Home » VintageKustom.com » Repairing Kustom Amps » kustom lead3 head (how to repair?)
kustom lead3 head [message #14243] Tue, 31 August 2010 06:23 Go to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
Junior Member
Hi, I'm new at this forum........Since a few weeks I became owner of an old Kustom lead 3 head with a 4-12 speaker cabinet.
I got it for a bottle of red wine Razz I was told the amp doesn't work anymore. Usually I do tube amps ( I own a VOX AC 50 '67 and a Fender Bassman '71)so I don't have any experience in transistor amps. When I switch on the amp I hear a deep pop in the speakers and its hums a bit. So the power amp must be working. Opening the amp learnd me that 1 of the two 200 ohms resistors in the power supply for the pre-amps was fried. After replacing I mearsure 12,5V + to ground and 13V - to ground.
Capacitors looks like new! Still I got no sound on de amp......
When I put a signal from an MP3 player on the power amp I get music out of the speakers but working my way up to the imputs in both pre-amps soon I get out of sound. The question is: can the fried resistor be the cause of the dead IC's on the pre amp boards? (both channels are dead) but the tremelo and reverb still work a little. Please anyone with suggestions: help!
By the way it is a later model Lead 3 with the main switch on the right below the master volume pot.

GREAT SITE!

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Re: kustom lead3 head [message #14244 is a reply to message #14243] Tue, 31 August 2010 14:11 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
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Hello and welcome!
The first thing to do is to confirm that the driver/power amp section is truly ok.
Do this by connecting a volt meter across the speaker output jack and confirming that after the initial power up that you read less than .125 VDC (125 mv) on the meter.

These amps use Moldex connectors to connect from board to board, as such they develope poor connections from dirt and tarnish on the aluminum pins and also by the Female portions swelling open.
Cleaning and retentioning of all these connectors is advised.

The blue wires going into the driver board bring in the audio signal from the other sections of the amp.
If you lift these wires at eitheir end and inject test signal into the board you will produce output, but it takes well over 5 volts to drive the power section to its full 130 watt RMS output into a 4 ohm load.

Although I have never seen a schematic for these amps, I do know that they are quite like the earlyer 150 and 250 Kustom amps circuit board wise.
In this regard the first channel output of the 150/250 models passes thru a section of the IC chip on the second preamp board.
If this is the case than one of your preamps may be good and the 14 pin IC chip on second preamp could be bad, this is just a guess on my part, but who knows!

These 14 pin chips are at this point in time only found thru semiconductor surplus supplyers, or out of other Kustoms amps that used them.
I have some spares for sale should you need them and are not able to trace them down.

You can get a Schematic for this amp from Broadwaymusicco.com.
I

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Re: kustom lead3 head [message #14248 is a reply to message #14243] Tue, 31 August 2010 17:11 Go to previous messageGo to next message
pleat is currently offline  pleat
Messages: 1452
Registered: June 2004
Location: Belding, Mi
Senior Member
Can you post the circuit board numbers? I may have some. I'm no tech but most ever kustom used a +8 -8 voltage to the pre amps.
It sounds like you have the silver control panel model. Early 74 to when they changed to the all black control panel amps.
pleat

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Re: kustom lead3 head [message #14254 is a reply to message #14243] Wed, 01 September 2010 06:18 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
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Thanks for the quick reply guy's ,I will check the connectors again (I already sprayed them with contact spray) and clean them with fine sandpaper. About the voltage on the speakeroutput DC or AC ? speakers connected or not? About the 14-pin IC: which one is pin 1 and which one pin 14? normally there is a small cutaway in either one of the edges, but there is no one on the SL 22211/NTE 725 ? Board numbers are: 5128 5129 rev2 on the pre-amps and 5065 on the power amp......

I'll keep you posted on my efforts...

Thanks in advance

Peter Razz

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Re: kustom lead3 head [message #14255 is a reply to message #14243] Wed, 01 September 2010 09:22 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
Senior Member
Hi.

