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Old 15th Jun 2021, 9:40 pm   #1
Spencervs
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Default Signal generators

Does anyone know where (or if they have one to sell) I can find a signal generator that I can use to realign my radios? I'm quite new to the idea and currently my equipment is an AVO, DDM and a cap reformer I threw together. What would I need to realign my LW, MW, FM radios and how would I calibrate any generator that I may come across? This is all new territory for me!

Thanks,

Spencer
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Old 15th Jun 2021, 9:48 pm   #2
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Default Re: Signal generators

If you look on ebay, this item is the kind of thing that would suit you very well indeed https://www.ebay.co.uk/itm/203493250...4AAOSw7YNgxvyD, though that exact one has the wrong frequency range. You need one that will go down to at least 1MHz, preferably 100kHz.

However, comparable generators usually sell for something around 50-60 I'd guess.

That type of generator is typical of those used in TV /Radio repair shops in the past. It's the kind of thing you might have been able to pick up at a good price at a radio rally, but they are mostly off until next year

B
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Old 16th Jun 2021, 1:33 pm   #3
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Default Re: Signal generators

Hi Spencer. As others say Ebay is a good place to look... in my view a Marconi TF995 would do all of what you need, but it is "old tech".... Valve operated, a warm up time would be required for reasonable stability. However attaching a frequency counter to it would provide an accurate reading, and highlight the stability. Re aligning domestic radio's is more an "art" than a science.
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Old 17th Jun 2021, 12:28 am   #4
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Default Re: Signal generators

So I now am the owner of a tektronix 2201 oscilloscope and was wondering if this would help me in alignments? (given it by work for free!) or calibrating a generator even?

I found these generator adds on ebay and they seem a good cheap price to get me into the hobby: https://www.ebay.co.uk/itm/274829623...AAAOSw2XVgwyaW or http://ebay.co.uk/itm/274829623458?h...AAAOSw2XVgwyaW

are these ok? I was thinking of finding a cheap small signal counter and using this to help calibrate them... but is there a general way to calibrate a signal generator? will the oscilloscope help?

thanks
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Old 17th Jun 2021, 2:44 am   #5
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Default Re: Signal generators

It's 3.40am, so I may be mistaken but I think both your links go to the same item; the "ALL wave sig gen" with the white front. It has no maker's name, but looks very similar to one I had many decades ago. If that works, or could be got going, it would certainly meet your needs for LW/MW radios, and would get you going with FM radios. It's a signal generator in a proper metal box and it's got a level control and a meter and that is all very nice. If it doesn't work, it will not be rocket science to make it work, though you may end up tracing the circuit diagram (tedious but do-able).

Including p&p, that's 27.50, I'm surprised that Hermes want 8.50 for economy delivery.

19 is not outrageous, but it will very probably fetch appreciably more. I note that he's open to offers; may be better than going through auction. You got your scope free; spend some cash now

B
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Old 17th Jun 2021, 4:37 am   #6
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Default Re: Signal generators

In your first post you included the need to realign FM sets. The one you linked to doesn't do FM.

There were many signal generators made with only AM modulation, and fairly crude output level control. They were good enough for repairers to trace down the structure of normal AM broadcast radios to find where a stage wasn't working so they could fix it, and to perform re-alignment. Their very basic output level control didn't include a comprehensive enough attenuator to be able to take the signal down low enough to be able to measure the radio's sensitivity. Even if they had that much attenuation, they weren't screened well enough to prevent leakage overruling the attenuated output.

These basic sig gens are still in circulation, and cheap.

There was a better class of sig gen, usually thought of as 'lab grade' which might have better frequency control, but the big difference is they tended to be huge and heavy because of all the screening. They could be used to make very small signals and check sensitivity. Repair shops didn't bother with these. They were too expensive back in the day and they were mostly bothered with fixing gross faults. Nowadays these things are seen as 'boat anchors' and you can pick one up for not much money if you look around and can wait for one to turn up. They were more complex, and you'll find plenty of threads on here of people having to put some work into fixing them and getting them reliable.

So far, no FM!

FM showed up on some lab grade sig gens, and some of those didn't tune low enough in frequency to be able to service long and medium wave AM receivers. So you wound up needing two sig gens.

Philips made some more comprehensive generators aimed at more modern repair shops, with all the facilities you need to handle AM and FM broadcast receivers. Some Marconi models could as well.

An extra difficulty with FM sets is that the IF needs to be wider because of the width of the signal. The IF adjustments aren't simply peaked on a spot frequency as AM sets are. They employ what's referred to as 'stagger tuning' What you need to handle this is a sig gen which can be swept (a 'wobbulator' is the old name) and an oscilloscope (You landed lucky there!). This allows you to see the shape of the IF selectivity and you can adjust it to get the right width with a flat top (not a sharp peak). If you peak-tune an FM set, you can hear the difference, high notes sound distorted and speech sounds distorted or sibilant. On top of this, you also need to adjust the coils in the FM discriminator so that it gives a linear response when swept. Easy when you have a sweeper and a scope.

