Kustom 150 power supply 50% high

[jbreher]

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!

[J M Fahey]

Hi jbreher.
What you write is impossible .... yet I believe in you, so there must be an explanation, however improbable or ridiculous it may seem now.
1) change your multimeter's battery
2) borrow another multimeter and check with that one too, see if measurements match
3) all Kustom schematics posted here refer to boards; please post the schematic of your power supply.
If not available, draw one yourself.
No fancy design software necessary, a clear hand drawn scan will do.
A couple topside and bottomside board pictures will help too.
Be specially clear about wire colors and what is written on the board (silkscreen and/or copper side) where each of those wires is soldered.
I'm intrigued by those "filter inductors", what's written on them? Show them clearly in the pictures.
Good luck.

[nashvillebill]

I've got an old Kustom 250 which I recapped some years ago. It had a conventional power supply: transformer, brige rectifier, filter caps....but maybe the 150's were different.

[jbreher]

@ J M -

Can't be impossible, 'cause it's happening.

Just went back and verified.

1) & 2) - I did not change the battery in my meter (Fluke 8020A). I trust you'll accept verification with another meter as sufficient? Voltages measure the same with my old-skool Simpson 260-7PM.

3) Schematics attached:

K150 (A).jpg - schematic is a PC5033 rev 0 (?), my power amp board is a PC5033 rev 1. There are slight differences before the signal hits Q1, otherwise identical.

K150 (B).jpg - as I said, all wire colors match the schematic. Note presence of L1 -- two mutually coupled inductors in a single package -- between the rectifier CR3 and the bulk filter caps C1, C2. It physically looks like a transformer.

overview.jpg - shows amp, variac, meters. You can see that the output of the filter caps to the power amp board has been disconnected. Fluke DMM attached across C1 - positive rail. The negative rail measures about one volt lower (though obviously negative polarity). The Simpson VOM is attached across the line cord hot and neutral -- right where the line cord makes its first connections.

measurement.jpg - you can clearly see the Fluke indicating 41.1 V dc (positive rail), and the Simpson indicating just shy of 80 V ac (line). Yes, if I swap the meters , the indicated voltages are the same.

power-supply.jpg - better detail view of the power supply. Inductor L1 is the transformer-looking thingy in the center of the picture. That is a separate terminal block behind it -- not part of the inductor. The terminal block holds the zeners for the low voltage supply. The black things bolted down to the chassis are stacks of two power resistors -- one stack per rail. One of each stack is the dropping resistor before the zener, and the other is a current limiting resistor for an indicator light inside the power and polarity switches. As I mentioned in the first post, the power switch has been replaced.

I know I'm gonna feel stupid when the aha moment hits. But right now, I have no explanation for what I think I'm seeing.

[Steve Conner]

Are the pilot lights working and fitted with the right bulbs?

I ask, because I think this is a choke input power supply, so it needs a minimum load. If the load goes below the critical current, the choke will demagnetize and the output voltage will shoot up from the average to the peak, a factor of about 1.5, which is about what you're seeing.

And, I think the pilot lights are part of the minimum load (why else would they use two, one on each rail, with big 10 watt ballast resistors?)

Or maybe the choke L1 has some shorted turns. That would stop it from doing its averagey-thing.

Another possibility: Someone replaced L1 or the mains transformer with the wrong part. Or L1 got burnt out during a previous output stage meltdown, and someone just wired round it.

If it were me, I'd try shorting across the pilot lights to let the 400 ohm ballast resistors draw current. And maybe bias the output stage a bit hotter too...

[jbreher]

Cool! I'm going from frustration mode to learning mode!

I guess I've never before thought about the implications of having an inductor *before* the first filter cap. Thanks to Steve, and some subsequent googling, I now understand that the voltage will regulate down to a lower value in an inductive input filter, as opposed to a capacitive input filter. And it makes sense that the inductor cannot do it's job of filtering if insufficient current flows.

