Meet my friend Rusty the Fisher KM-60 Tuner

Oh. Another item....

I found the voltages inside were too high. I presume that is in large part because I replaced the selenium bridge with a silicon bridge.

I put in the normally recommended 100 ohm resistor (mine is 7W). I don't think this tuner draws enough current to drop the voltage enough. I was measuring 218V right after the bridge, The 100 ohm resistor is dropping about 5V, but the diagram shows that the first tap off the power supply is supposed to be 192V. I'm thinking that I should increase the dropping resistor from 100 ohms to about 500 ohms. I can back calculate the right size knowing that the current through the resistor is about 0.05A. I'm going to wait on that until I'm sure that all the heaters are firing on all cylinders and the current draws throughout are where they are supposed to be.
 
Use a 1K trimmer or pot to dial in resistance, then get real close or dead on with a resistor. But check the B+ with the line voltage @ 117VAC 1st. Your line voltage may be higher than spec'ed. 15% over or under is ok per FISHER 192+ 5%= 220.8 so you're within the 15% threshold. You're at a hair under 13.6%.

Get the heaters on the tubes to read btwn 6.0 and 6.3V (or 12.3-12.6V) and everything else should be fine.
 
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Tim; If you're not time constrained with the tuning cord, I can dig mine out of storage and take some detailed pics for ya.
 
Tim; If you're not time constrained with the tuning cord, I can dig mine out of storage and take some detailed pics for ya.

Larry,

Thanks very much for the offer, but I just don't think I've got a mechanical problem with the way it is strung. I have the assembly manual to compare it against and it looks fine down to the way it is wound around the tuning knob shaft and through each pully, but I will need to move the needle to line it up with one of these 3 known stations. (Doubt that will fix the real issue, but I'll start with baby steps.)

After that I'll start chipping away at these two issues starting with verifying that all sides of the tubes are lighting up. I did some of that just to get the mono sound through it, but there may be a tube or a switch that isn't making good connections.

It won't be until this coming weekend before I get back into it.... Work gets in the way sometimes.
 
When the dial string is pulling on the capacitor pulley to open the plates (higher frequency), then the dial pointer must be moving up (also higher frequency). If that is reversed, it HAS to be a stringing error of some sort. It is possible that the instructions are incorrect. The path from pulley to pointer is usually the shortest direct path so any tensioners will not affect the calibration. It should be possible to simply reverse the string direction on the capacitor pulley.
 
.....
But check the B+ with the line voltage @ 117VAC 1st. Your line voltage may be higher than spec'ed. 15% over or under is ok per FISHER 192+ 5%= 220.8 so you're within the 15% threshold. You're at a hair under 13.6%.

Get the heaters on the tubes to read btwn 6.0 and 6.3V (or 12.3-12.6V) and everything else should be fine.

Does anyone here have both a "regular" meter AND a true RMS meter? A non RMS meter assumes that the AC is a sine wave and the badly clipped sine wave allegedly has a higher RMS voltage than would be indicated on the non RMS meter. That would stress the tube heaters. It would be good if someone could compare measurements.
 
When the dial string is pulling on the capacitor pulley to open the plates (higher frequency), then the dial pointer must be moving up (also higher frequency). If that is reversed, it HAS to be a stringing error of some sort. It is possible that the instructions are incorrect. The path from pulley to pointer is usually the shortest direct path so any tensioners will not affect the calibration. It should be possible to simply reverse the string direction on the capacitor pulley.

Hi Fred,

I’ll post pics and the figure from the assembly manual this weekend when I return from business travel. It’s really not an error with the routing of the cord though.

In short, the pointer is closest to either the top or the bottom of the tuning cap. It’s as simple as that and I think it’s correct right now. There’s something else wrong here. That’s the mystery.
 
Does anyone here have both a "regular" meter AND a true RMS meter? A non RMS meter assumes that the AC is a sine wave and the badly clipped sine wave allegedly has a higher RMS voltage than would be indicated on the non RMS meter. That would stress the tube heaters. It would be good if someone could compare measurements.

Fred,

You’re thinking that the AC on the heaters might be clipped? Or do you have some other reason for asking?
 

Thanks Larry. Interesting topic. Has this topic come up because I've said the voltage is high on the DC side? I presumed this is mainly due to the change to silicon in the rectification. I wasn't planning on anything more than checking that everything is fully powered and then increase the dropping resistor to compensate. I can't imagine that the PS is clipping, but maybe you guys know something that I'm missing.
 
Fred,

You’re thinking that the AC on the heaters might be clipped? Or do you have some other reason for asking?

If the line is clipped, then the heater AC will also be clipped. This is not a problem as long as the true RMS voltage is correct, but without some way of knowing what we are reading without a true RMS meter, we really don't know the RMS value, unless someone with both meter types can compare.
 
Yeah. Due to "MODERN" switching power supplies, increased use if florescent lighting, and such that only operate at the peaks of the sine wave. A true RMS DVM will read the PEAK voltages, where a non RMS meter will top out at the clip on both sides.
 
The RMS meter must do some averaging over time because simply reading a peak value will be incorrect. That peak value, depending on the amount of clipping, could be close to the sine wave peak or it could be quite a bit less. Correct RMS measurement requires sampling the peak value "N" times over a cycle, then compute the value by:

Vrms = SQRT [ (v1 + v2 + v3 + v4 + ... vN) sqared / N]

This is probably physically accomplished with some sort of internal integrator.
 
Finally got back from a business trip and took some pics of the string. These didn't turn out very well, but we'll give its a try.

If you take a look at the assembly diagram, the route for the string is pretty simple. There is only one way that it fits properly and it's strung per the diagram.

IMG_0981.JPG IMG_0982.JPG IMG_0986.JPG
 
Also, powered it up tonight for a short period while wiggling the audio tube and one of the multiplex tubes. Now I'm getting sound out of both channels in both mono and stereo. I think this can be easily saved if only I can figure out what is happening with the dial. Work is going to get in the way for a week or so.
 
Dial string routing appears logical and the short path is from the top of the tuner pulley direct to the pointer. One slight possibility: Remove the top cover from the front end and verify that the capacitor plates are opening when the dial is moved up. The front end will typically be gear driven so the actual capacitor turns in the opposite direction of the pulley. Is there any chance that the front end could have been replaced with a different unit that does not have the reversing gear?
 
I'll pull mine out this weekend from storage and take some detailed pics of the stringing and of the cap with the cover OFF. This will give Tim another unit to compare with.
 
Dial string routing appears logical and the short path is from the top of the tuner pulley direct to the pointer. One slight possibility: Remove the top cover from the front end and verify that the capacitor plates are opening when the dial is moved up. The front end will typically be gear driven so the actual capacitor turns in the opposite direction of the pulley. Is there any chance that the front end could have been replaced with a different unit that does not have the reversing gear?

Fred and Larry,

I tried to take the cover off the tuning cap a while ago to see about dirt and corrosion. It seemed to be soldered down underneath. I'll give it another shot.
 
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