Need a manual for Heathkit SG-8 sig gen

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Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Tue Jan 10, 2012 10:49 pm

Hello. I have a Heathkit SG-8 signal generator that I've never used. It is missing the leads for one thing, but I can fabricate something. I picked it up with some other equipment back in the 90's. It has a couple of tubes in it. I found a schematic.

I also have a B&K frequency counter. I've tried hooking the sig gen to the FC to see what the output looks like. But I need to learn more. The readings I get make no sense, so either the signal is screwed up or my setup is. Or both. Smile
Packed away, I have an Elenco dual trace O scope which I used a bit back (when repairing transistor stereo equipment was worth the effort).
I will check out the sig gen first (2 screws, nice copper chassis, 2 tubes).
Then I could get the o scope out and see what the output is?
I have a B&K function Generator, but it only puts out 100 Khz max.

Can somebody suggest a test with this equipment to diagnose the output of the Sig Gen?
Heathkit signal generator, B&K frequency counter, B&K function Generator, Elenco dual trace Ocilloscope.
Does anyone have a manual or a source for one, for the Heathkit Signal generator?

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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Thu Jan 12, 2012 1:36 am

OK, I can reply to myself. Smile
I found a manual at www.mods.dk
(In fact, I found 9,000 manuals there)(Danke, Erik. Vas ist das, dk?).
But everything I need for now is in the Heathkit manual. I think.
I have recapped and added diodes and a 3-prong cord. I hope a grounded cord doesn't upset the works. I know a grounded chassis can sometimes make a radio uncomfortable. Smile

Once I've tried a couple of AM alignments, I will tackle the big Marconi FM.
But the best is still to come for the SG-8. Mien own little AM transmitter, Ja?

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Re: Need a manual for Heathkit SG-8 sig gen

Post by Bill Cahill on Thu Jan 12, 2012 1:48 am

If the radio is transformer operated, grounded cord o.k.
If not, you are taking a dangerous risk because B- is usually on chassis, one side of line, or, with a capacitor to chassis.
This could be a dangerous shock hazzard.
Congrats. on finding the mannual. Not easy to find.
Glad you found it.....
Bill Cahill
May we see pictures of the set??

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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Thu Jan 12, 2012 11:35 am

Thanks Bill. Yes there is a power transformer in the SG-8.
I am struggling with the pictures part. I have them in Photobucket, so that's a start.

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Re: Need a manual for Heathkit SG-8 sig gen

Post by Bill Cahill on Thu Jan 12, 2012 12:28 pm

Don't copy the html code. Do the code at the very bottom. INsert the code in your post while posting. I open two windows at the same time, and, go back, and, forth.
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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Thu Jan 12, 2012 2:24 pm



OK. Here is my Heathkit SG-8. I am still not sure if it will do what I need. The manual gave me a voltage chart. The voltage on pin 1 of the 12AU7 is way high at 107 vdc. The chart says it should be 78 vdc.
Another curiosity - the filament voltage is only 5 vac. I expected to see 6.3 vac. So the filaments are cool and the plate voltage is high.
I've installed an RCA jack in place of the original. I know that's wrong for high frequency test equipment. Oh well, the lead to the original jack wasn't sheilded,so I think this will work.



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Re: Need a manual for Heathkit SG-8 sig gen

Post by Resistance is Futile on Thu Jan 12, 2012 2:41 pm

There is nothing wrong using an RCA jack, if you can find an BNC all the better.
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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Thu Jan 12, 2012 2:51 pm

Thank you, sir. I went and looked at the schematic again and the first thing I saw was the selenium rectifier, which has been replaced by 2 diodes in series. I think we need a dropping resistor in there. That explains the high plate voltage.
I will add another resistor and see what that does.

Will the 12AU7 and 6C4 operate with only 5 vac on the heaters?

