Marshall IBS 3520 200 watt problem

[DrGonz78]

Well so far the amp passed the dim bulb test up to 100 watt bulb inserted... I then took voltage on the speaker leads to find about 7vdc on the output... Now I am pretty sure that is telling something on the board is off and that just maybe the output transistors are fine. I found R8 and some dude had soldered the legs of the resistor to some wired extensions. Very strange that you could easily put the resistor in place w/ out these extended legs, but that is the least of it all. There are three joints that connect on this exact pad and it had a nasty cold soldering job too. The schematic says that R8 is 220 ohm resistor and the other dude had put in a 2.2k resistor...? Looks as if there is a mod on the power board as well but not sure if it was factory or "some dude" mod. Anyway I am pretty sure that I need to replace the 2.2k R8 resistor w/ a the proper 220 ohm one, but just wanted to start this project here with all you guys. Any feedback and thoughts are encouraged to help here on this as I am working tonight on the amp. Thanks.

I got into this one now because there has been some recent activity w/ this type amp as of late, and there is always good advice here as I go into the project.

[DrGonz78]

So I am not sure about one thing... The preamp board that I am working on is JM90D and the schematic is JM90E? So, that resistor might have been correct or is this preamp board identical is the basics? I think looking at the schematic that R8 traces out to ground and is pretty much the same (have not found it to be any different).

Anyhow I have the power amp board off and while inspecting the legs of the output transistors I have come across one that is barely soldered onto the board (cold solder). I will test the transistors, but not sure what I am looking to measure. I will test them to ground w/ diode/continuity setting to see the readings. Will reflow the solder joints and then test the output to see if the 7.5 vDC is still present.

Edit: I have tested the transistors and it appears that they are not shorted out to ground. I have definite readings that indicate to me that they are good. So that is a relief for now unless my readings are not right. Two pairs were reading. 1.4 and .230 approx. and the other two pairs were 1.3 and .300 approx.

[Jazz P Bass]

Post the schematics?

[DrGonz78]

Sorry

bass_200w_3520.pdf

[g1]

On that schem. R8 is 100 ohm.???
From the position it is in, I'm wondering if someone was attempting to affect the bias?

[J M Fahey]

1) having 3 legs, you have 3 pairs.
Identify D/G/S and post all 6 pairs for each transistor.
Remember to use diode setting, not a resistance scale.
For good measurement without pulling them , lift 1 leg of R16/17/22/23/18/19/20/21/13/26
2) put R8 back to 100 r.

[DrGonz78]

Yes g-one R8 is 100ohm on the power board page but I am pretty sure that I am referring to the R8 on the Preamp board on page one. This shows a 220ohm as the value... I really hate it when they reuse component numbers on multiple boards, but this one does. I think I was right on this but please let me know if you look at it again, thanks. Here>>> page 1



Yes Juan I will do a better check on those transistors and report back here. Sorry, I was running out of time last night and will spend more time on this week, thanks.

Edit: I will have to look at the names of each board and track this down. I will look at the other R8 and check it's value... Also, page 2 of the schematic shows (I think) JM90D on that list for the components it is referencing, so maybe it is 100ohm R8 but it's so confusing.

[MarkusBass]

R8 in the preamp is not critical for the amp and it cannot cause any problems with the power amp (especially 7V DC on the output). It can slighly influence mains hum - put there 220 Ohms and leave it. You have problem with the power amp and this is something that you should look at.

Mark

[J M Fahey]

Dr Gonz, I think the *big* problem here is a dead power amp.
For now, let's stick to it and forget the preamp.

[DrGonz78]

Juan & Markus you are more than correct.

Here's the other problem: I have looked over this schematic on the power board and it just is not adding up at all. That R8 on the power board says 100ohms on the schematic and it measures 10K ohm. That is w/ the leg pulled too. If you look at TR5 and TR6 on the schematic they are complete the opposite on my board. Schematic reads TR5 BC182 and TR6 BC212 which is the exact opposite of the what is on this board. I don't see anything that says RV1 anywhere. Measuring tons of resistors that are not matching anywhere close to the schematic. Looking at the board and matching the resistor color codes is really proving that this board does not match the schematic in so many different ways. So to really troubleshoot this amp I am going to need the right schematic. Especially when >>> Read on...

To be honest four resistors and two transistors got fried after a solder re-flow. I had not changed anything but after the soldering I think the power flowed through the circuit stronger or something. I know the Dim bulb would have been useful here but I had done it before the soldering. I was kicking myself at this point!! I looked at the soldering that I had done and nothing was wrong. So trying to figure out the fried resistor values seems nearly impossible now since the schematic is proving to be a bust for me on this power board (JM89C).

There is some sort of mod on this board too w/ a resistor and a Zener diode running in series off of R4 leg to ground side of R5 leg. ??? Weird stuff...

Two of the output transistors TR 7 TR 8 read K135 and the other two TR 9 TR 10 say J50. Are those the same ones on the schematic???

This is turning into a horrible repair on this one lol! Oh well can't win em all, but just looking still for somebody who might have an idea on the correct schematic. This might have to sit in the closet for a while until then...?

TR 2 and TR 4 fried up which might have been failing to begin...? Also, R9 R11 R13 and R19 are burnt... Good thing the amp was free, but not looking like any profit coming out of this guy.

Edit: Kicking myself one more time for not firing up w/ dim bulb after soldering...

[DrGonz78]

So do you think someone modded up this amp completely or is just a different board version?

Edit: I did some test on the output transistors but not sure it they are any good? I think they are toast now if they weren't before the disaster struck. Any advice on testing these transistor is greatly appreciated.



EDIT: Definitely that was not a mod as per another pic I found but reading this schematic has me confused none the less. Transistors have different models but are probably the same really. Noticed the larger wire wound resistors too.

[MarkusBass]

So the board does not match the schematic. Doesn't it tell you that you don't have a correct schematic or this is a different amp model than you think? A quick search on drtube web site shows that this is the 3540 power amp and the schematic of this power amp is available here: http://www.drtube.com/schematics/marshall/3540pwr.gif
Please change the subject of this topic in order not to confuse people reading the topic.

Mark

[DrGonz78]

Markus you are the Man!!! I swear to God on the back of the amp it say 3520. I will look again and see if it looks like 3540... Even the guy that I got it from had 3520 on the item. I can tell right away looking at the schematic that you have set me on the right path and I really can't thank you enough for the correction...! So, now it is all about rebuilding this poor old power board slowly and correctly.

I have no idea how to change the title of the post... Can anyone help with that cuz I really want this thread to reach the right point of interest. Let me know how if you know...

[J M Fahey]

1) Markus =
2) I *think* that Marshall had to deliver a few 3520 and were short of Power PCBs .... spare 3540 PCBs to the rescue.
Quite logical from a Mfg's point of view.
I do it all day long: many 60W combo customers unknowingly received a 100W PWR , PSU and even Transformer sometimes.
Good thing is now you are in the correct path.
As to testing the power transistors, refer to post #6 ... although given that you are not so sure as to which resistor is which, it might be better to (carefully) pull them and measure outside.
Mark them with a sharpie so they go back to same position (if good).
Have new TO3 micas on hand, of course, plus a little grease.

[MarkusBass]

Yes, this maybe the case. 3540 was a 2x200W amp and 3520 was 1x200W. 3540 was introduced 1 year after 3520. You can read more here: Marshall Amps Info & Schematics (in the middle of the page).

Mark