Marshall bass amp 3520 (1988)

[g1]

What is the voltage rating on the large blue caps?
On the rectifier board there is double black, double blue, and double red wires. What is the resistance reading from black to blue and black to red?

[rudutch]

What is the voltage rating on the large blue caps? 10,000UF, 63Vdc
On the rectifier board there is double black, double blue, and double red wires. What is the resistance reading from black to blue and black to red?

open, open, open

[g1]

93VAC into 35 ohms is just over 2.5A through the PT secondary. And it is not blowing the fuse? Can you measure the current at the fuse holder with the same load on the secondary? It doesn't make sense that you were blowing fuse and arcing with no shorts at the rectifier etc. yet no problem with a 35ohm load. (By the way, that 35ohm load is running about 250watts with 93VAC across it).
Best case scenario would be that the rectifier had a short, and bypassing the fuse burnt it to an open condition, but that is just a hopeful guess.
You should remove the rectifier and see if all it's diodes are ok. Also see if you can get a price on that transformer from a Marshall dealer.

[J M Fahey]

Dear rudutch. Both g-one and me are still worried about that 35 ohm load you mention, since, among other things, it means a power consumption of over 250 Watts, not a minor matter.
Would you be so kind as to draw a simple schematic showing the transformer, its wires (indicating their color) and what those 35 ohms are connected to?
Otherwise offering useful suggestions is impossible.
Thanks.

[rudutch]

there are 4 primary and 2 secondary windings - the secondary winding go to a rectifier. I isolated the rectifier,
Used a 27w 120 incandescent lamp to check the xfmer with a 'load'. That is where the reading came from

I am convinced the power section is shorted

[Steve Conner]

Well, get the power transistors out and check them for shorts.

The Hitachi originals are long out of production, but the BUZ901 and BUZ906 are modern equivalents, available from Newark. You might even like to try the dual die versions, the BUZ901D/906D. They can handle more dissipation, and might make the amp more robust.

There should never, ever, ever be sparks coming from inside a transformer. It implies a serious fault and a fire risk. If you find that the transformer is busted, Antek sell reasonably priced toroidal transformers on Ebay USA, in a choice of voltages suitable for both tube and SS amps.

You could also replace the whole power section with something else that used cheap bipolar transistors, which is what I'd do.

[J M Fahey]

Now some of your strange readings start to make sense.
You spoke of 35 ohms without qualifications and calculated dissipation was over 250W, current over 2.5A and so on.
A tungsten LAMP is a PTC, *most* non linear resistor.
It *might* be 35 ohms, *cold* , but with 120V applied it will be over 530 ohms. Go figure.
There must be a 3rd secondary wire, the center tap.
It should go to the ground, which is also the centerpoint of the big 10000uF capacitors.
Now connect the transformer to the rectifier and filters but disconnect the + and - wires from the power board, to check that the PSU is working ... or not.
Always with the bulb lamp limiter in series with mains.
Before that, solder a 4K7 1W resistor across each capacitor, to safely discharge it after tests.
Measure output transistors for shorts; if in doubt pull them out and re measure.
The beauty of lateral MosFets such as those is that big power amps are very simple.
Even if parts are relatively expensive they will be cheaper (or equivalent price) to buying a new power module, plus the hassle of adapting it to the available space.
If *I* had it on my bench I would convert it to cheap Vertical (switching) MosFets and call it a day, but I don't suggest you doing so because thermal stability tests should be made and I don't trust "remote control" for that.
But those BUZ something aren't that expensive.
The originals, of course, are out of the way.
And the transformer *looks* fine, so far.
Good luck.

[rudutch]

Now some of your strange readings start to make sense.
You spoke of 35 ohms without qualifications and calculated dissipation was over 250W, current over 2.5A and so on.
A tungsten LAMP is a PTC, *most* non linear resistor.
It *might* be 35 ohms, *cold* , but with 120V applied it will be over 530 ohms. Go figure.
There must be a 3rd secondary wire, the center tap.
It should go to the ground, which is also the centerpoint of the big 10000uF capacitors.
Now connect the transformer to the rectifier and filters but disconnect the + and - wires from the power board, to check that the PSU is working ... or not.
Always with the bulb lamp limiter in series with mains.
Before that, solder a 4K7 1W resistor across each capacitor, to safely discharge it after tests.
Measure output transistors for shorts; if in doubt pull them out and re measure.
The beauty of lateral MosFets such as those is that big power amps are very simple.
Even if parts are relatively expensive they will be cheaper (or equivalent price) to buying a new power module, plus the hassle of adapting it to the available space.
If *I* had it on my bench I would convert it to cheap Vertical (switching) MosFets and call it a day, but I don't suggest you doing so because thermal stability tests should be made and I don't trust "remote control" for that.
But those BUZ something aren't that expensive.
The originals, of course, are out of the way.
And the transformer *looks* fine, so far.
Good luck.

