Advance LF Generator Type SG66

[XTC]

The point about Wien Bridge and I think twin T oscillators is that the loop gain has to be set to exactly 3. You can't do that accurately enough by adjusting resistor values. Less than 3 and the circuit won't oscillate, greater than 3 and you get clipping and distortion. The lamp or thernistor has a very non-linear voltage/current curve and thermal inertia.

As David says, it's a subtle scheme. Before Bill Hewlett's patent, there were oscillators to create pure sine waves but they relied in heterodyning two RF signals. Big, clumsy, expensive and not so good. Audio inductors are another way to go, with the same problems.

There are other ways of stablising Wien Bridge oscillators such as using a FET or a LED/CdS combination which can produce better results and eliminate the bounce. The humble lamp can give 0.01% THD or less over most of the useful range and avoids a lot of complexity.

The general principle of setting up a Wien Bridge with lamp or thermistor stabilisation is that you are balancing eliminating the bounce against distortion. You want the sig gen to oscillate reliably on all ranges when switching ranges. Some people find the bounce very annoying.

Distortion meters can be found on the junk circuit for £20 or less. The Marconi TF2331 is not too big and is a good reliable piece of kit. For some reason, Distortion meters are more common than ultra low distortion AF sig gens.

It really depends what you want the oscillator for. By the sound of things the SG66 is not really up to hi-fi testing because it doesn't produce a pure enough sine wave.

Pete.

[davidgem1406]

I picked it up for just audio amp testing as it has sine and square waves in one box. I wanted the square wave for something in particular at the time but now cant remember what that was.. But then I only paid £0.99p for it..
I'm also working on an SG 63F, the meter was o/c. That's under a parts wanted thread.

At the moment it seems to oscillate reliably on all frequencies and without clipping.
I will know better when I have put the oscillator panel back in place and added the screening.

I could put a request for a distortion meter in parts wanted and see what happens, but is it really worth that sort of attention?

I just remembered what I wanted the square wave for scope calibration.

Dave.

[XTC]

For £0.99p and so many pleasurable hours spent messing with it, and I'd say it was a bargain.

You could even do the unthinkable and use the case, variable cap and tuning network as the basis for a solid state sig gen. There were a few Linsley-Hood designs in the mags, particularly EE, which used easily available parts and produced vanishingly low distortion. Some used LED/CdS combinations rather than the now very hard to find R53 thermistor.

One of my sig gens is Philips PM5110. It produces less than 0.01% THD over most of the range as checked with an HP8903 and has a square wave output and a dedicated TTL square wave output. It's about 3"x6"x8" a nice neat little unit. Just under £30 off ebay a few years back. Ebay is always worth thinking about.

As for the distortion meter, it depends. I've enjoyed playing with distortion meters.

Is the square wave good enough for scope calibration?

A lot of it comes down to what you want. Is it something to do a job or is it something you fancy messing about with?

Pete.

[davidgem1406]

Just found a Marconi TF2331 on Ebay at!! wait for it!! £375.00 + delivery.
I always look there for things like this but everything seems to be rather a rip off these days.
I now need an RF voltmeter to set up the SG63F, aint that just like it.

If I was going to build one I would not be using the SG66 case, it's about 4 times to big I would think.

I did buy it to be able to make use of it but will just have to see what the end result is. It was a bargain at that price so at the end of the day not a loss if it turns out to be not so good.
The SG 63F was also a bargain at £26 and that is absolutely pristine, it even arrived in its original box and half the original packaging, shame about the meter though.

I'm not sure as yet what the square wave is like as been dealing with the oscillator so far.

I will get it all back together in the next few days, have a better idea of things then.

Dave

[XTC]

Look at the radio rallies, auctions and swapmeets as well as ebay.

I paid a fiver for mine, which was the asking price. I've seen a few more for from £2 to £20, as well as Dymar and others for about the same.

Stewart of Reading are selling them for £40 and £95 + carriage +VAT.

http://www.stewart-of-reading.co.uk/Sale.htm

Pete.

