First time use of sig gen TE20D - UK Vintage Radio Repair and Restoration Discussion Forum

sobell1980 · 18th Sep 2014, 9:04 pm

Hi all,

Just acquired my first sig gen. I understand following the service data of various manufacturers on how to set it up for alignment and inject signals etc. But the set I am testing is dead, all valves being powered up etc and I just want to inject a signal that can be heard through the speaker and see where my signal is being lost. I assume I need a modulated signal, what settings do I need to set my sig gen on etc. Another new learning curve for me! What type capacitor do I need to fit in line with my probe? If any one can advise on my new test equipment and how to use it that would be great.
Many thanks,
Dave.

Radio Wrangler · 19th Sep 2014, 9:45 am

To get a signal out of the speaker of a dead set, you need to work backwards along it. Start with injecting an *audio* signal from the generator's modulation source through a 0.01uF, 0.1uF or so capacitor into g1 of the output valve.

If that works, then you change to injecting *modulated RF* at the IF frequency and work your way back to the mixer.

David

Skywave · 19th Sep 2014, 10:31 am

Quote:

Originally Posted by sobell1980

But the set i am testing is dead, all valves being powered up etc

In which case, a check of the voltages at critical points on the circuit - such as valve electrodes - is the thing to do first. When you are confident that all is well in those areas, then that is the time to inject signals to locate the faulty areas / components.

Al.

HamishBoxer · 19th Sep 2014, 12:20 pm

Is there any HT on the dead set around the rectifier and smoothing caps?

Station X · 19th Sep 2014, 12:35 pm

What's the set?

sobell1980 · 19th Sep 2014, 1:30 pm

The dreaded Roberts RMB Graham. The thread is in domestic radio section. I decided on investing in some more kit to finally sort this out. Many thanks.
Dave.

Station X · 19th Sep 2014, 1:59 pm

Assuming the valves light up, the first thing to do is check for HT between the negative rail and both primary terminals of the output transformer. This will tell you whether HT is present and whether the O/P primary is intact.

Almost all faults on a domestic receiver can be found using just a DMM.

winnem1 · 19th Sep 2014, 2:12 pm

Hi
If you have not used a sig gen before beware of connecting the signal earth from the generator to ground of the radio as it may be a LIVE chassis. You should be alright with the Roberts RMB as it has a mains isolating transformer.

19th Sep 2014, 2:32 pm

This is a battery/mains set with quite fragile valves (the heaters anyway) check the LT and HT voltages first, I doubt any of the radio components are duff, they lead an easy life with only 90V of HT. If the HT and LT are OK it is probably a duff valve, I had a set that would only work on the 220V setting, the mixer/oscillator valve was a bit weak on emission, the lower voltage setting gave it just enough extra heater volts to work. That was the only battery valve set I have owned that didn't work a bit from the start, it was my first in 1972! A Vidor 396 if memory serves correctly or at least one with the dial pointer held both ends by the dial cord, that fascinated me at the time.

sobell1980 · 19th Sep 2014, 7:50 pm

Hi,
Below is the thread if you wish to follow the repair and see what i have, and have not tested etc. It was advised to me to expand my kit as i was working in the dark. It has been an extensive repair so far. I would like to use the sig gen to diagnose if it is a valve as i do not have replacement or substitutes. I also simply want to practice using the sig gen and be efficient using it. How do i set the attenuation etc and the modulated signal. What range do i use to inject an audio signal. I can see what the data sheets say for alignment which is pretty staight forwards, but what do i do to inject a signal before alignment to test components and the circuit? Any help advice greatfully received, just wish to practice using my new kit.
Kind regards
Dave.

https://www.vintage-radio.net/forum/...ad.php?t=95682

Sideband · 19th Sep 2014, 8:53 pm

Hi Dave.

As you have not used a signal generator before might I suggest that you practice using it on a working set? This might sound counter-productive BUT at the moment, you have a set with an unknown fault/faults. There is nothing more frustrating than injecting a signal into a set, getting no signals and not being sure if it's you doing something wrong or the set that is wrong. At least with a working set, you know that if you inject an appropriate signal you should get an output. If you don't, you know it's you that's done something wrong.