As I posted the voltage to be measured is DC voltage (vdc)
and should be measured with the speaker(s) un-hooked.

Note that for testing the amp can be run with a speaker load safly, and infact all voltage show on Kustom schematics are taken with NO speaker load, and all controls up full unless other wise noted on the schematic.

The preamp section of these model amps are powered by 12VDC and regulated by two Zener diodes most times mounted off of a two post terminal block on the floor of the amp near the power transfomer.

Now on to the 14 pin IC chip and the lead reference.
With the lettering on the chip facing you, pin #1 is on the lower left, and pin #14 is on the upper right.
If atleast one of your two 14 pins ICs are good you can install 14 pin sockets and swap the chips from board to board and may be get one of the preamps up and running if its is just a chip problem.

Just as a reference. On other electronic gear that use this IC chip I have hard wired in two 8 pin IC sockets above the board and used two 6 pin or 8 pin newer ICs (depending on how many gain stages I was replacing)with ratings and spec`s that where close enough to work and in the end go the gear up and running again.

Keep us informed on you progress!

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Re: kustom lead3 head [message #14256 is a reply to message #14243] Wed, 01 September 2010 09:25 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
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Note that when firing up the amp, that pop sound thru the speakers is a spike of DC voltage for a quater of a second, and is a normal thing at power up with these amps.

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icon1.gif  Re: kustom lead3 head [message #14259 is a reply to message #14255] Wed, 01 September 2010 10:09 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
Junior Member
Tonight I'll have the time to check these out, now I know the pin numbers, I can Check the voltages Thanx.....
BTW I know the reason for the pop in the speaker, it was just my reason for concluding the power amp works

I'll keep you posted! Smile

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Re: kustom lead3 head [message #14260 is a reply to message #14255] Wed, 01 September 2010 12:33 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
stevem wrote on Wed, 01 September 2010 08:22
With the lettering on the chip facing you, pin #1 is on the lower left, and pin #14 is on the upper right.

If pin #1 is at the lower left, then pin #14 will be on the upper left directly above pin #1. The pins are numbered in sequence around the chip:

1...14
2...13
3...12
4...11
5...10
6....9
7....8

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Re: kustom lead3 head [message #14266 is a reply to message #14260] Thu, 02 September 2010 02:57 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
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Registered: August 2010
Location: rotterdam holland
Junior Member
Thanx Bill, I tought so too........

After cleaning the connector pins still no output.
There is a mall DC voltage on the speaker output when switched on after switching of again, the output voltage goes up to approx 13 volts due to the capacitors I recon......It takes about 30 seconds the voltage to go back to zero.

Is there a trick to test the IC's without removing them from the board?

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Re: kustom lead3 head [message #14267 is a reply to message #14243] Thu, 02 September 2010 09:34 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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Sorry about the mistake on pin #14 location on the chip, thanks Bill once again for being on top of things, as always its great to have you hear on the Board!!

In regards to testing the IC, I do not have a schematic with me at the moment, but I would say that other than pin 14 being at a high posititve DC voltage and pin 7 reading a high negitive DC voltage all other pins should read no higher than 1 volt DC + or - if I recall correct.

Comparing the voltages on each chip would be a way to start out for now.

I am not sure about how to read you in regards to discribing how and where you are seeing the 13 volts you posted about, but since the amp is not massivly huming, or blowing fuses I would than concider at least the high current section of the output stage good.

These amps I belive have a Module output jack you can connect to which will let you send any preamp signal you may have to another amp.
If it turns out your preamps are working but the input stage to the driver board has a problem you will have a large amount of signal out of this jack to feed into another amp or stereo for testing use.

Note also that the blue wires in these amps are the signal output wires.
You can snip these open and connect between them and the amps chassie with a cable to feed these outputs into another amps in order to see what you have or do not have.

I will get back to you with some other info once I get my schematics back in hand.

