Marshall IBS 3520 200 watt problem

[g1]

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...

Go back to your original post and see if you see the "edit" button. If so, maybe you can change the post title? If there is no edit button, then you are past the time frame for editing that post.
That being said, why change the title? There are possiblyy many 3520's with the 3540 boards, the schematic problem may come up again for others.
Sounds like this amp is of fairly unknown origin. You will want to give it a real good going over before you run it.
You may see some low resistances when measuring the output devices, this can be normal for power Fet's, so don't be too alarmed.

[DrGonz78]

Well I did double check the model # on the amp and it is 100% a 3520 amp, so you guys are spot on about 3540 power modules. No need to change the title of the thread since this might happen to others working on this model amp.

Anyhow I am now replacing burnt resistors and checking drift on others as well, so far so good. Also, I am looking to replace the TR4 (SA93) and the TR2 (A06) and I happen to have three A06 on hand so probably will put all new ones in on this board for safe measure. Checked all the zener diodes and they are good.

So now for the K135 and J50 mosfet transistor readings:

****Note anytime that I say "Nothing" that means the diode did not read anything nor did it beep for continuity. D = Drain || G = Gate || S = Source***
****First reading is on each is always Black and Red clips then reversed Red and Black clips of the multimeter... So it will read like (G&S = .227 and then G&S reversed = Nothing). First in the series the probes will always be Black clip then Red clip then reverse clip leads for the next reading between pairs*****

K135 # 1 (first Transistor)
G&S = .227 || G&S reversed = Nothing
G&D = Nothing || G&D reversed = Nothing
D&S = Nothing || D&S reversed = Nothing

K135 # 2 (second Transistor)
G&S = .199 || G&S reversed = Nothing
G&D = Nothing || G&D reversed = Nothing
D&S = Nothing || D&S reversed = Nothing

J50 # 3 (third Transistor)
G&S = Nothing || G&S reversed = .369
G&D = Nothing || G&D reversed = Nothing
D&S = Nothing || D&S reversed = Nothing

J50 # 4 (fourth Transistor)
G&S = Nothing || G&S reversed = .318
G&D = Nothing || G&D reversed = Nothing
D&S = Nothing || D&S reversed = Nothing

I thinks I have the G S D references correct...? I have included a pic that shows my referencing of these transistors.

So this is where I am at testing these transistors. I have removed them from the board so they are being tested freely. I would also test J50 # 4 G&S reversed = .318 then I could touch the Drain (finger) to manipulate the readings and then touch the Source again to switch it back and forth... That would make it go up and down on the diode function of the DMM.

I also tested this on Vdc to see it raise or drop the voltage through the transistor. Thanks again to all that have helped or will help on this project. Please let me know what you think about these readings and the best place to order new mosfets if I need them... Thanks

[J M Fahey]

Lucky guy, those expensive MosFets look fine.
Put them back carefully, with new micas and grease (I don't trust removed and replaced ones very much), tighten bolts and nuts reasonably and resolder them.
Recheck that your board matches exactly the 3540 schematic and we'll continue testing.
Obviously, if there are "mysterious wires" or components added or parts missing, let us know.
Good luck.

[DrGonz78]

Great to hear this as those are really expensive mosfets!! The micas are actually in good condition still but yeah since I am ordering some misc. parts already might as well get a fresh start.

I think I am also going to pull off the BR to test it freely (diode setting) as well but might need to even look at this power transformer too at some point. Is there something I could do as far as testing what the B+ and B- rails are at before hooking them back up to the power board? Will wait for parts to continue and follow your instructions to make sure this beast does not suffer anymore disasters. Thanks

[J M Fahey]

Start by plugging the amp into the bulb lamp limiter , with PSU unhooked from the power amp board, and see what happens.
If you get +/- rails you have just tested transformer/bridge/filter caps as a system, no need to test "part by part".
Solder a 2K2 2W (or 4K7 1W ) across each filter cap or they may never discharge on their own.

[DrGonz78]

Sorry in advance to ask so many questions but I have a few points of interest on this amp before I start my testing. Just being extra careful with this one...!

So I decided to replace all the NPN and PNP type transistors on the power board, since TR6 (BC182) looked a bit stressed on the case. Anyhow, did a lot reading to come up with replacement parts for these transistors. I already had the three MPSA06 NPN so I put those in already since I knew they were exact matches. I got the A93 transistor replaced by way a 2N6520 part that was the best solution, over rated now at 350v max instead of original 200v. So these were all cross referenced and it all seems to have been good fits for the now obsolete A93.

My question is about replacing BC182 & BC212... Here is the original datasheet archive from MicroElectronics. The BC182 was made by ME and the BC212 was made by ITT. I found this datasheet in conjunction with these manufactuers.

BC182&BC212.pdf

I was just wondering if these are good replacements. BC212 = BC556 and BC182 = BC546. I think now they are over rated but these are the only replacements that were cross referenced that I could find. Let me know if they are good, thanks.

http://www.onsemi.com/pub_link/Collateral/BC556B-D.PDF

http://www.onsemi.com/pub_link/Collateral/BC546-D.PDF

Also, Juan had told me how to read the B+ and B- voltage. So basically light bulb limiter plugged in and I would just disconnect both the B+ and B- wires that connect to the power board. All other wires connected to the power board correct? You said put those resistors on the caps to release the charged caps when it is then powered down. I am gonna try this soon after I finish putting all the parts on the power board. So I would ground meter to the chassis and just read each wire B+ and B- accorrdingly?