[davidgem1406]

That's a good suggestion Pete but I don't know of any radio rallies or swapmeets that take place anywhere near me.
As for auctions the nearest to me I believe is Maidstone but I don't think they deal with that type of equipment.

I had a look at the Stewart list it gives no idea as to what that equipment is like, just a price, always makes me a bit apprehensive.

Dave

[ppppenguin]

Originally Posted by davidgem1406

I had a look at the Stewart list it gives no idea as to what that equipment is like, just a price, always makes me a bit apprehensive.

Stewarts are a decent and reputable company who wouldn't deliberately be selling bad kit. Phone and ask.

[XTC]

http://www.southgatearc.org/rallies/index.htm

TBH Sheerness looks like a bit of a rally blackspot, but there are ones at Ipswich and Brighton and to the West of London.

As for auctions, I was thinking of the Radiophile and BVWS ones in particular.

That is Stewart's clearance list. They'll have powered up the items up and checked them. As Jeffrey says, Stwart's are a well-established and reputable company not known for selling rubbish. A lot of people on the forum have dealt with them, including me, and found them perfectly satisfactory.

The only reason I mentioned them was to give a reference point, bearing in mind that they are dealers, have overheads and have a living to make.

Pete.

[Radio Wrangler]

Sheerness isn't a blackspot! it all depends on your viewpoint.

Radio rallies are dying out North of the Border. The only rally I get to with much interesting stuff is an annual trip down to the G-QRP doo (Was Rochdale, now Rishworth) but that's a 500+ mile round trip. Several people come up from Dover, Folkestone, and Maidstone for that event.

Over the years I've picked up some amount of stuff there inc a Tek 7000 frame and plug-ins for £20, a set of Marconi Q meters...


THe HP8903 mentioned a few posts ago is rather a nice low distortion audio source and distortion/SINAD/level meter. All remote-controllable. You find thee mostly in racks full of radio test systems.

Most of the AF generators and distortion meters we're talking about don't equal the performance of a better grade computer sound card. The sound card lacks the nice wide tange output level and sensitivity switching.

HP lost the professional audio market to a firm called Data Precision who did it with what was essentially a super-duper soundcard with associated software.

All that said, simple dedicated instruments are much handier on the bench.

David

[davidgem1406]

Thanks for all the information, I will just have to keep an eye open for the items I need as traveling 100's of miles to rallies and auctions is not a proposition at the moment. The pension wont stretch that far the way things are at present.

Even Brighton and Ipswich are a fair run to make, Brighton at 70 miles and Ipswich at 100 miles each way.

Something will turn up somewhere! we hope.

Sheerness is rather a black spot there is virtually nothing there and nothing happening there. If you want paint and wallpaper you pretty much need to go of the island. But that's all another story!

I will just carry on and do what I can to have this generator working as best I can for now.

Any further information is much appreciated.

Dave

[XTC]

It depends what you are trying to achieve.

If you want a sine wave which looks OK on a scope, just do that.

If you want to make sure this sig gen is performing to within spec, Radio Wrangler David's suggestion of software and a sound card is a good one. I must admit that I haven't looked into such software. I'm sure I've seen it sold for a tenner. It may even be available as open source these days. Google is your friend.

If you want a distortion analyser, keep an eye on ebay. The prices vary enormously. You can often see several identical items from the same seller sell for very different prices at about the same time. If an item is sold as faulty the price is depressed, and if you find an item for local collection only, which happens to be within driving distance, it limits the number of buyers and depresses the price, together with which you eliminate delivery and damage in transit.

There's also the For Sale and Wanted sections on the forum.

I find the rallies are a pleasant day out and usually worth a drive. You can't go expecting to find a particular item, but they are generally interesting and you often come away with things such as connection wire and valves, or some piece of gear you didn't know you wanted til you saw it.

Pete.

