Marshall 8080 - Lack of volume on boost channel.

[J M Fahey]

Hi OCD, you're welcome, discussions here are wide open, and, besides, 30 brains think more than one .... or at least should
What you post is excellent, I should also like to add that that *very* thin wire needs not be scratched, because it's the "self-soldering" type, and the enamel becomes soldering flux when heated.
Just unwind a little, wrap a couple turns around the post or through the eyelet (whatever you have), solder , and you're done.
Don't let it be tight, allow some slack, or vibration will cut it.

[Trevor]

Thanks for all your help here. I'm pleased that I have tracked down the faulty component at least!

I will try and remove the auto transformer and look for the small wire you speak of. I have read on the internet that it is uncommon for these transformers to fail like this. What are the chances that there is a short somewhere else in the pre-amp circuit that caused the secondary winding to burn out? Is this something I can test for etc? If that is the case and I fix the winding or replace the component it will just burn out again the second I power it up.

One other thing I would like to ask regarding the small auto transformer - what is the correct way to test the primary and secondary windings? I have been measuring the resistance between the input leg and the ground leg (30ohms) and the high voltage output leg and the ground leg (no continuity)

[oc disorder]

"What are the chances that there is a short somewhere else in the pre-amp circuit that caused the secondary winding to burn out?"

Pretty Slim.. apart from being a gay country singer there is not much
that could short maybe C59 .. its more likely from vibration or being dropped,
unlikely short in tube a very long shot.

"Is this something I can test for etc?"

Try the tube in something else - with autotransformer out of circuit
measure resistance/continuity between ground and D3.

"If that is the case and I fix the winding or replace the component , will it just burn out again the second I power it up?"

Doubt it, if you get a good join , but its down to lady luck or Pretty slim...
Tell us after you do it.....

"One other thing I would like to ask regarding the small auto transformer - what is the correct way to test the primary and secondary windings?"

You're doing ok so far - its just one big coil (winding) with taps
see autotransformer: Definition from Answers.com

Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer

Cheaper than an extra winding on the mains transformer apparently
can an accountant explain this...?

The only problem may be if there is not enough slack to get a decent termination or if there was a weak point internally.

Although J M is confident the soldering process will melt the insulation
in my experience this is not always the case.

It's micro surgery - toothpicks tweezers and emery paper - you may have to
move the mountain to Mohammed , ie extend the terminal to the wire if you get my drift.. so there is minimal strain on the fine wire ...

Good luck on your mission...!

[Jazz P Bass]

Marshall power transformer

Keep in mind that you are measuring a parallel resistance if the transformer is mounted on the circuit board.
(Look at the schematic.)
Out of circuit tests will differ as far as winding resistance goes.

Mileage may vary.

[Trevor]

Thanks for all the valuable info.

Today I removed the autotransformer from the PCB and inspected it visually. I could not see any broken wires on the underside. I smelt it also and it did not smell like it had overheated and burnt up. Who knows what's wrong with it. Maybe it just died of old age?

I decided to call Marshall and order a new one of these autotransformers that luckily they still carry and have in stock.

Watch this space! When I get the item in the post I will re-fit the new unit and post feedback!

Thanks again,

Trevor Keast

[J M Fahey]

It might be something as simple as corrosion.
On a humid place it does not take very long to eat through fine wire as that.

[Enzo]

It can be something as simple as one of the fine winding wires snapping from vibration.

[Trevor]

OK, so today I received my new autotransformer from Marshall in the post. I replaced the old one with the new one in the amp and powered it on.

The first thing that struck me was that the main transformer started to make a slight buzzing sound as if it was working quite hard all of a sudden that I don't remember it making before I replaced the part.

I took my digital multi meter and measured the voltage on the DC side of the diode that comes after the autotransformer and I did indeed have a high voltage this time that I didn't have before.

However, as I took the measurement, there was a sudden crack and a puff of smoke from the other end of the board and I noticed that the board mounted fuse had blown!! I'm not exactly sure where the smoke came from but I'm hoping that it was the fuse and not another part of the amp that is now destroyed.