Lastly, there is C11 on the power board and it is right next to TR4 A93 that flared up. I think it needs to be replaced as it was probably stressed by the heat from the flared up transistor. It say 220P. Which I think means 220 pf and I have a replacement 220 pf ceramic cap. I am just thinking it is probably more like 100v or 200v and I have a 50v part here. There is nothing saying what the voltage is on this part. Does anyone have a good recommendation on the changing of this C11 part?

Thanks to all contributions to this thread so far and thanks for reading.

~DrG

[J M Fahey]

Yes, disconnect only +/-B wires, but leave ground connected or you won't have reference to chassis.
Since power consumption will be very low, you'll *almost* get proper voltages, mayve 5V less.
The discharge resistors are needed or caps will hold their charge and when reconnected to the board may fry something expensive.
Don't replace transistors at random, "looks fried" does not mean much, unless the case exploded like a bomb.
Every part you replace introduces new doubts and may make repair impossible.
Trust your multimeter more than anything else.

[MarkusBass]

Yes, you can use BC546 and 556. Just make sure that they have pins in the same order. MPSA93 can be replaced by MPSA92, which is available in almost every shop I know:
http://www.datasheetcatalog.org/data...ens/MPSA92.pdf
The capacitor - I would buy 200V rated.
BTW, what exactly is the problem with the amp now?

Mark

[DrGonz78]

Points well taken Juan I went back and took out the new A06 transistors and put back in the old ones. I had to change out one A06 since it fried up. I also put in all the burnt out resistors and have soldered over the caps the 4k7 1 watt resistors too. It is all put back together and I am ready to go check voltages while hooked up to the bulb tester. I have yet to replace the BC182 and BC212 transistors so those are still in the amp.l

Yeah Hey Markus... Yeah the amp is still waiting for a check of the voltage of B+ and B- rails to see if power is making through BR. Also, testing in this way to check the whole system with PT and Filter caps. I used a 2N6520 in place of the A93... Do you think this is a good enough match figuring it is not far off from the A92 you suggested and the legs match on the 2N6520?

http://www.onsemi.com/pub_link/Collateral/2N6515-D.PDF

Thanks again guys

[DrGonz78]

Well here is the outcome so far as to the testing the BR, Caps, and PT as a whole. Nothing. No voltage detected on either rail... Not even a mv. So, first guess is that the BR84-8432 is bad or shorted I will take it out tomorrow and test it independently. Let me know what you all think too. Thanks for the guidance so far and can't wait to get this one in the bag.

Edit: BTW when I turned it on it passed the bulb test... Got bright and dimmed out quickly w/ a 100 watt bulb in place.

[Jazz P Bass]

In the original post, you had voltage.
Now you have 'none'.
Check the fuse?

[J M Fahey]

when I turned it on it passed the bulb test... Got bright and dimmed out quickly w/ a 100 watt bulb in place.

Sorry but this is typical of a properly working PSU.
Where are you placing your test leads?

[DrGonz78]

In the original post, you had voltage. Now you have 'none'. Check the fuse?

The fuse is good T4A-250v checked it w/ meter and it's doing fine. The first postings voltage was detected while the output board was attached and no light bulb limiter. Yes, 7 vdc on the output. I went and flowed solder over the BR connection since the solder joint was eroding away. Right after the soldering the thing lit up the two transistors and 4 resistors. Could that BR have been at the end of it's life? I wish I had tested what the B+ and B- voltage was prior to re-flowing BR solder joint.

Sorry but this is typical of a properly working PSU. Where are you placing your test leads?

I included the part of the passing of the dim bulb test to make clear that PSU is probably fine. I hope. Anyway I am grounding my probe to the chassis right where it clips to ground from the power input plug, right on the clip. I always a take a few minutes for continuity checks and to make sure that it is grounded before voltage checks. I have "both" the Red (B+) and Blue (B-) wires disconnected from the power board circuit, but everything else it connected up on the amp. I then clip the other probe to either the Red or the Blue wire to test the voltage or each one.

[J M Fahey]

Just to reduce variables, please measure AC volts between PT secondary center tap and winding ends and DC volts across capacitor legs.
*Maybe* through some connection problem chassis is not a reliable reference point.
In these problems "it *should* work ... but it does not", I try to dumb myself big time and check even the most stupid stuff, twice or more if necessary.
It's not wasted time.
And ... good luck

[DrGonz78]

Okay let me just make sure I am doing this right and I got pictures for reference. I will put probe on the CT connection (2 brown wires - Secondary CT) for my ground and then measure Black wire (BK~) and then Red wire (RD~) for AC voltage to see what I am getting coming off the PT. Then I can stay grounded to the CT and then test DC volts B+ and B- right there on this board. Also, then check the legs of the filter caps too. After looking at this all I am starting to see the power staged circuit in a different light. I like the point to point wiring to understand what is really going on as the PT steps down and converts into DC voltage. I am going really slow on this to really digest all that I can on this one. THanks