Marshall Valvestate VS30- Hum/No Output

[glstarr]

Help !!!! Borrowed a friends amp for a little practice and during the night had an electrical storm that took out a few electronics in the house including my friends amp. When powered up I get a loud 60 cycle type hum and no output. I have some tube amp experience only. Checked dc voltages on chips and all are ok. All components look good. Is it safe to use a 1950's signal gen. and or tracer on these cicuits? Any help would greatly be appreciated!!! Thanks!!!

[oc disorder]

Hi glstarr and welcome to the forum.
The most common fault with this type of amp is the output chip
which can have the exact symptoms you describe.
It's a TDA2050 .
Providing the sig gen you have is safe in itself I see no reason
you can't use it.
The signal should be ok up to C48 which goes to pin 1 ,the input to the TDA 2050.
Can't see how an electrical storm could blow it .
More likely to affect the power supply but if you have checked the dc supply
(+15volts and -15volts) and thats ok and the channel 2 led is functioning ,
I would think it is the output chip.
The easiest way to remove it is to cut the legs off then unscrew it from the heat sink taking care not to damage the thin insulating washer between the back of the chip and the heat sink. Also the little insulator on the mounting bolt shouldn't be damaged although it may use one of those clips instead.
Then its easy to remove the debris cleanly without too much heat not damaging the pcb tracks and pads.
Fit the new one physically to the heat sink first before you solder it in.
That way there wont be any strain later on on the solder joints .

[Enzo]

If it has that TDA2050, and there is a loud hum, check for DC voltage across your speaker. There should not be any. Or watch the speaker copne. When you power on, does it move one direction and stay there? That would be DC. It will burn up a speaker, so don;t leave it on.

And if there is DC there, then just replace that TDA2050.

[glstarr]

Thanks!!!

WOW !!! Thanks to both of you for the great info. First thing I did was check for dc on speaker, and sure enough there was 15 volts. went to radio shack and picked up a low wattage iron and took out the part and it was a breeze to do the way you said to take it off the board! I will go to the parts house and pick one up tommorrow and reinstall. I've recapped and troubleshot quite a few tube type jukebox amps but this dang circuit with the chips, etc sure had me lost. Guess i best get busy and learn some new circuit theory!! Again, Can't thank you guys enough!!! I'm sure glad I found this forum. Will Post my results when I pick up the part.

[Enzo]

Hey I am an old jukebox guy. REbuilt more old tube Seeburg amps than I can count, plus old AMI and Wurlitzer amps. DOn;t see many Rockola tube amps around this part of the country.

Of course I have also rebuilt a ton of solid state Seeburg and Rockola amps

[glstarr]

Yeah, I did jukebox restorations and repair full time for about 10 years. Quit about 12 years ago when it got too hard to get chrome done. This area seemed to be all ami's in the forties, fifties and sixties,but my favorite has always been seeburgs!!! There must of been alot of seeburgs up your way,I see trashcans from michigan on e-bay all the time!! I made a trip up your way a while back and got a nice blonde trashcan and a seeburg 9800 to restore one of these days!! Well my part won't be in for a week. Also, OC Disorder, thanks so much for the schematic! I'm going to do some tracing this weekend and see if the rest of the amp is ok.

[Enzo]

Yes, Michigan was a big Seeburg market.

Rowe came along long after tubes to become a major player here.

As a company trainer, the 1970s Rockolas were a favorite, I could take those apart not looking.

[glstarr]

If you have the time, you should write an article for jukebox collector magazine!! I'm sure they would be very very interested to here what it was like to be a rockola insider!!And so would us readers!!!!

[Enzo]

I used to have a monthly column in Startech Journal. I wrote a few columns for Playmeter, and a few of my pieces from Startech have appeared in Replay. All long ago.

[glstarr]

Still need help!!!