[davidgem1406]

Hi Pete

I keep a constant lookout on eBay for things, however I have found that over the last 12 to 18 months people are wanting the earth for vintage stuff, they seem to assume vintage means rare.
In this case (Distortion meter) non working is not the way to go as who knows what other equipment may be needed to get that working.

I understand what you say about rallies and such like and I'm sure they are an excellent day out in their own right.

For now I will just set it up with a scope but it would be good to have it within spec. I may have found a distortion meter from my thread in the wanted section.

Dave

[Radio Wrangler]

A distortion meter is simply an audio millivoltmeter and a notch filter.

Most usually the notch filter is another wien bridge circuit. However the HP8903 just mentioned is a higher-order "state-variable" active filter to give a sharper notch along with electronic tuning.

To fix/setup a distortion meter, first sort out the audio millivoltmeter. Then you can either use a fixed freq audio source and tune the distortion meter notch filter across it, or else leave the filter fixed and tune a variable freq audio source across it. You can plot the notch shape from the tuning scale and the audio millivoltmeter readings. The interesting parameter is how deep the notch is.

The distortion meter simply notches out the fundamental and measures everything else. The big big flaw in most distortion meters is that they do not make a true-RMS measurement of the distortion residue. Because of its uncertain wave-shape classical peak to RMS fixed calibration factors don't work properly.

There's a lot of info on how test gear works in the Hewlett-Packard Journal (Including one bit I wrote...) and it's all on the web and all free. Google HPJournal and HPlabs (whose servers host it)

Cheers
David

[davidgem1406]

Hi David,

Thanks for that info, bit over my head at the moment but I will take a look and read up on it.
I have a Marconi TM2331A coming my way thanks to a member of the forum.

Not sure what you are saying about Audio Millivoltmeter, unless its part of the distortion meter? I'm looking for an RF Millivoltmeter. 2 different jobs going on.

Dave

[XTC]

Distortion meters contain an audio millivoltmeter and you can usually use a distortion meter as an audio millivoltmeter. As David says, it should ideally be a true RMS meter, rather than calibrated to read RMS assuming a sine input. however, when instruments such as the TF 2331 amd HP 333 were made, true RMS meters were expensive instruments in their own right.

The TF2331 manual discusses the difference between an averaging meter and a true RMS meter.

A few years back I had some good buys with instruments on ebay. Prices were very variable. To be honest I haven't looked for them there in the past couple of years. It can be good for components, washing machine spares, perspex offcuts and things like that.

Pete.

[davidgem1406]

Hello All.

Here is where I am with this generator now.
I have reassembled it and wired in the oscillator module sensibly now and it is all working.
I have set up RV6 as stated in the manual but not having an RMS meter I used the P to P having converted it from the RMS figure of 1.8V, I then adjusted RV6 to give 5.09V P to P across R26.

After I made a check on the frequency settings for all 5 bands setting the top and bottom ends as near as possible (not to easy as the adjustments are anything but easy and fine to move). However I could not set the top most band neither high or low ends. If I adjusted the low end that moved the high end way of and vise versa. Also I can not get to the maximum frequency of 150KHz as per the scale markings. The oscillator will run well beyond the 150KHz by adjusting RV6, but that then changes the setup for the thermistor.

I have been most of the day trying one way and another to rectify this problem by adding and subtracting resistors and capacitors, even fitting pots that I can adjust instead of constantly soldering resistors in and out, all to no avail. Whatever I do to correct one end throws the other to far out to adjust back in.

Here is a table showing the settings I have obtained.

LOW HIGH
Band 1 5Hz to 50Hz -- 5Hz 50Hz
Band 2 50Hz to 500Hz -- 49Hz 501Hz
Band 3 500Hz to 5KHz -- 496Hz 5008Hz
Band 4 5KHz to 50KHz -- 5.009Hz 48.997KHz
Band 5 50KHz to 125KHz - Not set

Although the top frequency is stated as 125KHz the scale goes to 150KHz so we need the 150KHz to put the 125KHz in the correct place on the scale.