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Re: Need a manual for Heathkit SG-8 sig gen

Post by Resistance is Futile on Fri Jan 13, 2012 1:46 pm

No!
The 12AU7 requires 12 Volts let me take a look at that as it may have a center tap on the filament and could just use 6 volts for each leg. Suspect
AHA! just as I thought 6 volts on each leg at .3ma total.
And the 6C4 requires 6.3 Volts .15ma

The dropping resistor is always needed, because the selenium rectifier has an internal resistance that the silicon replacement diodes dont have.

The photos are missing in your post.
Please look at this video---> CLICK HERE
Ignore the voltage in this video for the filaments as it is taken off the B+ and it doesn't apply in this case.
But to arrive at the correct B+ voltage for your Generator, measure the B+ as you increase the variac voltage (if can do it this way)


Last edited by Resistance is Futile on Fri Jan 13, 2012 3:39 pm; edited 1 time in total
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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Fri Jan 13, 2012 3:30 pm

Yes it is wired for 6 volts to the center tap.
The tranformer is only putting out 5 volts from the filament winding. Weird.

I had to scrap Photobucket. I will return.

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Re: Need a manual for Heathkit SG-8 sig gen

Post by Resistance is Futile on Fri Jan 13, 2012 3:42 pm

I dont think that the voltage is correct on the filaments either. You may have a short in the filament transformer, but that is not likely. you could have a cold solder joint or somthing is loading the voltage down somewhere. Cold solder joints can add unwanted resistance.
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Re: Need a manual for Heathkit SG-8 sig gen

Post by Resistance is Futile on Fri Jan 13, 2012 3:49 pm

Look at the two caps that are in the primary winding of the power transformer, they could be going bad, and check the continuity of the on-off switch, a litte resistance there could also drop the voltage. You should only have o to 1 ohm max.

Heres a PDF manual for the Generator:
----> Click Here

There is a voltage chart that you need to check out to help in the back of the book. If you can't get the correct filament voltage you may have add a small filament transformer as a replacement or the whole power transformer.
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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Fri Jan 13, 2012 7:54 pm

Thanks. I found a manual and tons of others at www.mod.dk
Erik in Germany. His site is worth checking out.

I had a 100 wirewound ohm resistor in there. Used parts, looks like I did it when I added the power cord. I upped it to 330 ohms just because. That brought the B+ down a little.

I set up the oscilloscope, sig gen and freq counter and found a nice waveform coming out of the generator. It is a little squished at the bottom but this is lo-fi. Smile

I can pick up a small buzz with the Stromberg Carlson AA5 setting across the room, and I can zero-beat the two to a radio station. I have a transmitter!!
And both tubes in the Signal generator are running with 5.2 vac on their heaters. Cool. Smile

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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Sat Jan 14, 2012 2:01 pm

Here's an interesting article on transmitting with a signal generator. That is a Heathkit SG-8 on the left.

http://www.ohio.edu/people/postr/bapix/heath_sg.htm

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Re: Need a manual for Heathkit SG-8 sig gen

Post by Bill Cahill on Sat Jan 14, 2012 2:28 pm

Thanks for the tip, and, the link. That is a great idea...
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Re: Need a manual for Heathkit SG-8 sig gen

Post by westcoastjohn on Thu Mar 08, 2012 1:55 am

I want to report that a) The signal generator works pretty well for doing a radio alignment by the book. I happen to have a B+K frequency counter I picked up somewhere cheap. The counter helps me set the sig gen more accurately than the dial pointer. I give it some time to warm up and it stays on frequency pretty well.

b) It works pretty well as an AM transmitter too!
I use a CD discman as an audio source. From the headphone jack, I brought the left and right side together and hooked them to one secondary lead of a small output transformer. The ground wire from the headphone jack goes to the other secondary lead. The primary leads go to the input of the signal generator. On the Sig gen output, I plug in a 10 foot stranded wire antenna, leaving the ground floating. With this rig, I can play CD's on my AM radios, but it doesn't have any range, just covering my shop, which is probably a good thing. Without the transformer, there was an impedance mismatch and I was not getting enough power to transmit more than a few inches.

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