Well, Finally had time to install tda 2050 and amp immediatly started working but I have 40 volts on pin 3 & 5 of tda 2050 and resistor r18 is getting really hot! Checked darn near every component in power supply circuit and can't find anything amiss. Anybody got any ideas? ( circuit schematic is on earlier thread)

[oc disorder]

Sounds like something wrong with the power supply too.
See on the last page of the schematic where at the top and bottom
there is a +'ve and -'ve 15 volt supply.
Its a typical zener circuit where ZD1 and ZD2 provide the 15volts and
R18 and R17 take up the slack .
Measure across ZD1 and ZD2 and see if you get 15 volts on each.
Now this should not affect the power amp as from the diagram it connects to the positive and negative rails before it is dropped to 15volts.
As they don't say on the diagram what that is I have to guess so as the main filter caps are 35v (2200uF) its got to be between 20 and 35 volts.
So your measurments gave 40v across the output IC so between pin 3 and ground you should have minus 20volts and between pin 5 and ground you should have positive 20volts .
Getting back to the fault it seems something is drawing too much current on the negative rail causing r18 to overheat from the strain.
This could be a faulty pre-amp IC or even a shorted D2 at the input.
Also anything that is between the minus 15volt rail and ground could be suspect (C15).
Often an IC with a short in it will get hot so you could compare the surface temperature of the IC's to see if that gives a clue.
I would expect something else not to work like switching , overdrive or reverb
if there was a faulty IC.

[glstarr]

Still troubleshooting

First I can't thank you enough for taking the time to reply. I hope I can get on this sight someday with answers instead of ?'s. !! Ok, 24 volts on one side of r18, 4 volts on other. Everything seems to check good up and down minus 15v side. replaced zener zd2 and c15 with no change. checked resistors and caps all down circuit & didn't find any problems. read 4volts across zd2. I guess the next thing to suspect would be an ic pulling the voltage down? maybe IC2 or Ic3 do to the fact they get -15v? also d2 checked ok. Can't leave amp on long enough to heat up an ic because r18 really heats up fast. Also replaced r18.

[oc disorder]

Well getting closer now is the time most people would be plugging in the lightbulb limiter probably with a 60w bulb/globe.
Ok there's 5 IC's and I guess 1 of them has a fault.
If you have an accurate meter with a low ohms scale after the amp has been off for a while so everything is discharged (so not to upset the readings)
measure between pin 4 and ground of the IC's and see which one is the lowest, keeping the same ground connection for all.
If that is inconclusive the quickest way is to cut the -15v track where you can easily bridge or join them neatly later.
A sharp knife required .

I can't tell from here how the track runs there may be some wire links in the design which will allow you to isolate the various sections.
If you pick a mid point so say 3 are disconnected and 2 are connected turn the amp back on and see if ZD 2 has gone back to -15v.
If it has then one of the 3 disconected must be faulty so you need to reconnect them 1 by 1 by cutting tracks or lifting links.
Of course you could fit sockets to all the IC's but that would mean you may have to replace all too !

If it hasn't gone back to -15 v then the fault must be in one or both of the two still connected and they need to be individually isolated as well.

Tedious I know but as you can't leave it on for very long there is not much option apart from fitting a 5 watt 150 ohm resistor away from the board
(without cutting the leads) instead of the 1/2 watt one and letting it get hot
then touching all the chips with your finger or if you have one of those aim type thermometers the hottest one is the most likely culprit !
Watch you don't burn your fingers !

[glstarr]

still at it

Ok. when I cut the trace on ic 2 everything went to pretty much normal !!!! Then when I hooked ic 4 back up,resistor r17 on the b+ side started heating up and voltage dropped to 11 volts on both sides. So I guess I'll order these two ics and see what happens. It takes at least a week or more for the supply house to get the parts in, so I'll post as soon as I get the parts. Is there a good place to order on line? Also I can't thank you enough. this has been a very educational repair. !!!!! Gary

[oc disorder]

Good.
I often fit sockets in case there's a next time.
The M5201 is one Marshall use a lot for switching.
Attaching a reference.