well.  In any case, good luck!!

[Ronsonic]

Have you checked the bridge rectifier? It's been some years, but I remember finding one with a faulty diode bridge. That could cause what you're describing. Are the caps getting hot?

Ron

[shredgd]

I did some measurements (please look at the schematic):

- the current draw across the fuse slot starts at about 160mA as soon as I turn the preamp on, then lowers and settles around 140mA in about one minute.
- voltage drop across R85/R86 is from 31.7VAC (i.e. the value for that transformer secondary) to 29.6VAC at the input of bridge BR1.
- BR1 takes the voltage to 32.7VDC, so I have 16.4VDC across C16, -16.3VDC across C17.
- voltage regulator IC5 transform the voltage to 14.6VDC, while IC6 does that to -15.0VDC

- the HT secondary is at 57.5VAC, which become 74.4VDC right before R37, with 71.5VDC after R37.

- the LT secondary is at 10VAC, which become 10.5VDC after bridge rectifier BR2. Voltage regulator IC7 gives 4.97VDC at its output, IC8 gives 4.98VDC

- if you look at page 3 of the schematic, you can find R200 and R201: these two resistors transform 14.72VDC to 5.66VDC and -15.04VDC to -5.60VDC respectively. This big voltage drop is completely normal as you can read on the schematic, but it impressed me how these two resistors account for about 20mA current draw each!

After these measurements I checked for warm/hot components: the voltage regulators kept room temperature, as well as the electros I replaced yesterday. The transformer gets fairly warm, even though not really hot, but that's what any power tranformer does, isn't it?

So it seems to me at least there is no voltage/current overloading to the following circuit, or are my conclusions too quick?
Could it really be the transformr now? Or should I exclude/measure something else?

Considering, as I said before, that the preamp works perfectly (until it burns the fuse) and that it looks like one components is "simply" leaking current to ground, what about raising the fuse value and forget about it?
I know this is totally unorthodox, but...

Giulio

[Ronsonic]

Looking at those, your +/-16.3ish volts on the input of a 7X15 regulator is low.

The HV supply looks very low, that should be in the hundreds of volts. Do check those diodes.

That LV supply using resistors and zeners (R201,200 - D13, 14) that cook the PCB is pretty typical construction.

This isn't a unit with a history of having an under-rated fuse. Most of these live in racks where they stay for decades without trouble.

Ron

[shredgd]

Looking at those, your +/-16.3ish volts on the input of a 7X15 regulator is low.

The HV supply looks very low, that should be in the hundreds of volts. Do check those diodes.

That LV supply using resistors and zeners (R201,200 - D13, 14) that cook the PCB is pretty typical construction.

This isn't a unit with a history of having an under-rated fuse. Most of these live in racks where they stay for decades without trouble.

Ron

Good observation, the voltage at the input of those regulators is definitely low. I'm going to replace BR1!

Regarding the HV supply voltage, considering that quite little power transformer, I think this preamp is not built to "cook" those tubes, so I am assuming "High Voltage" is just to indicate "higher than the other secondaries"! Also, I should have experienced a volume loss if from hundred of volts the plates of my tubes suddenly received 70 volts. Or am I wrong?

My idea of raising the fuse value was just a "symptomatic" remedy to a suspected but unidentifyable leaky component, I didn't intend it to be underrated. I was simply prone to the leaky transformer hypothesis and had no intention to bother searching for a replacement, etc (unless it fused, of course...).

But thanks to your observation maybe I'll cure this baby just by replacing that tiny bridge. I'm crossing my fingers!

Giulio

[MKB]

Regarding the HV supply voltage, considering that quite little power transformer, I think this preamp is not built to "cook" those tubes, so I am assuming "High Voltage" is just to indicate "higher than the other secondaries"! Also, I should have experienced a volume loss if from hundred of volts the plates of my tubes suddenly received 70 volts. Or am I wrong?

Judging from the voltage rating of the first filter cap on the HT (C15, 10uF @ 350V), I would expect the voltage on the cap to be around 275-300VDC or so.  That would make the HT transformer winding around 210VAC or thereabouts.  58VAC is WAY too low.  It might be best to completely replace D4-D7, perhaps first remove them completely and check the HT winding voltage with no load.  If you still have a low HT with no load, that might be your problem (perhaps an internal short in the PT).  If the no load HT voltage looks OK, then replace D4-D7 and disconnect one end of R37, then check the voltage across C15 (Note: don't run it long with R37 disconnected as you might exceed C15's voltage rating).  The HT circuit should be easy to troubleshoot as the current requirements aren't very high and the circuit is very simple.

If the HT winding is bad, remove D4-D7 and see if the preamp will run correctly (except for the tubes of course) and the operating currents go back to normal.  You could replace just the HT winding with an additional transformer, small 1:1 transformers are available and maybe you can find space to fit one in the case.  That might keep you from having to replace the entire PT with a Marshall part, which might be expensive.

[Ronsonic]

Looking at those, your +/-16.3ish volts on the input of a 7X15 regulator is low.

The HV supply looks very low, that should be in the hundreds of volts. Do check those diodes.

That LV supply using resistors and zeners (R201,200 - D13, 14) that cook the PCB is pretty typical construction.

This isn't a unit with a history of having an under-rated fuse. Most of these live in racks where they stay for decades without trouble.

