Can anyone Identify this mod chip installed in a JCM2000?

daddysouldonut

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First post here so please be gentle. I am a long time guitar circuit tinkerer, and trying to expand my knowledge a bit. Recently acquired a DSL100H with no tubes or output transformer for 150. I know what youre thinking: Bias drift. More on the amps history after my main question for those who are interested. I have found a small PCB installed that sits between CON3 and CON10, with one wire headed to RL3. Some Google searches lead me to believe this is something possibly related to the FX loop, which was met with many complaints it seems. Does anyone recognize this, or understand what it might be accomplishing? I never use FX loops...
MODCHIP.jpg
CON10CON3.jpg
RL3.jpg
RL3 CON.jpg

So... the story behind this amp. It was owned by a guy named Larry who founded Conquest Sound, a cable manufacturer. He had this among other amps in his shop to test cables against competition. So I imagine it was never ran super loud or for long periods. In 2019, Larry sells the company and the shops contents to a new owner. Within a year the new owner ran the business into the ground and Larry bought this amp BACK at the liquidation auction. It was then he noticed the OT and Tubes were removed, and it wound up in my hands for 150. Is it possible the second guy was removing valuable components out of inventory before they were sold off??? The only evidence of any issues other than the missing components is the main fuse (not HT) was missing. Someone was definitely in this amp considering the mod chip and all the labeled connections. But it has the stock 60-00 board, with no visible alterations, or charring for that matter. I have the exact stock Dagnall transformer on the way, and will be using Tung sol 12ax7s and EL34Bs. I had some tubes on hand, altogether at this point I am in $350. Once the hurricane passes I will be picking up a loaded 4x12 Marshall cab from a kind gentleman for $85.

Yes, I know the only *correct* fix for the bias issue is replace the board completely or remove the bias circuitry off of it. Just seems like... a lot of people claimed to never experience an issue. From what I understand the bias doesnt start to runaway until things get pretty hot, so is it dependent on type of use? It was used in this guys shop for over 15 years, although who knows what the second owner did.

I really appreciate anyone who has taken the time to get through all of this. I just want to know as much as I can before I load up any of the new parts. If all goes well Im looking at a DSL100H with 4x12 for investment of $435.
 

Jon Snell

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I cannot see what the IC is, probably a 4558 or TL072 type and is probably a buffer or slight gain stage maybe.
Should be easy enough to work out ... there is ground, + 15v - 15v no connection and in and an out.
Pins 1 is Out, pin 2 is IN pin 3 N/C pin 4 ground pin 5 +15v pin 6 -15v.

In my experience, over the last 50 years, bias issues only arrive when the amp has been played very loud, for long periods at a time or without service intervals and worn out, over heating EL34s.
Always check for dry joints, poorly crimped power connectors that get hot, they heat up the board and are the start of your troubles.
 

daddysouldonut

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I cannot see what the IC is, probably a 4558 or TL072 type and is probably a buffer or slight gain stage maybe.
Should be easy enough to work out ... there is ground, + 15v - 15v no connection and in and an out.
Pins 1 is Out, pin 2 is IN pin 3 N/C pin 4 ground pin 5 +15v pin 6 -15v.

In my experience, over the last 50 years, bias issues only arrive when the amp has been played very loud, for long periods at a time or without service intervals and worn out, over heating EL34s.
Always check for dry joints, poorly crimped power connectors that get hot, they heat up the board and are the start of your troubles.
I appreciate the info, but in all honesty my knowledge isn't anywhere near robust enough to comprehend the first paragraph. I'm basically just wondering if, based on the part of the circuit it sits at, If it is of any consequence to normal operations without the fx loop. I've tried to read up as much as I could on the flaws of these amps, and how it might explain the missing components. However, the story given could explain that as well, so I'm just kind of anxious at the moment because I don't want to fry the new OT or tubes trying to diagnose it. A lot of threads recommended upgrading the stock Dagnall OT, but I wanted something drop in that I could just fit the existing fittings right onto.
 

daddysouldonut

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See that's what's curious to me, why would you go in and install an FX loop mod but not also fix the bias drift- if it did in fact exhibit that issue?

