| Kustom IV Bass Head Schematic? [message #22106] |
Thu, 15 January 2015 16:43  |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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 #22126 is a reply to message #22106] |
Fri, 16 January 2015 13:34  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22131 is a reply to message #22106] |
Fri, 16 January 2015 16:44 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 #22134 is a reply to message #22106] |
Fri, 16 January 2015 18:01 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 #22207 is a reply to message #22106] |
Wed, 21 January 2015 17:28 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 #22209 is a reply to message #22106] |
Thu, 22 January 2015 06:23 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22219 is a reply to message #22106] |
Fri, 23 January 2015 06:50 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22227 is a reply to message #22219] |
Sat, 24 January 2015 11:57 |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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.
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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22422 is a reply to message #22106] |
Sun, 08 February 2015 23:52 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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 #22427 is a reply to message #22426] |
Tue, 10 February 2015 14:09 |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22444 is a reply to message #22209] |
Mon, 16 February 2015 12:54 |
corona blue
Messages: 25
Registered: January 2015
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Junior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22449 is a reply to message #22106] |
Tue, 17 February 2015 06:11 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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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] Report message to a moderator |
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| Re: Kustom IV Bass Head Schematic? [message #22450 is a reply to message #22106] |
Tue, 17 February 2015 13:35 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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 #22454 is a reply to message #22446] |
Wed, 18 February 2015 10:06 |
kustomoholic
Messages: 227
Registered: March 2009
Location: Tulsa,oklahoma
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Senior Member |
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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...
Craig
[Updated on: Wed, 18 February 2015 10:06] Report message to a moderator |
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