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Kustom IV Bass Head Schematic? [message #22106] Thu, 15 January 2015 16:43 Go to next message
corona blue is currently offline  corona blue
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Hi everyone-
First post here. Very very glad to have found this site!
A friend of mine recently picked up a Kustom IV Bass head and 2 x 15 cabinet.
It is in fairly good cosmetic shape, appears to be vintage 1977 or so going by the date codes on the pots.
He brought it in for some service. It sounds pretty good but when it warms up it begins making a "pink noise" sound, like when a resistor gets noisy in the signal path.

This thing is a beast and worth fixing as it sounds pretty sweet, but I am having some problems finding a schematic for this thing. I have not yet pulled any of the circuit boards to retrieve the board numbers (printed on the under-side) but I am hoping someone here might have some experience with these units.

Is it possible to post pics here?

Thanks for any assistance,
Allen

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Re: Kustom IV Bass Head Schematic? [message #22122 is a reply to message #22106] Fri, 16 January 2015 11:24 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Steve???

[Updated on: Fri, 16 January 2015 11:24]

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Re: Kustom IV Bass Head Schematic? [message #22123 is a reply to message #22106] Fri, 16 January 2015 11:41 Go to previous messageGo to next message
stevem is currently offline  stevem
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Musicparts.com has the schematic you need for 20 bucks.

It sounds like you may have a thermal issue going on so a hair dryer set on low and some freeze spray from radio shack may just help you pin down what's going on, just do not heat up any transistor it IC chip to more than about 150, I guess a thermal gun would be handy also in that regard!

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Re: Kustom IV Bass Head Schematic? [message #22124 is a reply to message #22123] Fri, 16 January 2015 12:00 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Steve. I got the MusicParts schematic yesterday. It may be helpful, but it is date 1974 and I think this amp is later by a few years, going by the pot date codes. The freeze spray sounds like the ticket.

Any tips for me about removing/reinstalling the transistors from the back? They are set down with sil-pads it looks like.

Thanks!
Allen

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Re: Kustom IV Bass Head Schematic? [message #22126 is a reply to message #22106] Fri, 16 January 2015 13:34 Go to previous messageGo to next message
chicagobill
Messages: 2005
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Those transistors are in sockets on that amp right? Just remove the two mounting screws and pull the transistor out.

Sometimes the Sili-Pads will stick a little and you will need to grab the transistor case with a pair of pliers. If you try and pry them out with a screwdriver be very careful not to damage the pad.

Oh yeah, welcome to the place.

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Re: Kustom IV Bass Head Schematic? [message #22127 is a reply to message #22126] Fri, 16 January 2015 13:40 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Yes, they look like they are socketed.
Are those transistors a TO-3 package? (before my time).
I thought I would have some spares on hand in case.

When going back together, is there a proper torque spec for the transistors.
I have heard it's a common mistake to overtighten transistors.
Or is that more for when using mica insulators?

And thanks for the welcome. Glad to have found this great resource.

[Updated on: Fri, 16 January 2015 13:41]

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Re: Kustom IV Bass Head Schematic? [message #22128 is a reply to message #22126] Fri, 16 January 2015 13:40 Go to previous messageGo to next message
pleat is currently offline  pleat
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Kustom made two versions of the Bass IV. The first issue was the 1974 era that actually had two 138 watt power amps into a 4 ohm load each amp. It also featured a 7 band Graphic EQ.

The second Big K version of the Bass IV used a mono 300 watt amp into a 4 ohm load.

pleat

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Re: Kustom IV Bass Head Schematic? [message #22129 is a reply to message #22128] Fri, 16 January 2015 13:42 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks pleat.

This would appear to be the latter one.

Anyone have a schematic for it??

I think the schem I got from MusicParts is for the earlier one.

[Updated on: Fri, 16 January 2015 13:43]

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Re: Kustom IV Bass Head Schematic? [message #22131 is a reply to message #22106] Fri, 16 January 2015 16:44 Go to previous messageGo to next message
chicagobill
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Sorry, no schematic in my files.

I'm sure that there is a torque rating for transistor mounting, but I never bother with it. The only time it is really important is with plastic case transistors, too much torque and they can actually crack.

There really is no need to over tighten transistor mounting screws. Once the case has made good thermal contact with the mounting surface, tightening it more doesn't make the thermal transfer any better.

The Sili-Pads are more prone to compressing than the mica washers, so I'd just tighten them until they are snug. I have seen the earlier versions of the pads squeezed so much that they short out. Most of the newer ones seem thicker.