My plan of attack is... Install the new OT, plug it in and see if any fuses blow, smoke etc. Then install the tubes, set bias, wait about an hour and check bias again to see if it's moved. As stated I'm new so I understand the safest approach is take it to a tech but for the insane value I'm getting here I'd like to try to do it at home. Amp will most likely NEVER be pushed to it's limits, I barely go above 3 on my Egnater Rebel 30w.
 

Dogs of Doom

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HI, :welcome:to the forum...

re: bias/effects

you can't leap from 1 to the other, in trying to figure anything out, at this point. They are totally unrelated, & w/o knowing the true history of this amp, anything could have happened.

Quite possible the studio guy pampered it & added the effects loop mod. Sold it off & the next guy trashed it... :shrug:

Maybe studio guy had someone came in & trashed it & sold it as-is to the next guy, who bought it as a project, like you, but never got it going, so he dumped it. :shrug:

There's no indication as to why the OT was pulled. Maybe to fix another amp? :shrug: or maybe it got fried due to a blown tube, or, the motherboard went south. :shrug:

It could be anything from 1 end to the other & anything inbetween...

These amp's are notorious, for the 16 ohm speaker out jack failing, causing a ground issue, if you use the 4, or 8 ohm output, which could blow tubes & trans'...

But, we don't know if the trans' was blown, or if it was just pulled... :shrug:

Good luck w/ it though, I'm not a tech, so, I won't have much to offer, except my experience & what I've seen, here on the forums.

:cheers:
 

Dogs of Doom

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also...

do note:
WARNING! these amp's contain high voltages that can cause death or other permanent damage. While this amp model has a mechanism to drain the cap's, if that mechanism is faulty, then you could gat shocked w/ lethal voltages.

you're a new member here, we'd hate to lose you due to you trying to save a buck, or not knowing what you're getting yourself into...
 

daddysouldonut

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HI, :welcome:to the forum...

re: bias/effects

you can't leap from 1 to the other, in trying to figure anything out, at this point. They are totally unrelated, & w/o knowing the true history of this amp, anything could have happened.

Quite possible the studio guy pampered it & added the effects loop mod. Sold it off & the next guy trashed it... :shrug:

Maybe studio guy had someone came in & trashed it & sold it as-is to the next guy, who bought it as a project, like you, but never got it going, so he dumped it. :shrug:

There's no indication as to why the OT was pulled. Maybe to fix another amp? :shrug: or maybe it got fried due to a blown tube, or, the motherboard went south. :shrug:

It could be anything from 1 end to the other & anything inbetween...

These amp's are notorious, for the 16 ohm speaker out jack failing, causing a ground issue, if you use the 4, or 8 ohm output, which could blow tubes & trans'...

But, we don't know if the trans' was blown, or if it was just pulled... :shrug:

Good luck w/ it though, I'm not a tech, so, I won't have much to offer, except my experience & what I've seen, here on the forums.

:cheers:
Nothing to disagree with there, I'm just working myself up without the parts yet to truly figure it out. I do plan to fix the output jack grounding issue regardless of anything else. According to Larry nothing was ever done to it before 2019 when the company was sold. Then the mystery period. I did notice the lugs on the Ohm selector are a little dark, but the solder joints underneath look okay, all the board material looks good. If I can drop in the parts set the bias and be good to go I will be ridiculously pleased.
 

daddysouldonut

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Thank you for the concern. I've been getting 1000% more help already than any of the shops I called. Discharge caps and one hand rule is what I'm working with. Any other safety tips?
also...

do note:
WARNING! these amp's contain high voltages that can cause death or other permanent damage. While this amp model has a mechanism to drain the cap's, it that mechanism is faulty, then you could gat shocked w/ lethal voltages.

you're a new member here, we'd hate to lose you due to you trying to save a buck, or not knowing what you're getting yourself into...
 

daddysouldonut

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More poking around revealed R71 completely removed... From what I can tell this is a bleed between the two filter caps? Uh...pretty important? I ordered a 270Ohm 5w replacement
 

FleshOnGear

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More poking around revealed R71 completely removed... From what I can tell this is a bleed between the two filter caps? Uh...pretty important? I ordered a 270Ohm 5w replacement
It’s a common mod to replace R71 with a choke. Seems like the previous owner was at least planning to do that, maybe along with replacing the transformer?
 

