tubeworks mosvalve 500 killer hum

[ilyaa]

got one of these from a friend to fix

its the mosvalve 500 (942)

i havent done anything yet but the problem as my friend has described it is:

turn it on, VERY loud hum, turning gain all the way down does nothing - hum does not stop.

would checking DC volts on the output be the right place to start, assuming everything inside looks normal once i open it up? is that what normally produces an unstoppable hum? or would DC getting to the output create other symptoms?

[DrGonz78]

So this hum is very audible, but is it passing signal? Don't bother trying to test for signal again if you did not already. I would disconnect it from the speaker and definitely test to see if there is DC on the output. Better safe than sorry.

[ilyaa]

im also having a hard time finding a good schematic. the only one around is illegible.

there are no tubes in this thing, so i assume no output transformer. is it safe to turn on without a load?

heres the schematic:

http://img.photobucket.com/albums/v202/xr3drumx13/MV500Schematic.jpg

(i know how can you read that?!?)

whats that coil thing on the output? a relay?

[Roly]

Yeah, well any amplifier called a "MOSvalve" is having a bit of an identity crisis.   :duh

Any amp that doesn't have an output transformer, and that's the vast majority of S.S. amps, can be operated without a load.  So yeah, you're cool with this one.

The "coil thing" lower right is a relay controlled by a speaker protector circuit (to its left) that only connects the speakers if there is no DC present.

A really loud hum normally means a boofed output device or three (but if you can hear it that means either that the speaker protection isn't working or the hum is originating earlier).  Yes, check for DC offset on the outputs.

[J M Fahey]

1) build a lamp bulb limiter
2) plug amp into it, turn on, without speaker or any load connected.
3) check for presence of DC voltage at speaker outputs, then all rails (+/-  A/B/C/etc.)
Post results.

EDIT: the schematic is barely readable, most values can be seen and the few which don't, can be guessed, it's somewhat standard for a MosFet power amp.

[ilyaa]

plugged it in with a limiter:

initial power on the limiter glowed really bright just for a second - i guess kind of a surge of current - and then quickly went down to just faint reddish glow. after that if i powerd on/off it did not light up bright again, just stayed faint reddish glow.

with limiter inline, i get between 100 and 200 mV on the speaker outs (normal), with occasional spikes up to around a Volt (is that normal)

if i plug the amp in WITHOUT the limiter, i get 0 V on the speaker outs.....

A+ im getting +60
A- im getting -40 (!)

B+ im getting +16
B- im getting -14 (!)

its hard to read the schematic, but im assuming A+/A- should be equal but opposite polarity riiiight?

ill post the other voltages later because ill have to spend some time with the schematic to figure out exactly where they are.....anyone every seen a more legible version?!

[Roly]

The circuit has that lumpy look like it has passed through a fax machine at some stage.

initial power on the limiter glowed really bright just for a second - i guess kind of a surge of current - and then quickly went down to just faint reddish glow. after that if i powerd on/off it did not light up bright again, just stayed faint reddish glow.

That initial bright glow will be the main power supply caps charging up - expected.

The fact that it stayed dim over on/off cycles suggests that the caps are remaining charged and that therefore there is no excessive current draw, which looks good for the output stage.

Yes the power supply rails should be equal, so give the power supply a close going over.  If you can disconnect the power supply from the rest of the amp you can then run the power supply alone and see if reasonable voltages are restored.

The symptoms so far (loud hum, no excessive current, low -ve volts) might be consistent with the filter cap on the -ve rail going open circuit, either internally but more likely at one of its solder joints.

The output voltage jumping up and down (with the volume at zero) is not normal.  You may have an intermittent such as a dry solder joint, so with the Limiting Lamp and your meter on the output try tapping around the power amp with a chopstick o.n.o. and see if the fault is mechanically sensitive.

[ilyaa]

wow you guys got it!

one of the -ve filter caps had de-soldered itself. when i measured across it after taking the board out, i got a little spark and 50 volts (amp off). i discharged it, re-soldered it, and now im getting +/- 60 or so volts on the rails and a steady -24 mV on the output.

only thing is, plugged in with a limiter, the bulb glows red - dull but not as dull as it was glowing before. before i could just see the filament glowing, now its glowing bright enough to make the whole bulb kind of dim red. does that indicate too much current or is it okay? should i plug it in for real and see if it works?

[J M Fahey]

Dull red or red-orange is fine.

[Roly]

wow you guys got it!

Some of us have been at this game for a very long time now - musta learnmed somthin. 


