Homelab RF sig gen... maybe? - UK Vintage Radio Repair and Restoration Discussion Forum

Adrenaline · 30th Sep 2008, 11:14 pm

http://www.ethicalscience.co.uk/pilot/homelab.jpg

Also found along with the pilot reciever (see the domestic radio forum for details) I found this - I think it's a homelab model 10 RF signal generator.

I've not powered it up yet, but i'd like some help ascertaining if it's working (or potentially wil work after a recap) - I have no oscilloscope here, and cant find a circuit diagram for it.

Brian R Pateman · 1st Oct 2008, 6:01 am

That's what it looks like to me - although it is a long time since I saw one in the flesh.

I don't think that I have any information but it will be a fairly simple instrument using standard circuitry. Front panel controls fairly self explanatory.

Run it up on a lamp limiter (See Paul's main site for details if you don't have one). Check that you don't have any DC at the output terminals before connecting anything to them for testing.
Test by setting to AF and see what you can hear with a small high impedance speaker or earphones across the output terminals.

Good luck,

Then use a loop of a couple of turns of wire adjacent to the aerial of a working radio and set to MCW and gradually increase the output control and see what happens.

Adrenaline · 4th Nov 2008, 6:50 pm

As i've nearly finished the restoration of the 'Blue Peter' radio, I decided to try and get this thing running so that I could align the MW and LW stages of the radio.

As I have no circuit diagram for this beastie, i decided to start at the beginning before powering the circuitry - as I recognised an old electrolytic (2-in-a-can) that would probably need replacing (date on can is 1948!)

Anyhow... looking inside the box, I disconnected the valve heaters, and drew the attached circuit.

There is a long grey finned thing (indicated on the attached diagram as a diode) that I assume is a Selenium rectifier. There are two contacts on it, and also what appears to be a central mounting bar threaded at each end and used to mount to chassis.

I powered the transformer up with the circuit broken where indicated, and get 33v between A and B (measured on DC 600v range, digital meter).

Switching to AC volts, and measuring from C to B i get the expected 205v AC.

My questions:

This seems very low for an anode voltage - is it indicative of a dead rectifier, if that's indeed what it is?

Is the circuit that i have drawn realistic for a supply for such a piece of equipment?

<any thanks in anticipation!

Ed

Station X · 4th Nov 2008, 7:19 pm

Between A and B the waveform will be one half of a sine wave ie alternate half cycles will be missing. You need to connect everything up and measure across the smoothing capacitor where the waveform will be much closer to pure DC. Assuming of course that the electrolytics aren't leaking.

The circuit as drawn is a bit strange using an autotransformer to get the HT. I assume the secondary winding of the mains transformer is just for the heaters?

Adrenaline · 4th Nov 2008, 8:16 pm

yes, the secondary winding is only used for the heaters, it struck me as an odd circuit as well.

I'll try and get some replacement smoothing caps - I dont hold out much hope of the original being any good.

The valves in it are a pair of 6SN7GT's, so i'll hunt down some specs for them to see what anode voltage i should be aiming at.

Thanks for your help so far.

Ed

Dickie · 4th Nov 2008, 9:21 pm

If that really is the circuit it suggests that it is not isolated from the mains, which would make it rather dangerous to use as a bit of test gear. I wouldn't like to put on a pair of headphones connected to it

Mike Phelan · 5th Nov 2008, 9:19 am

I agree with Richard there - a live chassis on test equipment is something I have never come across, and is potentially lethal.

I wouldn't worry too much about getting the maximum Va on the 6SN7 specs.
Most of these things have a very low HT for long life and cool running - 80 -100v DC is not unusual. (I know that's not the reason for it on your sig genny, though.)

30th Sep 2008, 11:14 pm	#1
Adrenaline Retired Dormant Member Join Date: Sep 2008 Location: Halesworth, Suffolk IP19 Posts: 32	Homelab RF sig gen... maybe? http://www.ethicalscience.co.uk/pilot/homelab.jpg Also found along with the pilot reciever (see the domestic radio forum for details) I found this - I think it's a homelab model 10 RF signal generator. I've not powered it up yet, but i'd like some help ascertaining if it's working (or potentially wil work after a recap) - I have no oscilloscope here, and cant find a circuit diagram for it.