So, if you only want to do AM, you can pick from a great many basic sig gens, and there is a steady flow of them on auction sites, at radio rallies (when they get held again) and they crop up in the for sale section of this forum.

Sig gens with FM capability are less common. You have to look a bit to find one.

In my radio shack I have two sig gens. One is a Marconi model TF2008 which is a lab grade job about 50 years old. All-transistor, does FM and sweeps. It can align just about anything and has attenuators and screening good down to very low levels. I bought it in the late 1980s and paid 80 quid for it back then, in non-working condition, and fixed it. I was lucky, all that was wrong was the cord for the frequency scale pointer had come off a pulley and jammed. The other sig gen is a much later beast. A frequency synthesised one with AM and FM facilities by Hewlett Packard. This qualifies as rather expensive and a lot more than needed for fixing broadcast sets. Oddly, it's less comprehensive than the feature-packed Marconi.

The best fit for what you've asked for is probably one of the Philips models aimed for doing just what you've said, AM and FM receiver fixing in a better equipped workshop of the later sixties and seventies. Other people on here are more familiar with these... I worked in the labs at HP, so you can guess that almost all our test gear was our own brand and we were famously at the very expensive end of things.

David
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Old 17th Jun 2021, 6:37 am   #7
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Default Re: Signal generators

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Originally Posted by Radio Wrangler View Post

The other sig gen is a much later beast...This qualifies as rather expensive

I worked in the labs at HP, so you can guess that almost all our test gear was our own brand and we were famously at the very expensive end of things.

David
Yes, the word "expensive" seems to cropping up a lot; David probably has the most expensive set of "amateur radio" toys of anyone on this forum. Like me, he started off in amateur radio well over 50 years ago, but his professional background affects his perspectives; he likes to have the pretty much the same standard of kit at home has he had at work . This tends to be true to some degree of many/all of the professional radio engineers on the forum.

As for FM, judging by Spencer's comment that, "This is all new territory for me", I don't think he's going to be doing fine tuning of FM discriminators any time soon. I currently have two signal generators, but I don't have, nor ever had, a proper sig gen for FM. I suspect that the majority of forum members don't have one. Perhaps we should do a poll?

B
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Old 17th Jun 2021, 7:48 am   #8
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Default Re: Signal generators

Yes exactly! My situation is I have a few AM radios and one FM but I don't know if I will restore many FM radios as I currently only have the one, so it may be a waste! So it seemed like a better alternative might be to get something that would do LW and MW, maybe SW.

Sorry for posting the same twice! Hazzard of being up so late...

https://www.ebay.co.uk/itm/SIGNAL-GE....m46890.l49286

That's the other. It's clearly some kind of hand made so I'm dubious on its reliability but the seller actually offered it for 20 to me earlier. But unsure if I can find a circuit diagram...
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Old 17th Jun 2021, 9:01 am   #9
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Default Re: Signal generators

Most FM alignment in vintage receivers can be done using an ordinary AM signal generator, a multimeter and maybe two or three resistors.

Lawrence.
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Old 17th Jun 2021, 9:04 am   #10
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Default Re: Signal generators

I rather like the idea of the homebrewed one! It could be quite a good learning aid too.....

If nothing else it would provide some nice vintage knobs and switches and probably a useful airspaced variable capacitor which if sold separately would likely recoup its purchase price at 20!

A trawl of '50s '60s radio mags might even turn up its details in an article there

This: https://www.ebay.co.uk/itm/154495167261

might be worth a punt. Only just out of production!
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Old 17th Jun 2021, 9:15 am   #11
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Default Re: Signal generators

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Originally Posted by Bazz4CQJ View Post
David probably has the most expensive set of "amateur radio" toys of anyone on this forum.
I defer to Jeremy!

Spencervs is probably best off with a basic AM job, and then see later if he needs anything further.

Nowadays you can pick up some rather good hifi FM tuners for very little money. If the phantom twiddler has been inside them (they crop up on the forum from time to time) then a sweeper and scope really are needed to get them back to the performance they should be giving. Without that gear you're feeling around in the dark, you might be able, with care, to get a basic broadcast set quite listenable using a simple generator or even listening off-air.

But for sweeping, you don't need a full sig gen, you can lash one up as a breadboard on a scrap of raw copperclad PC board. All you want to do is scan across the usual 10.7MHz IF. It's not difficult, and very educational. The scope is the difficult part.

David
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Old 17th Jun 2021, 9:31 am   #12
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Default Re: Signal generators

A little Marconi TF2016 with an external 10 frequency counter from ebay would be a nice choice if you can find one. This little sig gen covers 10kHz to 120MHz and does AM and FM.

It has a proper step attenuator and a decent ALC system as well. The main letdown is the poor frequency scale but there is a rear panel connection to drive an external counter. The Marconi TF2016 seems to cost about 50-80 these days but they don't appear very often.

