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The Heathkit SB-620 “Scanalyzer” is an IF spectrum analyzer. It graphs the amplitude of signals around the one that your receiver is looking at, so you can see all of the signals around you at a glance. Problem is, it has to be setup for the particular IF frequency of your receiver. The original kit included parts for a wide range of IF frequencies. Only one frequency could be installed though and my unit had the wrong frequency for the classic Heathkit SB-301 receiver I have.
The pdf article describes how to convert it to the IF frequency common to many Heathkit receivers, including: SB-100, SB-101, SB-110, SB-102, SB-104, SB-300, SB-301, SB-303, HW-100, HW-101 and HW-104. The photo above shows it operating with actual 80m signals. Get the article and schematic below:
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IF booster amp and new coils in the SB-620
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Reader Comments
| Posted by
Steve L. |
January 22, 2023 - 04:55 pm | |
"Wow thank you for your efforts looking into this!"
--- Happy to help.
"I have RG62 connected directly to the...V7-1 (grid) because the 610 manual 'preferred' the grid circuit. The RG62 couples to the spare phono...via a 2.2pF capacitor...Side Note- You mention smaller coax? I wonder if I could get away with coax that is smaller... RG62 is a bear to work with, but I know the cap/ft. is desirable."
--- I see the preference for the grid mentioned in Fig.9 on p.33 but they do show it on both plate and grid. Since we're scratching for gain, I'd try the plate. Yes, I used small coax for the short run inside the SB-301. Seemed the only reasonable thing to do. But it's good you used RG62 as it has only half the capacitance as (say) RG58. There is no doubt that the cable capacitance (as opposed to resistive loading) is by far the major part of the load. Signal level will be inversely proportional to that load cap. Thanks for including the pics. That brings to mind the fact that it's important for the 2.2pF to be at the site of the circuit board tap. That way, the loading on the receiver circuit is no more than 2.2pF. As it is, the receiver sees the load of the coax run from the board to the site of the jack. It would be good if you could pick up a ground for the coax at the circuit board instead of at the jack. (Yeah, I know it must be awful working with the RG-62 under there.)
"...The scope's vertical deflection with the signal generator on its highest setting was weak even then (but the signal gen. is an Eico 324 in poor health, so I chalked it up to a weak signal from the generator at the time)."
--- I built an Eico 324 kit as a kid and still have one. I just cranked it up, many years after restoration and it works. At 3400kc, its max output reads about 1.5Vpp, which is comparable to the 0.5Vrms sensitivity mentioned in the '610 specs at that frequency. That's supposed to give an inch of vertical deflection. Mind you, the only load I had on the 324 was the 46-inch RG58 cable which came with it. I used a 10X scope probe on the alligator clips on the end of the cable. Have you checked your 234 output with a scope? Incidentally, the 324 only has two tubes--12AV7 RF osc & cathode follower (CF) and 12AU7 audio osc/amp & CF. The audio CF supplies the whopping 54VDC which runs the 12AV7. (That's the average DC voltage. With 100% modulation it swings something like 0V to 108V.)
"I should mention that I hooked up the signal generator DIRECTLY to the VERT jack on the scope... Should I be using a coupling capacitor to perform this task?"
--- Connecting the signal generator directly to the '610 is fine.
"I had considered tapping at the plate to provide the scope additional gain, but, of course, I was afraid of loading the IF stage down. Would you recommend tapping at the plate instead?"
---Yes, tapping at the plate. Tapping at the grid or the plate will load the IF, unfortunately, but with the 2.2pF at the circuit board, it should have minimal effect on the radio.
If you could get the output of the 324 as it should be, that would clarify whether there is a problem with the '610 and indicate how how much change is needed. Moving the 2.2pF cap should help the receiver and should not impair the signal at the '610 grossly. |
| Posted by
Dan M. |
January 22, 2023 - 01:04 pm | |
Wow thank you for your efforts looking into this! I have included two images to show my IF tap arrangement in addition to my responses below.
http://www.tronola.com/misc/Tap_at_the_PCB.jpg
http://www.tronola.com/misc/Connection_to_jack_at_back_of_SB301.jpg
1. I have RG62 connected directly to the foil of V7-1 (grid), because the 610 manual "preferred" the grid circuit. The RG62 couples to the spare phono behind the audio transformer via a 2.2pF capacitor (I think this is what they call a "gumdrop").
a. Side Note- You mention smaller coax? I wonder if I could get away with coax that is smaller... RG62 is a bear to work with, but I know the cap/ft. is desirable.
2. 24" long RG62 cable from spare phono to SB610 VERT jack.
