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Repairing a Kasino Natural Bass Amp [message #15480]

Fri, 01 April 2011 17:23

jeffamm
Messages: 6
Registered: April 2011

Junior Member

I'm repairing a Kasino Natural bass amp and would like to compare notes with someone else who knows this amp. The schematic in the chassis shows it to have a 5104 preamp and a 5033 power amp. My first focus is the power supply. Someone had tried to repair this amp before and installed one of the filter capacitors in reverse, so it outgassed when I powered it up (got a nice photo of the geyser). Beyond that, the AC transformer output is 100VAC, center tapped. That seems high to me, and the schematic doesn't indicate what it should be. Does someone know for sure? There are two reasons why it seems high to me - the filter caps are rated at 50V on the schematic, and usually I see a bigger margin. Also, the pilot light ballast resistor is rated at 5 watts and runs very hot. I measured it as having 40 volts across it, which is about 8 watts of power it is dissipating. Any comments from experience?

By the way, the guy I bought it from (non-working, $20) said that he has a lot of Kustom schematics. Are there any missing from your group collection? Maybe he has one.

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Re: Repairing a Kasino Natural Bass Amp [message #15481 is a reply to message #15480]

Fri, 01 April 2011 17:52

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the place.

Every vintage Kustom amp I have ever seen including your Kasino uses a bipolar power supply of + and - 40vdc. So yes, you are correct in assuming that the power transformer is suspect if it is running at 100vac ct. The original ones are typically 60vac ct.

Do you think that the power transformer has been replaced?

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Re: Repairing a Kasino Natural Bass Amp [message #15482 is a reply to message #15480]

Fri, 01 April 2011 19:06

jeffamm
Messages: 6
Registered: April 2011

Junior Member

Thanks for your reply Bill. I haven't been inside one of these amps before so I don't know what it should look like, but the transformer does look original to me. It is riveted to the chassis, if that's any indication. Here's an image:

http://guitarkitbuilder.com/img/xformer.jpg

The serial number is 14535 if that helps.

I can't think of how the transformer fails by putting out more voltage than it should.

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Re: Repairing a Kasino Natural Bass Amp [message #15483 is a reply to message #15480]

Sat, 02 April 2011 11:19

stevem
Messages: 4739
Registered: June 2004
Location: NY

Senior Member

Hello and welcome also!

Here is some more info for you to go by.
On all Kustom amps which run - and + 40 volt power rails like yours the power Transfomers will output 99.4 to 101.8 UNLODED volts dependant on your local line voltage.I assume you where measuring this voltage with that popped filter still out of the circuit (unloaded), so I would deem your PT to fine.

In all my many decades of servicing gear I have never come across a bad PT that put out too much volatge, plus it looks like you fuse did not blow which more than likly would have happended should the current draw have been anywhere near high enough to short the PTs windings in the way it would have to to change the ratio of the windings to pump more voltage out of it.

Some other things to note would be in reagrds to the preamp board.
If this board uses a IC chip which I belive it does, than these semiconductors as used in these amps need a bipolar power supply to feed to them all the time when being powered up.
In short I am saying that your IC may now be shorted due to the dead side of the 12 volt power supply rail when that filter blew up.
I would start out trouble shooting by unplugging the Molex connector to the preamp board and installing a derated fuse by a factor of 1/2 to 3/4s of a amp.
This will let you power up the amp, idle it and prove out that the 5033 board and the rest of the output stage is OK. You can than plug the preamp board back in and see if the - and + power supply rails that feed that board hold at around 12 volts.
If you find a - or + 12 volts on any other pins of the IC but the power input ones than that Chip is bad.

When you first power up the amp have a meter set to read 50 volts DC across the speaker output jack with no load, if you see any DC voltage but for a breif spike at power up than you also have blown output and or driver Transistors in the output stage.

Also note that all voltages shown on your schemtaic where I belive taken with no load on the output stage and all controls up full.
This is normally noted on the schematic(s)

The K150 amps that use basiclly the same power supply use a 10 watt 400 ohm resistor to each lamp, so yours are 5 watt, are there two 5 watts stacked on top of each other?

Let us know how things progress!

[Updated on: Sat, 02 April 2011 11:30]

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Re: Repairing a Kasino Natural Bass Amp [message #15485 is a reply to message #15480]

Sat, 02 April 2011 15:55

jeffamm
Messages: 6
Registered: April 2011

Junior Member

Steve,

OK that's really good news on the PT, and I was hoping that that was the answer.

Yes my board has an IC identified as 4739CP. I had the molex connector disconnected for some of my testing but not sure at what point I did that, so perhaps it is damaged.

http://guitarkitbuilder.com/img/kasino_b.jpg

On the lamp there is just one resistor. The markings on it have faded but I believe I can read it as 5 watts, and the resistance appears to start with a "2", so I think it is consistent with the schematic at 200 ohms.

http://guitarkitbuilder.com/img/kasino_a.jpg

The one inconsistency I've observed from the schematic is on the resistors associated with the zener diodes, which the schematic shows as 400 ohm, 10 watts. The ones I see, mounted stacked, one on top of the other, are marked 200 ohm, 10 watt. I can easily see that as having been a design revision.

http://guitarkitbuilder.com/img/kasino_c.jpg

I'll keep this thread informed with progress.

thanks again

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Re: Repairing a Kasino Natural Bass Amp [message #15488 is a reply to message #15480]

Mon, 04 April 2011 12:54

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

First off, I need to correct my earlier post, as I did some real research on your amp. Your amp has the large filter choke at the input of the power supply. I pulled out my Sidewinder chassis which shares the same power supply and measured the power transformer output at the bridge rectifier, and I got 99vac ct. The final DC at the filter caps was plus and minus 45vdc.

