SG Frequency Change With Modulation
Posts: 807
Threads: 49
Joined: Feb 2013
City: Shenandoah,
State, Province, Country: Pennsylvania
Hi all, I have the typical service grade Heathkit signal generator from the 50s, it was my dad's. I have done the typical electronics restoration on it years ago. I usually use AVC voltage as my indicator for peaking things during alignment of a radio. I noticed if I peak things without modulation and then turn on the modulation I need to readjust things just a tad to reach maximum response again. It's as if the signal generator's RF frequency changes some with modulation. I have a frequency counter that I use but when I turn on the modulation the counter goes crazy and I can't read the true frequency. Anyone ever notice this happening? I guess one of these new digital generators might be better but they look to be beyond this old brain's comprehension level to understand. I'm still stuck in an analog world!
Ron
Bendix 0626. RCA 8BX5. RCA T64
Philco 41-250. Philco49-500
GE 201. Philco 39-25
Motorola 61X13. Philco 46-42 Crosley 52TQ
Philco 37-116. Philco 70
AK 35 Philco 46-350
Philco 620B. Zenith Transoceanic B-600
Philco 60B. Majestic 50
Philco 52-944. AK 84
Posts: 358
Threads: 4
Joined: Aug 2015
City: Monteith, Ontario CA
I have had a couple of Heath SG series signal generators come thru' my shop over the years. I completely redid one (just for the heck of it) in an attempt to make it usable, a model SG8, and it had to have been the absolute worst generator I have ever seen as far as lack of stability. I don't recall any issue with the frequency changing due to the addition or removal of modulation, but then I couldn't keep it on a frequency long enough to do an alignment anyway. I went back to my General bench oscillator ....
EDIT : for the last decade or so I have been using a BK4040A. .... love it !!
(This post was last modified: 12-06-2019, 09:01 AM by John Bartley.)
Posts: 1,186
Threads: 49
Joined: Oct 2017
City: Allendale, MI
Perhaps the counter is confused by the side bands??
Posts: 23
Threads: 2
Joined: Dec 2018
City: Coopersville
State, Province, Country: MI
Hi All!!
The counter isn't confused. It's trying to read the RF AND AF signals together. You can't set a signal generator with a freq counter when the generator is running a modulated signal, AM or FM. IF you have expensive equipment, i.e. HP or TEK, THEY will give you accurate freq measurements with AM modulation. That's because the Counter, built into the generator, is looking at the oscillator output, before it's AM modulated. That said, if your power supply is being loaded more when the modulator in the generator is running, then I'd expect the oscillator to change frequency a tiny bit. Try looking at the power supply line in the generator and see if it's stable between loads. IF it's not, then you've found your culprit. I suspect that's what's going on here. The frequency is changing due to power supply fluctuation.
You can try pulling signal from the oscillator tube via a cathode tap. You might have to amplify the signal to get your counter to read, but THAT will give you a freq reading that should be accurate. That said, I haven't looked at the SG-8 circuit to see where the modulator is at, in relation to the generator output. The freq read scheme I just described was used by HP in their 606B signal generator. You could hang a freq counter on that port and read freq while running the modulator. That generator was using an RF amp for output with a modulator and mixer stage between the oscillator and the RF amp tubes
Kim Herron W8ZV
w8zv at goldenradioservice.com
1-616-677-3706
Posts: 807
Threads: 49
Joined: Feb 2013
City: Shenandoah,
State, Province, Country: Pennsylvania
Thanks all, this is a cheap basic generator from back in the day and isn't in anyway stable. That's one of the reasons I bought a counter. I figured the counter was getting confused with the modulation and side bands and all. So I have no real way of checking if the RF osc. is changing when adding modulation but I suspect it is by the fact that I need to slightly retune the radio when adding modulation. It's been awhile since I had it apart but if memory serves, it has 2 tubes. One is the RF osc and the other the audio osc with a selenium rectifier!
Ron
Bendix 0626. RCA 8BX5. RCA T64
Philco 41-250. Philco49-500
GE 201. Philco 39-25
Motorola 61X13. Philco 46-42 Crosley 52TQ
Philco 37-116. Philco 70
AK 35 Philco 46-350
Philco 620B. Zenith Transoceanic B-600
Philco 60B. Majestic 50
Philco 52-944. AK 84
Posts: 1,130
Threads: 79
Joined: Jan 2014
City: Annapolis, IL
You have to have a strong enough signal to the counter to be stable. If it is too weak, it jumps around. I use an Eico and it is very stable. I think it was one of the better units back in the day. The radio repair books will tell you to have the gen on for about 30 minutes before alignment. Now, in the days before those digital freq counters, all you had to go by was a known good radio and the pointer on your dial, and seems like they did pretty good.
I note that on my AM radio transmitter that the freq jumps around some when I hit it with modulation. Steady when things are quiet.
If I could find the place called "Somewhere", I could find "Anything"
Tim
Jesus cried out and said, "Whoever believes in me , believes not in me but in him who sent me" John 12:44
Users browsing this thread: 2 Guest(s)
|
Recent Posts
|
Jackson 715 not working
|
Now that you posted the schematic, I don't know why that Sprague electrolytic cap is across the meter, as it is not indi...MrFixr55 — 05:51 PM |
HiFi (Chifi) tube amp build - but my own design.
|
What may be lacking in the PP Tube amps may be the 2nd harmonics, which some, especially RCA back in the day called &quo...MrFixr55 — 05:32 PM |
Jackson 715 not working
|
Usually in an emission tester, the tube under test is measured as if it were a diode. So, some testers connect all the g...RodB — 04:17 PM |
Restoring Philco 37-604C
|
Yep. F5 is green, D5 is Red.
Red is Bad.
Green is Clean.morzh — 01:30 PM |
Jackson 715 not working
|
I did start to do that but I stalled out because I could not figure out how the grid and plate get voltage. In this diag...daveone23 — 11:52 AM |
Restoring Philco 37-604C
|
(Insert Homer Simpson "DOPF" Here.)
When all fails, look at the can. Took the Ron Ramirez advice, red Caig D...MrFixr55 — 09:23 AM |
Philco 91 Speaker Replacement
|
From your text I am not sure if you intend to use the existing speaker with a resistor instead of the field coil.
It wo...morzh — 08:44 AM |
Philco 91 Speaker Replacement
|
My field coil is bad. I am still hoping to find an original, but if I can't I will go with a fitting Philco speaker, 125...dconant — 08:34 AM |
Philco 91 Speaker Replacement
|
As Rod said, it is OK to use a fitting speaker, and then look for an original one.
If you buy a Hammond 125 output tr...morzh — 08:15 AM |
Philco 91 Speaker Replacement
|
Yes, I often have to substitute, then keep an eye out for an original. In the meantime, the radio is working and being e...RodB — 08:02 AM |
Who's Online
|
There are currently no members online. |
|
|