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 Post subject: Heathkit SB620
PostPosted: Jun Mon 10, 2019 11:18 pm 
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Location: Portage le Prairie, Manitoba
I'm hoping this is the right place for this thread.

With dozens of projects mid way through completion, I've jumped back into one that I never did anything with. A Heathkit SB620 panoramic adaptor. I bought it a few years ago now and powered it up very quickly one day before letting it sit in the pile. It powers up and the CRT has a solid display. Bright enough without the intensity control at the max. The sweep works as well in all positions.

Image

I'm not super familiar with these devices, but I think I have a rudimentary understanding of how they function. Perhaps I am wrong in my assumption that feeding a signal from my RF generator of the proper center frequency will show a pip? I'm not sure what frequency it is set up for, so I tried this at 3395KHz, the most likely frequency I would think, but I go nothing. So then I tried 455KHz, I imagine the second most likely frequency and still nothing. I then tried tuning from 400 up to 6000KHz as slowly as I could while watching the trace but no pips. I did get instances of the entire trace rising though and I marked down the frequencies. 570KHz, 635KHz, 1270KHz, and 7920KHz when I went even higher.

I also get a steep response that raises the entire trace within the IF bandwidth. 349.850-350KHz. I guess the IF is working!

My goal is to set this unit up for 455KHz in one way or another to use it with my fleet of SP-600 receivers. I do have an SB-301 as well that I may try seeing as it has a wired IF output. I also have the manual and schematic for this unit, so perhaps a browse through it may help me identify the coils and such to narrow down the frequency range.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 11, 2019 3:50 am 
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Well by checking the schematic diagram and notes in the manual pertaining to the various coil, capacitor, and resistor combinations, I can tell that this particular SB620 is not set up for 3395 or 455KHz, but somewhere in the range of 1600-2215KHz. Again, sweeping within this range did not give me any pips on the CRT.

I tested all the tubes, most of which had mangled pins that needed to be straightened. All tested good aside from the triode section of the 6EA8. I replaced it with a better scoring tube but that did not make any difference.

While sweeping the 1600-2215KHz range hoping to find a pip, I turned my back for a few seconds and the AC hum of the unit changed to a high pitched squeal. Sure enough, I turned around and the trace was gone. It wasn't lost, however. Playing with the vertical position control brought it down, but not low enough to sit on the horizontal line of the scale. Poking around under the chassis, I found two terminals, both feeding the CRT socket that were not soldered! It seems that I forgot the "kit" in "Heathkit". Somebody else put this unit together and I'm suddenly having doubts that it ever worked properly at all. I soldered these two terminals and it's back to working as it was before.

I think I'm going to have to go through it one terminal at a time making sure that everything is wired up to where it's supposed to be. I'll also check continuity of all the coils, tolerance of the resistors, and replace the few electrolytic and paper type capacitors.

Does anyone happen to have the section of the manual that covers building the unit? My copy is missing that part.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 11, 2019 4:52 am 
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Location: Albuquerque, NM 87123
With Heathkits you need to check every connection and replace every electrolytic. No exceptions.


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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 11, 2019 12:15 pm 
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Great advice on replacing those electrolytic caps. Also, there is an article covering the SB-620 in the April 2019 copy of Electric Radio.

You can find the assembly manual at ebaman.com



Todd
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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 11, 2019 2:36 pm 
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I do have the coil data, probably from when the Heathkit stuff was still on the edebris site. PM me your email if you need it. I am not sure if that is on the ebaman site also, what I have was originally in txt files and converted later to Word format.

Your SB-620 was probably originally set up for the Heathkit RX-1 or one of the Hallicrafters selectable sideband receivers both of which used a high IF in the 1600 Khz. range.

It is critical that you replace the high voltage capacitors in the CRT circuit before going further. The original HO-10/H0-13 and later SB-610/620 all share a very fragile power transformer and leakage or breakdown in these caps will take it out immediately. I am getting ready to build an original HO-13 kit which will get new capacitors before I start the build.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Wed 12, 2019 2:42 am 
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I've replaced five of the electrolytic capacitors so far, the others I will have to order as well as the high voltage capacitors. I also found an improperly wired resistor. The 10K resistor from pin three of the pip center control was wired to the wrong spot on the diode bridge for the filaments. I noticed this when I realised that the control had no effect. Said resistor was also tied to the improper terminal, but not soldered.

That's three unsoldered terminals and one mistake in the wiring. I assume there are more to be found. The unit is still not functioning correctly. It seems to be acting like an oscilloscope instead of a spectrum analyser. Displaying voltage vs. time rather than voltage vs frequency. So far, I have not been able to get a steady pip. Just a rise in the entire trace.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Wed 12, 2019 3:29 am 
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I really wouldn't operate it any more until you replace those HV filter caps. They have taken out a lot of transformers in these instruments and yours are definitely a ticking time bomb if they are original. The new film type will provide a safe and long-lived replacement. You will find a lot of dead Heathkits in this family at hamfests due to transformer failure.

You can get them from Mouser.

