Opinions on Blown Power Transformer

[Blake F]

Hi folks, looking for expert opinions on an issue. Recently bought a 1969 Plexi 100. It has a weird issue and I want to see if I'm getting anything wrong in my theory:

It has a 120hz buzz that varies in volume with both volume knobs. I'm not a novice on amp repair; I've swapped good tubes, replaced ALL the filter caps, checked voltages, diodes- everything. Filtered DC doesn't have this buzz on scope anywhere. However, my scope does show a 120hz blip ALL over the preamp section, from components to the actual jack terminals and their 68k resistors with no input! I checked all the jacks etc. and they're fine. Pulling the PI tube kills the buzz on the speaker output. Removing V2 (only) buzz still present but doesn't vary with volume knobs. Removing V1 (only) buzz still present and varies with volume knobs. It's not blowing fuses. Pic below is what the buzz looks like and I confirmed it's 120hz from the waveform (8ms).

Thanks in advance.

 

[paul-e-mann]

Hi folks, looking for expert opinions on an issue. Recently bought a 1969 Plexi 100. It has a weird issue and I want to see if I'm getting anything wrong in my theory:

It has a 120hz buzz that sounds most like a sawtooth wave that varies with both volume knobs. I'm not a novice on amp repair; I've swapped good tubes, replaced ALL the filter caps, checked voltages, diodes- everything. Filtered DC doesn't have this buzz on scope anywhere. However, my scope does show a 120hz blip ALL over the preamp section, from components to the actual jack terminals and their 68k resistors with no input! I checked all the jacks etc. and they're fine. Pulling the PI tube kills the buzz on the speaker output. Removing V2 (only) buzz still present but doesn't vary with volume knobs. Removing V1 (only) buzz still present and varies with volume knobs. Again, I see the 120hz blips all over the preamp section- it doesn't appear to be coming from any one point. Upon looking at the fuses, I discovered someone had modified a fuse to be a short so the fuse wouldn't blow. The amp does blow a 2A fuse (the spec'd mains fuse value) but won't blow a 3A.

I will give my theory next, but I want the experts to let me know if I've missed anything: I believe what's going on is that the power transformer HV winding is arcing internally once it gets past a certain voltage as it charges the filter caps. Once it arcs, it put's out an EMF spike that's big enough that it's picked up on the highly sensitive preamp components and then amplified. To me, this explains why the power section doesn't pick it up when the PI tube is pulled. I can physically hear something 120hz that sounds like it's coming from the PT area. I know you can hear a little normally on a working amp especially on startup charging but this seems a bit louder. Just checking if I've missed anything- the PT's in these old amps are major surgery to remove and replace.

Thanks in advance.

I'm no expert, matter fact I know very little lol, but are your primary and secondary voltages good? Welcome to the forum, there are tons of experts here, hang out a bit and a few will be here shortly.

 

[neskor]

60hz is your AC. This can pollute your ground and your amplifier circuit.

120hz is a byproduct of rectification.
So this comes from the amplifier power supply or a power supply nearby.
Look for a bad ground connection on the input side, make sure all the solder joints and connections are sound. Its possible but unlikely a filter cap has failed.

 

[dtier]

The fact that it exists at the input jacks which are chassis grounded, with no instrument plugged in) suggests that it is in the ground plane (chassis) at that point. Do you see it there? Were are the rectifier anodes connected to the chassis? Is your scope isolated or grounded? ie: is the scope probe getting its ground from the wall plug or the circuit that its connected too.

 

[PelliX]

Does the amplitude of the wave change at all if you touch or press down on the transformer?

is the scope probe getting its ground from the wall plug or the circuit that its connected too.

Aye, and the amp itself, is that grounded or floating?

 

[Blake F]

The fact that it exists at the input jacks which are chassis grounded, with no instrument plugged in) suggests that it is in the ground plane (chassis) at that point. Do you see it there? Were are the rectifier anodes connected to the chassis? Is your scope isolated or grounded? ie: is the scope probe getting its ground from the wall plug or the circuit that its connected too.

The rectifier anodes are running a ground wire to the buss of the pots, not grounded to the lug of a filter cap pair like some drawings I have- but it looks factory original. As for the ground plane, how would I see the hum with the scope if I'm grounding the scope probe to the buss? I am grounding the probe to the pot buss- I don't get any hum signal if I leave the scope probe ungrounded.

 

[Blake F]

Does the amplitude of the wave change at all if you touch or press down on the transformer?



Aye, and the amp itself, is that grounded or floating?

That's an interesting issue I never thought of. I will check the ground and make sure the amp is wired correctly- the plug was changed to a U.S. plug from one of those British 240 plugs and the person could have miswired it.

 

[Blake F]

That's an interesting issue I never thought of. I will check the ground and make sure the amp is wired correctly- the plug was changed to a U.S. plug from one of those British 240 plugs and the person could have miswired it.

Well it's wired correctly and ground is working. Was hoping it was that simple. : (

 

[Blake F]

60hz is your AC. This can pollute your ground and your amplifier circuit.

