SB-220 Meter Repair

I'd decided to work on a Heathkit SB-220 Linear Amplifier with a bad Plate Voltage meter (as well as a couple of smoked resistors and a bad zener diode).

The simplest way to fix the meter would be to find a replacement. Unfortunately, I couldn't find any on the web (although I suspect they show up on ebay now and then). However, I'd read posts on the Internet about how others had substituted a meter movement from a Heathkit SWR meter for a blown SB-220 meter. I had a couple of old HM-2102 meters that I'd picked up at various swapmeets, and I thought I'd give it a try.

First thing to do, though, was to characterize both the meter that I was replacing and the meter from the HM-2102. Because the SB-220 Plate Voltage meter was shot, I characterized the SB-220 plate current meter (assuming both meters used identical movements), and I discovered that the meter was 1400 ohms and that it was 200 uA Full Scale. I measured the meters in the two HM-2102 SWR meters, and they each measured 1000 ohms and 100 uA Full Scale.

So, to make the HM-2102 meter's characteristics equivalent to the the original SB-220 meter, I would need to modify it so that it had 1400 ohms resistance and 200 uA full scale.

1400 ohms and 200 uA means that, for a voltage of 0.28 volts across the meter, the meter should read full scale. To make the 100 uA meter read full-scale with 0.28 volts across it, I would need to insert an 1800 ohm resistor in series with the meter. Then, to make its overall resistance 1400 ohms, I would need to connect a 2800 ohm resistor in parallel across the meter/1800 ohm resistor combo.

(Click on image to enlarge)

So, I took apart one of the HM-2102 meters so that I could start modifying it, only to discover that it had a huge magnet, and this magnet blocked the SB-220's meter-illumination light bulb from being inserted into the back of the meter.

I wondered if the second HM-2102 meter would have the same problem (per other internet postings, it seemed that Heathkit often used different meter movements for the same product). I took apart this second HM-2102 meter and discovered that it used a different magnet structure, and that this magnet provided room for the light bulb. So that was the meter to use!

Here's a photo showing the different meter movements (from their backs). The meter on the left is the first HM-2102 meter with the too-large magnet (it also has a series 1800 ohm resistor that I'd installed before I discovered the problem with the lamp installation). The second meter is the HM-2102 meter that I used, and the third meter (at the right) is the original SB-220 meter. (Click on photo to enlarge.)


Here's the final modified meter. You can see the 1800 ohm resistor in series with the meter, and a 2740 ohm resistor (close enough to 2800 ohms) in parallel across the meter terminals. (Click on photo to enlarge.)


Neither of the HM-2102 meters had their mounting holes for the faceplate in the same position as the SB-220 meter (and each was different from the other), and so I had to add two new holes to the original SB-220 faceplate (these are the two lower holes). (Later, when I mounted the faceplate, I discovered that I had to cut away some material from the faceplate because of interference with two other screws on the meter movement, but this isn't shown in the photo below.)


The HM-2102 meter movement also has a pivot-point that is lower in the meter than the pivot point in the original SB-220 meters. This means that the needle isn't a true "radius" of the meter scale, but instead has some angular offset at either extreme of its travel, as can be seen here, in this photo of the finished, modified meter (the needle crosses the meter mark at an angle, rather than being coincident with the meter marking).


All in all, it isn't an ideal solution, but at least the meter now works!