So now I see a problem with my methodology - with the power amp disconnected from the rails, no bias current is being drawn. I am tempted to add a purely resistive load on the output rails, so I can load down the supply, while still leaving the power amp disconnected. I really don't want to make another set of RCA 40409 / 40410 transistors out of NTEs.

@Steve - the negative rail has a pilot light. However, with the previously replaced power switch (as mentioned above), the positive rail has no pilot. In power-supply.jpg, you can see the red wire heading to the power switch is unterminated. I'll load this resistively, and see if the supply regulates.

The polarity switch looks stock. The integral lamp measures ~ 13 ohms. Fairly negligible in relation to the 400 ohm dropping R. Accordingly, I can eyeball that the pilots should present about 100 mA load on each rail.

The inductors L1 and the power tranny look stock. The solder joints look untouched, as do the rivets holding them to the baseplate. Accordingly, I don't think they've been replaced. I had thought about the possibility of several shorted turns in the primary of the tranny. This would reduce the turns ratio, thereby increasing the output voltage. However, as you say, the 1.5x factor I am seeing tends to implicate an unloading of the inductors as the issue.

However, if anyone has a K150 open, I'd appreciate a voltage measurement on the secondary.

So here is my plan of attack:

- Before I rebuild the power amp a third time, I want to make the power supply operate properly. To this end:

- simulate the 2nd pilot light with a 12 ohm R. See if the supply regulates down

- if not, add resistive loads across filter caps to simulate the power amp being attached and drawing current.

- if I still cannot get the power supply to regulate down to 40V, I know the power supply itself is bad. Otherwise, start in on the power amp rebuild again.

Anyone have any idea what sort of load a properly biased power amp might present to the power supply? I would imagine significantly less than the ~100 mA/rail presented by the pilots, no?

[Steve Conner]

So now I see a problem with my methodology - with the power amp disconnected from the rails, no bias current is being drawn. I am tempted to add a purely resistive load on the output rails, so I can load down the supply, while still leaving the power amp disconnected. I really don't want to make another set of RCA 40409 / 40410 transistors out of NTEs.
...
Anyone have any idea what sort of load a properly biased power amp might present to the power supply? I would imagine significantly less than the ~100 mA/rail presented by the pilots, no?

Yes, you're right, and your plan of attack seems fine to me.

You can probably replace those old RCA transistors with some modern ones, though. For instance some of the service guys on this forum just replace all TO-3 output devices with the MJ15024 (NPN) and MJ15025 (PNP) which are easy to get, not too expensive, and meet or exceed the specs of just about every other TO-3. As far as I know they're what Peavey and Crown used in their biggest amps.

For drivers I like the MJE1503x, the MJE340/350, and in TO-92s I like the 2N5551, 2N5401, MPSA42 and MPSA92. We don't have NTE in the UK, but I've heard they're basically junk.

The power amp bias current depends heavily on the power amp design. Some PA and musical instrument amps, famously Peavey's old ones, run in Class-B with practically no bias current at all, and may even have no bias adjust pot. But others can run as hot as 50-100mA per transistor pair. Heck, you get Class-A hi-fi amps that run AMPS of idle current and double as room heaters.

[J M Fahey]

WOW !!! A CHOKE input supply !!!!!!!!!!!!!!!

I thought they only lived on Electronics 101 books, as example of things that *might* be built.
Seriously now: they have some justification in tube amps, because tubes are often biased to 60% max. plate dissipation, and anyway can survive 50% higher +B voltages; but on *transistor* amps that's crazy!
Specially because the idle current for a normal power amp is measured in milliamperes, power transistors become lossy negative-impedance Zeners a few volts over their maximum rated voltage, and to boot are mortally wounded by second breakdown.
And tubes, in a worst case, can redplate and live on, but SS junctions became gobs of molten metal above 150 or 200ºC.
First time in my life I see a choke input SS amp supply.
I see it as an unsafe system, personally I would junk both pieces of iron (or save the PT for another project) and mount there a nice 30+30V, 150 or 200 VA transformer, toroidal or EI, your choice, and only then go on repairing the power amp.
The amp won't notice the difference yet weigh quite a few pounds less.
PS: you should replace those 2200x50 caps with 4700x50 which probably will be the same size or even smaller.
And the possibility of beefing-up the power transistors is excellent.
PS2: you can replace those RCA40409/40410 with TO220 types, but remember to link those pads where the heatsinks were soldered because often they are used to join tracks. Been there
Good luck

[jbreher]

Back to head scratching

Well, after embarking full of hope on the project of getting the supply to work properly, my spirits are now sinking.

