The Repair Bench: Kasino Natural Bass Amp

In this episode of The Repair Bench we are troubleshooting a Kasino Natural bass amp from the early 1970s. The Kasino brand was made by Kustom Electronics, maker of Kustom Amplifiers. If you're not familiar with them, the Kustom amps brand was famous for covering their amplifiers in "tuck and roll" style upholstery (Photo 1), previously used in hot rod cars.

Photo 1 - Typical Kustom Amplifier Featuring Tuck and Roll Upholstery

While these amps were famous for their unique look, they also had a large following by notable artists for their great sound. Kustom was a leader in high-end, solid-state amplifier design and famous artists using their equipment at the time included Credence Clearwater Revival, Johnny Cash, Carl Perkins and Waylon Jennings. Their position as a high-quality amplifier, with fancy upholstery made their product expensive and limited the market size. To expand their dealer network and sell a less expensive product, Kustom created the Kasino amplifiers brand, using the same designs but with traditional Tolex-type coverings.

THE KASINO NATURAL BASS AMP

Photo 2 - A Heavy Duty Cabinet

The Kasino Natural bass amp (Photo 2, above) is an early 1970s, solid-state amplifier rated for 75 watts RMS (root-mean square) and 150 watts peak. It has two input jacks for one channel with controls for volume, drive and three, count'em, three bass controls (Photo 3) - this is our kind of bass amp!

Photo 3 - Three Bass Controls

One of the first things you notice with this amp is that it's built like a tank! It is very heavy due to its double-cabinet construction. The inner cabinet is a fairly standard sheet metal box enclosure of heavy duty construction. This box slides into a secondary cabinet (Photo 4) made with cast-metal sides and a tolex-covered top and bottom. This design was clearly overkill, as the amplifier industry developed cabinet/chassis designs that weighed less and still offered adequate protection.

Photo 4 - Outer Cabinet with Cast-Metal Sides

Beyond the cabinet, the Kasino Natural Bass Amp is a fascinating trip into the early history of solid-state guitar amplifier design and construction. It features a hand-drawn (Photo 5) printed circuit board and a variety of metal and plastic transistor types (Photo 6). The construction of this amp is much more suggestive of a custom "one-off" build rather than a product that was manufactured in numbers.

Photo 5 - Printed Circuit Board

Photo 6 - A Variety of Transistor Types

INITIAL INSPECTION

Before opening this amplifier we gave it a try and got no sound or output of any kind. As we began to remove screws we observed it externally to be in good condition, but obviously not in original condition. The schematic on the chassis showed it to have a 5104 preamp board and a 5033 power amp board. Kustom/Kasino standardized their circuit boards, and then used them in different combinations in different models. This amp had a lengthy orange power cord which replaced the original, and once the case was open the internal components showed various signs of having been repaired or modified. For example there were screw holes in the bottom of the cabinet that were no longer in use. We noticed that the filter capacitors had been replaced and the mechanical mountings had been modified. With the Kasino Natural bass amp powered up again, we observed that front panel lamp was illuminated and that its ballast resistor (Photo 7) was running very hot - too hot to touch.

Photo 7 - Interior View of Front-Panel Lamp and Ballast Resistor

It was then that we noticed that one of the electrolytic capacitors was outgassing (Photo 8).

Photo 8 - Filter Capacitor Outgassing

Seeing the capacitor in this condition we took a quick photo and then turned the power off. This type of outgassing is generally due to the applied voltage being in reverse polarity to the markings on the capacitor. So the initial assessment was a bit perplexing. An outgassing capacitor and a ballast resistor running too hot are odd symptoms. After studying the schematic we could see that the outgassing capacitor had indeed been installed in reverse. This was a bit troubling as it indicated that this amplifier was not working the last time it was closed up, and someone inexperienced had worked on it.

TROUBLESHOOTING THE KASINO NATURAL BASS AMP

The troubleshooting began with the power supply. Since a replacement capacitor wasn't immediately available, the one that outgassed was tested to see if it could be temporarily reused. This would only be a temporary situation, as the assumption is that an electrolytic capacitor that has been subjected to a reverse polarity voltage will have been damaged to some extent. The capacitor was tested by first discharging it, then using a digital multimeter to test its resistance. The characteristic of a good capacitor is that the resistance will initially appear as a short circuit (low ohms) and then the resistance will rise to very high or open circuit as the capacitor charges. In this case the capacitor passed the test so we decided to use it temporarily to continue troubleshooting. After reinstalling the capacitor with the correct polarity we were now seeing +/- 44 volts from the power supply, which was in the correct range. Re-connecting a speaker and test signal we found that the amp "works" now, in that an amplified signal comes out to the speaker, though the audio output is not real loud, and doesn't sound particularly good. We injected a 400 Hz, 72 millivolt peak-to-peak (mVpp) signal from our audio generator and began to trace the signal with an oscilloscope. Referring to the schematic in Photo 9 (click to enlarge) we measured 1000mVpp at transistor Q3 with the drive and volume on full, indicating that transistors Q1, Q2 and Q3 were amplifying the signal. However the 1000mVpp was reduced to 40mVpp at the output of IC1a (pin 1), indicating a problem between Q3 and the IC.

Photo 9 - Schematic for 5104 Preamp Board (click to enlarge)

Testing the DC voltages in the Q4-Q8 tone circuits we 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. Since this point connects directly to the +12v supply through R21, a 150 ohm resistor, we realized that something was pulling this line to near ground. After isolating sections of the circuit by cutting the printed circuit board traces we found that C2, the 100 ufd filter cap, was bad (very low resistance). With C2 removed we got a lot of volume and good tone, though with still a bit of low level noise, scratchiness and hum. We believed that these symptoms were likely to go away once capacitor C2 and the outgassed filter cap were replaced, and the boards were remounted and properly grounded. With the needed capacitors on order we turned our attention to the front panel lamp and ballast resistor. This turned out to be a simple problem. The proper bulb for this amp is a #1829 28-volt lamp, however it had a #47 6.3-volt lamp installed. This left a much larger voltage across the ballast resistor, which was making it run very hot. Replacing the bulb fixed the problem. We ordered a pair of replacement filter capacitors, even though only one was known to be bad. Given the inexperience of the previous repairer, we didn't know what might been done to the other filter capacitor, so the decision was to replace both of them. However these capacitors were too long to mount vertically, so a new clamp was used to mount them horizontally, as shown in Photo 10.

Photo 10 - New Filter Capacitors Mounted Horizontally

With the new filter capacitors, and capacitor C2, installed and all boards mounted in place, the amp was tested and found to be in perfect working order once again. Time to rock out and enjoy the Kasino Natural bass amp. By the way, the Vintage Kustom Amp forum was an invaluable resource for fixing this amp. Check them out at www.vintagekustom.com for literature and information on the tuck-and-roll vintage Kustom amps from the 60's and 70's, as well as their related products such as guitars and organs . They provide a webboard for help with kustom gear history, technical information and repairs as well as discussions with other collectors. The site also has a classified page to buy and sell Kustom gear.