This is how I learnt to use a signal generator and a scope. No point in using it on a duff set until you know what you are doing and can be certain of expected results.

Besides, are you sure that your signal generator is actually working properly?

sobell1980 · 19th Sep 2014, 9:00 pm

Sig gen definately works. It was demonstrated briefly when i collected it. I have a couple of transistor sets that work i could try it on. All my valve sets are inoperative. I agree Sideband with your idea. If some could talk me through it please that would be wonderful. I have to change frequencies through different sections of the set? Rf and af? what do i set attenuation to? Many thanks for your time.
Dave.

Radio Wrangler · 19th Sep 2014, 9:22 pm

OK,

With a dead radio, the best place to start is to check all the voltages marked on the circuit diagram of the service sheet. I gather that this is a somewhat recalcitrant set and you've already done this. I also understand that this set has a proper mains transformer and doesn't have a live chassis. Using grounded(=Earthed) test equipment like signal generators and oscilloscopes is difficult to do safely with live chassis sets.

So first check that the chassis is indeed safe. Use a multimeter to check insulation between the chassis and live and neutral (with the switch on)

On the panel of your sig gen, you should find a terminal for audio out or modulation out.

Insde the sig gen there is a wide-range tuneable oscillator to make the radio frequency (or IF frequency) signal. All alone this would just make a clean signal which would not produce a sound from an AM radio set. It would actually make the normal background noise fade.

So the signal generator has to apply a sound signal to itself. It is a small AM transmitter and it needs some audio to transmit. To do this there is a second oscillator inside the thing, and this one is tuned to an audible frequency. 400Hz and 1000Hz are popular frequencies. Posh signal generators may have the audio frequency variable, but basic ones don't and that's perfectly fine for fixing radios. This 'modulation signal' is always available from a sig gen because it's very valuable for checking audio stages.

Once you're sure your chassis is safe connect a lead from the signal generator ground terminal to the chassis. Connect another lead through a capacitor - the exact value isn't important 0.01 to 0.1 microfarads would be OK, but choose something with a high voltage rating... higher than the HT voltage of the set. A simple sig gen without an output level pot for the audio output might just come out via an internal capacitor, but it's a good habit to always use an external one. You'll find it more convenient if this test lead has a probe on the end. Voltmeter lead sets are cheap and you can use one, or make a probe from an old BIC ballpoint pen case. I use an old scope probe (they still work with signals going in the reverse direction)

There are two places you need to probe. One is the first grid of the output valve, the other is the top end of the volume control pot (with the volume wound up a bit)

It doesn't matter what frequency you have the RF section of the generator set to... you aren't using it (yet)

You'll have some sort of mode switch offering a plain carrier (or 'cw') and modulated carrier, it may have a specific position for audio only, but if not, select modulated signal because the audio oscillator is often powered off when you select a plain carrier.

That should get you over a first step

David

sobell1980 · 21st Sep 2014, 9:00 am

Hi David,
Many thanks for your lengthy reply. It has been most useful. I have now familiarised myself with all the sig gens functions and controls, they are no longer alien to me. I have found that when i turn the knob to INT modulation it powers up both the RF output, and the AF output. Both AF anf RF on seperate jack posts. On INT MOD it also modulates the RF output with a 400 Hz signal but if i swop my leads onto the AF jack posts it outputs just the 400 Hz signal as audio frequency. This is variable up to 8 Volts but there are no markings on the variable volt control

so where abouts should i have the voltage set for AF output? When EXT MOD is selected, the 400 Hz signal is cut off and a external signal can br applied through the same AF posts i mention earlier. Many thanks for your help, it has certainly cleared things up. I have the power switch to solder back into the RMB set, then i shall give the sig gen a whirl. Thankyou for your continued interest.
Dave.

