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Kustom 150 power supply 50% high [message #12967] Sat, 13 February 2010 22:23 Go to next message
jbreher is currently offline  jbreher
Messages: 11
Registered: January 2005
Junior Member
I am trying to repair a buddy's Kustom 150 - one of the old tuck-n-roll units with an aluminum faceplate.

I've rebuilt the output stage twice now, and promptly blown it up both times.

I now learn that the supply rails are ridiculously high. I understand they may sag a little with a working output stage drawing current. However, with the output stage not connected, the rails are at ~ +/- 62V, when they are spec'd from the schematic at +/- 40.2V. That would be a lot of sag!

So I am thinking that the high voltage is the reason the output stage keeps blowing.

Funny thing is, other than the power switch having been replaced at some point, the power supply looks untouched. All wiring matches the schematic (including color coding), all joints look 50 years old, tranny and inductor look stock, caps look stock, rectifier looks stock, etc.

With the supply unloaded (disconnected the rails to the power amp board), I dialed up the voltage on the variac to the point where the rails were at the schematic's spec of +/- 40.2 V dc. At this point, the tranny was outputting 64.4 V ac (as measured at the rectifier). The input to the amp was 78 V ac (as opposed to 120).

The coupled filter inductors in the power supply are a bit confusing to me. Can anyone give me a clue as to whether the coupling would drop any more voltage? Or are they really just simple filter inductors?

Any hints on how to fix this are most welcome - thanks!

[Updated on: Sat, 13 February 2010 22:23]

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Re: Kustom 150 power supply 50% high [message #12973 is a reply to message #12967] Mon, 15 February 2010 06:10 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Hi.
I have never had any of my power transformers un-hooked in any of my k150 or k250 heads to know what they sail up to voltage wise unloaded, but the main filter are only rated for 50 VDC,and 50% regulation for that transfomer seems way out of spec.
I will take some measuerments from mine tonight and post them up tomorow.
My best guess now would be that the PT has a shorted turn on the primary side, just enough to kick up the volatge, but not pop the mains fuse.

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Re: Kustom 150 power supply 50% high [message #12978 is a reply to message #12967] Mon, 15 February 2010 12:44 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
I've never seen or heard of this one before.
What is the AC voltage going into the rectifier?

I'd try to figure out if the higher voltage was a result of too much ac from the transformer secondary or if the choke was somehow responsible for the increased dc.

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Re: Kustom 150 power supply 50% high [message #12979 is a reply to message #12967] Mon, 15 February 2010 13:29 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
I can not see how the choke could be the cause as the volatge is already DC so it can not be transfomed, about the only thing that could happen would be a short which would more than likly blow the fuse, or in the least kill off some power supply volatge.

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Re: Kustom 150 power supply 50% high [message #12980 is a reply to message #12967] Mon, 15 February 2010 13:51 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
After re-reading the original post, I see that there is 64.4 vac out of the power transformer. So I would assume that the problem lies there.

Were there export models of the metal fronts made?

Edit:
Is the 64.4vac reading the full secondary? (Which I believe would be normal.)

[Updated on: Mon, 15 February 2010 13:59]

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Re: Kustom 150 power supply 50% high [message #12982 is a reply to message #12967] Mon, 15 February 2010 13:59 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
I guess, I know some K250s have turned up over seas, but a 240 volt euro transfomer would put out only 20 of the normal 40VDC on our line voltage no?

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Re: Kustom 150 power supply 50% high [message #12983 is a reply to message #12967] Mon, 15 February 2010 14:04 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
No, I was thinking about a Japan version with a 100vac primary.

Now I'm not sure if he meant the 64.4 vac was with the variac down to 80 volts or at the full line voltage.

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Re: Kustom 150 power supply 50% high [message #12985 is a reply to message #12973] Mon, 15 February 2010 15:24 Go to previous messageGo to next message
jbreher is currently offline  jbreher
Messages: 11
Registered: January 2005
Junior Member
To reply to each in turn:

@stevem:
I had the same thought about the primary having a few turns shorted. That is why I asked about the secondary voltage.

@chicagobill:
As indicated, when the line (and consequently the tranny primary) is at 78 V ac, the secondary (an hence the rectifier) is at 64 V ac, with a valid center tap.

The choke! The choke! I've since learned a bit about power supply filters with an inductive input (as opposed to a capacitive input). When operating properly, they regulate down to the *average* dc level -- not the peak as would be the case with a capacitive input filter.

Proper operation requires sufficient quiescent current be drawn, such that the inductor may actually induct, rather than merely act as a resistor. With insufficient quiescent current, the inductor is for all intents and purposes out of the circuit, and the filter thereby acts as a capacitive input filter. This causes the output voltage to climb from average to peak -- a factor of about 1.5x - just about what I am seeing!

As an aside, other correspondents have noted that they have *never* met a production amp with an inductive input filter in the wild. Just another thing to love about these Kustoms, I guess.

AAR, dc resistance measurement shows a difference between the two windings in the dual choke. I don't have a millohmmeter, but I measure 1.0 ohm across one coil, and 1.5 across the other.

I started disassembling the choke, and found evidence of overheating.

I am currently (no pun intended) trying to rewind the choke. This is not a task I've done before. Wish me luck!

