Heathkit SG-8 RF Generator Modifications. *** SG-8 ON Steroids ***

Yes, another Heathkit SG-8 RF generator modification video. But unlike others, this one has some existing modifications that smooth out the waveform along with a regulated B+ supply and a built-in frequency counter for accuracy!
Here is a link to the schematic and photos used in the video.
drive.google.c...
I wasn't clear on where I connected the 4.7K resistor, right across the Band "A" coil. Screen shot here.
drive.google.c...
I also connected the frequency counter to pin 2 of the 12AU7 through a series 560 ohm resistor and a ,47uf capacitor.

[@dri50]

Perfect timing seeing this video today. I had the cover off my SG-8 to see if I could improve the waveform for 455KHz. I use a separate Heath digital counter to set the frequency. Thanks for posting this. I'll try the waveform mods tomorrow.

[@wesleymatthews137]

My resto is complete. Thanks for the tips. My audio waveform is right on! The RF waveform a hair fuzzy, but certainly acceptable. 100% functional. Frequency drifts lower as it warms up but stabilizes nicely after about 15 minutes.

[@AngusLepus]

Thanks for sharing your mods - picked up one of these at a hamfest and thought of at least dumping the selenium rectifier and putting on BNC jacks.

[@wesleymatthews137]

The video at 06:13 states that the "blue resistor" is the 4.7 k resistor that is across band coil A and the Capacitor is "one of the older caps replaced in the unit." This is all incorrect. There is a photo linked in the description to the Coil A resistor and it's not the one described at 06:13. See that photo for correct placement. There is no "older Cap" to replace in that area either. The "blue resistor" appears to go from the 12AU7 pin 2 to ground (and it's value cannot be determined from the video)? The "Cap" appears to be part of the RF feed circuit to the frequency counter. I would grab RF prior to the attenuating circuit I suppose so that any counter will get the full and constant output from the generator, but I'm not sure what the function of the cap is that appears to drain/filter the RF feed for the counter to ground? Possibly this prevents the counter from pulling down the generator frequency? Was the 100 ohm dropping resistor behind the diode that replaced the selenium rectifier measured or estimated? I'm going to measure just for my own educational purposes, but I've seen lots of controversy surrounding dropping resistors and was just curious. Half of the guys measuring, the other half estimating. Thanks for the schematic. It's handy. I wish the counter install was depicted on it though. That would answer lots of questions. You've really managed to clean up the waveform nicely!

[@wesleymatthews137]

I replaced the Selenium Rectifier with a 1N4007 and tested for the additional dropping resistor using a trusty old Heathkit Resistor Substitution Box, model RS-1. Pre-mod, under load, I measured 125 volts prior to the 3.3K 1W original resistor and 80 volts after it. Post-mod, the voltage, as expected, was high...by about 20 volts. Using the substitution box to add resistance, starting at 100 ohms as the video suggests, I found 470 ohms was required to match voltages to the original specs and nailed about 80 volts, post dropping resistor. Certainly close enough to the original voltage. On mine, Pin 1 of the 12AU7 was 95 vs 65 volts and Pin 6, 55 volts versus 85 volts on the voltage chart. I may need to investigate further.

[@docholliday3150]

Late to the game here but was curious if the higher band frequencies are derived from harmonics of the lower band frequencies. That is true of many of the vintage RF sig gens. If so, did your modifications (cleaning up the waveform) have any deleterious effects on the highest frequency bands? Thanks for the nice video. doc

[@electricroo]

I added the switch on the 200pf capacitor because with the capacitor in circuit it did affect the higher frequencies.

[@Mr71chevyvan]

my SG-8 is factory built. there is a 10K 1/2watt res in the plate circuit of the 12AU7 pin 1 and my reading is 70vdc. i have noticed diff construction and parts used between kits and similar factory built models on this and other Hk products. another example is my filter cap assy is a metal can mounted sideways under the chassis. its caps are 50-30 mfd @150vdc. my selector knobs are chicken head. tuning knob is large brown. the soldering and construction are too perfect even by a good kit builder standard.

[@DaedaWerx]

Your schematic shows 200 and 220pf. Which value is right? Also where is the 4.7k connected?

[@electricroo]

Oh sorry about that. It's 220pf. Look in the description of the video I added a photo.

[@rgoodleaf]

Hello. I'm looking at purchasing an SG-8 and I'm doing some research on restoration. I was wondering why you need the 100 ohm resistor in series with the 1N4007 if you are using a regulator to set the plate voltage at 78v. I would see why you need the resistor if you were connecting point A directly to point B.

[@electricroo]

When you replace a selenium rectifier on old equipment you may need a resistor in series with the diode to keep the voltage with in range of the original selenium rectifier. That being said, I inserted it on the schematic in case someone might not want to use the regulator circuit and we're not aware that a dropping resistor may be necessary when replacing a selenium rectifier

[@rgoodleaf]

Good point!! Better to be safe. Thanks for a very informative presentation. What frequency counter did you use? It looks like a necessary addition.

[@wildstar1063]

Where did you take the input for the frequency counter? And did you couple it through a capacitor or just direct connect it question

[@electricroo]

Oh, I am sorry I didn't see your question. For some reason I missed the notification. I believe I took it directly from the output as testing indicated that it did not interfere with the signal.

[@wildstar1063]

@@electricroo Thanks for the info, I have one of the Heathkit signal generators and wanted to add the same frequency counter to it.

[@carl112466]

I added the 200 PF cap on pin 2 but when I flip the switch my Sin wave is cut in half from 455hz to about 225hz . Any ideas why.

[@electricroo]

Not sure why it should not do that as that capacitor only reduces the drive on the tube for a cleaner waveform at lower frequencies.

[@carl112466]

I see the 4.7k res but I can not see where you connected it. I know it is somewhere on coil A but I cannot see where on coil A and the other end I am thinking it connected to pin 1 of the 12AU7

[@electricroo]

Read the description, there is a photo. Right across band A coil.

[@jjedi1416]

Where did you get the frequency counter?

[@electricroo]

Sorry for the delay, I missed your message notification. Here is an eBay link to the counter. www.ebay.com/itm/Blue-RF-Signal-Frequency-Counter-Cymometer-Tester-0-1-60MHz-20MHz-2400MHZ-case/122300257852?hash=item1c79a9d63c:g:G1oAAOSwnHZYb1Dk