Rodger WQ9E


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 Post subject: Re: Heathkit SB620
PostPosted: Jun Wed 12, 2019 7:11 am 
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Not really sure if they are original. There are a lot of different types of capacitors in this thing. All I know is, I cannot resist the urge to play around with this thing. I will be placing an order for new capacitors tomorrow.

I went through the entire schematic, tracing the wiring and double checking it all. I found one more mistake. One of the pins of the horizontal position pot was tied to the right terminal but on the wrong terminal strip. Two identical terminal strips, albeit, with different components attached placed about two and a half inches from each other. And honest mistake. I noticed it when I found a resistor that was not connected to anything on one side. I was hoping it would be the missing piece that would make this thing work correctly, but it was not. I had noticed that the horizontal position pot had to be just about fully CW to center the trace, yet the 12AU7 tested strong on both triodes, so I knew something was wrong somewhere.

Anyway, that is fixed now and there are no other issues with the wiring that I have found. I highlighted everything as I went over it on the schematic to be sure.

I also tried a bit of the alignment procedure. L1 and L2 have little effect when adjusting them. I'm not sure if it's always this subtle a change int eh height of the trace. I know that I am in the 1600-2215KHz range. I double checked the numbers on the coils. But still, no matter what I do, I do not get a pip. I can get some interesting looking traces with noise though.

Image

Tomorrow I will test resistors for values, ceramic caps for shorts, and coils for open circuits.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Thu 13, 2019 5:34 am 
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At around 10pm tonight when I should have been in bed, like I should be right now for that matter, I decided to poke around a bit more in this panoramic adaptor. I tested a few resistors and all were pretty much within spec. One 100K that was at 111K, not terrible. I was decently certain that I was not going in the right direction. Suddenly I had the thought to test grounds seeing the terminal strips fastened with nuts and screws. By the fifth or so terminal I tested, I had found an open circuit. The oscillator coil ground connection. The wire was looped through the ground lug on the V2 socket, and the lug was soldered, but no solder adhered to the lead connected to the coil. I soldered it up right and tested the unit again...SUCCESS!!! :D

The CRT is now displaying proper pips! Namely, (after a little adjustment) a 1600KHz pip. It was at 1580 ish before I adjusted it.

Image

It's far from working perfectly, but it is functioning! Now to order the capacitors and do my best not to touch it until they have been replaced! Then I'll check the rest of the resistors and do an alignment and all that and see how the display looks. Then I'll see about converting it to 455KHz either by changing or modifying the oscillator coil or building another oscillator and using the front end as a mixer.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Sat 15, 2019 3:48 am 
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That's a neat toy; I'll have to keep my eyes out for one. I have a SB-610 on deck for repair. Just Radios does have those 1600 volt capacitors for the HV supply.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Sat 15, 2019 5:09 am 
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Location: Portage le Prairie, Manitoba
It definitely is neat! I have used my HP 8557A as a panoramic adaptor before, but it drifts terribly even after warming up for an hour. This particular 620, although not in the greatest shape seemingly from poor storage, seems to be low hours. With the various wiring errors that I found, I imagine that it may not have worked properly from the start and that the original builder may have given up on it because of that. The CRT has plenty of life left, but I'd still like to pick up a second one if only to have a spare CRT.

I have placed an order for the capacitors from Sal's Capacitor Corner. I have never bought from him before, but remembered seeing the adds here. I look forward to getting this unit all recapped, then I can really start playing around with it without having to worry about the high voltage caps failing.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Mon 17, 2019 8:36 am 
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Well I now have a Heathkit HO-13 on the way as well! It'll be interesting to delve into that one when I get it. I like the chassis layout on it from what I've seen. I've downloaded and printed the manual for it already and plan to read through it in the next few days.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 5:00 am 
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The capacitors I ordered from Sal's have arrived and I have replaced the HV caps thus far. Below is a picture of the underside of the chassis with 11 new capacitors and one resistor that was out of spec. I have not tested all of them yet.

Image

I also got a spare CRT that should work in this unit and will definitely work in the HO-13 that I have on the way. A 3RP1. A coworker of mine gave me a truck load of ham radio gear (mostly in terrible shape) that he got from a friend of his who is currently cleaning out some sheds on his property, meaning, there is more to come! Anyway, one of the items was the matching SB610 monitor scope. I wanted to save more parts from it, but it was not really worth it. The transformer was a chunk of rust and it would take 100 years to get the mouse urine smell out of the rest of it. I did save the CRT though. Some of the cement that holds the ceramic insulators inside the electron gun assembly has cracked off and is rattling around inside the tube, but it does not seem damaged beyond that. There is a phosphor burn line from the trace as well, but it's not terrible.

Image

I will test it for shorts and if all seems good, try it out just to see what it looks like.