120hz is a byproduct of rectification.
So this comes from the amplifier power supply or a power supply nearby.
Look for a bad ground connection on the input side, make sure all the solder joints and connections are sound. Its possible but unlikely a filter cap has failed.

By input side, what do you mean? Power input or guitar signal input?

 

[dtier]

The rectifier anodes are running a ground wire to the buss of the pots, not grounded to the lug of a filter cap pair like some drawings I have- but it looks factory original. As for the ground plane, how would I see the hum with the scope if I'm grounding the scope probe to the buss? I am grounding the probe to the pot buss- I don't get any hum signal if I leave the scope probe ungrounded.

It sounds like your isolated then. Most bench scope probe commons are at ground potential, (if the scope is grounded), and if so will introduce another ground connection to the chassis where it's connected. That could confuse readings if your having chassis grounding issues.

 

[playloud]

The rectifier anodes are running a ground wire to the buss of the pots, not grounded to the lug of a filter cap pair like some drawings I have- but it looks factory original.

That sounds like a recipe for ground loops. I would experiment with grounding rectifier at mains filter cap.

Some pics wouldn't go amiss.

Where/how well is your heater CT grounded?

 

[dtier]

Pots don't always conduct well to the chassis where there mounted. Black oxide lock washers or loose nuts, etc. If the best conducting one is on the input side that's where the rectifier diodes will be drawing current from as connected.

Just throwing ideas out there.

 

[neskor]

you can try Larry grounding scheme, just to be sure
also double check rectifier diodes

 

[Blake F]

That sounds like a recipe for ground loops. I would experiment with grounding rectifier at mains filter cap.

Some pics wouldn't go amiss.

Where/how well is your heater CT grounded?

I'll re-route the ground to the lug and check the heater CT. However, I've been experimenting with a ground jumper at various other points with no improvement. I would think a jumper would improve the situation if that was the problem since electricity always takes the easiest path to ground. I've also taken several ground lugs apart, scraped the chassis under them and the lugs and resoldered with no improvement- very frustrating.

 

[Blake F]

Pots don't always conduct well to the chassis where there mounted. Black oxide lock washers or loose nuts, etc. If the best conducting one is on the input side that's where the rectifier diodes will be drawing current from as connected.

Just throwing ideas out there.

wouldn't experimenting with a jumper wire to other ground points (like i've done) point this out though? But part of what you're saying is on the right track... the question seems to be going in the direction of 'how is this 120hz blip getting or reflecting back into the preamp section'? Correct me if I'm wrong, but it has to be coming through the wires or through EMF I would think. I appreciate you throwing ideas out there- that's exactly what i'm hoping for, getting my mind moving in a different direction because I can't seem to make any progress yet. I originally posited that it might be the power transformer (and thus the title of this article) but it's not blowing fuses; but do you think it's possible the power transformer is arcing internally and creating EMF spikes large enough to get back to the preamp section? I know there's lots of distance and a shield over the transformer but the blip is pretty small that i'm seeing in the preamp section and the filter caps aren't taking care of it so i'm wondering if it's EMF.

 

[KraftyBob]

I don’t know what caps are in a Plexi, but I recently swapped out a few bad radial caps in an Egnater Tweaker 15 and when I powered up I had an awful 120Hz hum. When I removed the bad caps I didn’t realize I also removed the little sleeve in the through hole that connects the bottom pad to the top so neither cap was grounded. I ran a flying lead across the cap grounds to the ground lug they were supposed to connect to and problem solved.

Just thought I’d mention this since you recently replaced the caps.

Tip: I use a guitar tuner app on my phone to determine the frequency of hum coming out of the speaker. Quick and easier than setting up my scope if that’s the only thing your checking.

 

[Pete Farrington]

The rectifier anodes are running a ground wire to the buss of the pots, not grounded to the lug of a filter cap pair like some drawings I have

The rectifier negative output needs to connect directly to the reservoir negative lug.
The arrangement you describe will tend to cause the symptoms you describe.

 

[Blake F]

Woohoo!!! Fixed!!!

This was indeed the rectifier negative output being connected to the pot buss. Re-routing the wire to the ground lug of the first set of can caps (the set with the bleed resistors) solved the issue! As mentioned in earlier in this thread, this looked to be a factory deviation as it had the red dye on it. THANKS SO MUCH TO ALL WHO CONTRIBUTED!!

 

[Blake F]

The rectifier negative output needs to connect directly to the reservoir negative lug.
The arrangement you describe will tend to cause the symptoms you describe.

Thank you. I was already attempting this when you posted but you were spot-on. Weirdly enough, the ground to the pot buss looked factory, red dye and all.

 

[playloud]

Excellent! It's not that surprising if you think about it. What happens is that the 120hz starts flowing in the ground plane (i.e. between points at different potential) and then it gets into your signal. Basically if your input signal is (input tip)-(input jack ground) and your output signal is (output tip)-(output jack ground), and (input jack ground)-(output jack ground) includes the 120hz buzz (of audible magnitude), then your output signal will include it (possibly with reversed polarity).

wouldn't experimenting with a jumper wire to other ground points (like i've done) point this out though?

No. All you're doing here is introducing new potential paths for ground loops