I grounded the low end of the B+ bleeder - the one that would be connected to the (now missing) lamp. This added 40V / 400 ohm = 100mA of load to the B+. Still have the same issue - B+ @ 40V with line = 80 V ac.

Thinking further, the low voltage is generated off B+ by a 200 ohm R in series with a 12 V Zener : (40V - 12V)/200ohm = 140 mA load on B+ - more significant than the lamp bleeder.

So total load on B+ = 100mA + 140mA + whatever_the_power_amp_quiescent_is

Load on B- is symmetric to this.

So thinking I just needed to account for the missing power amp quiescent load, I strapped 250 ohms from B+ to B-. (40V - -40V)/250ohms = 320 mA additional load on both B+ and B-.

The supply still is way out of bounds. At a B+ of 40V, line in is 85V ac.

Can anyone truly imagine the actual power amp drawing more from each rail than a third of an Amp at quiescent? I should be safe to assume not, correct?

I think I need to resign myself to admitting that the inductor has failed. I just want to see if anyone has any other ideas first.

I doubt I'd be able to find a similar inductor. This leaves me with the option J M forwarded, with replacing all the iron with a tranny of lower voltage, and stiffening up the capacitive regulation to make up for the lost averaging of the inductors.

However, the downside is that this makes the amp something other than what it once was.

I don't own an LRC bridge. Even if I did, I have no spec for the inductor. Sure, I have a Kustom part number, but they've been defunct for decades. How should I go about testing the inductor itself to see if it has failed?

Anyone see anything I may have missed?

[olddawg]

You are overthinking this. Just rebuild the output section and run it up on a variac. Monitor the rails so you don't exceed the indicated supply voltage. Are you sure the filter caps arent shot and you are reading peak voltage from the rectifier? Have you looked at the rails with a scope? That's what I would do. Variac and scope it.

[Steve Conner]

The supply still is way out of bounds. At a B+ of 40V, line in is 85V ac.
...
Anyone see anything I may have missed?

Yes, take it up to 120, the B+ may stop rising once the choke magnetizes.

I still think it's likely that the choke was cooked by an output stage failure and has some shorted turns. RG Keen's famous neon bulb test will do to check it.

If you install the transistors I suggested, the output stage should withstand 65V rails fine, and you can throw the choke away. The 5401/5551 are rated 160V, and the others are 250V or more.

[J M Fahey]

As another opinion/option, I would just throw a new 30+30V, 200VA transformer there and forget about that choke which very probably, as was said above, is shot.
Although a fascinating possibility, rather than Upgrade it would stick to making it work like any other "normal" Kustom.
It is technically possible; I would do it if it were on *my* bench, maybe adding a fan for the almost tripled power output (65/40 squared= 2.65 x the original power), but given the amazing capacity for frustration this amp has already shown, I would play it *very* safe .
Just my two cents.
PS: yet I would use better/modern components as suggested, not to increase power but to make it more reliable

[jbreher]

@olddawg - earlier in the thread, I indicate that I have already rebuilt the output stage - twice - with rather immediate failure upon running it up to full voltage. I wasn't monitoring the B+ / B- at the time, but it did not draw excessive current up until the point when the line voltage was near 120, and the outputs failed.

@Steve - I previously took it up to 120 - you may recall earlier in the thread where I indicated that the rails were at +/- 60V at a line of 120. This has been verified in yesterdays test, with the B+ bleeder grounded and the 250 ohms across the rails.

I'll try to find 'RG Keen's famous neon bulb' test. May be a little late for that however. I removed the choke. While I don't have a good meter for very low resistance, the windings measured 1.0 ohm for one coil and 1.5 ohm for the other. Further, under the bell covers, there is significant 'ooze'.