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Re: Kustom IV Bass Head Schematic? [message #22132 is a reply to message #22131] Fri, 16 January 2015 17:17 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Great info Bill. All of that will help a lot.
Do the sili-pads require thermal compound too (like mica)?
I have no experience with them.

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Re: Kustom IV Bass Head Schematic? [message #22134 is a reply to message #22106] Fri, 16 January 2015 18:01 Go to previous messageGo to next message
chicagobill
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No compound needed, that was the big selling point. No muss, no fuss, an insulator and thermal compound all in one. But like I said, don't tear them or squeeze them too hard.

If you are replacing them, make sure you get the ones that are in fact insulators. I've heard that some of them are just good for thermal transfer and will not stop shorts.

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Re: Kustom IV Bass Head Schematic? [message #22135 is a reply to message #22134] Fri, 16 January 2015 18:17 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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chicagobill wrote on Fri, 16 January 2015 18:01


If you are replacing them, make sure you get the ones that are in fact insulators. I've heard that some of them are just good for thermal transfer and will not stop shorts.


YIKES! OK, that is golden advice. Thanks again.

I will revisit this thread as I move through the trouble shooting.

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Re: Kustom IV Bass Head Schematic? [message #22136 is a reply to message #22106] Sat, 17 January 2015 07:35 Go to previous messageGo to next message
stevem is currently offline  stevem
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Note that the output transistors in any amp are not very likely to be the root problem of your hiss issue!

If on the rear of the amp it has a preamp out Jack and a power amp in Jack plug a cable into either Jack as this will break the signal path from preamp to power amp, if this action stops the hiss than the isue is not in the driver or power amp section, but in the preamp amp section and then we can help you deal with that.

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Re: Kustom IV Bass Head Schematic? [message #22140 is a reply to message #22136] Sat, 17 January 2015 11:59 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Steven... not sure if this unit has a preamp out but that's a great suggestion. I will check.
Thanks for your suggestions by email too. The Molex connectors are priority one.

Just want to say thanks to everyone for the warm welcome and the generous assistance.

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Re: Kustom IV Bass Head Schematic? [message #22206 is a reply to message #22140] Wed, 21 January 2015 15:47 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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UPDATE:

I got some time on this Kustom yestday.

I unplugged and cleaned every board connector with a toothbrush, pipecleaners and DeOxit spray.
Then removed the three pre-amp boards and sprayed out the pots with DeOxit and worked them.
Cleaned all the jacks thoroughly.

Re-assembled, plugged in the cabinet, powered it up...

No problems! Let it run for about an hour, no excessive heat. I didn't have an instrument there, so just ran the pots and made sure everything seemed to respond without issue... so far so good.

I will get a guitar down there this weekend and try it out but seems the cleaning was the ticket.

Question: is there a online parts resource for these old amps? Something that is specific to the vintage Kustoms? Like Fliptops is for the old Ampegs? This amp would clean up real nice with some new knobs.

And I am also wondering if you guys recommend shotgunning the electrolytics on the pre-amp boards? Looks like it would be very easy to do.

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Re: Kustom IV Bass Head Schematic? [message #22207 is a reply to message #22106] Wed, 21 January 2015 17:28 Go to previous messageGo to next message
chicagobill
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Unfortunately no parts resource.

Glad to hear that it's working again. DeoxIt is amazing stuff.

As for the electrolytic caps, are any of them out of spec or showing signs of failure? Personally, I prefer to leave them if they are working, replace the ones that are bad, and only replace them all when there seems to be a lot of them that are bad. But that just me, if it ain't broke don't fix it.

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Re: Kustom IV Bass Head Schematic? [message #22208 is a reply to message #22106] Wed, 21 January 2015 17:42 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Bill-

On the caps, I was just thinking while I have it all out and exposed... "why not?" But I hear you.

I guess I was appealing to any experienced Kustom technicians here... do those caps have a bad rep for failure??
They are just 25V and 35V Elytics, and they don't look puffy. If I had a schematic with voltages around the ciruit I could better know if they are in spec... I still have hopes of finding the right schematic.

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Re: Kustom IV Bass Head Schematic? [message #22209 is a reply to message #22106] Thu, 22 January 2015 06:23 Go to previous messageGo to next message
stevem is currently offline  stevem
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As you know all Elytics will fail sooner or latter and the longer they sit around un- powered up the sooner they will fail!
The two big main power supply rail caps hold up very well most times and if you do not hear any 120 hz hum with the amp idling then they are good, for now that is!
It's the tantalum type Elytics caps within the circuit that fail the most in these era amps and I have found that they will go leaky at first and run hot in short order, but this means that you can pin the bad ones down by using a thermal gun on them!
In regards to replacement knobs, little bear electronics and Rickenbacker guitars carry the knobs.
Oh! And glad to hear that the hiss was just a high resistance issue due to a poor connection!

