[Boatanchors] wtb/wtt: heathkit sb620 scanalyzer

[Eugene Hertz]

Hi all, I posted a message on the heathkit list with some problems I'm having with an sb620, but realized there may be some one this list with experience with these.  In the end, I don't think I am equipped to figure out the problems I have on my unit.  So I thought I'd offer a swap of mine and anyone elses that might work on 455khz. Otherwise maybe for cash.

>From the responses I got on heathkit list, it looks like the general recommendation is to rebuild it from scratch. As I don't have time for that, I was wondering if anyone would want to do a swap. I believe mine could be made to work pretty easily if someone knew what they were doing.  I can get signals and pips on it from an rf generator at different freqs, so I suspect the basic functions are working, just not sure about the IF stuff. The difference in cash could be discussed. 

Alternately, I am looking for any other panadapter/sdu for 455 if any one has non-heatkit (non-hallicrafters--just dont like the way that one looks!). 

I will paste my post to heathkit below in case anyone has some words of wisdom (yes the 620 has all original caps and I know its a ticking time bomb, but I got two kids whose combined age isn't 5 so I have to pick and choose my projects :)
thanks 
Eugene 
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Hello, 

First let me start by saying I am only a novice with electronics, but I know many of the principles. Several years ago, I purchased an sb600 on ebay. The seller said it was rigged for 455 IF. Well as things happen, it sat on my shelf for many years. Well, I finally dusted it off to see whats what. I decided to try the alignment procedure in the manual (which I have as well). I think I have several problems that I might not be able to fix, so I thought I'd run it by the list here and determine if I can fix it, or be forced to sell it (or trade it with a working 620 on 455). 

Seems the first step is to set an rf generator to 350khz. I did so and sure enough the trace jumped from the baseline to about the 35 on the log scale. But as I was tuning around I saw that I had a much bigger jump to about 15 on the log scale when the rf gen was set to about 364Khz. So I wasnt sure if a) the frequency (trap?) was off, or whether the jump at 364 was normal (ie some image response or something-I dunno). 

So the next step in the alignment procedure is to take a little non-conducting screw driver (tweaker) and adjust L1 until the line came back down to the baseline. First, this is not very intuitive to me, but I followed. Now it seems like no matter how much I turn L1 in or out, there is almost NO change in the vertical position of the trace line. I also tried to adjust L2 (as directed in the manual) almost no affect on the trace. It seems that the 364Khz signal was a little more affected by tuning L1 and L2, than was 350Khz. One other thing, 350Khz seemed much steeper, in other words, tuning the Rf gen slightly above and below 350 caused the trace to drop to the baseline. However, 364Khz was very linear as I moved closer to 364 the trace rose slowly until arriving at its top height at 364Khz, and again beyond 364 the trace would gradually lower back to the baseline. 

So Do I have a problem? Where do I begin? 

I did some other investigation. I checked the manual to see if I did infact have a 455Khz IF 620. What I found was strange. It seems I do infact have: 
L1: 40-590 
L2: 40-590 
L3: 40-775 

so far so good. Here's where things get strange. 
R1: 1M (should be 820k) now I am not sure if this is a big deal, I havent analyzed the diagram carefully, but this may simply widen or narrow the 10Khz sweep rate compared to what it should be. 

R2: Correct at 100K 

C3: Should be two 130pf resin caps in parallel for 260pf. Instead I have two ceramic discs (is that ok?)one says "RMC 220 20%" I take this to be a 220pf cap. The second cap in parallel with this says "RMC 50 5% NPO" I take this to be 50pf, so in total I have 270pf instead of 260pf. Any issue here? Or can L3 simply be adjusted to compensate? 

C4: Should be 75pf ceramic disc cap. Instead I have again two ceramic disc caps. One says "271 N750" which I took to be 270pf (27x10). The second says "RMC 50 N150 5%" could this be 50pf? If so, I have 320pf where the manual says I should have 75pf. This sounds like a major departure. What would the effect of this have? 

Further down the list (pg 28 of the assembly portion) notes that there should be a 470pf disc cap between lug 1 and 2 of coil L2. Instead, I have a "resin" cap with the words "510 +/-5% 500AE" Does this mean 510pf? is this ok? 

Any way, I haven't investigated every single component, but this is what I found. Can someone help me continue to trouble shoot this? Should I first put every component back to stock (which means selling this albatros because I dont have the time) or could these caps be netted out with proper adjustments of the L's? 

HELP! 
Eugene