FleshOnGear

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More poking around revealed R71 completely removed... From what I can tell this is a bleed between the two filter caps? Uh...pretty important? I ordered a 270Ohm 5w replacement
Also, the schematic calls for a 7W resistor there.
 

daddysouldonut

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Also, the schematic calls for a 7W resistor there.
You're right, I had cancelled the 5w. I'll be looking into chokes now, makes perfect sense why that was missing but no char marks. Nothing was drilled.

Also noticed something really strange... On the small PCB bias board, they seem to have added very small wires along two of the tracks. They aren't bridging anything, just run from one end of a track to the other, almost like they're... Reinforcing them? Tracks themselves look fine.
 

D-Max

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See that's what's curious to me, why would you go in and install an FX loop mod but not also fix the bias drift- if it did in fact exhibit that issue?

The question is whether this particular amp suffers from it.
On the earlier DSL/TSLs this issue could develop.
At some point Marshall redesigned the PCB, so eleminating the risk of bias drift due to a PCB issue.
 

daddysouldonut

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You're right, I had cancelled the 5w. I'll be looking into chokes now, makes perfect sense why that was missing but no char marks. Nothing was drilled.

Also noticed something really strange... On the small PCB bias board, they seem to have added very small wires along two of the tracks. They aren't bridging anything, just run from one end of a track to the other, almost like they're... Reinforcing them? Tracks themselves look fine.
 

daddysouldonut

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We will see, I went ahead and ordered an appropriate choke to fill R71. Other than that I've just done a few (usual?) things like snip C46 and C12. I'm also working on a little backplate with a 40mm fan for each power tube. Will not be running off amp power.
 

Dogs of Doom

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what year is this model?

why did you snip C46? was it fried? what was it's value?
 

daddysouldonut

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what year is this model?

why did you snip C46? was it fried? what was it's value?
2002... It has the 60-00 board. I snipped it because I've read it's one of many culprits contributing to the biasing issue on these. Read that most people just remove it. Mine didn't look so good physically, though I didn't test it.
 

Dogs of Doom

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it doesn't contribute to the bias drift, but... the early models had an under rated one. they changed it from 500 to 1K & it seemed to fix the problem. The 16 ohm speaker out issue would cause C46 to blow.

Some people say it doesn't do anything, but, I've heard that it helps reduce/stop oscillation. YMMV...

Not sure what the 60-00 board means. They are all 60-00. The board, should have an ISS number. That will tell you more of what matters.

look at the red arrows:

1664519985644.png

when you look for schematics, you will look for the ISS#. The one above, is the latest, ISS# 21. The 21 is blocked out & there is no ISS# 22...

So, look at your board & see what the visible # is, & yours is that # -1... So, if 8 shows, yours is 7...

if you look at this one (ISS 19)

jdepu61srzxodf6lrtyt.jpg


also note R9, can be the culprit that causes bias to go out of whack. It, & it's pair R6, need to be in spec, to make the bias right. Usually, if they fail, the bias can read volts, not mV, or bias will be in the hundreds & you can't get it anywhere close to 40mV...

The other culprit, is the bias pots. Those cheap little pots can cause runaway bias.

Best bet though, is to fire it up w/ a lightbulb limiter & just see if she starts up, once you get the trans' in. No power tubes at 1st, then if everything seems ok, then put the tubes in.

You might want to hardwire the ground on the 16 ohm output as well, while you're waiting...

watch this for the next couple minutes after 27:54...



I know, it's a TSL vid, but, same series, almost identical board, same problems...
 
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