Ron

Good observation, the voltage at the input of those regulators is definitely low. I'm going to replace BR1!

Regarding the HV supply voltage, considering that quite little power transformer, I think this preamp is not built to "cook" those tubes, so I am assuming "High Voltage" is just to indicate "higher than the other secondaries"! Also, I should have experienced a volume loss if from hundred of volts the plates of my tubes suddenly received 70 volts. Or am I wrong?

My idea of raising the fuse value was just a "symptomatic" remedy to a suspected but unidentifyable leaky component, I didn't intend it to be underrated. I was simply prone to the leaky transformer hypothesis and had no intention to bother searching for a replacement, etc (unless it fused, of course...).

But thanks to your observation maybe I'll cure this baby just by replacing that tiny bridge. I'm crossing my fingers!

Giulio

Why replace the low voltage bridge? If there were something wrong with it you would have a very different problem, like one side missing or the caps getting hot, etc. Whatever is pulling down that voltage is probably elsewhere. Here's why I say that: your LV supply is a couple of volts low, maybe 20%. Your HV supply is about 200 volts low!! Marshall (Korg) doesn't waste money on caps, if they're rated at 350V it's for a reason.  There are a few possibilities, check the diodes first also make sure C27, 36 aren't shorted.  But you've definitely got a problem in the HV supply. See if it pulls back up if you remove the tube.

Ron

[shredgd]

Poor me!! 

I lifted one side of D4-7: the now unloaded HT secondary is still only giving me 60VAC!
This means my transformer has an internal short!
This really sucks! It will take me ages and a lot of money to get a replacement from Marshall!

If the HT winding is bad, remove D4-D7 and see if the preamp will run correctly (except for the tubes of course) and the operating currents go back to normal.  You could replace just the HT winding with an additional transformer, small 1:1 transformers are available and maybe you can find space to fit one in the case.  That might keep you from having to replace the entire PT with a Marshall part, which might be expensive.

Unluckily the current draw is still 130mA! That's to add... to ... (what did the proverb say in English?...)! 

Anyway the patience you all had in reading this and losing your time for me is really priceless!
You are generous people, thank you!!

Giulio

[MarcoMike]

Where do you live in italy? You may find a "transformer producer" (I know one here in Modena) who may reproduce a PT following your requirements. I know original marshall tr. are around 150 euro. I guess it's gonna be less than 50 in this diy (or "do it himself") way...

[shredgd]

Vivo a Perugia.

The problem is the JMP-1 is not a conventional Marshall amplifier. It's an hybrid rackmount preamp, so it's got a PCB-mounted and quite small transformer which needs an exact replacement. In fact I'm afraid I won't even find a replacement, I did a quick search over the web and only big hammond-like transformers for Marshall heads came out. If I was VERY lucky I would find an used and broken (for other reasons than the PT...) one on ebay and I would cannibalize it...

Anyway, thanks for your suggestion, I will keep it in mind if I don't find an original replacement. Maybe he can "simply" replace the windings if I give him the "metal"?

[MetalGuy]

Judging from the voltage rating of the first filter cap on the HT (C15, 10uF @ 350V), I would expect the voltage on the cap to be around 275-300VDC or so.  That would make the HT transformer winding around 210VAC or thereabouts.  58VAC is WAY too low.

You have an obvious problem in the high voltage section! You should have approx 300Volts after rectification.
The power transformer can be replaced but you'll need to custom order it. Since you're from Italy I would recommend you to contact your famous Hi-Fi tube amp builder  Andrea Ciufolly - I hope he'll be able to help you. For contact info checkout his page at:

http://www.audiodesignguide.com/

[MKB]

Sorry to hear about the overloaded PT, that has to be the hardest thing to fix.  Unfortunately even if you disconnect the HT winding, it will still load down the transformer due to your primary current remaining the same, so your fuse problem might remain.  It's a little strange that the PT doesn't get hot due to the internal short.  You have three secondaries; the 200V HT, the +5V LT winding, and the +/- 15V analog winding.  The +5V LT and +/- analog supply windings are most likely separate to provide proper grounding isolation so you don't get digital nasties in the signal chain.  Unfortunately the best and easiest (and perhaps most expensive) solution would be to contact Marshall and try to get a replacement.  It might be difficult to find an aftermarket replacement transformer, and adding separate transformers to make up the voltages you need might hack up the unit too much.

[shredgd]

Actually the transformer got pretty warm quite quickly, maybe it would have gotten definitely hot if I waited some more (but I didn't, because I still hadn't made my diagnosis, I was afraid I would burn some valuable component).

The good news of today is I called the main Italian authorized Marshall repairer and I can have the PT (actually I've already ordered it, and they have it in stock) for 68 euros + 12 euros for shipping, payment on receiving. As I've seen classic Marshall heads PTs priced around 125 euros (excluding shipment), I'm feeling quite lucky mine is a JMP-1...

What scare me a bit, now, is succeding in pulling out the old PT from te PCB. I think I'll have to patiently and repeatedly use my solder pump until all those solders are as clean as possible: pulling out a capacitor by touching one contact at a time with my soldering pen while rocking the cap is definitely easier... But I once removed the 4558 from my SD-1 to socket it, so I'm optimist 

Thanks again for your support.

Giulio