Last year I acquired a very nice Heathkit SB-620. It was complete, clean and very good cosmetically. I thought that it would make a fun addition to use with my vintage receivers so I undertook the project of getting it working properly. Here is the how the interior looked when I received it (other than the big electrolytic capacitor that I replaced before taking this photo):
These units are basically a narrow bandwidth spectrum analyzer designed to take a feed from the IF of a receiver and display a pass-band of a width selected via the controls. When the kit was assembled the user would choose the frequency of the IF to be used. The frequency chosen affects a number of different components in the circuitry. Heathkit provided all the components needed for a wide variety of Intermediate Frequencies and the builder would use the ones needed. The rest could be put into the old junk box.
The unit I purchased was built for an IF of 3395 kHz which was used by Heathkit equipment of the era as well as some other receiver manufacturers. As I intended to use this unit with my Collins 51S-1 oy maybe my Collins R-390A I’d have to do something about the incompatible Intermediate Frequency. But that was a problem for later.
Right now the goal was to get the unit into proper working condition. To that end I ordered a capacitor kit from Hayseed Hamfest and added the optional resistor kit. That seemed prudent given the small additional cost and give that 60 year old carbon resistors could easily be out of tolerance.
While I awaited the arrival of said kit I checked the tubes and replaced any that were suspect. Then I gave the wafer switches a good cleaning with DeoxIT and MG Chemicals Nu-Trol contact cleaner. Bringing the little unit up on a Variac showed that there was a nice trace on the CRT and the CRT itself looked to be in excellent condition. Based on that and the excellent condition of the face and cabinet it appears that this unit had seen not a lot of use and had led a soft life in someone’s ham shack somewhere.
Once the capacitor/resistor kit arrived I set about replacing all those components. This was a relatively straightforward and enjoyable endeavour. Who doesn’t enjoy the smell of a hot soldering iron and flux vapour in the shack, punctuated by the occasional “thwap!” of the ol’ Soldapullt?
There’s the underside of the chassis after the capacitor and resistors had been replaced:
Here you can see the nice chrome-plated multi-section capacitor can provided by Hayseed Hamfest. The can contains four electrolytic capacitors in this unit.
Once the new R’s and C’s were installed and an initial smoke test showed no problems and a nice display it was time for the alignment process. The manual offers two methods for this, one with test equipment and one without. As I am fortunate enough to have good test equipment I used that method.
As this little spectrum analyzer is basically a super-heterodyne receiver the alignment process is similar. I normally do two passes through the alignment process to get it spot on but I just did one in this case as I may end up modifying it for a different IF anyway. I just wanted to make sure it was working properly and it seems to be.
I am pleased with the result and now I need to figure out how I will get it hooked up to the 51S-1 which has an IF 0f 500 kHz. The 51S-1 does have an “IF out” jack on the rear apron so that part is solved.
Here is mine running during the alignment process.
The cabinet, knobs and yellow display filter are all in excellent condition. The SB-620 is not nearly as common as it’s transmitter mate, the SB-610, is so I was lucky to get one and in such good condition.
The CRT is a 3Rp7 which is no longer available and uses a high-persistence phosphor to compensate for the slow scan rate of the beam. I was doubly grateful that the CRT was in excellent shape! While a substitute could likely be found it would not be the high-persistence 3RP7 that is really needed for proper use of this machine.
The P7 phosphor gives the high persistence display needed for spectrum analyzers and radar displays. P7 has two colors giving two different persistences: there is a short-to-medium one via purple-blue fluorescence and a long one via yellow-green phosphorescence. By using a color filter after the screen either color, and therefore either persistence, can be selected. In the case of the SB-620 they wanted to use the longer persistence and so you have a yellow filter in front of the CRT.
There are also two NE-83 neon lamps used in the circuitry. One you can see on the front panel. But it is more than a power indicator. It’s main use is actually as a voltage regulator for the local oscillator. The other NE-83 can be seen on the underside of the chassis. It is part of the relaxation oscillator circuit in the horizontal sweep section. Because neon lamps degrade over time and because they perform important functions in this unit I’d like to replace them. That is also on the to-do list for the rejuvenation of this fine little Heatkhit Scanalyzer.
More to come on this project as it progresses.