vintage Marshall 2203 HELP its been modded...redplating all 4 tubes

[Smitty02]

Hey all I have some issues with a 2203 it has been modded but i did a cap job and it was working great for about 20 minutes then went into redplating all 4 NEW tubes I caught it though before they were fried, tested them on tube tester and all test good still. I also converted it to EL34 spec's on the bias supply. It biased up right and then went south with the red plating.. I have it on my bulb limiter and am having some issues I was hoping I could get some input on, here is the schemo seems R8 and R29 that run in series are getting very hot and I noticed that on V1 the .68 C1 cap was loose I dont know if it was a result of the others getting hot.. I did a recap all went well I did a good job and have rechecked all joints but cant figure this out everthing looks good. I am not sure if the choke is shot or what. NOW for the mods it has had several old torres mods done to it 1 the did a PPIMV and this bullet proof mod where they added some resistors to the speaker outputs (a bunch) and added some kind of (diode resistor) thing on the tube sockets along with changing the 1k grid resistors to 2.4k 5 watters ?? and also put in a mid boost mod wrong which I had already fixed. They put it in the mid pot instead of the treble location per instructions, currently my main concern is the 2 10k cooking, on the layout they are the big ones next to the bias pot I can not leave it on to long before they start to get too hot to touch.. Have I got a short some where ? all looks good and worked fine for about 20 minutes after I biased it up I was going to let the tubes burn in and caught it red plate.... ANY help would be welcome guys, thanks I am adding a layout for help with any instruction

[Jazz P Bass]

Check the polarity of the capacitors.

The bias cap especially. (it installs positive to ground.)

[xtian]

Yes, start with the basics. Pull the power tubes out and measure the bias voltage on pins 5. I can't quickly find any mention of recommended voltage, but you should see around -40 to -50vDC at pin 5 of each power tube.

[Smitty02]

Yes the bias caps are in right. I got into it after I made the post as I have not been able to work on it due to bad health but I fired it up and took stock again... with power tubes in my bulb limiter glows bright... also I am not sure if using a bulb limiter that the bias voltage will be up to snuff I got the bias pot turned all the way down and am only seeing -8v though the bais tap AC coming in is almost 100v it goes - after the diode as it should... This has been hacked on and was not in great shape when it came to me I will post a pick of this thing and all these mods, its a shame... before it red plated the pin 3 voltage was only 450v which seems a bit low to me... for it to red plate all 4 tube I expected the bias supply maybe a bad joint but all looks good.. may be possible that a trans crapped out but it has some sort of protective mod on the inner 2 power tube sockets to protect the trans which I suspect could be a issue and it has resistors for the OT protection.. As I said it worked for about 20 minutes then red plated all the caps are installed right, I did each one at a time and one I replaced had a dead side... the layout i pretty much exact only point to point rather than circuit board. I will be looking into it again later today here are several pics of these mods and grand pa's little helper I know it a mess and still dont understand the ppim mod? why ? it has a master volume and a gain what help is this mod ?

[xtian]

OK, you've got to start from scratch, here, in terms of first-time startup procedure. So pull all the tubes and keep your bulb limiter in-line to prevent any destruction.

Power up with no tubes, and bulb limiter. Bulb might flash, but should go dim quickly, because you should not be pulling any significant current with no tubes installed. (If bulb never goes dim, you've got a short circuit to ground, possibly bad e-cap; I'd think a real hard short to ground would pull way more current that you're seeing...)

Bias circuit first: measure AC between 100vAC bias tap and chassis. Good? Next measure DC voltage after the reverse-oriented diode. Do you have negative DC voltage around -40? If not, check the value of the resistors and adjustment pot in the circuit, and if they're ok, replace the pair of electrolytic caps in the bias circuit.

[gui_tarzan]

What a mess. All of the great information given so far aside (and I would look at those first), have you compared the layout with the modded build? The schematic is the OEM, not the modded version, correct? If so that's not going to show the mods and you'll get confused at the wiring. Right or wrong, if I were looking at that my next step would be to trace each connection and make the adjustments on the schematic. Without knowing for certain what is connected to what it's going to be tough to troubleshoot. That's exactly how I learned how to fix mistakes I made on a custom amp I built for myself last winter.

if that were mine I'd rip out all of the Torres "mods" and return it to stock. That would eliminate those being the cause of your grief. If it's a customer's you may not have that option.

