Friday, December 9, 2016

Repairing the power switch on the Kenwood KA-8011 (a.k.a. KA-801) amplifier

Back around 1990 my brother mentioned to me that there was an amplifier, in a box, in pieces, in the back room of the TV/electronics store where he worked at the time and that if I made an offer I could probably get it for cheap.  Dropping by one day I saw that it was a Kenwood KA-8011 Integrated DC amplifier (apparently the same as the KA-801, except with a dark, gray front panel) laying in a box from which the covers were removed with a bunch of screws and knobs laying in the bottom.  I also noticed with some surprise that it had a world-wide voltage selector switch on the back and that the power cord had a Japanese 2-pin wall plug and U.S. adapter - and still does!  All of the parts seemed to be there so I offered some cash ($50, I seem to recall) and walked out with it and a receipt.
Figure 1:
 Spoiler alert:  This is the KA-8011 with the repaired power switch.
As noted in the text, the original, blue-painted panel meter lights were
replaced long ago with blue LEDs.


When I got home with the amplifier I knew that I had my work cut out for me particularly since, in those days, before the widespread internet, I had no schematic for it and no-one that I contacted seemed to be able to find one.  Powering it up I noted that the speaker protection relay would never engage, indicating that there was a fault somewhere in the amplifier.

A visual inspection of the awkward-to-reach back panel's circuit board revealed several burned-looking leads sticking up from the circuit board where transistors had exploded near several burned resistors.  After a few hours of reverse-engineering a portion of the circuit I realized that the majority of the circuit at fault was one of four identical phono preamp input circuits (there are two separate stereo phono inputs) and associated low-level power supplies.  Between the intact amplifier sections and being able to divine the color bands on the smoked resistors - along with some educated guesses - I was able to determine the various components' values and effect a repair.
Figure 2:
The power switch, with a broken bat.
Click on the image for a larger version.

The amplifier now worked... sort of.  I then had to sort out a problem with the rear-panel input selector switch, operated by a flat, thin ribbon of stainless steel in a plastic jacket that was engaged from a front-panel selector.  I managed to cut off the portion at the front that had been damaged where it was pulled-on from the front panel having been loose in the box, punch some new holes in the ribbon, align the two (front and rear) portions of the switch mechanism and restore its operation.

Snap!

Having done the above, the amplifier was again operational and I have used it almost every day in the 25+ years since, needing only to replace the blue-colored incandescent meter lights with LEDs, powered from a simple DC filtered supply.  In the intervening years I also had to replace some of the smaller electrolytics on the main board that had gone bad, causing the speaker protection circuit to randomly trip on bassy audio content and with slight AC mains voltage fluctuations.

Figure 3:
Comparing the old (top) and new (bottom) switch components.  In order
to prevent it from interfering with the body of the switch some of the
metal on the new bat would have to be removed.
Click on the image for a larger version.
I was annoyed when one day, a few months ago, the power switch handle - which had been bent before I got the amplifier - and then "un-bent" during the repair - broke off in my hand when I turned it on.

Using the "bloody stump" of the power switch for a few months  I finally did a search on EvilBay to look for a new switch.  While I didn't find a power switch I did see a "tone control" switch for the same series of amplifier - so I got that, instead.  When it arrived I noted, as expected, that most of it did not mechanically resemble the power switch or look as though it would easily mount in the same location, but it did have essentially the same metal bat on the end as the original that I figured I could fit onto missing portion that had broken off the power switch.

Comment:
Even though the "new" switch was much too small - of insufficient current rating - to have been used to switch the mains (AC input) power, it would have sufficed to operate a relay.  To have done this would have required that new holes be drilled in the front sub-panel to match those of this new, smaller switch. While this would not be "original" circuitry, it would have looked the same from the front panel.  This is a possible option should the power switch itself become unreliable some time in the future due to the heavy currents at turn-on.

Removing the original power switch I laid the two side by side and made notes of the differences between the metal bat of the new and original - which was narrower in some places to clear parts of the switch body - and taking a file to the new one I took off some metal to clear the possible obstructions.  I then noted on a crude drawing the length and orientation of the new bat based on the axis of the switch's pivot point.  Because the bat of the original switch was embedded in a block of molded Bakelite I knew that I would have to somehow attach a portion of the new switches' bat to the old, so I carefully disassembled with old power switch, cutting off and saving the original rivet on which the switch pivoted, noting where everything had gone and saving the small springs, contacts and some small Bakelite pins.
Figure 4:
The new bat, butt-soldered on the old switch.  Note that the bat from the
"new" switch has been filed to better-resemble the shape of the
original bat to clear the switch body.
Had I not been able to find a "similar" switch on EvilBay I could have
probably measured the original switch, found some scrap
steel of similar thickness and made a suitable replacement entirely
by hand with careful filing using the bat of another switch as a template.
Click on the image for a larger version.