Go back and follow the logic.

- VERY loud hum
AC gets to the power stage either because a device is drawing excessive current, or a loss of filtering.  "VERY loud" suggests the latter.

- turning gain all the way down does nothing - hum does not stop.
So it's certainly not the preamp, must be in power amp/power supply area

- you can hear it
i.e. the speaker protection isn't operating, again suggesting that there isn't a DC offset at the output, in turn suggesting the OP devices might be okay.

- "occasional spikes up to around a Volt"
It's intermittent, suggesting it's physical and not a boofed device

- "0 V on the speaker outs"
That's why the protection isn't firing, again suggesting the OP devices are okay

- "A+ im getting +60, A- im getting -40 (!), B+ im getting +16, B- im getting -14 (!)"
Both main and preamp -ve rails are low on a DC meter, suggesting you are looking at unfiltered DC

consistent with the filter cap on the -ve rail going open circuit, either internally but more likely at one of its solder joints

Filter caps are heavy and often give their connections a hard time if they are PCB mounted (which is why you find them glued or strapped to the board with a cable tie).

one of the -ve filter caps had de-soldered itself. when i measured across it after taking the board out, i got a little spark and 50 volts (amp off). i discharged it, re-soldered it, and now im getting +/- 60 or so volts on the rails and a steady -24 mV on the output.

"Your best test instrument is your eyes".  Good observation, particularly the spark.  :dbtu:

A large proportion of repairs are like this, e.g. PCB-mounted input sockets with fractured solder joints or adjacent tracks.  You can often smell a burned component before you see it, and over time you learn some characteristic smells.  Working on valve amps you learn the difference between normal burning dust smells and abnormal smells.

Now, what have you done to stop this happening again?  Cable tie?  Glue?

should i plug it in for real and see if it works?

Generally; if you can be pretty sure that you have found the fault and fixed it, as here, then you can risk going directly to full mains power, but where you may still have doubts, such as when cancer has rampaged through an output stage taking several devices out, a more cautious approach is to step up through lamp wattages.

I have a collection of lamps ranging from a few watts up to 100 watts, and if it's possible you may still have a fault such as a dead transistor you have missed*, then you need to step up the wattage cautiously, taking measurements as you go and being suspicious, not optimistic - if you see something you don't like power down and have a cup of coffee and a good ponder on the situation.  Can this be explained purely due to the reduced voltage, or could it be an indication of something more serious?  Amplifiers don't have Human Rights and don't get the benefit of the doubt.

* I'm sure that every tech here who has serviced solid state amps has at some time replaced a bunch of dead devices, powered up ... and blown the new ones.  When you find a dead device in a solid state output stage you must expect that other devices may have been damaged as a result, and frequently the only way to be really sure you have found them all is to pull them and test them out of circuit.  In-circuit tests can identify things like a shorted transistor, but they can't ensure that a device is good.  Output transistor failed?  Test the drivers.  Drivers failed?  Test the pre-drivers, and so on; and by "test" I mean remove and test out of circuit.  It's slow and a chore to do it properly the first time, but it's a bigger chore, much slower, and more expensive to do it right the second time.

I've seen people blow up two or three sets of new output transistors (or one guy go through a whole drawer full of expensive Nixie drivers) because they didn't stop and think.  When you have come back to the same point a second time it's time for a break and try and find somebody to discuss your problem with.

I've found that seriously confusing problems can be solved simply by explaining it to another tech, your wife, even the cat, because the process of explaining your logic, putting your assumptions into words, can expose to you what you have overlooked.  Assumptions are a constant trap, but by definition it will be the bit you assume is okay that will be where the fault is.

Putting it to one side and sleeping on it has worked for me too, if not providing an answer then often providing a new question to ask, something different to try, a different perspective on the problem, a different test that will separate this from that.

Ego is a problem here; a faulty bit of gear simply is what it is, and if you can't see it then you have to shift your view until it aligns with the reality.  Nowhere is this more confronting than with a bug in some computer code you have written - the machine is only telling you you're doing it wrong, that you don't understand what is going on.  A warning sign is the comment (often from a trainee) that "it's doing something strange/silly", it means that the real nature of the fault is not yet understood.

Be suspicious, be very suspicious.

[ilyaa]

thanks for the exhaustive response, Roly! it's really helpful for a n00b like me to get as much info as possible.