Where should I go from here??

Many thanks,

Trevor Keast

[Enzo]

The part that blew up was part of the original problem SOmething was shorted, adn it caused the old transformer to overloaqd and fail. Now you have replaced it, so there is once again power in the faulty circuit, and since that circuit was never repaired, stuff still burns up.

I'd be looking for shorted rectifiers and filter caps in that high voltage supply.

[J M Fahey]

What worries me is that the autotransformer, even if fully shorted, does not "eat" enough power to blow a mains fuse all by itself.
Current (and power) is limited there in two ways:
1) R114 (100r/5W) is in series with the primary. At worst, with a shorted winding, it would dissipate 13W, more than enough to toast it in, say, a minute; that didn't happen.
2) If C59 (33uFx350V) or D3 (1N4007) were shorted, (check both), R111 (1K/¿?W) would burn.
Lift one end of D3 and check the AC voltage coming from the autotransformer.
You should have 100 to 200VAC there, it's unspecified on the schematic.
Good luck.

[Enzo]

What if the short was a meter probe inadvertently touching where it ought not?

[Trevor]

I was quite careful with my meter probes. Also as I mentioned above the MAIN transformer was buzzing slightly as if it was handling a large load even before I touched anything with the meter.

I will remove diode D3 next and measure the voltage coming out of the HV side of the autotransformer.

I will test all the caps in the HV circuit also however I can confirm that the larger of the two caps in that circuit was holding a good high voltage charge as I needed to discharge it before I could finish up with my testing yesterday.

First thing I need to do is obtain a new fuse...

I will report back!

Many thanks.

Trevor Keast

[J M Fahey]

Well, you are pre-answering something, even if by chance: the fact that one HV capacitor still had "a high voltage on it" already tells me that
a)your transformer supplied HV AC
b)the diode works as intended
c)the capacitor too
d)there are no shorts on the HV line.
Conclusion: you probably have a problem, even an important one ... but not just there.

[Trevor]

Today I got some new fuses from Radio Shack. They were not the exact same T2A 250V as was in there before, they were simply 2A 250V slow blow from Radio Shack. I'm guessing that would be acceptable.

The first thing I did was remove the diode D3 in the high voltage circuit to measure the voltage coming out of the autotransformer. After removing it and powering up the unit, the fuse blew instantly.

I then decided to go one step further and remove the autotransformer. Again, after powering up the (second new fuse) blew instantly.

This concerns me as I was running the amp just fine with the autotransformer removed last week.

At this point I'm lost. I have no idea where to look next. Have I killed it?

Many thanks,

Trevor

[Trevor]

I have now calmed down from the initial shock of thinking I have killed the amp and continued with testing. It does appear that more damage was done when the inital fuse blow occured.

The fist thing I did was to disconnect all the secondary coil connections from the transformer and turn it on with a new fuse. This works! So the main transformer is OK! I measured the voltages on the secondary coil and they were all fine.

So, the next step I took was to test the resistance between the legs of the power transistors mounted on the heatsink. Now these SHOULD be ok because the power amp was working just fine on the clean channel however my tests indicate otherwise. On BOTH transistors I have a reading of 1.2 ohms between the center leg and the right most leg. This cannot be right.

So, next step, I removed both the power transistors from the board and re-tested. The results were the same...

So, what I decided to do next was to power up the amp with both the transistors removed to see if the fuse still blows. I re-soldered all the wires back onto the secondary coil of the main TX, I put in the new fuse, and I wired a 60W mains light bulb in series with the mains live into the amp.

I turned the amp on and the fuse was OK! Upon pressing the power button the mains light bulb glowed very faintly for a fraction of a second and then faded to no light.

Please also bear in mind that I have currently got the autotransformer and the diode D3 removed so the HT preamp is not powered up....

Where should I go next from here in your opinion?

Many thanks,

Trevor Keast