[Smitty02]

I had a bad day and just had a chance to go over a few things before I called it a night.... Thanks for your input guys I am going to really try to get into it tomorrow I put a set of el34's in it and fired it up for a short time and i do have a bright bulb , I am thinking a short some where....I tested all the caps which are new after I drained them and all are new and test good.. I do agree and will look into the bias supply again as it drops off sharply when I get the the 15k and 47k to only -8vdc. it is good coming in though at the supply to the board and diode with bulb limiter inline. all resistors measure correct also... It is a shame that someone hacked this up even the cab was stripped to bare wood... I have been studying the layouts and original schematic and am currently looking for a voltage chart for the model I found a nice one for a 2204....thanks guys hopefully I can get to the bottom of this as I did get this running before I did the cap job but it was a mess but sounded like wow I want one ! LOL that works right of course....I wan to build one so hopefully my experience will help me to make my own one day as they dont look to bad at all.... But they usually dont ha ha till you get knee deep in them scratching your head.. My health is been bad and it tough for me to put alot of time into it without taking breaks, Hey Grandpa's helper is due back tomorrow maybe she can help me fix it... I really would like to strip all the mods out I wish i had better oics for you all its a mess really someone did a # on it for sure Thanks again guys I will be posting some more findings soon... I dont get the 2.4 k 5 watters and no 1k's as called for and suspect those little black shrink wrapped voodoo amp savers they installed... no shrink wrap on those ppimv mod leads....

[gui_tarzan]

Smitty, I'm in Michigan too, where are you?

[Smitty02]

Hi guys just a update i checked the bias circuit and noticed that I got correct bias well close to correct -39vdc after the diode but it drops down to -12vdc after going through to the bias trim pot to -12vdc BUT if I switch the standby off to the run position it goes to +4.5vdc and red plates and dont pass no signal at the input but it does hum audibly and does NOT blow any fuses... any ideas ?? all caps and resistors test good, I know I need to get the bias voltage up to about -35vdc but this is with the stock resistors in place, I switched them back to original setup

[Jazz P Bass]

The only place a positive voltage can come from is the B+.
I would remove the PI coupling caps (C14 & C16) & try the measurement again.

One or both may be leaking.

[Smitty02]

They tore those out Jazz and put in a torres MV kit, Dual gang pot and 2 sets of .047 caps... I just got this beast up and running again granted it 3am .... I ripped it all apart and flipped the board to really get at it as I done it originally just tilting the board out I had to replace the conversion parts for EL34 tubes and put it back to stock 150k's for the PI and 15k for the ac dropper new 4007 bias diode 15k and 47k now I am too high and have to put a piggy back resistor in to drop it but its running -48vdc solid stable... This amp had many bad joints and I think my doing it without pulling all the pots and flipping the board stressed a few plus the naggled up Master vol mod ... Can I switch that back to stock or would you rather ?? I see no sense in it its got vol and a gain I dont know the benifits of this master vol mod, am I missing something here ? it is a old beat up thing but all original trannies for some reason the conversion 220k's didnt play nice with it maybe due to the MV mod I dunno but I put the old 150k's back in seems better ?? as they cut out the .022 PI caps but its rocking again I need to fine tune it and such bias it put the NEW tubes in, this was a hack job on a real nice amp I will try to post better pic's of the thing you will laugh and cry some one would do such a thing to this amp... I guess he got it cheap though so ?? to those out there with 1978 JMP's if you run into issues pull the pots and flip the board and redo the solder joints they are pretty weak, its worth the time to just flip the board rather than scratch your head wondering why it pooped out... Nice amps I want to build one now LOL any how thanks for the help guys I got good bias now and fixed several bad joints that were not my doing, some were hanging by a thread literally.. I still want to remove that Master volume mod... What you all think ?

[gui_tarzan]

As I said earlier, I would return it to stock and then see what it runs and sounds like and go from there. But that's just me.

[Smitty02]

Well I am still waiting to hear from its owner I would like to take it out.... I do have a bad hum and found switching tubes helped alot it was on the master vol untill I swapped the PI tube to V1 now its on the gain control... using good sovteks ax7's wxt's I think some sheilded leads would also help the lead going from gain to board if very touchy as is V1 pin 1 and 6 leads as usual.. I tried a JJ ax7 in it and it was WAY microphonic the sovteks are very quiet but I got that hum ... could be a lead dress or shielded or lack of shielded issue, also could be just picking up noise from the outlet in my work room lights and soldering iron.. I noticed the biggest difference swapping tubes though..... the .68 coupling cap to v1 was unsoldered.. From heat or just bad weak joints but that could be a issue too, if it was a heat problem, it was loose as heck I did leave some of the joints that looked good alone but they are thin with the solder I didnt want to go nuts as i found sometimes it more trouble in the end.. I resoldered about half the board from the PI back left the right side alone except for a few obvious bad ones I did notice that I get no voltage reading from the top of the 47K bias resistor and the bias pot but it does work when adjusting, should I get a reading from the top of the 47k ? Another thing .. should I ground the tone stack (pots buss) to the cap under the board like in the posted layout cause it is and was never grounded there... AND why on the posted layout does it say 330UF next to that cap under the board (shown on the far left of layout) Its coming along though thanks guys

[52 Bill]

Well I am still waiting to hear from its owner I would like to take it out....

While I'm not a fan of Torres mods, if they are installed correctly they will work. It sounds like the person that installed the mods was the real cause of the problems.

If the owner wants it back to stock, then that's easy enough to do. If the owner was the one that wanted these mods installed, then go back and install them correctly to make him happy.