JMP 2203 -78 master model 100w lead

[niilopi]

Hey,
Can you guys help me spot the wrongs in my amp. Mods in the circuit, preamp stage sounding electric and bad. New tubes and bias done but no help.

Main wrongs i spotted two of the main tubes have 3W 1KJ resistor instead of 5KW 1KJ. Input jacks, preamp pot and circuit have some wrong resistors and caps.

pictures :
https://postimg.org/gallery/n816md36/

[neikeel]

Hello and welcome.

It has certainly seen some work, not necessarily by a professional.

From the front end, the treble peaker (470k/470pF between the first two stages has been replaced (can't see the value - often a 68k).
The V1b bypass cap is usually a 0.68uF has been replaced by a larger electrolytic - value?)
The 33k slope resistor has been replaced with a HUGE (2W rated) 56k like a bass amp.
Most of the coupling caps have been replaced with EROs and the ceramic disc caps in the tone stack and second treble peaker across the 470k on the board likewise.
The PI anti-fizz cap had been modded (possibly two caps in series.
Presence cap replaced
Filter caps replaced
Rectifier diodes replaced with heavily over spec ones, bias caps replaced with radials, piggy backed MF resistor in bias circuit presumably to get it in range for biasing
Both master and gain pots have been replaced as have V5 and V6 sockets and screen resistors.
Can't tell about the transformers.
Need to know a little bit more about the bad sounds before making comments n how to fix it, other than taking it all back to stock

[niilopi]

Hey, i double posted this thread.. heres some more info about my current situation. Ill look further soon

Hey,
About 7 years ago i bought this amp via ebay UK. That time i didnt really know about sounds it was all about max gain and was fine for me. Now i realize the amp has sounded like shit for the beginning and i cant really make the great gain sound usable for me. The master vol sounds good clean but the pre amp ruins the sound for good.

I recently got new tubes for it changed the EL -> KT77. Went from 100W->50W by leaving out two outer main tubes and changing the impedance output to 4ohm with a 8ohm cab. Also changed preamp tubes to V1 5710 and V2,3 Tungsol 12ax7. I biased the amp taking the voltage from one tube 471V and got the bias up to 32.4mA which was max value i could get out of it.

Ive been trying to figure out what the problem is. Master vol sounds pretty clean even at high vol and preamp at minimum. Preamp sounds very bad, electric, breaks up at treble and muddy. Theres also a slight background buzz which goes up with volume.

So after the tube job didnt make a difference i started looking into the amp. Amp seems heavily modded, amateur connections and weird looking components. Problem is i didnt find a Marshall JMP 2203 schematic with EL connections only the one with 6550. So ive been looking trough the schemes of JCM800 but i guess theres some differences there also?

Biggest alarming differences i found were:

From left to right on circuit board:
-4 Diodes left side of circuit are changed, much bigger in size than original and the anode-kathode are installed in ways that are way different than in the JCM800 schemes.
-Theres two 10uF caps i measured 32uF with the multimeter even tough it reads 10uF on the component.
-Under those two caps on the right two resistors are "piggybacked", Original value 15k, i measured 9k.
-In between the yellow caps below them are two small caps in between two resistors. These caps should be 47pF in the JCM800 scheme, instead there are 2x 100pF caps (101k 1kv).
-On the right of the right side yellow cap is supposed to be a 100k resistor which i measured to be 4,5k.
-The huge resistor below the two white caps says 55k/33k, and on mine its 55k. I believe on the JCM it should be 33k.
-Between the last two yellow caps on the right side of the circuit board is two resistors. On both of the resistors stock value should be 100k but the left one i measured 150k.
-The last black cap on the right stock value 0,68uF, is now a 1uF component.

On the input jack and pots:
-Between LO input and Pre-Amp pot there should be a 470k resistor, i measured 320k even when it says 470k on the component. Also on the same connection stock 470pF cap has been removed.
-On the HI input between + and - should be a 1M resistor but instead it has 2M.
-Also the wiring of the lo and hi seem to differ a lot from the schemes.
-On the Pre-Amp pot between pin 1 and 3 there is no capasitor.

Tubes, (havent had time to look very carefully yet):
-One main difference i noticed.. On the center 2 main tube inputs that i am using now there are white 3W 1KJ resistors between pins 4 and 6. I believe these should be 5W 1KJ as in the scheme?

