Some Common Problems and Solutions
 
There are many facets to a Kenwwod Hybrid Transceiver.  It is virtually impossibe to go through all possible problem areas that might turn up in a page or two, but I've included several common areas that need attention on many rigs.  This should help to give you an idea of things to look for if you are performing your own restoration on one of these wonderful Kenwood rigs.  I hope you enjoy the photos.   73, Ken, K4EAA
Soft Bandswitch Operation
You have to wiggle the bandswitch to get a selected band to respond properly.   This is most often caused by a cracked coupler between the front panel control and the final cage bandswitch.  Easy to identify, sort of hard to get at.   It will restore perfect bandswitching if you do, however.
Coupler crack 1
The arrow points to the guilty culprit, a broken plastic coupler that Kenwood used on later rigs
How do I fix it? I replace that plastic coupler with a bronze coupler with 4 setscrews that will last for the next 100 years . . .  It's all at ground potential, no need for an insulator at this juncture . . .  Hard to reach, but well worth the effort.    A much closer view appears in the picture below.

Coupler crack2
Here you can clearly see the crack in the coupler, it will never again work properly.



Cracked Tuning Cores
Even though the Kenwood owner's manuals explicitly warn about using metal tools to try tweaking the tuning cores, some people have to learn for themselves.  During an alignment,  any time my plastic wand won't budge a core means there's a cracked slug.  The slug is nearly impossible to remove without damaging the fragile and brittle coil forms, so I have to stop and change the entire coil.  Here's some pictures showing what to watch for:
Picture of coil pack
The slug in the coil form in the lower right (pink) is cracked. 
The one just to the left of it has already been removed, as it too was cracked.  The shield has been removed for access.
Cracked core
Here's a close-up showing the cracked (and immovable) core.
Damaged Cathode Resistors
If a rig appears to load up properly, with sufficient plate current and good high voltage,  but suffers from low output, the answer is many times the cathode resistors.  Kenwood uses these resistors as fuses, to protect the finals and power transformer if the rig is mistuned for any length of time.  The resistors will overheat, start to climb in resistance, and maybe even burn and rupture.  In the picture below, the larger 1W 10 ohm resistors are in series with the cathodes of the 6146B's.  In a rig that's been abused, these resistors may look fine, but actually be high in value from being overheated.  The only way to tell is to measure them.  Being in parallel, they should measure a total 5 ohms to ground.   The plate current metering is derived from these R's, and will read much higher than it's actual value if the resistors are damaged.  I replace them with 10 ohm 1W metal film units, to maintain the safety that they provide as pseudo-fuses.
Cathode Resistors
These cathode resistors look and measure fine.  They do not need replacement.
Note the bandswitch section (bottom center) that has been revealed by removing the shield plate.
It is important to clean and lube these contacts as well.
VFO Tuning Capacitor Noise & Dropout
After 25 to 35 years of use, or certainly after a few years of storage, the VFO capacitor will become noisy, intermittent, even quitting all together because the mechanical sliding contact has tarnished and corroded.  The picture below shows the access door on the side of the VFO unit on a 520S.  A right angle screwdriver, plenty of light, and a steady hand allow me to remove this cover for access to the sliding contact area on the VFO capacitor.  A health dose of contact cleaner,  working the cap between its extremes several times, followed by a quality lube applied to the contacts will prepare this important cap for the next 25-35 years.

VFO Cover
You can see the plate held by two phillips head screws on the side of the VFO can


Don't Forget those Variable RF Amp, Mixer, and Driver Capacitors
Less obvious than the VFO cap, these 3 units for peaking the receiver and driver circuits require cleaning and lubing as well.  Each sliding contact must be sprayed and the capacitor worked between its extremes to allow smooth operation and tuning.
Driver Capacitors
The arrow points to one of 3 clean & lube points on the Driver control
 

TS-820 DG-1 Access for Repair
One of the most common, yet the most difficult problems to diagnose on a TS-820 concerns problems in the digital display.  The Mixer and Counter boards are housed in a metal box next to the IF board, and troubleshooting it can be hard.  The picture below shows the digital unit with its cover removed and laying on an insulating piece of cardboard while being troubleshot.

In this case, the mixer board passes the test, showing strong output and stable frequency on all bands.  That means the problem HAS to be on the digital board, which has been known for it's questionable plated through-holes.

The yellow circle shows the DG-1 with cover removed, being probed.
 
 
 
Continue on to Bandswitch Repair

 
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All Photos and content copyright 2005, K4EAA, Ken Kemski.

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