Help with a Kenwood SM-200 station monitor.

[KC2ZPK]

I purchased this unit about a year ago, mainly for the bandscope and the signal monitoring features, but I have some difficulties with it. First, the bandscope does work ok, drifts for a while, but I hear that is normal. What I am having problems with is the transmit and receive monitor functions. When all the switches are in the proper position for monitoring the SSB signal I attempt to monitor the tx signal, into a dummy load. No matter how cranked up the mic gain is, I only get about a 1/4" high waveform on the screen, maybe 1 division on the graticule. Much less the  almost 3 divisions show in the manual. The waveform is virtually useless. There is also a setting for monitoring a received signal, but have no idea if it works, all I get is a flat line, no matter how strong the other station is, and no matter what 'RF ATT' setting I use.

I did do some trouble shooting, and found that R1, a 560ohm 3W resistor in the RF sampling portion of the monitor to be about 3X the resistance it should be, and slightly burnt. I can only guess the SM-220 saw some time behind an amp of legal limit power, or more, as the unit is rated at 2KW  PEP for 5 minutes.

The unit also was modded to allow automatic switching from transmit monitor to receive monitor, QST 11/88 Automatic TR Switching For Kenwood SM-220 Monitor Scope, I removed this as the relay was way too loud, and was done messy. While removing this mod I found the burnt R1 resistor, and some other issues in the RF sampling 'box' in the SM-220. I plan on replacing the components in this box, but I am having a hard time sourcing the 1p, 3p, and 5p 500v ceramic capacitors. I have been able to find the rest either on Digikey, Mouser, or Kenwood parts( for the diodes)

If anyone has some advice on these issues please let me know.

Thanks
73
John

[K8AC]

Hi John.  I had an SM-220 many years ago but I'm afraid that I remember little of the operation.  I was just looking at the schematic, focusing on the RF sampling area.  I'd replace R1 and C1 first as the tap at C1 will get you the maximum output voltage.  The other caps are switched in series with C1 and of course each one that's switched into the mix will reduce the sample voltage when the switch is turned.  Those are not critical values, and you could just series some other small caps to substitute for each of the ones you can't find.  Or, another thing you could do is to build what we used to call "gimmick" capacitors.  Those are constructed by twisting together two wires (something like enameled #20 or so) and using the capacitance between the two wires.  The number of turns required would depend on the size of wire, the insulation, etc.  so if you have a way to measure the capacitance produced, that would be useful.  If not, you could just make maybe 10 twists and then snip off bits of the wires until you achieve a value that produces the output you need for the power level you run.  Sounds hokey, but works very well. 

You might also want to check the values of R2 and R3 and the diodes D2 and D3.

73, Floyd - K8AC

[KC2ZPK]

Hi John.  I had an SM-220 many years ago but I'm afraid that I remember little of the operation.  I was just looking at the schematic, focusing on the RF sampling area.  I'd replace R1 and C1 first as the tap at C1 will get you the maximum output voltage.  The other caps are switched in series with C1 and of course each one that's switched into the mix will reduce the sample voltage when the switch is turned.  Those are not critical values, and you could just series some other small caps to substitute for each of the ones you can't find.  Or, another thing you could do is to build what we used to call "gimmick" capacitors.  Those are constructed by twisting together two wires (something like enameled #20 or so) and using the capacitance between the two wires.  The number of turns required would depend on the size of wire, the insulation, etc.  so if you have a way to measure the capacitance produced, that would be useful.  If not, you could just make maybe 10 twists and then snip off bits of the wires until you achieve a value that produces the output you need for the power level you run.  Sounds hokey, but works very well.  

You might also want to check the values of R2 and R3 and the diodes D2 and D3.

73, Floyd - K8AC

Yeah, I was going to start in the RF sampling area and see how that goes. Since R1 is about 3X too high, I figure I should get a signal on the scope 3X as much as I have now, once I get it replaced.

The feature I am still trying to figure out is the "monitor received signal" I have been going over the manuals and I believe I am hooked up correctly. The SM-220 was also modded to allow  V.Input to be connected in the back. I think I am going to take that out and bring the SM-220 back to factory stock and get everything straight, and see if it works better, or at all.


Also, any good ideas to free up sticky trim pots? A couple of the dials stick a little, and I think the resistance skips a little as well


Arrgh... I love projects, really I do....

[K8AC]

On the trim pots, I usually just give them a tiny blast of a cleaner and then work them back and forth many times.  Sorry I can't remember any more details of the SM-220 operation.  I guess it's been 27 years since I used one with me TS-820S!