[Updated on: Thu, 22 January 2015 06:26]

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Re: Kustom IV Bass Head Schematic? [message #22213 is a reply to message #22106] Thu, 22 January 2015 17:10 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Steven-
Interestingly, I didn't see (or recognize) any tantalum caps on these boards. I will look again.
I saw ceramics, e'lytics, and blue film caps (look like sprague OD).

The ceramics and film will probably outlive me.

The small e'lytics on the preamp boards I figured could be worth doing to avoid issues down the road, although these show no obvious signs of physical failure. Still, they are pushing 40 years old. I will probably just replace them.

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Re: Kustom IV Bass Head Schematic? [message #22219 is a reply to message #22106] Fri, 23 January 2015 06:50 Go to previous messageGo to next message
stevem is currently offline  stevem
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I was just taking a guess on the tantalum's as I did not go back and look at your photos of the gut shots!

As with any of these amps power supply improvements in sound can be had by replacing the rectos with fast recovery types, or at least adding .01 @ 200 volt ciramics across the current rectos you have in the power supply and also placing across each main rail filter a .22 uf and a .022 uf cap to let the audio bypass them better is a good mod also!

[Updated on: Fri, 23 January 2015 06:55]

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Re: Kustom IV Bass Head Schematic? [message #22227 is a reply to message #22219] Sat, 24 January 2015 11:57 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Steve. I took a quick look again and did not see any tantalum on the preamp boards.
However, I think I found one on the output stage board. It is bright orange in color.


http://i863.photobucket.com/albums/ab198/sixtringrazorboy/Amps/Kustom4Bass2_zps4e2ee090.jpg


On the bypass cap suggestion you made: Are you suggesting bypassing the e'lytic caps that are holding up a localized dc rail?
Or... are you suggesting adding bypass caps to inter-stage (blocking/coupling) film caps? (Typically done in hi-fi mods)

The thing for me is, since I don't have the schematic, I am uncertain if Kustom is running signal through any of these e'lytics.
I guess I could reverse engineer the boards... might as well draw out my own schematic, lol.

I took in some other work this week that I am trying to turn out quickly, so the Kustom may not get much attention until next week.

[Updated on: Sat, 24 January 2015 12:22]

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Re: Kustom IV Bass Head Schematic? [message #22229 is a reply to message #22106] Sat, 24 January 2015 16:11 Go to previous messageGo to next message
stevem is currently offline  stevem
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Yup, that's a tantalum on that driver board next to the Ciramic cap.
I was taking about placing those two caps across each of the big main filter cans, you can try it out in the preamp section like they do for hi fi, but I think it makes for too sterial of a sound, but you be the judge!

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Re: Kustom IV Bass Head Schematic? [message #22353 is a reply to message #22229] Wed, 04 February 2015 13:04 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Steve-
Will putting those bypass caps across the big DC filters eliminate the 'pop' I am getting when I flip the power switch on?
It is a pretty loud transient spike.
I would love to eliminate it.

I have the blown EV-15B-SRO now out at Orange County Speaker for recone.
It should be back next week.
I will keep the thread updated.

Thanks for any help on the 'pop' question.
Allen

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Re: Kustom IV Bass Head Schematic? [message #22418 is a reply to message #22209] Sun, 08 February 2015 14:48 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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stevem wrote on Thu, 22 January 2015 06:23
As you know all Elytics will fail sooner or latter and the longer they sit around un- powered up the sooner they will fail!
The two big main power supply rail caps hold up very well most times and if you do not hear any 120 hz hum with the amp idling then they are good, for now that is!


So if I needed to replace these big filters, where would I find them?
The amp does seem to have a bit of 120 Hz hum at idle.
I can modulate the hum with the master volume pot.
The hum falls off with the MV all the way up.

The caps in the pic above are a General Electric 86F series 9800uf/100V (125V surge) cap.
Could they be replaced with modern 10,000uf Hi-Fi caps like Nichicons, Panasonics, or Elnas??
Or do these have a discharge rate that is critical to the design of this amp?

Thanks for hanging in with me guys.

Allen

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Re: Kustom IV Bass Head Schematic? [message #22419 is a reply to message #22106] Sun, 08 February 2015 15:06 Go to previous messageGo to next message
stevem is currently offline  stevem
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In regards to the pop noise on turn on that is normal and the use of the added caps I posted about will not change that.
In regards to the 120 hz hum you have do this check , turn the amp on and hook up a voltmeter set for dc across the speaker output jack and report back with what that mv level is, than set the meter for ac volts and see what that mv level is.

