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Author Topic: AKG N6128/23 replacement transformer?  (Read 4909 times)

River

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AKG N6128/23 replacement transformer?
« on: October 25, 2013, 02:08:44 pm »

Hi Everyone -

One of our Neumann KM56s has a loud hum which has been narrowed down to the power supply.  All 4 of our KM56s are paired with AKG N6128/23 supplies.

We swapped the transformer from it's supply to another supply and the hum followed, meaning that the problem is in the transformer itself.

Can anyone recommend a source for a replacement transformer for the AKG N6128/23 power supplies?  According to the schematic it's a dual secondary with 155V and 22V on the secondaries.  Not sure about current draw offhand.

For future reference, is the transformer in this unit the same as the one used in the other later AKG supplies that are of a similar form factor like the N12a and the N28a?

Thanks!
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klaus

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Re: AKG N6128/23 replacement transformer?
« Reply #1 on: October 25, 2013, 08:07:41 pm »

First, I would be VERY reluctant to operate an AC701 tube with a power supply that has so much heater output that, if you don't carefully calibrate it, it will blow your tube. This supply was designed to drive a 6072 dual triode @6.3VDC, not the anemic AC701 with its fragile 4VDC heater.

Second, I am not sure how a power supply hum could be traced to a defective input transformer. If the transformer is blown, there will be no output. Might this not be a case of mismatched applications (see above) or defective rectifier, or leaky filter caps?
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

River

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Re: AKG N6128/23 replacement transformer?
« Reply #2 on: October 25, 2013, 08:55:03 pm »

Hi Klaus -

Thanks for the response.

These have been modified for the 4V heater voltage.  Apparently they've been used with all four of our KM56s for many years with no real issues up until recently.  If you're interested I can draw up the mods to the supply for posterity.

Honestly it took me a long time to find the schematic for the unmodified circuit for that supply.

I've seen the transformer hum occur in the past, although never on a Mic Supply.  Usually it's an issue with the the laminations or some mechanical looseness in the transformer, possibly from heat expansion over time.  I can only attest to the fact that it's happening here.  We have swapped the transformers between two units and swapped them back, narrowing the problem to the transformer itself.

It's a new one for me too.

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soapfoot

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Re: AKG N6128/23 replacement transformer?
« Reply #3 on: October 25, 2013, 09:14:14 pm »

I had an analog delay unit that hummed badly once. Its hum was also traced to the power transformer. It turns out that some laminations were loose and rattling; a bath in shellac fixed it. This hum was mechanically audible, but also created a hum that was audible and measureable in the signal.

Not saying that will help in your situation, but it IS possible for a power transformer to still function and yet cause noise further downstream in the audio path.
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klaus

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Re: AKG N6128/23 replacement transformer?
« Reply #4 on: October 26, 2013, 12:14:23 am »

Up to now I was under the impression that the objectionable hum in your p.s. was electric in nature, not mechanical vibrations. Hence my comments.

All AKG small box power supply transformers (as used in 6128 and related C60, C28, and some late model C24 supplies) hum, due to primitive mounting with overly thin brackets, plus deficient lamination techniques on the transformers themselves.

Solution I suggest: hold down the transformer in various places, until you find the exact position where the hum is noticeably reduced through finger pressure, then put some E 6000 on that spot.

WARNING: Dangerously high voltages are present inside the power supply, even when the power switch is off. If you are a qualified technician: make sure to keep your left hand behind your back, when you do this!
If you are not: let a qualified electric specialist deal with it.
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Klaus Heyne
German Masterworks®
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River

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Re: AKG N6128/23 replacement transformer?
« Reply #5 on: October 28, 2013, 02:53:17 pm »

Thanks Everyone -

To clarify, the hum is coming through the audio path.  I've not compared the intensity of mechanical vibration to the other, quiet supplies.

I've seen this before with other gear, but not tube mics. 
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klaus

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Re: AKG N6128/23 replacement transformer?
« Reply #6 on: October 28, 2013, 04:04:23 pm »

Audible transformer vibration is not related to electric hum in power supplies, at least not in my experience.
The filter caps in the N6128s and related AKG miniature supplies are notoriously underperforming, and suffer from early demise, especially the heater caps.

Test:
Parallel, with alligator clips a new, healthy 5k mfd or larger to each of the existing caps, one at a time. Put headphones on, then remove one of the clips (the ground clip, for example) and listen for increase in hum. If you find such cap, replace it. When there is not more increas in hum when you remove the paralleled cap, you are done.

WARNING: Please observe the caution/safety note I added to my last post!
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

soapfoot

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Re: AKG N6128/23 replacement transformer?
« Reply #7 on: October 28, 2013, 04:29:20 pm »

Audible transformer vibration is not related to electric hum in power supplies, at least not in my experience.

