Ampeg BA115HP (2003) humming

[okw]

Hello! I recently picked up a defective 2003 model BA115HP. It has a very loud transformer hum, noise in the speaker and a sharp bang in the speaker when I turn it off. I disconnected the secondary side of the transformer (to rule out the amp), still humming/almost vibrating, and a big "dang" when I turn it on. I removed the trafo from the "hot side" of the amp PCB (before the bridge rectifier), to rule out the two 0.0033 caps. Still humming. Is this a failing transformer, or will it do this anyway under no load?
I didn't dare to test more with the amp PCB connected. Any tips?

[Enzo]

Transformer? It sounds more like you have DC on the speaker, and that usually is the result of a failure in the output stages of the power amp.

[g1]

But the secondary is now disconnected and still transformer buzz? If it's not drawing current, could just need some creative shimming if something is loose.

[The Dude]

I, at first, passed on responding to this thread because it didn't make sense to me.

First, "It has a very loud transformer hum, noise in the speaker". Then, "I removed the trafo from the "hot side" of the amp PCB......Still humming."

So, if the hum is in the speaker (electronic), unhooking the transformer secondary would make it stop. If the hum is mechanical (transformer vibration), it wouldn't likely be heard in the speaker. We need some clarification or there's some weird magnetic coupling thing going on.

[Enzo]

Clearly we have a communication issue in the description.

[okw]

Thanks for all the quick answers. Maybe it was a little unclear, but as I wrote, the transformer is humming, regardless if connected or not (with or without load on the secondary side - it is humming when plugged in and making a big "dang" when turned on). When troubleshooting, I usually start at the beginning (intake->fuse->power supply->input caps->etc.->). That's why I disconnected the transformer to rule out this. Turns out the transformer hums without load. It might be ageing? But it's only from 2003 and should last much much longer than this. That makes me think something further up the chain is drawing too much power and deteriorates the trafo quicker than usual, especially since the speaker makes noise (when everything is wired up or course). The two 5600uF caps aren't bulging or leaking (as I can see) and I measured them both to be 5300uF and with very very low ESR with my MESR-100 meter.

[Helmholtz]

Mechanical transformer hum is often caused by vibrating, loose core laminations.
The root cause for core vibration is the magnetic AC flux produced by the magnetizing current.

As loading the transformer does not increase core flux, the load currents have no influence on core vibration.
(The load currents can increase transformer hum by winding vibrations though, caused by the repelling magnetic forces between turns.)

If the PT was designed for 60Hz, 50Hz operation will increase magnetizing current. Same with higher than nominal input voltage.
If the PT wasn't designed with some headroom, magnetizing current can rise considerably because of beginning core saturation, which lowers primary inductance.

So what I'm saying is, PTs can hum without any load but still work ok. The "dang" when turning on indicates a large inrush current, maybe a sign of a somewhat underdesigned transformer (core).

Does it blow the correct type primary fuse? What's your actual mains voltage and frequency?

For amp troubleshooting we need a schematic.

[okw]

I have 237V and 50Hz. The Ampeg is wired as 230V (blue/white PT cable goes into J17, which indicates 230V export wiring). I have the schematics for it (457SCH_0.pdf from ampeg webpage). The PT is marked SLME 94-015-40 CMEC 0103. It does not blow any fuse (neither onboard glass fuse nor the mains fuse box).

[Helmholtz]

It does not blow any fuse (neither onboard glass fuse nor the mains fuse box.

So I guess it delivers correct secondary voltages?
If available I would use the 240V primary taps.

Please make schematic available here.

[okw]

I disconnected the J14/J15 bridge. Totally quiet of course, since the primary side is an open loop. I measured J19/J20 to 76VAC and J19/J22 to 38VAC, which seems fine since the schematic tells me I should measure +/-50V after the bridge rectifier. So the TP output is correct (76VAC x 1,41 ==> 107VDC, minus the forward drop of the diodes)?
Btw, If I turn the PT off and then on again within a few seconds, then "dang" is much more powerful (probably cause the core is saturated). Does this tell me something, or is it normal?
https://ampeg.com/support/files/Sche...P/457SCH_0.pdf

[okw]

Just measured J29 pin 1 and 2 to be +35V and -35V. They should be +16V and -16V. Referenced to GND pin 4. Any idea what this could indicate? Are my 16V zeners (D11/12) gone? This is without J29 connected to J28 (the front module). Would it rise this high due to the missing load?
I measured the DC output of the bridge rectifier. It's100VDC bang on.

[J M Fahey]

I think your amp got a hard bump and slightly oversized bobbin dislodged from EI core.
It had been held together by dried varnish which cracked on the hard bump.

That explains both laminations being free and somewhat loose bobbin acting as an electromagnet pulling loose laminations together, making the "dang" sound.

hard that many times.

Your solution, if mechanical noise is unbearable, is to re-pot/varnish the whole transformer.

You need to remove it from chassis, take good note where wires go, plus a good "map" plus a picture just in case.

Get some can/box/tupperware where transformer fits in, fill it with some kind of varnish, wood type marine grade is perfect, and let it there overnight, so varnish gets to the smallest nooks and crannies.

Then place a broom handle or similar between 2 chairs, the varnish pot under it, lift transformer by the wires, tie them to broom handle, and let transformer drip extra varnish back into the pot, again overnight or a full day.

Place old newspaper generously on the floor to avoid a mess.

Unused varnish goes back into the original can and can be used on furniture, etc. ; let transformer hanging for at least other 24 hours so varnish is only sticky but does not make a mess.

When itīs as dry as you like, reinstall it on amp.

Noises shoud disappear or be GREATLY reduced.

[okw]

Thanks, J M Fahey, that's great advice. I'll try this once I get the amp sounding normal again. Any advice on the power rail (measured 35V vs spec'd 16V)?

[g1]

The zeners D11 and D12 should work whether or not the connector to front is connected. Check voltage at the zeners, check their solder and that the connection to ground is good.

[okw]

I've been searching and searching and couldn't for the life of me find D11 and D12. Apparently they are on the front board(!). Seems like a strange thing. Especially since the J29 connector says pin1 +16V and pin2 -16V. That's why I'm reading 35V without the front board connected. Seems like a strange design choice. I would expect the find the whole power supply circuit on the power board shown below, then J29 would be +/- 16V regardless if connected or not.
Anyway, I measured D11 and D12, they are indeed +/-16V now. Then something else is damaged. Any tips on how to get further? I have an oscilloscope, but no signal generator (I've ordered one, but 20-30 days away).