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Re: Kustom IV Bass Head Schematic? [message #22422 is a reply to message #22106] Sun, 08 February 2015 23:52 Go to previous messageGo to next message
chicagobill
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If the hum can be eliminated by changing the settings of the different controls, it points to problems other than the main filter caps. If the filter caps are allowing for ac hum on the power lines, the hum will be there all of the time regardless of the control settings.

If you look to replace the main filter caps, you will find that computer grade filters will cost maybe $30-$40 each. You can use smaller snap-in type caps, but you will need to find some way to mount them and connect them to the circuitry.

Is the pop you hear at turn on/off the amp circuitry or a dirty power switch? Try turning on the amp and then pull the plug out of the wall instead of using the power switch. Is the pop the same? Try turning on the amp by plugging the cord back into the wall. Is the pop any different?

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Re: Kustom IV Bass Head Schematic? [message #22425 is a reply to message #22422] Tue, 10 February 2015 13:16 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks guys for the great suggestions.
I apologize for being slow to get back to the thread. Been swamped at work.

OK. Got some time over the weekend and yesterday to go over this Kustom.

I inspected every ground star. Input, output (PT center tap), and safety/chassis.
DeOxit and Scotchbrite on the contact surfaces.
Found that the chassis star had corrosion in the screw hole, and also the screw itself made minimal contact because of the paint on the rear of the chassis. I scraped off paint around the hole cleaned out the corrision, used a star washer between the screw and chassis.

I was able to measure an improvement around the chassis.

Bill, the preamps don't seem to effect the hum. I think it might be eddy currents around the PT (??)

Steve, thanks for reminding me to look at DC offset.
I played the amp for an hour (sounded good) and measured neg .050V offset.
Not sure where these Kustoms should be set, but with the HiFi stuff I have done, usually as close to '0' as possible is the goal.

AC component at output (4ohm) was measured a couple different ways:

With all controls at minimum, I saw .005V
With all controls at maximum, I saw .050V
(most of what I heard at max was pre-amp hiss)

Bill, thanks for the good suggestion on testing the switch for problems.
Using your method, the amp responds the same either way.
There is a 'thud' when powering up or down.
When the amp is turn on from a 'cold' state, the thud is more pronounced.
Once it has run for a while, it's not too bad.
Maybe the main filter caps getting slow??

Overall, it seems to be shaping up.
Just the question on the DC offset.

On that, I noticed a trimmer wheel on the output board. I think it can be seen in the gut shot pic.
Is that where the DC offset is adjusted??

Thanks for your guidance on this beast.

Allen

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Re: Kustom IV Bass Head Schematic? [message #22426 is a reply to message #22106] Tue, 10 February 2015 14:04 Go to previous messageGo to next message
stevem is currently offline  stevem
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I would have to take a guess that the trim pot you see is for bias and that .005 volt is fine for a off set so I would not do a thing in regards to output transistor match to try and improve it!

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Re: Kustom IV Bass Head Schematic? [message #22427 is a reply to message #22426] Tue, 10 February 2015 14:09 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Hi Steve-

.005V was the AC component at idle, preamps an min.

The DC offset was negative .050V (50mV).


Edit: Ran the amp yesterday for a few hours.

After warm up, nominal DC offset at output(4ohm) observed at neg 50 to neg 66mV.
(The AC in my shop typically runs between 122-127VAC).

Is that DC offset number acceptable?

[Updated on: Thu, 12 February 2015 13:36]

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Re: Kustom IV Bass Head Schematic? [message #22444 is a reply to message #22209] Mon, 16 February 2015 12:54 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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stevem wrote on Thu, 22 January 2015 06:23

It's the tantalum type Elytics caps within the circuit that fail the most in these era amps and I have found that they will go leaky at first and run hot in short order


Steve, when you replace these old tantalums, do you replace them with tantalums, or do you like to use a modern box-film cap?

Just wondering if you have a preference.

Best,
Allen

[Updated on: Mon, 16 February 2015 12:56]

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Re: Kustom IV Bass Head Schematic? [message #22445 is a reply to message #22106] Mon, 16 February 2015 15:12 Go to previous messageGo to next message
stevem is currently offline  stevem
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Go the film route!

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Re: Kustom IV Bass Head Schematic? [message #22446 is a reply to message #22445] Mon, 16 February 2015 15:19 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Cool. I have a bunch of MKP's in my stock, but no tantalums! lol.