That's enough for one day I think

Dave

[Radio Wrangler]

Hi Dave,

You may be suffering from digitalcounteritis. That unit will never have been calibrated to that sort of accuracy, and strays will make the highest band the worst of all.

You can see the difficulty building up at the top end of band 4, thereafter it only gets worse.

The HP 651A is a wien bridge generator which goes to 10MHz, but it seems to have taken a lot of doing. An extra gang on the capacitor is used for phase compensation across the tuning range and there are more trimmer capacitors than you want to think about.

Band 5 is probably a bit of a optimism in that unit

Cheers
David

[davidgem1406]

Hi David,

I'm not sure about your comments on the accuracy.
The manual states that to do the frequency calibration the output should be set against a frequency standard having an accuracy of better than 0.1%.

As for the top band that is just to far out, at about 15KHz to be usable. The top end frequency (150KHz) can be obtained quite easily but by adjusting RV6.

I am now in two minds as to take the bull by the horns and rebuild the oscillator to the circuit in the manual, but I cant convince myself that doing so will change or better the situation. That would also mean finding some additional place for some components that at present don't exist.

At some time in the past somebody had added additional capacitors across the top end trimmer capacitors for each band with the exception of band 1. I have removed these additions.
These additional capacitors had the effect of reducing the top end frequency for each band. That tends to make me think that the basic frequency of the oscillator was set much higher, maybe by RV6.

Thanks
Dave

[davidgem1406]

I have now rewired the oscillator panel to the circuit in the manual.
The results are to say the least not good.

The frequencies were a mile and a half out even reaching 1.2MHz and that was band 1 the lowest. The other bands ranged from 300 to 500MHz.

My first thoughts were that I made an error in the rewire of the panel as I had checked all the switch ranges with a meter and they came out correct, but had little influence on the frequency.

I removed the panel again as once it is wired in it is impossible to work on it as the wiring wont allow it to be turned over. I fully checked it but found nothing wrong with the circuitry. I had used a wrong value cathode decoupling capacitor but reading the manual that would only have effect on the top frequency. I replaced it with a correct value anyhow.

After replacing all the wires so that they were longer I refitted the panel and checked again. Adjusting RV6 would make an improvement for say the low bands but that lost the top bands and visa versa. I could find a spot where I had all bands but the wave form was horrendous.

I attempted to set up the RMS voltage across R26 and destroyed the thermistor via the earth line of the scope. This discharged a large value capacitor through the thermistor, RIP!

I tried replacing the thermistor with the only one I had (reads 10Meg), followed by various values of resistor, they worked but basically gave the same results, except the frequency had reduced and on some bands was close to what they should be. I now wonder if the thermistor was at fault right from the start causing the original problems pages back??
The thermistor read 90K to 100K at ambient temperature, if held the value would increase.

Now the problem is to find another thermistor or a replacement. I am given to understand by an article I read that the R53 is unobtainable.
http://sound.westhost.com/articles/sinewave.htm

I could try one of these if there are any left:
You are bidding on 1x 100 KOhms B3952 (Axial) Thermistor.

AL03006-58.2K-97-G1
Package: DO-35
Size: 2mm diameter x 4.2mm (29mm with leads).
Glass bead NTC thermistor.
Ideal for RepRap style hot ends.

Specifications at

http://www.thermometrics.com/assets/images/ge1.pdf

I suppose time will tell what the next move is!!

Dave

[XTC]

Bad luck.

RA53 thermistors went out of production at least ten years ago and are very difficult find. They were quite special in that they were a tiny bead on two very fine wires in an evacuated bulb.
I don't think you'll have success with the device you suggest.

Search on the forum for R53 and RA53 thermistor.

I have seen threads where people have turned up similar devices used for amplitude limitation in telephone exchanges, which they found in their junkbox. Maybe one of those would do?

The other option is to rejig the circuit slightly to use a lamp. I believe the thermistor gives slighty better performance. The lamp has to be chosen quite carefully.

This is one site explaining the difference in the two techniques.

http://www.onegentleman.biz/Hardware...cillators.html

Pete.