marshall valvestate VS265 problem

[rockjunky]

Hi all

A friend of mine has a marshall valvestate vs265 amp that has had intermittent problems


Basically after a certain amount of time being on, a loud hum would come in and out through the amp , he said the previous owner had to get it repaired (mentioned resoldering stuff)


I found a schematic here, it does not appear to be 100% accurate (I will explain why soon )
http://www.amparchives.com/Amp%20Arc...265r_2x65w.pdf


heres what i found

*previous preamp tube had no glow to it , replaced
* friend had tried to resolder some of the points....whether it was him or from the previous repair some were in very bad shape , traces lifting or broken

I had to put silastic under the REG's and resistors because the trace had lifted so no ensure no more movement

I found REG 2 point 'com' to have had its trace completely broken, I managed to resolder it
I found REG 2 point 'I' to only have a circuit on one side and not passing through to the other , I resoldered this

previously R20,R21 and REG1 were getting VERY hot
since the resolder R20,R21,REG1 and REG 2 are all still very hot

now the amp has a loud pop when turning on and VERY loud when turning off
it seems the hum coming in and out has gone ....I do have the amp disconected fromt he speaker box so it is not getting vibration on it which may be part of the issue if there are bad solder joints
at the same time I don't want to leave it on for to long with the heat thinking it will fry these components

I noticed looking at the board, the O on REG 1 and REG 2 is going to C31 & C32 which are not shown in the schematic (seems to be C20 & C18)

I'm getting 41VDC on R21 if I remember correctly and I think -41VDC on R20...it shows 35V on the schematic so this seems quite high

what could be the problem?






showing 1 stamped on the revision? I think the schematic is Rev2

[Jazz P Bass]

Nice job cleaning up the mess!
Is the popping the only issue.
Some amps do the popping thing.
I take it the hum is gone.

[andrewkirsanow]

Is it doing the same popping when powered on or off, no matter which of the channels is active on the amp? If the voltage regulatrors are getting that hot, it's possible that they have generated enough heat to damage the solder traces under them. Do you still get the correct voltage on the regulator output pins? I did hear someplace that some of these boards suffered from bad soldering at the factory, and also some of the component legs where not pushed through the board far enough to make a decent solder joint. When humming creeps in and out, it can often be to do with bad solder joints on decoupling capacitors etc.

[rockjunky]

dammit just lost my message I was replying with!

did some resoldering last night and the popping is now gone!

I haven't left it on for long because of the heat on resistor 20&21 and REG 1 & 2
but no hum has happened

with this much heat could these components unsolder themselves!?!??

Jazz:I wouldn't say it was a nice job but I did the best I could

andrew: on REG 1
I +25VDC
COM 0VDC
O 12VDC

on REG 2
I -28VDC
COM 0VDC
O -12VDC

looks like has to be +15 and -15 VDC output?

[g1]

Strange that the schem. shows 12V regulators giving +/- 15V. Yet the commons are tied together. Also the resistors R20 & R21 are shown as 100 ohm but the photo looks like something else. Maybe colors are off due to heat but best to check them.
With the board labelled as V100 maybe that schematic would be a better match?
Could someone have swapped in a different board?

[rockjunky]

Strange that the schem. shows 12V regulators giving +/- 15V. Yet the commons are tied together. Also the resistors R20 & R21 are shown as 100 ohm but the photo looks like something else. Maybe colors are off due to heat but best to check them.
With the board labelled as V100 maybe that schematic would be a better match?
Could someone have swapped in a different board?

the resistors are 180 ohm
I take it the last two digits on the the REG is the voltage output? (ie 12V?)
Dont think it would be a different board...but the schematic is different in the capacitors on the output ...it shows 18 to 21 but as far as I can see its only two 31 & 32
maybe this is a different revision?

[g1]

Yes, it may be a different version. The +/- 41 volts you were measuring before must have been on the supply end of the resistors? I don't see 35V shown on the schematic aside from cap. ratings.
The regulators and resistors will run quite hot. You said the popping is cured, if everything else is now ok then you are probably good to go.
Yes the 12's on the regulators indicate 12Volts. This is why it is odd they show the voltages as + and - 15 volts. Maybe a typo, hopefully they are not supposed to be 15Volt regulators that someone replaced with 12's because of a typo!

[Jazz P Bass]

The VS100 has the same goofy notations on the opamp rail voltages.
+ - 12 volt regulators.
+ - 15 volts out.
Marshall Magic!

[rockjunky]

Yes, it may be a different version. The +/- 41 volts you were measuring before must have been on the supply end of the resistors? I don't see 35V shown on the schematic aside from cap. ratings.
The regulators and resistors will run quite hot. You said the popping is cured, if everything else is now ok then you are probably good to go.
Yes the 12's on the regulators indicate 12Volts. This is why it is odd they show the voltages as + and - 15 volts. Maybe a typo, hopefully they are not supposed to be 15Volt regulators that someone replaced with 12's because of a typo!

yes the 41V was on the supply side of the resistors.....I've done some electronic stuff but many years ago so I'm very rusty in terminology etc
speaking of.....I messed up the cap rating with the voltage there!

one thing I did find is that there are little black clips that go thru the chassis and hold these plastic connectors in place which hold the PCB still.....both these were missing which I can imagine weren't helping with vibration etc

I'm really suprised by the heat thou ........I know someone with a similar valvestate so maybe I can compare.......
I'm more worried when it will be put back in the speaker cabinet, heat wont be able to escape as easy


should I be concerned with the difference the difference on the REG input side? (-28 & +25) should these be the same?