[Updated on: Thu, 02 September 2010 09:43]

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Re: kustom lead3 head [message #14269 is a reply to message #14243] Thu, 02 September 2010 12:46 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
The way that I check the IC's is by reading voltages on the input and output pins. Except for any special circuits like oscillators and switching circuits, in any bipolar powered circuit there should be almost no voltage on any input or output pin.

You should as Steve noted, find +12 volts on pin 14 and -12 volts on pin 7. Check pins 8,9,13 and pins 5,6,1 there should be less than say 0.5 volt on all of these pins. Please note that these numbers relate to the 739 chips only. By the time the III's were being made, I believe that Kustom was also using standard 8 pin, 1458 dual opamps as well.

I assume that the 13 volts on the speaker output was gotten with no speaker attached. This 13 volts is the turn on thump that we all hear when we turn on the amp. Normally with the speaker attached this 13 volts bleeds off quickly and should not be a problem.

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Re: kustom lead3 head [message #14270 is a reply to message #14243] Thu, 02 September 2010 16:32 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
Senior Member
I did some schematic snooping and on anything other than the + and - power pins you should not find anything more than .033 volts plus or minus.
So like Bill posted anything over .5 volts is suspect.

Suspect also is why you are finding a -13 volts as these supplys are regulated by a zener diode.
This leads me to think that the negative supply is unloaded like one of the semiconductors or chip is blown open, not shorted.

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Re: kustom lead3 head [message #14272 is a reply to message #14270] Fri, 03 September 2010 08:26 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
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Well, I'm a bit further now....Yesterday I measured the voltages on the pin and they were way out of line! I biult out the little pre-amp board and inspected every connection on the board and re-soldered all of them. I let the board out of the amp and reconnected the connectors. Much to my surprise I suddenly heard my guitar through the speakers! tonepots were not working but the volume did... When I builded the board back into the amp, it was again dead as can be..... So now I think there is a shortcut in the powercircuit and I suspect the 2 zenerdiodes mounted to the chassis. Does anyone know the specs of these diodes? By the way, the amp wasn't very loud, but I think I'll have to build out the other (big) pre-amp too. One more question? can I disconnect the zeners without danger? Thanx in advance!

Peter Very Happy

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Re: kustom lead3 head [message #14273 is a reply to message #14243] Fri, 03 September 2010 09:32 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
The two Zeners are a 1N5349,which are a 12 volt 5 watt model and should not be hard to come by if one is toasted.

If you disconnect them you can not power up the amp with out unplugging the connectors that carry the regulated + and - 12 VCD into the two preamp boards and the effects board, the driver board and the power amp section how ever run on the 39.5 volt + and - supply and can be left hooked up.

You had found a open 200 ohmm 5 watt resistor, was this the one that feeds the 12 volt minus supply rail that is now reading 13 volts?

Any semiconductor that is power off of a differential supply like those preamp boards and may be also the effect board in these amps should not be powered up with only one leg of the supply working, as most times this will blow a bunch of stuff.

With out having the proper schematic in front of me I can not comment as to why the preamp board worked somewhat when you had it un-bolted from the chassie.
One guess may be that you have a shorted semiconductor that was no longer shorted to ground with the board dismounted from the chassie.

[Updated on: Fri, 03 September 2010 09:38]

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Re: kustom lead3 head [message #14274 is a reply to message #14273] Fri, 03 September 2010 09:48 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
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Registered: August 2010
Location: rotterdam holland
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Thanx Steven! I think it is the pole on which I replaced the resistor (it should be 200 ohm but I only had one of 220 ohm) so there is a slight difference between the poles. I think i will replace the zeners and the resistor for proper ones and see what happens.............

Peter

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Re: kustom lead3 head [message #14281 is a reply to message #14243] Sat, 04 September 2010 09:42 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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If you can not find a 12v 5 watt Zener, you can always string two or three lesser rated ones in serise as long as you come up really close to the 12volts needed and be at 5 watts or more.

