Kustom Bi-Amp Slave Model: VIII BAS

[gopher7]

Hello,

Many thanks to all that share info in this forum and at this site - this is my first post. I have a Kustom Bi-Amp Slave, model VIII BAS (part of a PA system). It was made at the Kustom factory in Chanute Kansas during the mid 70s (guessing). It powers up but there is no output. I have not found any information on the net for this amp. Tried doing some basic trouble shooting with no luck. My guess is that the problem is on the control board, since it had been previously repaired. A few of the original Tantalum caps have been replaced with aluminum electrolytic caps. I am looking for the control board schematic PC5258 and any related schematics for this amp.

[Enzo]

COntrol board as in preamp? What are all the board numbers?

I have replaced old tantalums in Kustom amps myself.

If it powers, test the power amp first. Are all power supplies present? If you hit the input pin of the power amp, does sound result? Even if you don't know the circuit, on the power amp card, if power is present and signal applied, signal should be present on transistors through the amp, so is there signal in there?

Likewise you can test the preamp circuits even if the power amp is shot. If they are powered, do they send signal as far as the wire over to the power amp?

[gopher7]

Hello Enzo,

My thanks for your reply and suggestions regarding the signal paths to check. The preamp is a PC5258 Rev-0. The amp boards are PC5033 Rev-1 and PC5065 Rev-2. I found the amp board schematics and parts layouts on this site. I wasn't able to download the parts layout for the PC5065 - can't figure out why...(suggestions?) Any thoughts on finding the preamp PC5258 schematic?

Also, did you replace the Tantalums with Tantalums or with aluminum electrolytic? The Tantalums are marked .1 100v, .01 100v, and .0047 600v. I have checked the net for a source for the caps and it does not appear that they will be easy to find.

[Enzo]

I hate tantalums, and never order them, when I have perfectly good lytics in my drawers already.

[gopher7]

I wonder why Bud Ross and his engineers preferred the Tantalums? Any thoughts on the other questions in my previous reply

[52 Bill]

The Tantalums are marked .1 100v, .01 100v, and .0047 600v.

Those are not Tantalum caps, they are mylar or polyester caps, like an Orange Drop.

Kustom Tantalum values were more like 6.8uF and 33uF

[gopher7]

Hello Bill,

I believe that they are Tantalum - very clearly marked with "-" polarity. Just refurbed a Kasino Club-L amp with 4x10s. Had to replace one of the Tantalums on the "clean channel preamp board - used an aluminum electrolytic in place of the Tantalum cap. Guessing that Kustom used the Tantalums in the mid 70s because they had better specs than the "can" electrolytic, were easier to obtain, and maybe cheaper. The aluminum electrolytic caps have evolved a lot since the 70s.

Do you have a source for the Kustom model: VIII BAS - Bi-Amp Slave, preamp board PC5258 schematic? Hoping that maybe Enzo or ? has a source. I've spent considerable time on the net with no success.

[gopher7]

Bill,

Correction - the amp I refurb'd was the Kustom Club-L not Lounge-L.

[Enzo]

Those would be extremely unusual values for tants. And tants would have a + sign, rather than the negative, in most cases. They would have a + sign on the schematic.

The tantalums usually were these little teardrop shaped things dipped in epoxy.

I will look for 5258 next time I am at the old shop.

If you have the 5033 and 5065, those are the power amps, and the input to it is a three pin connector. Have you injected a signal there?

[gopher7]

Bill,

First, my apology. My wife told me to put my glasses on and look again at the caps again. You are correct, the caps are not tantalum. They are the old ITW caps and appear to be Mylar Caps. What I thought was the "-" polarity mark was the bar mark on the cap. Enzo pointed out that the tantalums have "+" marked not "-". I checked the schematic for the clean channel preamp on the Club-L and saw the cap that I replaced had both the "+" a "-" marked. The cap was missing from the Club-L preamp board, so I had no physical cap - just the schematic and tantalums at other points on the preamp board as a reference. Thanks for steering me in the right direction.

[gopher7]

Totally messed-up. You are correct - they're not tantalum.

52 Bill also pointed out the values that I indicated for the "tantalums" would be unusual. So, with reading glasses on I rechecked. As noted to Bill in a separate reply, the caps are the old ITW brand and I believe Mylar.

The PC5258 schematic will be much appreciated. As you can tell - I need the help.

[52 Bill]

The PC5258 schematic will be much appreciated. As you can tell - I need the help.

I checked my binder and I don't have a copy, sorry.

Enzo mentioned the values as being odd for tantalums. He also noted that you can test each individual power amp and the input/crossover circuits to try and isolate the problem areas.

My experience with this series of amps is that often the boards are interconnected by Molex connectors. These connectors become oxidized and will cause all sorts of problems. Make sure that if it has them, that they are all clean and tight. And also watch out for broken solder joints on the pc mounted connectors.

Kustom tried to isolate all of the grounds of the boards and used the Molex connectors to make the ground connections. I've seen a few power amp circuits blow up due to a complete loss of the ground connection at one of the Molex pins.

[gopher7]

Many thanks for checking your binder for the schematic. PC5258 preamp has one 3 pin Molex connector going to the PC5033 amp and one 3 pin Molex connector going to the PC5065 amp. I will clean all connectors.

Checked preamp for bad runs and solder joints - didn't find any. I haven't checked the 5033 or the 5065. They will each take some time since both are mounted to the back of the case with a spaghetti bowl of connections (Molex, spade, and hard wired) from the input, output, and line out jacks, preamp, and the power transistor rails. Some of the wires are so close to the front of the amp boards that the boards can be moved only slightly forward after disconnecting them from the case.
Thanks again for your help.

[gopher7]

Just wondering if you have had an opportunity to check at your "old shop" for the PC5258 schematic? I have a signal out of the each of the amp boards PC5033 and PC5065. It appears that the preamp PC5258 is not passing signal to the amps.

[52 Bill]

It appears that the preamp PC5258 is not passing signal to the amps.

Check the power supply for the preamp, typically Kustom used a plus and minus 12 volt power supply for the op amps. This supply was normally reduced from the main high voltage supplies via resistors and Zener diodes. If both supplies are present and stable, start checking the op amps for voltages on any output pins.

Early Kustom designs used a 14 pin dip uA739 style chip, but later on they started to use more common 4558 type dual op amps in an 8 pin dip.

Of course you can always send a signal into the input and then trace the signal path through the preamp circuit until you find where it dies out. If both high and low freq. outputs are dead, then I would expect to find the problem earlier in the signal path before the freq. separation.