[olddawg]

I would try to get a manufacturer's parts list for that model and serial number. Have you called Marshall? If you are using 12v regulaters for 15v rails it's going to cook. If it's a mis-print, everything checks out (no excessive current draw), and you're just worried about the heat you can:
1. Replace the resistors with higher wattage ones.
2. Replace the regulators with higher current subs.
3. Heat sink the regulators.
4. Add a whisper fan.

I would also flip that board over, flux, and resolder every connection. Also, I would hardwire any suspect connectors. If mounting hardware is missing, use rubber grommets, screws, and high temp silicon glue to secure the board.

BTW, I find the best way to repair broken traces is to hard wire them point to point over the damaged area with solid wire. You usually have to scrape the trace clean and use flux. If the board has become carbonized, it may also be conductive. In this case you need to cut the bad part out with a dremel tool and jumper the traces.

[km6xz]

It appears that those regulators were replaced since the entire preamp circuit is intended to run on +/- 15v. They probably saw the schematic and assumed it was right and the originals were the wrong parts. The 7812/7912 are needing to dissipate more heat dropping the additional 3volts at the current through it. The resistors will still get hot and hotter now that they are sitting in a non-heat conducting pool of glue. The heat of the regulators will go down if replaced with the proper ICs but they are intended to be used with a heat sink. There are small clip on and other forms of light TO-220 heat sinks available probably at the same place the ICs are purchased.
And get rid of the silicone based glue if you do not want the resistors to burn up. Fix the traces and support the ICs by their legs or mount them on the chassis and wire them into the pc board traces, the silicone glue will cause heat build up.
Silicon has pretty good heat conduction, about 35% of that of aluminum and 12% of copper but silicone glue which has silicon particles in it is a poor heat conductor, less than dry air which means the resistors and ICs will get hotter than if left with not heat sink in mid air.

[olddawg]

It appears that those regulators were replaced since the entire preamp circuit is intended to run on +/- 15v. They probably saw the schematic and assumed it was right and the originals were the wrong parts. The 7812/7912 are needing to dissipate more heat dropping the additional 3volts at the current through it. The resistors will still get hot and hotter now that they are sitting in a non-heat conducting pool of glue. The heat of the regulators will go down if replaced with the proper ICs but they are intended to be used with a heat sink. There are small clip on and other forms of light TO-220 heat sinks available probably at the same place the ICs are purchased.
And get rid of the silicone based glue if you do not want the resistors to burn up. Fix the traces and support the ICs by their legs or mount them on the chassis and wire them into the pc board traces, the silicone glue will cause heat build up.
Silicon has pretty good heat conduction, about 35% of that of aluminum and 12% of copper but silicone glue which has silicon particles in it is a poor heat conductor, less than dry air which means the resistors and ICs will get hotter than if left with not heat sink in mid air.

Absolutetly...When I said use silicon, I meant use a dab if necessary to secure the board mounting if there was no other way. You definitely don't want to bury regulator circuitry in it.

[rockjunky]

hey guys
I looked at another valvestate combo schematic (8240) and has a similar section 7912 & 7812 and it shows 12V output
so they either did a typo at the regs or the voltage :S
what worries me is looking at the resistors! (g-one tipped me off to!) I never noticed till now but they have a different colour code to each other making me think these werent original

when I mention earlier about the caps coming off the output side being wrong I relooked over it and it all looks fine .....my eyes missed the traces heading off everywhere!

I'm going to compare the heat on the resistors on the 8240 valvestate combo just as a guide

I got my hands on a hot glue gun......would it be better to get rid of the silastic and use this instead?

I think the chassis clips will help alot once it is back together

[rockjunky]

The VS100 has the same goofy notations on the opamp rail voltages.
+ - 12 volt regulators.
+ - 15 volts out.
Marshall Magic!

whoops! I just checked out that schematic and yes seems like the same error!

so looks like the REG's may be ok.......

I'll compare the heat tomorrow

thansk for all the help so far!

[km6xz]

Are they different value resistors? Their colors will change, a lot, when overheated. Check them out of circuit.

The heat will continue using the resistors and regulators to drop that difference between 41 volts and 12. They will run cooler, the pre-amp with have 6 volts p-p more headroom and everything will be as the designer intended if the regulators are 15 volt. A production line swap is not unusual and it either comes with a production revision instruction or just put into force by supplying the wrong parts to the placement machine.
You can calculate the heat that should be generated using 12volt regulators versus 15 based on the values you have measured. What is the drop across the resistors?