Clamping the old part in a vise I cut off most of the original bat, leaving about 5mm of metal remaining.   Carefully comparing the old and new piece I then marked where, on the new bat, that I would have to cut to allow the repaired piece - consisting of the new and old butted and laid end-to-end - have the same length as the intact original.  Doing so, purposely cutting the "new" bat slightly long, I did some fine tuning with a file until the two pieces laid down precisely lined up as they should.

Attaching the new piece

Using some silver solder intended for stainless steel I applied some of its liquid flux - apparently a mixture of chloric and hydrochloric acid - and using a very hot soldering iron I "butt-soldered" the two pieces together in careful alignment and then filed the surfaces flat to remove excess.  While the bakelite switch body can handle a brief application of a soldering iron, I knew that it would not tolerate the heat from a proper, brazed joint.

This (weak!) solder joint was intended to be temporary, needing only to be good enough to allow a sleeve to be made by wrapping an appropriately cut piece of thin, tin-plated steel (from my junkbox) around the joint.  Once this sleeve was checked for proper fit and folded tightly, additional flux was applied and the entire joint - sleeve and all - was soldered, the result being a very strong repair with the restored bat being of the same length and at the same angle as the original.

Reassembly:

The trick was now to get every thing back together.

Figure 5:
The steel sleeve being installed over the butt solder joint,
before final folding and soldering.
Click on the image for a larger version.
Reinstalling the pivot and making a few clearance adjustments to the original switch's frame with a small needle file, the original rivet was then soldered into place and the entire assembly washed in an ultrasonic cleaner to remove the remnants of the corrosive flux from the bat and switch body.

In the base of the switch, the contacts, which were the same as those had it been an SPDT switch, were reinstalled - this time, rotated 180 degrees so that the previously unused contact portions would now be subject to electrical wear.  These contact were then "stuck" into place with a dab of dielectric grease so that they would not fall out when the switch body was inverted.

Figure 6:
The repaired switch, reassembled,  with the new bat spliced on.
Click on the image for a larger version.
After reinstalling the springs and pins, the rear part of the switch with the contacts was placed over the top of the moveable portion, held in the mechanical center, and the base was carefully pushed into place, compressing the internal springs and pins.  Holding everything together with one hand the proper operation of the switch was mechanically and electrically verified before bending the tabs to hold everything into place.

In reality the reassembly didn't go quite as smoothly as the above.  During one of the multiple attempts to get everything back together the smaller-diameter rear portion of the small, spring-loaded Bakelite pins used to push on the contacts snapped off.  To repair these pins the front, larger-diameter portions - that which pushed against the metal contacts - were placed in the collet of a rotary tool and a shallow hole was drilled into the rear portion where the broken pieces had attached to fit short pieces of 18 AWG wire:  By rotating the piece into which the hole was to be drilled, the exact center is automatically located.

The pieces of wire were then secured using a small amount of epoxy - a process accelerated by placing the pins in a 180F (80C) oven for an hour.  After the epoxy had set the wires were then trimmed to the length of the original sections that had broken off and the ends smoothed over with a small needle file to prevent their snagging on the spring.  The result was a repair that was stronger than the original pins that easily survived switch reassembly.

The results:

Figure 7:
After reassembly it was noted that the gray "skirt"
was hitting the front sub-panel frame, preventing it from
being set to the "off" position.  A bit of heat was applied to
set a permanent bend so that it would clear this panel.
Click on the image for a larger version.
The amplifier was then put back together - very carefully.  The only real issue that I noted was that the gray plastic skirt/escutcheon on the bat ended up about half a millimeter farther away from the switch body and closer to the sub panel than before, causing it to snag on the front sub-panel's cut-out when I attempted to move it to the "off" position.  Careful softening of the plastic with the rising heat of a soldering iron and bending it very slightly allowed it to clear.

Putting all of the knobs back on, tightening the bushing nuts and screws as necessary before doing so, I then tested the amplifier on the bench and was pleased to find that I'd not managed to break anything.

Finding that everything was working fine I put it back on the shelf where it belongs where I continue to use it often.

[End]

This page stolen from "ka7oei.blogspot.com".

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