shorted rectifier..
all is good now

thx for the assistance

[Jazz P Bass]

Glad you got her fixed.

[marklim]

Hi, I really need some info. I am repairing an IBS 5510. It's got a blown transformer (Drake 7-200/6).

Could anyone tell me the secondary voltage of the tranny and also the power rating required (VA)?

Many thanks.

[J M Fahey]

Hi, I really need some info. I am repairing an IBS 5510. It's got a blown transformer (Drake 7-200/6).

Could anyone tell me the secondary voltage of the tranny and also the power rating required (VA)?

Many thanks.

How do you know the transformer is blown?

[marklim]

How do you know the transformer is blown?

Tested it. No inductance between both primary and secondary leads. Also scorch marks on the input leads. Isolated from rest of circuit. Totally no Vs from secondary when connected to mains.

Hope one of you guys can advise me. Just need to know the secondary voltage. Then I can get a replacement and rebuild both the PS and Output sections.

These are the preamp and PS/output schematics:



Thanks yall...

[J M Fahey]

Tested it. No inductance between both primary and secondary leads. Also scorch marks on the input leads. Isolated from rest of circuit. Totally no Vs from secondary when connected to mains.

Hope one of you guys can advise me. Just need to know the secondary voltage. Then I can get a replacement and rebuild both the PS and Output sections.

These are the preamp and PS/output schematics:
[ATTACH=CONFIG]20144[/ATTACH]
[ATTACH=CONFIG]20143[/ATTACH]

Thanks yall...

OK, now we're talking.
Sorry for asking for a *measured* confirmation, but *every* noob starts by asking "my transformer is dead, isn't it?"
So much so that there is a long thread talking only about that, go figure.
Funny thing is that only 3 or 4 (including yours) out of almost 100 were actually bad.

Back to the problem: +/- 55V DC rails were mentioned in reference to your amp, which sounds quite reasonable given its power output (in the ballpark of 200W into 4 ohms).
That points to a transformer with (0.7x55) = 38.5 + 38.5 V AC ; around 300/350VA .
AC voltage is acceptable between 36+36 to 39+39 .
This to ease your search.
Good luck and please post results.

PS: what about the power amp itself?
If power transistors are dead, good modern (UK made) replacements are available.

[Jazz P Bass]

Here is a DYI Audio link with some of the spec's & the Drake part #.
Link: Marshall Toroidal Power Transformer - diyAudio

[marklim]

OK, now we're talking.
Sorry for asking for a *measured* confirmation, but *every* noob starts by asking "my transformer is dead, isn't it?"
So much so that there is a long thread talking only about that, go figure.
Funny thing is that only 3 or 4 (including yours) out of almost 100 were actually bad.

Back to the problem: +/- 55V DC rails were mentioned in reference to your amp, which sounds quite reasonable given its power output (in the ballpark of 200W into 4 ohms).
That points to a transformer with (0.7x55) = 38.5 + 38.5 V AC ; around 300/350VA .
AC voltage is acceptable between 36+36 to 39+39 .
This to ease your search.
Good luck and please post results.

PS: what about the power amp itself?
If power transistors are dead, good modern (UK made) replacements are available.

Hahaha
I've been doing electronics for over 20 years, but only recently doing it for a fee
The amp is rated at 100W into 4ohms... After feedback from you guys and Dr. Tube, I got my answer.
Planning to change the tranny, it's confirmed to be 35V X 2. Also gonna change the 2 filter caps to Vishay units and the pair of 'FETs too...
Customer said since it's on the bench, might as well...
Will update you guys once I get it done. Hoping to get the parts in a couple of weeks.

THANKS AGAIN TO EVERYONE.