The new photos show an original transformer and a replaced 4739 chip. I don't think that I've ever seen XR branded chips in a Kustom product that was factory installed. But I may be wrong. In any case the chip is dated to the 29th week of 1978, which is probably after your amp was manufactured.

As for the hot resistor, what number lamp do you have installed? It should be a 28 volt #1829. If you have a more common #47 lamp in there, it will overheat the resistor as it will draw way too much current.

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Re: Repairing a Kasino Natural Bass Amp [message #15490 is a reply to message #15480]

Mon, 04 April 2011 14:20

jeffamm
Messages: 6
Registered: April 2011

Junior Member

Bill,

Thanks for doing the research, your info is very helpful. Here's what I've done and the results:

1. Reinstalled the capacitor with correct orientation (I tested it with DMM) so that I could continue troubleshooting and am now getting +/- 44 volts from PS. I am also now getting an amplified output from the amp (more on that below)

2. Of some concern, the regulated supply is delivering +13.3v/-12.6 volts. The schematic shows these ballast resistors to be 400 ohm, but my actuals are 200 ohms. I'm wondering if the lower value is keeping the zeners from regulating down to 12 volts? Or is there still not enough load?

3. I'll check on the pilot lamp bulb and report back.

4. The amp "works" now, in that an amplified signal comes out to the speaker. I don't know the speaker impedance this is designed for. It's not marked anywhere that I've found. But the output seems low, even if I'm at the wrong impedance, and doesn't sound good (though this is with a guitar into a single 8 ohm speaker).

5. On the 5104 board I'm feeding in a 400 Hz 72mv p-p signal at input. At Q3 I can get 1000mv pp with drive and volume on full. However this 1000mv is reduced to 40mv at output of IC1, and even lower at input of IC1. I'm suspecting a problem between Q3 and the IC, and will trace more tonight. I don't think the Q4-Q8 section should remove that much signal, but let me know if you do please.

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Re: Repairing a Kasino Natural Bass Amp [message #15491 is a reply to message #15480]

Mon, 04 April 2011 16:39

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I wouldn't worry about the 200 ohm resistors in place of the 400 ohm value. The 200R value is the most common used in these amps, so I don't know if the 400R was a production change or an error on the schematic.

I think that the low voltage supply readings that you are getting are fine. It's not that critical of a value, and in any case the voltages are within a 10% tolerance range.

It does sound like there is a problem in the preamp tone control section. There will be some loss through any tone network, but not as much as you are getting. Check Q4 thru Q8 as well as IC1 and any capacitor in that section of the board. Have you checked for bad solder joints, especially at the pots? The boards are mounted by the pots, and will often have cracked solder joints from vibrations. And some of those large value Mylar caps will break themselves loose from the vibrations as well.

According to my book, the Natural Bass amp is listed as an 8 ohm load.

Keep us updated.

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Re: Repairing a Kasino Natural Bass Amp [message #15493 is a reply to message #15480]

Mon, 04 April 2011 22:37

jeffamm
Messages: 6
Registered: April 2011

Junior Member

OK I made a lot of progress this evening. I started to do some testing in the Q4-Q8 tone circuits and found that the collector voltage on Q4 was only a few tenths of a volt. Likewise the voltage on the other side of its load resistor R20 was only a few tenths. This point connects directly to the +12v supply through R21, a 150 ohm resistor. I realized that something was pulling this line to near ground. After isolating sections with some trace cutting I found that C2, the 100 ufd filter cap was bad. With the capacitor removed I'm now getting a lot of volume and good tone. I'm not sure that I'm done troubleshooting though. There is still a bit of low level noises, scratchiness, hums, all of it down pretty low. I won't be able to tell for sure until I re-mount the board so it is firmly grounded, and install good caps (the C2 just discussed, and the power supply cap discussed previously).

On the pilot lamp front, your guess was correct, it's got a 6-8 volt lamp in it instead of the 28 volt required, so the resistor is getting hot.

I'll order all the replacement parts and get them installed, and let you know what the result is. I'll also be writing up the whole case in www.guitarkitbuilder.com.

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Re: Repairing a Kasino Natural Bass Amp [message #15494 is a reply to message #15480]

Tue, 05 April 2011 11:59

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

That's great news! Very good job of finding the problem. Logic will always win out, at least when it comes to troubleshooting.

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Re: Repairing a Kasino Natural Bass Amp [message #15495 is a reply to message #15494]

Tue, 05 April 2011 18:55

stevem
Messages: 4739
Registered: June 2004
Location: NY

Senior Member

Jeff it may help you to know that when I test Kustoms out on my test rig I have found that a input signal of .150MV is more than enough to drive these amps to full peak power output.
Driving you amp this way you should see and easy 23 volts (66 watts) of RMS power into a 8 ohm test load resistor.

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Re: Repairing a Kasino Natural Bass Amp [message #15497 is a reply to message #15480]

Tue, 05 April 2011 20:43

jeffamm
Messages: 6
Registered: April 2011

Junior Member

Steve, thanks for the reference point. I assume you mean 150 millivolts (.15 volts)? It will be handy to know what output level to look for.

I've got the parts ordered from Mouser so in a few days I should know where I'm at.

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