I have a collection of lamps ranging from a few watts up to 100 watts, and if it's possible you may still have a fault such as a dead transistor you have missed*, then you need to step up the wattage cautiously, taking measurements as you go and being suspicious, not optimistic - if you see something you don't like power down and have a cup of coffee and a good ponder on the situation.  Can this be explained purely due to the reduced voltage, or could it be an indication of something more serious?  Amplifiers don't have Human Rights and don't get the benefit of the doubt.

what's the real procedure, though, as i approach the real-life test stage? ramping up bulb wattage and just taking measurements, or actually plugging the amp in - to a speaker - and trying it out with a limiter still in-line? if things are working at that stage, presumably im ready to remove the limiter - but if im not getting any sound then back to the drawing board - is that the right process?

[teemuk]

The limiter itself may obscure some tests with the amplifier because, after all, it limits current. And it limits current whether the amplifiers is broken or in perfect working order. With the bulb there the amp, for example, will not reach its maximum output power because current limiting kicks in harder the harder you load the amp.

I basically go with the light bulb's dimness (or brightness) in the idle state of the amp as a sole indicator. If it's dim a chance is the amp works fine. If it's bright the amp is short circuited or at least drawing significant amounts of current (e.g. oscillating output stage).

Current draw shouldn't be excessive when the amplifier is in idling state (except in class-A amps, which are an example of one of those devices which's operation a light bulb limiter may obstruct a big time) so if the bulb is bright when the amp idles it's usually a sure sign of problems. Dim is fine, the amp is always drawing a bit of quiescent current. A high power amp with many parallel transistors may in fact draw lots. Amp biased hotter naturally draws more than amp biased colder.

If in doubt, then you can always take measurements of the circuit with multimeter. Excessive quiescent current draw? Voltages out of place / not right at some places in the circuit. Any other obvious signs of faults that may not cause excessive current draw to which the bulb would react?

There's basically not a "yes or no" -answer we could give based on the bulb itself, other than that when it's dim we know the amplifier isn't drawing excessive current at the moment. Note those words. The bulb doesn't clearly indicate faults that have to be "triggered" somehow, like output clamping to one of its rails during clipping, or oscillations starting in condition x. The bulb just indicates whether the amp is or isn't drawing excessive current. At the moment. That's all.

You have to advance from that point with other means. Whether they are trusting you fixed the amp completely, getting rid of the bulb and testing the thing (e.g. with "plug and play" method), or keeping the bulb and making further measurements in an attempt to pinpoint other possible issues. Most likely when the bulb is dim it can be removed and you can still perfrom the second option. You get more "accurate" results then too because the bulb is out of the circuit.

[Roly]

thanks for the exhaustive response

Well you seem to be tackling a few repairs and interested in learning how, and I'm always willing to pass on what I've learned to anybody with an interest.  You can learn electronics from a book, or these days for the net, but being a tech is also about how to approach problems and is more like a craft that has to be handed down, and this is my way of repaying my debt to those who mentored me.

Also posts are indexed by Google so it's as much for the curious who will come later as it is for you.


A key point with a solid state repair is when you get the output stage to "rebalance" the half rail to the mid point between the supply rails.  It is only safe to reconnect the speaker when the residual voltage on the output is about 100mV or less (and it's typically much lower).  This is a major milestone when you have been replacing dead devices because it's a pretty good sign that things are back under control.

Teemuk mentions a couple of exception cases such as latch-up and parasitic oscillation that may only appear under drive conditions, but these are thankfully rare; however these and the odd component that may only break down at full voltage are the sort of reasons that a methodical, cautious and progressive approach is intended to catch before they cause damage.

Generally speaking a healthy amp will work reasonably at low levels with a limiter in circuit, but if you try and push it the lamp will light and the amp may appear to play up, perhaps motor-boating with a very low frequency oscillation until you reduce the drive again.

I generally like to see a nice clean full rail-to-rail output voltage before I go for full mains, and then again at full voltage before I reconnect the speaker, and normally do a power output check into my dummy load before I do that.  Until you connect a load the amp is only putting out voltage, but once a load is connected it is required to deliver current, so I like to go step by small step.

If at any point something seems to be still amiss, then yes, you need to loop back and find out what is going on before pushing things any harder.  The full voltage test is often called the "smoke test" with good reason, but if the amp has passed through all the hoops as you cautiously increase the limiter wattage it's rare to strike trouble at that stage.

[ilyaa]

coooool

checked and all V's looked good. plugged it in, turned it on, and everything sounds fine!

i'm going to glue the culprit caps in a little better to make sure it doesn't happen again, but looks like we're good to go!

if something comes up i'll be back but i think we can put this thread to rest!