I figured films would be better. Back in the day they didn't have these nice, compact film caps! A tantalum was a small step up from an e'lytic.

I also might have had some good news regarding documentation on the big K IV heads:

A fellow Kustom officianado I recently met has discovered he has some service data on the K IV Bass and Lead amps.
No schematic (I'm still looking) but there may be some helpful service data coming my direction.

I will let you know what comes in. If it looks to be helpful, I can scan them as .pdf's and share them here.

Best,
Allen

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Re: Kustom IV Bass Head Schematic? [message #22449 is a reply to message #22106] Tue, 17 February 2015 06:11 Go to previous messageGo to next message
stevem is currently offline  stevem
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Cool, that would be great !
I have not been into my Baldwin era Kustom bi-mp in a while, but that power amp board layout looks a lot like the board layout in that,as all the power amps just have more outputs s the wattage goes up.
The one thing that stands out to me about that power amp section is the use of the T066 driver transistors as that amp is a latter model then my bi amp I am talking about, but yet mine uses flat pack To220 type plastic drivers!

[Updated on: Tue, 17 February 2015 06:19]

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Re: Kustom IV Bass Head Schematic? [message #22450 is a reply to message #22106] Tue, 17 February 2015 13:35 Go to previous messageGo to next message
chicagobill
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Steve, the TO-66 cased transistors would usually dissipate more wattage than the older TO-220. Now the TO-66 is just about obsolete. Thankfully the TO-220 will fit in the sockets as a direct replacement.

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Re: Kustom IV Bass Head Schematic? [message #22453 is a reply to message #22106] Wed, 18 February 2015 06:26 Go to previous messageGo to next message
stevem is currently offline  stevem
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Yup,I just find it interesting that since the 66 came into the transistor world before the 220 type that they used so much that they would step back to that, but I guess they needed / felt better with the higher dispipation ratting of that.

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Re: Kustom IV Bass Head Schematic? [message #22454 is a reply to message #22446] Wed, 18 February 2015 10:06 Go to previous messageGo to next message
kustomoholic is currently offline  kustomoholic
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Allen.... That is going out this morning...woke up late for work yesterday and forgot to stick it out to go... Remember the sections where all the data is together for the Bass/lead IV heads are missing from my manual so you'll have to do some mental assembly of the diagrams to get a complete picture ...Parts list are also missing from these sections but between your boards and the diagrams I've sent you should be able to get a good idea of whats up. I've never taken mine apart since it works great and don't need no fixin...yet. My head was exposed to fire but other than being a little cosmetically marred. It's ok For a 10 spot I can't complain...Very Happy

Craig

[Updated on: Wed, 18 February 2015 10:06]

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Re: Kustom IV Bass Head Schematic? [message #22455 is a reply to message #22454] Wed, 18 February 2015 11:48 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Thanks Craig! I will continue to look for the actual service schematic and keep you guys posted if I find anything.
If anyone has an idea about who might have this info, please by all means let me know.

Best,
Allen

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Re: Kustom IV Bass Head Schematic? [message #22509 is a reply to message #22106] Wed, 25 February 2015 15:43 Go to previous messageGo to next message
kustomoholic is currently offline  kustomoholic
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Allen..... did those diagrams arrive yet???????

Craig

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Re: Kustom IV Bass Head Schematic? [message #22516 is a reply to message #22509] Thu, 26 February 2015 11:40 Go to previous messageGo to next message
corona blue is currently offline  corona blue
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Hi Craig!
Yes!! I got them yesterday, and just now opened them as I am having coffee.

These are just what I need!

These all appear to cover the boards in this K4 Bass.

The board prefix is different in these drawings. It shows them as a 300 prefix, where mine are 009.
But the circuit design appears to be the same as does the layout.
So I don't know what to make out of the difference in the prefixes.

I will see if I can put away some time this weekend to make .pdfs of these copies and pass them up to the forum.

By the way, I delivered the amp back to my friend, who was very happy in the way it turned out. He has a couple GK bass rigs in his studio and this big K just kills them.

Thanks for taking the time to look up those boards Craig!

And to everyone who chimed in to help, THANK YOU!

Best,
Allen

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Re: Kustom IV Bass Head Schematic? [message #22517 is a reply to message #22106] Thu, 26 February 2015 11:49 Go to previous messageGo to previous message
kustomoholic is currently offline  kustomoholic
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Allen.... I got the closest I could to the actual numbers....not sure what the 3 digit prefix refers to specifically but I'm glad it shed some light on the subject.. Any one have an Idea what the prefix numbers meant??????

Craig

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