Yes, I'm probably a contender for having the most exotic test gear. However, in terms of quantity of test gear items I suspect there will be some collectors who have collected enough test gear to fill a house and some outbuildings!
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Old 17th Jun 2021, 12:36 pm   #13
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Default Re: Signal generators

I'll admit to been the tightest member of the forum; most I ever paid for a signal generator was 9 in 1973. Of the two I have at the moment, one was a gift and the other was 8. My 20MHz scope was also a gift.

However, for years, I was perfectly happy with my Antex fixed-temperature soldering iron but then I began to use a Weller temperature controlled one at work... and soon I had to get a temperature-controlled iron. On the very rare occasions when I've used the old Antex since then, I really don't like it!

I agree that the sig gen Spencer is looking at is right at the bottom line, but he could probably learn quite a bit by refurbishing it and adding the odd improvement. My guess is that loads of RF will be going down the mains lead! And as David pointed out, the RF output control will be 'minimalist'.

Still, think of all the sig gens published in PW in the 1960's; they were every bit as bad if not worse.

B
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Old 17th Jun 2021, 10:34 pm   #14
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Default Re: Signal generators

Hi!

I never bothered with an f.m. signal generator – I used to set ratio discriminators by tuning the primary core for best sensitivity/a.g.c. output at the limiter grid or ratio detector electrolytic stabilising capacitor, then tune off the secondary core until I got severe distortion, turn the secondary core back the other way through the distortion–free region until the severe distortion appeared again, note the number of turns between each onset of distortion, then set the secondary core approximately half–way between them!

For ordinary A.M. only sets, provided you didn't get a set with staggered i.f. A.M. tuning (there were a few made!) peaking the i.f.'s for maximum a.g.c. line volts on a reasonable strength local signal was usually adequate!

Chris Williams
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Old 17th Jun 2021, 10:57 pm   #15
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Default Re: Signal generators

WendyMott said:
Quote:
in my view a Marconi TF995 would do all of what you need, but it is "old tech".... Valve operated, a warm up time would be required for reasonable stability. However attaching a frequency counter to it would provide an accurate reading, and highlight the stability.
Wendy knows I've said this before - but my 995A/2M does everything I want it to do - the metalwork takes a long time to come up to temperature and a stock of 6AK5's would be handy too. I gave up using a frequency counter to check the frequency and found a cheap-ish digital transistor radio with HF and SSB (Degen 1103/Tecsun PL600) much quicker and close enough to get into the passband of my receivers.
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Old 17th Jun 2021, 11:39 pm   #16
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Default Re: Signal generators

Note that I don't think the TF995 covers the LW and MW bands. I think it starts at 1.5MHz? Are there different variants of this sig gen that go lower?
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Old 18th Jun 2021, 7:32 am   #17
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Default Re: Signal generators

I think wider coverage including lower frequencies and FM only came together in Marconi's next generation, Like the TF 2008.

Making FM was limiting in frequency coverage because of the difficulties of the 'reactance valve' technique and the need for wide deviation. The arrival of varactor diodes and of more complex generator structures opened up a lot more possibilities.

The Marconi never had the performance of the HP8640, but nowadays the 8640s are plagued with disintegrating plastic gears and switches. The Marconis lumber on.

The signal generator linked to in post 4 rings a bell. I think they were made in the fifties/sixties by a firm in Acton, using war surplus valves and components. They were a reasonable but very basic instrument.

David
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Old 18th Jun 2021, 5:27 pm   #18
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Default Re: Signal generators

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Originally Posted by Radio Wrangler View Post
The signal generator linked to in post 4 rings a bell. I think they were made in the fifties/sixties by a firm in Acton, using war surplus valves and components. They were a reasonable but very basic instrument.

David
I'm sure that I was given one of those by the old chap who ran the local repair shop, but sadly, have no recollection of what happened to it. In those days TV was on ~220MHz and the harmonics from the sig gen went down the mains and obliterated the TV signal; my mum and dad really did laugh, being sure that men in uniforms would be at the door within minutes . That issue could be sorted, and with TV on 700MHz digital, may be a non-issue now. An external attenuator could be added for better control of output.
Genuine vintage radio equipment, but yes, very basic!

B
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Old 18th Jun 2021, 6:21 pm   #19
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Default Re: Signal generators

Hi Baz, Test Equipment (Acton) Ltd

Cheap and cheerful gear that used war surplus (including what looked like a loudspeaker assy to "wobble" the RF on FM.
It did though work surprisingly well

Ed
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Old 18th Jun 2021, 9:01 pm   #20
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Default Re: Signal generators

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Cheap and cheerful gear that used war surplus (including what looked like a loudspeaker assy to "wobble" the RF on FM.
It did though work surprisingly well

Ed
Well, has it happens, I know someone who communicated with another guy (a line of sight contact) on 2m by shouting very loudly at an old signal generator . Not sure what mode of emission went in the log.

B
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