18th Sep 2014, 9:04 pm	#1
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	First time use of sig gen TE20D Hi all, Just acquired my first sig gen. I understand following the service data of various manufacturers on how to set it up for alignment and inject signals etc. But the set I am testing is dead, all valves being powered up etc and I just want to inject a signal that can be heard through the speaker and see where my signal is being lost. I assume I need a modulated signal, what settings do I need to set my sig gen on etc. Another new learning curve for me! What type capacitor do I need to fit in line with my probe? If any one can advise on my new test equipment and how to use it that would be great. Many thanks, Dave.

19th Sep 2014, 9:45 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,904	Re: First time use of sig gen TE20D To get a signal out of the speaker of a dead set, you need to work backwards along it. Start with injecting an audio signal from the generator's modulation source through a 0.01uF, 0.1uF or so capacitor into g1 of the output valve. If that works, then you change to injecting modulated RF at the IF frequency and work your way back to the mixer. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

19th Sep 2014, 12:20 pm	#4
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: First time use of sig gen TE20D Is there any HT on the dead set around the rectifier and smoothing caps? __________________ G8JET BVWS Archivist and Member V.M.A.R.S

19th Sep 2014, 12:35 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: First time use of sig gen TE20D What's the set? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

19th Sep 2014, 1:30 pm	#6
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: First time use of sig gen TE20D The dreaded Roberts RMB Graham. The thread is in domestic radio section. I decided on investing in some more kit to finally sort this out. Many thanks. Dave.

19th Sep 2014, 1:59 pm	#7
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Capacitor question for Roberts RMB Assuming the valves light up, the first thing to do is check for HT between the negative rail and both primary terminals of the output transformer. This will tell you whether HT is present and whether the O/P primary is intact. Almost all faults on a domestic receiver can be found using just a DMM. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

19th Sep 2014, 2:12 pm	#8
winnem1 Tetrode Join Date: Nov 2009 Location: Romford, London, UK. Posts: 64	Re: First time use of sig gen TE20D Hi If you have not used a sig gen before beware of connecting the signal earth from the generator to ground of the radio as it may be a LIVE chassis. You should be alright with the Roberts RMB as it has a mains isolating transformer.

19th Sep 2014, 2:32 pm	#9
Guest Guest Posts: n/a	Re: First time use of sig gen TE20D This is a battery/mains set with quite fragile valves (the heaters anyway) check the LT and HT voltages first, I doubt any of the radio components are duff, they lead an easy life with only 90V of HT. If the HT and LT are OK it is probably a duff valve, I had a set that would only work on the 220V setting, the mixer/oscillator valve was a bit weak on emission, the lower voltage setting gave it just enough extra heater volts to work. That was the only battery valve set I have owned that didn't work a bit from the start, it was my first in 1972! A Vidor 396 if memory serves correctly or at least one with the dial pointer held both ends by the dial cord, that fascinated me at the time.

19th Sep 2014, 7:50 pm	#10
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: First time use of sig gen TE20D Hi, Below is the thread if you wish to follow the repair and see what i have, and have not tested etc. It was advised to me to expand my kit as i was working in the dark. It has been an extensive repair so far. I would like to use the sig gen to diagnose if it is a valve as i do not have replacement or substitutes. I also simply want to practice using the sig gen and be efficient using it. How do i set the attenuation etc and the modulated signal. What range do i use to inject an audio signal. I can see what the data sheets say for alignment which is pretty staight forwards, but what do i do to inject a signal before alignment to test components and the circuit? Any help advice greatfully received, just wish to practice using my new kit. Kind regards Dave. https://www.vintage-radio.net/forum/...ad.php?t=95682

19th Sep 2014, 8:53 pm	#11
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: First time use of sig gen TE20D Hi Dave. As you have not used a signal generator before might I suggest that you practice using it on a working set? This might sound counter-productive BUT at the moment, you have a set with an unknown fault/faults. There is nothing more frustrating than injecting a signal into a set, getting no signals and not being sure if it's you doing something wrong or the set that is wrong. At least with a working set, you know that if you inject an appropriate signal you should get an output. If you don't, you know it's you that's done something wrong. This is how I learnt to use a signal generator and a scope. No point in using it on a duff set until you know what you are doing and can be certain of expected results. Besides, are you sure that your signal generator is actually working properly? __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