3. The 610 is wired for 3000-6000 kc- I confirmed this by visual observation AND I hooked up a signal generator tuned to roughly 3395kc. The scope's vertical deflection with the signal generator on its highest setting was weak even then (but the signal gen. is an Eico 324 in poor health, so I chalked it up to a weak signal from the generator at the time).
a. I should mention that I hooked up the signal generator DIRECTLY to the VERT jack on the scope... Should I be using a coupling capacitor to perform this task?
4. I had considered tapping at the plate to provide the scope additional gain, but, of course, I was afraid of loading the IF stage down. Would you recommend tapping at the plate instead?
---Thanks, Dan |
| Posted by
Steve L. |
January 22, 2023 - 12:03 pm | |
Hi Dan, First, congratulations on getting your ticket and getting into electronics a few years ago! Heavens, it's not a simple question as to whether the booster amp for the '620 can help here. I haven't looked into the SB-610 before but have been studying the schematic and manual this morning. It's input circuits are quite different from the SB-620 so I wanted to see how that might affect your situation. This is also affected by the way the SB-610 is supposed to be connected to the SB-301. It's also different from how the SB-620 is supposed to be connected.
From the way you described the problem, it sounds like the '610 might be loading-down the IF of the '301. Hence you get a choice (by tuning) of getting enough signal into the '610 or leaving enough for the '301 to use. This isn't a problem for the '620 because it has another input amplifier stage which buffers its input from its filter circuits. That allowed me to design the booster amp with lower input impedance (Z). The tap point recommended for the '610 is also higher Z than the one used for the '620, making the loading more critical. So it doesn't look encouraging for using this booster amp with the '610.
But there may be other ways to fix this. In going over the schematics of the the '301, '610 and '620, I see that there are a number of pitfalls which could contribute to your problem and should be checked:- First, looking at the '610 manual p.33, Fig.9, Heath says to connect a 1-12pF cap to the plate of the last IF stage of the receiver. In your case, that would be V7-5 of the '301. Is that what you did? What value of cap? In my '301, the cap connects to small coax to get to a spare phono jack on the '301. Then shielded cable goes to my '620. Is that like yours?
- If so, does the shielded cable go to the VERT jack on the back of the '610?
- The '610 has three different vertical amp circuits which could be chosen, depending on frequency range desired. Is yours wired for 3000kc to 6000kc operation, which is needed to work with the '301 at 3395kc? If so, you should be able to locate L1 in the '610 and can check for the other specific components shown in the '610 manual in Fig.3 and Fig.6.
- It strikes me that, if you used as much as say, 10pF to couple to V7-5 of the '301, that could detune IF "can" T4. If it isn't readjusted, that could reduce receiver sensitivity.
- If you used only 1pF to couple to V7-5 of the '301 (as Heath recommends on p.34 of the '610 manual for >3000kc), you could get better deflection by increasing the cap to as much as 12pF. (Check T4 peaking in that case.)
Anyway, sorry that the booster probably can't help here but I hope checking the bullet items can turn up something helpful. |
| Posted by
Dan M. |
January 22, 2023 - 04:49 pm | |
[Ed note: This message and its thread above are posted from an email exchange, with permission.]
I've run across your article on the DIY IF pre-amp for the Heath SB-620 a few times now.
I recently repaired the Vertical Amplifier section on a 610 station monitor, but the vertical deflection is... not great. I've tried peaking the inductors in both my receiver (SB-301) and the 610, but either receiver sensitivity suffers, or the vertical deflection on the scope is nearly microscopic... I can't have both.
Would your pre-amp design for the 620 be a good candidate for this problem? Sorry if this seems like a simple question... I'm not an EE and I've been limping my way through self-taught electronics troubleshooting since receiving my ticket in 2020. Thank you, Dan --73-- |
| Posted by
Steve L. |
April 05, 2022 - 05:52 pm | |
| Hi Rick, Congratulations on completing the kit and the mod! Seeing the signals too high, even with the PIP gain all the way down suggests that the signal from the receiver is going to the RF Input. Rather, it should go to the IF Input. The Ham Scan switch on the back should be UP to select Ham Scan. (The way Heath has that designed and labeled seems confusing.) The Amplitude Scale switch should be set to LOG. I doubt you will need the trimmer caps unless you end up scratching for more gain. Please let us know how that goes. We'll get to the bottom of this! |
| Posted by
Rick |
April 05, 2022 - 03:04 pm | |
| A few years ago I was lucky enough to purchase a complete 620 kit! Over the past winter I completed and aligned the kit for my SB 102 and it worked. However, only the stronger signals seemed to pop up...couldn't see the noise level at all. I cam across this article about the pre amp and thought this might do the trick. I completed the amp and was now wondering if I need the trimmer caps or since I used the original coils for the 3395 IF I didn't need them. I hooked up the pre amp and signals are now too high even with the PIP gain all the way down. Haven't realigned anything yet with the pre amp. |
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