Tomorrow I will replace the remaining four electrolytic caps. They comprise the can on top of the chassis. I do not plan to restuff it as the capacitors I have will not fit and I am just not a fan of doing that. I'll either see about placing them under the chassis point to point or build a custom turret board and mount it on top of the chassis in place of the original can.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 5:57 am 
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The SB-620 uses a 3RP7 CRT.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 11:22 am 
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As Pete noted, you would need to look quickly to see a trace on the SB-620 with a P1 phosphor CRT. P1 is the common short/medium persistence green phosphor type used in regular scopes while the P7 is a long persistence phosphor used for analog spectrum analyzers, radars, etc. P7 has a very brief bluish/white response when excited followed by a slow decaying yellow trace.

But you do have a spare for a regular scope.

With any scope, it is preferable to keep the trace intensity reasonable to avoid burns and preserve tube life so try not to have the room excessively bright and avoid direct light on the CRT face. Never leave a single spot or repetitive bright trace on the display for a long period of time or the phosphor will get burn marks.

Rodger WQ9E


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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 5:31 pm 
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I'm aware that the phosphor is different, but the tubes are otherwise the same. The P1 phosphor will not be as useful for a slow trace speed, but when cruising the bands, I will want the trace speed most likely set to the max to avoid lag in the display. In that case, the P1 phosphor would likely be preferable since the fast decay rate would prevent ghosting of signals. I don't plan to keep the 3RP1 in there as I like the color and contrast of the P7 phosphor better anyway. Like I said, I'm just playing around with it.

That said, the HO-13 uses a 3RP1 CRT, so if this tube works, it will be a replacement for that one if I ever need it. I plan to restore and align the HO-13 as well and do a performance comparison between the two units.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 6:16 pm 
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The SB-620 is useful when monitoring a single frequency and having the ability to monitor "X" KHz on either side of that frequency without having to re-tune. Great for watching for band openings. I'm not sure what usefulness there is with the scope when "cruising the bands".
Neither scope (HO-13 or SB-620 (I have both)) is very accurate but it makes for nice eye candy for visitors.
Personally, the HO-10 and SB-610, to me are far more useful in that you can put them in the transmitted RF output line and monitor your transmit waveform.

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Tue 25, 2019 8:41 pm 
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I primarily used my SB-620 to check for band activity as propagation was coming alive (i.e. 15 meters during moderate sunspot years) and locating pileups to find DX. An Icom PCR-1000 took over that job but it had a major drawback because of it limited hardware "horsepower" the spectrum scope couldn't be used when demodulating SSB/CW so you had to exit spectrum scan mode to see what you have found. Now any of the cheap little SDR boxes will do a better job but the vintage analog CRT display still looks cool.

I use my Tektronix spectrum analyzer plug in when aligning vintage phasing type SSB rigs and checking transmitters for spurious output but it is a far removed from the simple SB-620/HO-13 boxes with a resolution bandwidth down to 30 hz for single signal analysis and calibrated scan width from 2 Khz. to 100 Mhz. per division making it easy to find and analyze signals.

Rodger WQ9E


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 Post subject: Re: Heathkit SB620
PostPosted: Jun Wed 26, 2019 4:42 am 
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I don't know how useful the 620 will be for cruising the bands either, but I intend to find out. It will be a while yet before I am able to use it with my SP-600 though. Even once I get it converted to work properly with a 455KHz IF, the SP-600 will need some modifications to feed into it.

I removed the multisection electrolytic can today and fashioned a cover plate to mount over the hole.

Image

Mounted on that is a terminal strip that I used to tie things together where the terminals of the can were used previously. The various electrolytics were relocated under the chassis where convenient. I replaced two 8-32 nuts with stand offs and mounted a ground lug on top (technically the bottom) of each of them to give me some extra ground points for the electrolytics. It's not overly congested and sturdy enough for my liking. Not as stylish as a custom turret board, but it will certainly work.

Image

A brief test showed the unit to function correctly with the center frequency around 1600KHz as before. Nothing much has changed performance wise. There is still a little bit of ripple on the trace. I figure that it must be picking up hum internally somewhere.

I'll be back at it tomorrow hopefully. I'm getting some color put on a tattoo, so my left arm might not be as useful as usual. :P

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 Post subject: Re: Heathkit SB620
PostPosted: Jun Wed 26, 2019 1:16 pm 
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For your SP-600, pick the signal off at the common plate line from between second mixer V6 and gate tube V7 which will provide the 455 Khz. output regardless of whether the 600 is operating in a single or dual conversion band. 6-8 pf should do for the pickoff capacitor because there is significant gain in the SP-600 by this point in the circuit. Don't forget to do a touch up alignment of T3 after you add the additional capacitive load of the SB-620.

T1 and T2 provide a little selectivity prior to the pickoff point when operating on the dual conversion ranges but it shouldn't be enough to matter.

You will find a little trace ripple with all of the Heathkit scope line but it should be insignificant enough not to matter. If the filter caps are all good then the most typical cause in a Heathkit is poor lead placement. Also make sure that the mu-metal shield is installed around the CRT to protect it from beam deflection via stray fields from the power transformer. I recall that the shielding properties of a mu metal shield can be reduced if the shield is subjected to a hard impact but I know of no way to test for this other than substitution with another shield.

Rodger WQ9E


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