I have observed it once-- in a DOD 680 analog delay. The 12V power transformer, internally mounted, had a vibration that manifested itself as a hum in the audio path (not just an acoustical hum, but an actual--very loud--hum in the audio as well).  Potting the transformer in shellac cured the issue.

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panman

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Re: AKG N6128/23 replacement transformer?
« Reply #8 on: October 28, 2013, 07:16:52 pm »

One of our Neumann KM56s has a loud hum which has been narrowed down to the power supply.  All 4 of our KM56s are paired with AKG N6128/23 supplies.
We swapped the transformer from it's supply to another supply and the hum followed, meaning that the problem is in the transformer itself.

Maybe it`s just me, but I don`t get it. Could you please be more specific! You say only one km56 has a loud hum, but you have four km56:s and four AKG N6128/23:s? I tried to read again and again, but you don`t mention trying out the other three km56:s with that PSU, that is supposed to have a humming trafo and thus excluding the km56:s out of the equation. Why to swap the trafos? If you pair the other three with that same PSU and the hum follows, then you have something.

Many years ago I remember once having a strange hum with a km56. Swapping the PSU stopped the hum, but a km54 that got the "humming PSU" did not hum. If I remember right, the solution had something to do with grounding or not grounding the pin 6. Cannot remember any more details though, but it clearly was a grounding problem.
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Esa Tervala

Jim Williams

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Re: AKG N6128/23 replacement transformer?
« Reply #9 on: October 29, 2013, 12:06:17 pm »

I have observed it once-- in a DOD 680 analog delay. The 12V power transformer, internally mounted, had a vibration that manifested itself as a hum in the audio path (not just an acoustical hum, but an actual--very loud--hum in the audio as well).  Potting the transformer in shellac cured the issue.

Seen here many times. One of the worst cases was a very large torrid power transformer used in a Soundcraft TS-24 'Large Console Power Supply'. I've changed out a few over the years.

The use of high current draw bulbs used in the solo/mute switches (like on SSL's) caused the power transformer coils to become loose from the ferrite core. If you were working in the machine room you could tell when someone pressed solo as that transformer would mechanically buzz loudly. You could feel the rack case vibrating. It also transmitted through the linear regulators and into the audio power rails. It was audible as a buzz/hum off the control room monitors.

In that case it wasn't laminates that came loose (there are none in a torrid core, it's a solid piece of ferrite material) but the coils themselves. There was no repair, only replacement for that situation.

In the KM situation, it is also possible coils are loose as well as laminates. Any attempted repairs are likely to be temporary. There are low cost power transformers available from Triad or Stancor. I would suggest a larger sized/current transformer to avoid that problem in the future.
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River

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Re: AKG N6128/23 replacement transformer?
« Reply #10 on: November 19, 2013, 03:24:18 pm »

Sorry for the late response - I've been caught up with other repair issues.

This particular issue is absolutely the result of the transformer in the power supply.  I suspect that the laminations have loosened over the years and this is the cause.

I've tried the KM56 with other power supplies and the problem is in the supply.  Swapping transformers between supplies shows that the hum is localized to that particular transformer.

Can anyone recommend a replacement that will meet the correct specs and not be too difficult to mount in the 6128/23?

Thanks!
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klaus

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Re: AKG N6128/23 replacement transformer?
« Reply #11 on: November 19, 2013, 04:41:53 pm »

Why not just first try to quiet the transformer vibrations down with application of some E6000? If that does not work, you then could explore more intrusive and costly alternatives.
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Klaus Heyne
German Masterworks®
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Tim Campbell

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Re: AKG N6128/23 replacement transformer?
« Reply #12 on: November 20, 2013, 08:06:46 am »

Wouldn't it be more prudent to buy the proper power supply as they're readily available on the used market?
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Jim Williams

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Re: AKG N6128/23 replacement transformer?
« Reply #13 on: November 20, 2013, 11:40:33 am »

It's best to localize and suss out the actual cause instead of applying guesswork. Get a scope and examine the power rails with it. If there is excessive ripple, you will easily see that. Low cost replacement filliment/high voltage power transformers are available from Triad and Stancor, look in the Mouser catalogs.
www.mouser.com

Beats throwing expensive underwear against a wall expecting it will stick.
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River

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Re: AKG N6128/23 replacement transformer?
« Reply #14 on: November 30, 2013, 12:51:53 am »

Well, in a strange manner of coincidence, I now need to replace ANOTHER transformer in an N6128/23.

This one has what I believe to be a non-standard power transformer with wire leads that disappear into the windings.  Today the power supply kept shutting off and when I found the problem it was that one of the transformer leads was physically loose.  It was making an intermittent connection but was no longer actually connected to the windings.

I've looked through the Triad and Stancor catalogs and haven't found anything that has dual secondaries with the correct voltages.  This supply, like all the small-box supplies, should have 155V and 22V coming off the transformer.

Can anyone recommend a specific transformer model?

Thanks!

Tim
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