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Author Topic: Frying resistors on a Marshall 120/120 Valvestate  (Read 5140 times)

CBD

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Frying resistors on a Marshall 120/120 Valvestate
« on: August 11, 2011, 08:46:41 PM »
One side works fine but when I plug the speaker cord to the speaker out on the other side it fries  R87,R88.
I originally replaced R87,R88,R89, D36 and TR27 all with the correct values.( they were toast)
The TR27's outer casing was broke so I am not sure if it was bad but I replaced it none-the-less
Something made them blow originally but I can't see anything else that looks bad.
I do not have a scope....and short of testing everything....is there any tell tale sign that points to something that I should focus on?

EDWARDEFFECT1

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #1 on: August 11, 2011, 10:04:51 PM »
sounds like you might have a shorted capacitor.it will cause parts to burn. had the same problem on a peavey classic 30 tube amp...should be able to find it with an ohm meter or cap tester.when voltage goes to ground it's not good.you start burning parts........
« Last Edit: August 11, 2011, 10:07:42 PM by EDWARDEFFECT1 »

J M Fahey

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #2 on: August 11, 2011, 10:37:00 PM »
Too tired now, going to sleep, but be warned that toast parts are often dead ... and perfect looking ones too.
You did right so far but sure have more bad parts, but don't worry, we'll find them .
I guess you have at least a decent multimeter and a good solder sucker/pump.
Search (here or Google) for "bulb lamp limiter" and build one, it's a simple fixture which will help you.
Have a couple lightbulbs handy, 40 and 75W are fine.
Good night.

EDIT: just go to http://www.ssguitar.com/index.php?topic=2093.0
« Last Edit: August 11, 2011, 10:39:19 PM by J M Fahey »

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #3 on: August 12, 2011, 11:25:00 AM »
Thanks for the help.
I will make one right away

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #4 on: August 12, 2011, 01:42:38 PM »
I made the Light Bulb Limiter and it turned out fine.

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #5 on: August 12, 2011, 08:21:55 PM »
Too tired now, going to sleep, but be warned that toast parts are often dead ... and perfect looking ones too.
You did right so far but sure have more bad parts, but don't worry, we'll find them .
I guess you have at least a decent multimeter and a good solder sucker/pump.
Search (here or Google) for "bulb lamp limiter" and build one, it's a simple fixture which will help you.
Have a couple lightbulbs handy, 40 and 75W are fine.
Good night.

EDIT: just go to http://www.ssguitar.com/index.php?topic=2093.0

One other thing I noticed when I powered up the amp before....The clipping light stayed on on the right channel even with no signal.  (the same channel that fried the parts

J M Fahey

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #6 on: August 14, 2011, 12:00:34 AM »
Dear CBD, didn't forget about you, Fridays and Saturdays swarms of those funny people called Musicians visit my shop overexcited claiming they *need* their stuff for some kind of Ceremony or whatever they do on weekends.
We'll continue when I have some time free.
Your answer will take a few back and forth questions and answers to find what else is broken, besides the clearly visible.
Good luck.

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #7 on: August 14, 2011, 12:05:31 AM »
Thanks! I appreciate any help as I am Kinda new at repairs.
But a Quick learner.

teemuk

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #8 on: August 14, 2011, 11:12:08 AM »
The "clipping" LED becomes illuminated when signal at speaker output exceeds a certain voltage threshold. Likely one or several of the power transistors are shorted, forcing the speaker output to DC potential of one of the supply rails. This would keep the clipping LED illuminated constantly.

Is R71 open? That could force all the short circuit current to flow through R87 and R88 and the transistor...

With violent failures like this it's almost always a case of failure of one or several power transistors, likely also coupled to similar failures of driver transistors as well as components within (eg. such as the current limiter circuit that fries in your amp). Persistently check them (even components that look ok) and I'm pretty sure you should find the culprit(s).
« Last Edit: August 14, 2011, 11:15:51 AM by teemuk »

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #9 on: August 14, 2011, 02:09:42 PM »
Yeah I am getting NO reading on R71

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #10 on: August 24, 2011, 09:58:43 PM »
Any more thoughts on this?
I will start with replacing R71 -but I am not sure how to properly check the transistors

teemuk

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #11 on: August 25, 2011, 06:50:35 AM »
Collector-Emitter should be a very high resistance value, closer to infinite, this will verify that there is no collector-emitter short circuit.

Base-Emitter junction should behave like a diode, high resistance one way, low resistance the other way. Orientation depends on polarity.

Base-Collector junction should also behave like a diode. Orientation again depends on polarity.

You have to lift the transistors at least partially off the circuit to get accurate readings, otherwise parallel circuit parts and sections will skew the measurements.


If in doubt, you can practice with some spare transistors you have laying around and that you know are working. The test results should be similiar for all transistors regardless of their rated power dissipation.

In some cases transistors may test fine off-circuit but will actually fail under stress when highish voltages are applied and high currents are conducted. This is something for that light bulb limiter thing you built to reveal. (Unless you want to build a test jig that loads the transistors, which IMO is usually just too much hassle).

Once you tested that everything should be allright, power on with the limiter in the mains circuit and if the amp is drawing too much current then you either missed something or the transistors you tested to be intact are in fact failing under loading. (The probability of just missing something in the torubleshoot+testing process is somewhat higher so I wouldn't immediately assume that culprit are the transistors). Anyway, quick measurements of DC conditions should then pinpoint the problem and the parts that actually are culprit. Likely this will not happen but if it does you'll be thanking the light bulb limiter of saving you from the trouble of doing the entire troubleshooting + repair process again from the scratch.

When in powered circuit, the base-emitter voltage difference should be always equal to one diode forward voltage drop (or in case of darlington transistors, two). This may depend on device, silicons measuring around 500mV - 700mV and germaniums about 200mV-400mV. In case of darlington device, multiply with two. Drastically less or more is a sign of a failed transistor.
« Last Edit: August 25, 2011, 07:10:17 AM by teemuk »

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #12 on: August 25, 2011, 06:39:42 PM »
OK now it will power up and even play without the clip light staying on.
But that same channel that had the fried parts does clip well before the "a" channel still.
I did replace the resistor(r71) so at least I am not frying anything now.
It sounds a little like it is rising and falling in volume when I try just that channel.

CBD

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #13 on: October 31, 2011, 08:40:18 PM »
Does anyone have any further advice or recomendations on this?
any help would be appreciated
 
 

phatt

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Re: Frying resistors on a Marshall 120/120 Valvestate
« Reply #14 on: November 01, 2011, 05:02:56 AM »
CBD,
I'd just replace all actives on that Pwr Amp section and the Zener as well.

And then *with light bulb limiter* fire it up and see if it tries to fry parts.

IMHO,, I've never relied on the current limiting of SS pwr Amps.
 (all the parts surrounding Tr27 Tr19)

I use PTC devices on all my SS power Amps which saves on pointless PCB tracks and less silly stuff to solder.

The circuit you have adds 9 extra bits to pcb and may or may not actually work. :o

It's a 50/50 bet whether the current limiting setup will save the Amp or not ,,, in your case those parts may actually be causing the failure. 

In truth,,, *IF*  you are 100% sure the speakers will never be **shorted out** then all that safety stuff on the PCB is not even needed,,, but with Music Amps that is often a big IF. LOL.

Hopefully others here know more than myself and They will likely see something that my limited mind will have missed.
Phil.


« Last Edit: November 01, 2011, 07:05:00 AM by phatt »