I am at a point where I am committed to this task - I cannot turn back. However, I'd still be interested in someone making a voltage measurement on the secondary of their K150-1 power tranny. I still haven't been able to rule out a shorted primary.

BTW, I thought about the Jap possibility as well. However, that would only be an increase of about 20%, not 50%.

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Re: Kustom 150 power supply 50% high [message #12986 is a reply to message #12967] Mon, 15 February 2010 15:49 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
I will pull off some measurements for you tonight from my 150.
You might find it interesting to note that only the K150 K300 and K600 use a choke, the K250 model does not.

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Re: Kustom 150 power supply 50% high [message #12987 is a reply to message #12967] Mon, 15 February 2010 16:04 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
I've not rewound one of those chokes either, but I have rewound outputs and other trannys before. Please publish the specs as you find them for future reference.

By the way, in a page of notes I found from working on a 150, I got a resistance reading of 1.4 ohms for each winding.

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Re: Kustom 150 power supply 50% high [message #12991 is a reply to message #12987] Mon, 15 February 2010 20:58 Go to previous messageGo to next message
jbreher is currently offline  jbreher
Messages: 11
Registered: January 2005
Junior Member
Yeah, my meter is only good to a tenth of an ohm. However, your 1.4 ohm squares pretty well with my readings of 1.5 for a possibly good coil reading, and 1.0 for a shorted coil.

So far, I've stripped one of the two coils off the bobbin. There were seven layers, with a total of 241 turns. This ended up being 132' of wire for the first coil.

I'm now trying to figure out the proper wire to use. I've never rewound any magnetics before.

I measured the wire diameter with a micrometer every four feet. I got an average reading of .0350", with a minimum reading of .0333" and a maximum reading of .0376".

Given the age, (and thereby wide tolerance) can I make the assumption that an even-numbered AWG was used to wind this?

According to MWS, (http://www.mwswire.com/awgsearch2.asp) if you don't know what insulation type you need, you most likely want Polyurethane & Nylon 155. Nominal diameter for Single-insulated AWG 20 is .0334, and Heavy-insulated AWG 20 is .0346. Would one of these seem appropriate?

What does one use to insulate the layers from each other -- would Kapton tape be suitable, or is there another recommendation?

Any tips you'd want to relay to a rewinding virgin?

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Re: Kustom 150 power supply 50% high [message #12998 is a reply to message #12967] Tue, 16 February 2010 06:29 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Here are some voltage checks I pulled from my 73 150 for you.
AC input volatge from line.
118.2
AC output voltage measured across recto input with output from choke disconnected at filters.
99.6 VAC
DC voltage output from choke with filters still disconneted.
+47.9
-48.2
DC voltage of rails with filter hooked back up/amp normal.
+42.5
-42.3

DC of low volatge power rails feeding preamp boards and effects board.
+12.00
-12.67.


Both the windings of my choke came in at 1.5 ohms.

I am a hands on guy, but I have never rewound a transfomer, guitar pickups yes!
I think I would sonner pick up two chokes from Hammond, like a 159ZC model.
I hope this helps you out.

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Re: Kustom 150 power supply 50% high [message #13003 is a reply to message #12998] Tue, 16 February 2010 11:56 Go to previous messageGo to next message
jbreher is currently offline  jbreher
Messages: 11
Registered: January 2005
Junior Member
@stevem -

Thanks for the voltage listings. This tells me that my power tranny is fine. It also tells me that in this amp, the power supply filter is indeed meant to operate as an inductive input unit, with the output voltage significantly below the peak rectified ac value.

I'd actually already reasoned the power tranny was fine. I had earlier speculated that the primary may have a shorted primary, leading to less turns in the primary, leading to a change in the turns ratio, leading to an increased output voltage.

I now think this is an impossibility. Here is my reasoning -- please chime in as to whether or not you agree.

If there is one or more shorted turns in the primary, the number of turns that have shorted are indeed effectively subtracted from the primary's turn count. However, those windings don't just go away. They are still there in the transformer. As they are no longer part of the primary, these 'orphan' turns essentially become an additional secondary. Further, the terminals of this secondary are shorted to each other -- the same as if they were a designed-in secondary, and you had soldered these leads together.

With any primary current applied, there is a voltage built up across this phantom secondary. The only load on this phantom secondary is the resistance of the winding itself. Accordingly, a huge current circulates in this phantom secondary.

The massive current is reflected to the primary, causing the primary to draw massive current as well.

The amp is not drawing such massive current. Accordingly, there is no shorted secondary. Accordingly, there is no shorted primary.

Make sense?

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Re: Kustom 150 power supply 50% high [message #13004 is a reply to message #12967] Tue, 16 February 2010 12:29 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
Yes, in all the small number of power transformers I have replaced in my life time, ones with a shorted primary, even if they have no current draw placed on them from a disconnected secondary side will always blow the line fuse within a half a second of power up.

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Re: Kustom 150 power supply 50% high [message #13010 is a reply to message #12967] Tue, 16 February 2010 13:34 Go to previous message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Here are my thoughts.

Yes shorted turns usually cause large current draws, but I imagine that there could be failure modes that wouldn't.

As for rewinding the choke, the size of the wire is less important than the number of turns. The wire size will determine the current handling capabilities of the choke. It would seem that either form of the 20awg should work fine.

Was there some sort of tape insulation between the layers of the original coil? I don't think that I've ever seen that before.

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