1st Oct 2008, 6:01 am	#2
Brian R Pateman Nonode Join Date: Jun 2005 Location: Western Lake District, Cumbria (CA20) - UK Posts: 2,136	Re: Homelab RF sig gen... maybe? That's what it looks like to me - although it is a long time since I saw one in the flesh. I don't think that I have any information but it will be a fairly simple instrument using standard circuitry. Front panel controls fairly self explanatory. Run it up on a lamp limiter (See Paul's main site for details if you don't have one). Check that you don't have any DC at the output terminals before connecting anything to them for testing. Test by setting to AF and see what you can hear with a small high impedance speaker or earphones across the output terminals. Good luck, Then use a loop of a couple of turns of wire adjacent to the aerial of a working radio and set to MCW and gradually increase the output control and see what happens. __________________ Brian

4th Nov 2008, 6:50 pm	#3
Adrenaline Retired Dormant Member Join Date: Sep 2008 Location: Halesworth, Suffolk IP19 Posts: 32	Re: Homelab RF sig gen... maybe? As i've nearly finished the restoration of the 'Blue Peter' radio, I decided to try and get this thing running so that I could align the MW and LW stages of the radio. As I have no circuit diagram for this beastie, i decided to start at the beginning before powering the circuitry - as I recognised an old electrolytic (2-in-a-can) that would probably need replacing (date on can is 1948!) Anyhow... looking inside the box, I disconnected the valve heaters, and drew the attached circuit. There is a long grey finned thing (indicated on the attached diagram as a diode) that I assume is a Selenium rectifier. There are two contacts on it, and also what appears to be a central mounting bar threaded at each end and used to mount to chassis. I powered the transformer up with the circuit broken where indicated, and get 33v between A and B (measured on DC 600v range, digital meter). Switching to AC volts, and measuring from C to B i get the expected 205v AC. My questions: This seems very low for an anode voltage - is it indicative of a dead rectifier, if that's indeed what it is? Is the circuit that i have drawn realistic for a supply for such a piece of equipment? <any thanks in anticipation! Ed Attached Thumbnails

4th Nov 2008, 7:19 pm	#4
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Homelab RF sig gen... maybe? Between A and B the waveform will be one half of a sine wave ie alternate half cycles will be missing. You need to connect everything up and measure across the smoothing capacitor where the waveform will be much closer to pure DC. Assuming of course that the electrolytics aren't leaking. The circuit as drawn is a bit strange using an autotransformer to get the HT. I assume the secondary winding of the mains transformer is just for the heaters? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Nov 2008, 8:16 pm	#5
Adrenaline Retired Dormant Member Join Date: Sep 2008 Location: Halesworth, Suffolk IP19 Posts: 32	Re: Homelab RF sig gen... maybe? yes, the secondary winding is only used for the heaters, it struck me as an odd circuit as well. I'll try and get some replacement smoothing caps - I dont hold out much hope of the original being any good. The valves in it are a pair of 6SN7GT's, so i'll hunt down some specs for them to see what anode voltage i should be aiming at. Thanks for your help so far. Ed

4th Nov 2008, 9:21 pm	#6
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Homelab RF sig gen... maybe? If that really is the circuit it suggests that it is not isolated from the mains, which would make it rather dangerous to use as a bit of test gear. I wouldn't like to put on a pair of headphones connected to it __________________ Regards, Richard, BVWS member

5th Nov 2008, 9:19 am	#7
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Homelab RF sig gen... maybe? I agree with Richard there - a live chassis on test equipment is something I have never come across, and is potentially lethal. I wouldn't worry too much about getting the maximum Va on the 6SN7 specs. Most of these things have a very low HT for long life and cool running - 80 -100v DC is not unusual. (I know that's not the reason for it on your sig genny, though.) __________________ Mike.