I think the choke is a goner. I have already started to disassemble it in order to see if I can rewind it.

[R.G.]

I noticed a couple of things.
(1) the schematic shows the main filter caps rated for 50V. If that's real, there must have been no way for the power to go up to +/-60 - it would kill the caps.
(2) if you do, in fact measure 80Vac - ct on the secondary of the PT, then yes, the PT would put out +/-60Vdc when full wave rectified. 60Vct gives you +/-40Vdc, about. So if the trannie puts that out - then the trannie is wrong.
(3) I wonder if that's a choke for the power supply input, or a ringing-rectifier snubber to keep rectifier buzz out of the works. This is because it is fundamentally a beginner's mistake to design a choke input power supply without an immediate minimum-load bleeder to keep it from jumping up to capacitor-input voltages when the load sags. It does things like blowing out your filter caps and so on if you haven't put the bleeders in. NO power supply can ever be sure it won't be tested at no load.
(4)If this was designed as a choke input supply, then L1 is a swinging choke, designed to have a massive inductance at low currents and a much lower inductance as part of the magnetic path saturates. This is usually done by butt-stacking the majority of the laminations, and then putting the outer two or three lams across the butt-stacked spacer. The outer lams make for big inductance at low currents, then saturate to let it run as a lower inductance, but bigger current. Look at the laminations on the edge of the inductor. A good swinging choke design can run with MUCH lower minimum bleeder load.
(5) choke input filters went the way of the dodo because they're much more expensive than doing the same thing with filter caps.
(6) Back at the filter caps - those look very small for a huge output voltage. Was this thing rated for more than 150Wrms? +/-40 is about right for 100Wrms into 4 ohms or lower.
(7) The inductor may be a swinging choke, but with a shorted turn. Tester is HERE.
(8) I wouldn't just convert to +/-60Vdc. The heat sinking won't take it, the filter caps won't take it, the driver transistors may not take it, and so on. You'll be rebuilding the entire thing.
(9) AS JM sez...

As another opinion/option, I would just throw a new 30+30V, 200VA transformer there and forget about that choke which very probably, as was said above, is shot.

This is a viable option, but finding a 60VCT may be tough. Put in a toroid if you do that.
(10) not to stress an indelicate point, but does the power transformer have primary taps? Secondary taps?

[jbreher]

(1) Agreed. I only surmise that the output Qs made better fuses than the Cs would.
(2) I must not have been clear. On a variac, I dialed up the line voltage, while monitoring B+/-. When B+/- reached +/- 40Vdc, I stopped raising the line voltage. At this point, the secondary of the power transformer was ~ 65Vac. At this same condition, the voltage on the *primary* was ~ 80Vac.
(3) In a fully functioning K150-1, B+/- are each loaded by 240mA, plus whatever the output stage will draw. The 240mA is the pilot light circuit (40V 40V rails / 400+lamp ohms), and the preamp supply ([40V rails - 12 V Zener] / 200 ohm dropper).
(4) The choke is built as per a transformer. Two coils wound concentrically over an EI core. As opposed to other EI trannys I've seen, however, all the E's are oriented in the same direction, and all the I's are on the same side. There are two additional 'E-like' pieces of thinner spring steel (or similar) one per side, oriented opposite to the E's in the majority of the core. These hold the whole thing together. I thought this was merely the air gap to keep the DC from saturating the core.
It sounds as if maybe you are saying that these outer pieces are meant to saturate early? Are we describing the same thing?
(5) I can believe that.
(6) I am told the '150' in the model name, as per Kustom convention, is meant to denote approximately 75W rms @ clip.
(7) I googled up one of your posts on this method. However, I had already been into disassembling the choke. I'll keep the technique on 'mental file'.
(8) I am trying the approach of rewinding the inductor. I don't really want to make the amp something it was never intended to be.
(9) If the inductor rewind fails (I've never rewound anything in the past) I'll be looking at this option.
(10) No offense taken. Exactly two connections on the primary side, and three on the secondary.