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Re: kustom lead3 head [message #14283 is a reply to message #14281] Sat, 04 September 2010 10:04 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
Junior Member
Thanx for the tip Stevem.............
I checked the zener yesterday and they are ok.....
I also checked the other pre amp board with the effects but this one is still dead unfortunately Sad

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Re: kustom lead3 head [message #14538 is a reply to message #14283] Thu, 14 October 2010 04:04 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
Messages: 11
Registered: August 2010
Location: rotterdam holland
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Victory boogie woogie!

Guy's believe it or not, I've got the amp working! After replacing the IC's MC 1458 and SL 22211 the whole thing came alive and kickin'!
The SL 22211 was hard to find but I found a french website which gave the UA 739PC as a replacement. I finally tracked them down via a chinese site to a store nearby my town............ I called them up and they send me the 3 chips for free as a sample!
First I mounted some IC sockets on the board to prevent the chips from frying when soldered.
I fired up the amp yesterday and a small hum appeared through the speakers................after pluggin'in the les paul I found out everything works as it should, also the effects! Tonight I'll turn it up loud and see(hear) what happens..........
My final conclusion must be that due to the storage in a moisture shack one of the resistors in the 15V power supply circuit broke down this results in an unbalanced voltage through the IC which they don't like.........
I've learnt a lot during this repair.
Everybody who gave me tips and help on this item THANKS A LOT !!

Peter Very Happy Very Happy Very Happy Very Happy

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Re: kustom lead3 head [message #14541 is a reply to message #14243] Thu, 14 October 2010 12:07 Go to previous messageGo to next message
stevem is currently offline  stevem
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Great news!enjoy that sweetie now.

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Re: kustom lead3 head [message #14542 is a reply to message #14541] Fri, 15 October 2010 04:16 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
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Location: rotterdam holland
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thanks Stevem............

I find the power transistors getting pretty hot when crankin up the volume. it is a pretty loud amp, but since i'm new to this amp i wonder if this is normal?

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Re: kustom lead3 head [message #14543 is a reply to message #14243] Fri, 15 October 2010 07:42 Go to previous messageGo to next message
stevem is currently offline  stevem
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Are all of the output transistors of the same manufactuer, if not the match from side to side (gain wise) could be off enough to make one or two of each sides pair work alot harder.

Do they all seem equaly as hot? if so make sure tha the metal cased diode is held tigth in its clip on the output transistor mountung bracket.

Also check to see that its lead connections are ok as they are pretty stiff and tend to break off with enough movement.

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Re: kustom lead3 head [message #14544 is a reply to message #14543] Fri, 15 October 2010 08:30 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
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Location: rotterdam holland
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manufactor is the same and they are well mounted and connected
(this was one of the first things for me to check.
One pair is getting hot the other one not..........

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Re: kustom lead3 head [message #14545 is a reply to message #14243] Fri, 15 October 2010 10:10 Go to previous messageGo to next message
stevem is currently offline  stevem
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What do your volatges measure across the main power rails of the two big main filters ( balance wise) +39.5 and -39.5?

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Re: kustom lead3 head [message #14546 is a reply to message #14545] Fri, 15 October 2010 10:29 Go to previous messageGo to next message
peterjeur is currently offline  peterjeur
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Location: rotterdam holland
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I only have an analogue volt-meter and it say's 40V approx...
So I recon this is good Smile

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Re: kustom lead3 head [message #14547 is a reply to message #14243] Fri, 15 October 2010 12:11 Go to previous message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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To trace this problem out it would pay to pick up even a cheap digetal voltmeter as you will need to compare voltages from one side of the output stage to the other.
First though I would just for kicks swap the two output from side to side just to confirm that the heat problem does not follow the transistors, if that checks ok than its time to check or change out ( its cheap to do) the serise bias diodes on the driver board and any high tolerence resistors that may be out of spec.

[Updated on: Fri, 15 October 2010 12:11]

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