19th Sep 2014, 9:00 pm	#12
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: First time use of sig gen TE20D Sig gen definately works. It was demonstrated briefly when i collected it. I have a couple of transistor sets that work i could try it on. All my valve sets are inoperative. I agree Sideband with your idea. If some could talk me through it please that would be wonderful. I have to change frequencies through different sections of the set? Rf and af? what do i set attenuation to? Many thanks for your time. Dave.

19th Sep 2014, 9:22 pm	#13
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,904	Re: First time use of sig gen TE20D OK, With a dead radio, the best place to start is to check all the voltages marked on the circuit diagram of the service sheet. I gather that this is a somewhat recalcitrant set and you've already done this. I also understand that this set has a proper mains transformer and doesn't have a live chassis. Using grounded(=Earthed) test equipment like signal generators and oscilloscopes is difficult to do safely with live chassis sets. So first check that the chassis is indeed safe. Use a multimeter to check insulation between the chassis and live and neutral (with the switch on) On the panel of your sig gen, you should find a terminal for audio out or modulation out. Insde the sig gen there is a wide-range tuneable oscillator to make the radio frequency (or IF frequency) signal. All alone this would just make a clean signal which would not produce a sound from an AM radio set. It would actually make the normal background noise fade. So the signal generator has to apply a sound signal to itself. It is a small AM transmitter and it needs some audio to transmit. To do this there is a second oscillator inside the thing, and this one is tuned to an audible frequency. 400Hz and 1000Hz are popular frequencies. Posh signal generators may have the audio frequency variable, but basic ones don't and that's perfectly fine for fixing radios. This 'modulation signal' is always available from a sig gen because it's very valuable for checking audio stages. Once you're sure your chassis is safe connect a lead from the signal generator ground terminal to the chassis. Connect another lead through a capacitor - the exact value isn't important 0.01 to 0.1 microfarads would be OK, but choose something with a high voltage rating... higher than the HT voltage of the set. A simple sig gen without an output level pot for the audio output might just come out via an internal capacitor, but it's a good habit to always use an external one. You'll find it more convenient if this test lead has a probe on the end. Voltmeter lead sets are cheap and you can use one, or make a probe from an old BIC ballpoint pen case. I use an old scope probe (they still work with signals going in the reverse direction) There are two places you need to probe. One is the first grid of the output valve, the other is the top end of the volume control pot (with the volume wound up a bit) It doesn't matter what frequency you have the RF section of the generator set to... you aren't using it (yet) You'll have some sort of mode switch offering a plain carrier (or 'cw') and modulated carrier, it may have a specific position for audio only, but if not, select modulated signal because the audio oscillator is often powered off when you select a plain carrier. That should get you over a first step David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

21st Sep 2014, 9:00 am	#14
sobell1980 Octode Join Date: May 2011 Location: Bromyard, Herefordshire, UK. Posts: 1,241	Re: First time use of sig gen TE20D Hi David, Many thanks for your lengthy reply. It has been most useful. I have now familiarised myself with all the sig gens functions and controls, they are no longer alien to me. I have found that when i turn the knob to INT modulation it powers up both the RF output, and the AF output. Both AF anf RF on seperate jack posts. On INT MOD it also modulates the RF output with a 400 Hz signal but if i swop my leads onto the AF jack posts it outputs just the 400 Hz signal as audio frequency. This is variable up to 8 Volts but there are no markings on the variable volt control so where abouts should i have the voltage set for AF output? When EXT MOD is selected, the 400 Hz signal is cut off and a external signal can br applied through the same AF posts i mention earlier. Many thanks for your help, it has certainly cleared things up. I have the power switch to solder back into the RMB set, then i shall give the sig gen a whirl. Thankyou for your continued interest. Dave.