Does anyone have a clue where i should start to fix my sound, is there anything very critical that could ruin my preamp stage?

[neikeel]

Most of the things you pointed out are things I mentioned in my list above. The 100k plate resistors for V1 are correct.

The two 100pF caps in series in the PI = 50pF (near enough) and green-blue orange is 56k (see my post above).

If you go back to stock you might find the amp quite bright. I would replace the 0.68uF on V1b and restore the input jack wiring to stock except for the shielded wire, tidy up those grotty solder joints. The 1nF cap on the gain pot is a matter of taste.

The long red wire on the input should be shielded wire, the filament wires to the premp tubes need twisting tightly to eliminate some hum and a lot of the preamp wires should be dressed to keep noise to a minimum, similarly around the output sockets (two have been wired bad to front - the notch normally faces rear of the amp.

Whilst you are at it check all the grounds. Have you done much of this stuff before?

[niilopi]

I gotta admit im pretty new to this but getting there. Im ok with soldering and basic electrics and im studying to be an electrician at the moment also.

Thanks for the tips they sound pretty clear ill start working on it this week and keep u guys posted. Meanwhile all new info and especially where to begin welcome! I dont really understand all parts of the board so well that i could tell which components would be critical at this moment and what components i could just let be even if they dont match original specs.

[niilopi]

Just changed major components to my marshall to get it back to original spec.

Found some cold joints under the circuit board and tried my best to fix anything that looked suspicious. Ground cable from the pot grid was missing.

Replaced :
-Input jacks hi and lo
-bunch of Resistors
-470pF / 500V ceramic cap
-47pF / 500V Silver mica
-F+T 10uF / 450V ELKO
-ERO / Vishay / Roederstein MKT 1813 680nF (0.68uF) / 250V polycap
I bought this Ero for now but ordered also a SoZo Next Gen 680 nF = 0.68 µF - 160 V because i read a lot of good about this cap.

Im not an expert with this yet so correct me if i got some wrong components before i blow up my amp

Heres the schematic i made before my changes with the wrong values written in red. Theres one thing bothering me when i measure the negative feedback resistor 100k the multimeter shows me 4.5k ohms like in the presence cap. Is this normal? Also the huge diodes in the rectifier worry me as i dont know if they will work normally it says LT 40 6A07 on the diodes and they are 6A. Should i just change back to 1N4007 which is 1A?

https://postimg.org/gallery/okfc6daw/

Tomorrow gonna get some shielded wire and put the last wires together. Then i can do some voltage measuring and finally some testing after bias.

[neikeel]

Not looked at the pics yet but it sounds like you are on the right track.

The diodes are over - not under spec so apart from looking odd will be fine.

The presence cap wil never read correct in circuit - dont worry about it!

[niilopi]

Finished the wiring. Also the shielded wires were regular ones before so i changed them to proper. Now the amp is all stock. And no noticable hums.

Bias went now way up to 37ma and would have gone even further than before the parts were changed top bias was 32ma.

Did a little test drive and the amp has changed dramatically from what it was before. No more mud fuzz shit but clear even when breaking up. My amp and guitar really came alive. Its a huge win for me as ive been playing this amp for 8years like this.

I also made myself an attenuator and tested that for first time. After the surprise that my amp is now killer sounding i plugged it in and set the masters to a place where the main tubes start driving and there it is. The sound ive been hearing on my favourite records and never been able to achieve. The amp is really driving the guitar for the first time. Huge sound and great tone.

Theres still a few questionmarks.
-preamp pot has a Blindspot from 0 to about 1or2. After this pot works normally. If i turn the preamp all the way down theres no sound at all even if master is up is this normal? The pot has been changed at some point not original. Value is original.

-Effects. Most of my effects sound ok before the amp on my pedalboard. But is there a way to get in the delays and fx that should be on a loop without a loop and without losing tone?

[wdelaney72]

Congrats on getting the amp up and running.
The Zero Loss FX loop offered in the Metroamp store is fantastic. It's completely transparent, easy to install, and inexpensive.

yes, if your're gain pot is on ZERO you are feeding ground out of the wiper into the signal chain and can expect silence. Nothing wrong with that.