Odd Circuitry in ADVANCE sig gen

[Bazz4CQJ]

A couple of years ago, I picked up an old Advance signal generator with the model No. “DIP2”. At a glance, this looks really pretty close to the well known D1D model, but when I raised this on the forum, and searched Google, I could find no trace of the DIP2 model.

I’m finally getting to the stage where I have unravelled the circuit of this sig gen. Just like the D1D, there is a valve rectifier, two 6J5’s and one half of an ECC81 as RF oscillator. In the D1D, the 6J5’s provide a choice of sine or square wave modulation. In the DIP2, one gives sine wave modulation (using the same transformer circuitry to give AM as the D1D) and the other runs a 2MHz quartz crystal. So far, so good.

Where things get “strange” is that the RF output on the D1D comes out of the decade attenuator switch and goes straight to the outside world. But in the DIP2, the RF comes out of the decade switch and then goes through a CS2A microwave diode which is also fed with an audio voltage from the modulation transformer. The circuit of this ‘diode mixer’ is attached. Can anyone tell me what this diode circuit does for the DIP2, that wasn’t deemed necessary for the D1D? There’s no switchability here, either the AM mod is on or off. I wondered if it was a way of getting FM, but if so, it’s AM and FM at the same time. Can anyone help explain this?
Thanks

B

[Skywave]

An attempt at pulse-width modulation. For that, D1 has to be very fast - hence a microwave diode. Perhaps the 'P' in 'DIP2' refers to 'pulse'

Al.

[Bazz4CQJ]

Possibly When I bought it, it was working but I just checked it out quickly without looking carefully at the waveform. In order to assist figuring out the circuit, I've done a certain amount of disconnecting. I'll have to put it back together to do any more investigation. The fact that this really does seem to be the only DIP2 in existence seems to suggest that some client wanted something a bit unusual and not many were made. For my simple needs, it may be that the diode circuitry is best removed but I think that changes the output levels, so the SET point on the level meter will need re-cal?

B

[Skywave]

DIP2: photos as attached. Regretfully, don't have any other info.

Al.

[Bazz4CQJ]

Didn't you get those pics from me? I think that if I look at details like scratch marks, I'll confirm that. My Serial No is 114.

B

[Maarten]

The serial number seems to suggest that at least 14 but likely double that examples were made. Could be a small or very small series indeed.

[Bazz4CQJ]

Originally Posted by Maarten

The serial number seems to suggest that at least 14 but likely double that examples were made. Could be a small or very small series indeed.

The build-quality of the sections which are non-standard (compared with the D1D sections) looks very "hasty" and not like Advance's usual stuff. I know nothing at all about pulse-width modulation, and Al's suggestion is probably more plausible than anything I have thought of.

B

[ms660]

Is the diode the AM modulator?

The Avo CT378a uses diodes to modulate the RF but that's before the attenuator:

http://bama.edebris.com/manuals/avo/ct378a/

Lawrence.

[Skywave]

Originally Posted by Bazz4CQJ

Didn't you get those pics from me? I think that if I look at details like scratch marks, I'll confirm that. My Serial No is 114.

Ha! I might have done: I don't keep records of from where or from whom I collect stuff like that. I've always been interested in Advance test equipment and have a collection of documentation (and photos) of much - but not all - of their test equipment. So if anyone needs anything from that, just drop me a P.M. Can't guarantee to meet your need, but I can look and try.

Al. / Jan. 2018

[Skywave]

Originally Posted by Bazz4CQJ

I know nothing at all about pulse-width modulation, and Al's suggestion is probably more plausible than anything I have thought of.
B

Hmm: I appreciate your faith in my analysis, but, on second-thoughts, I'm slowly coming to the conclusion that my idea of 'pulse mod.' is a load of rubbish!

So, on second thoughts, looks more like a form of A.M.: D1 acting as a simple mixer / modulator for the two input signals: RF (from the decade attenuator) and L.F. (from the mod. xfmr.). Thus, O/P consists of sum and difference of those two sigs.: m*f1 ± n*f2. As far as the cct. - as drawn - goes, it is not helping cct. analysis, since it is not clear if certain lines cross (unjoined) or are joined: the junction of Cx, D1, the 1 kΩ R., etc., being an example.

What, at first, threw me onto the 'pulse mod.' notion was the fact that A.M., as such, is produced elsewhere (if I've read your comments correctly). So why repeat that process? A supplementary / additional form of A.M., perhaps to assist mod. depth at V.H.F.?

Idea 2: Is it possible to switch the R.F. off and leave the L.F. from the mod. xfmr. still running? If so, the idea might be to deliver the L.F. only to the 'R.F. O/P' socket. (Idea arises since the sig. from mod. xfmr. is after the attenuated R.F. - so if it is a form of A.M., at low levels of R.F., signal will be badly distorted - almost like a pulse-width - or pulse amplitude - modulated signal! I'm clearly going round in circles now!

I'll do a bit of digging around the 'Net & elsewhere - see if I can unearth anything relevant - but don't hold your breath!

Al.

[ms660]

From W.W July 1957....."The new Advance D1P/2 is a special version of the D1/D for the alignment of narrow band receivers in the range of 2 - 190Mc/s"

Pic...Mag page 322.....Text...Mag page 323:

http://www.americanradiohistory.com/...ld-1957-07.pdf

Lawrence.

[turretslug]

Before looking at the circuit, I'd wondered if the 2MHz crystal was being used to provide harmonics for calibration points and that these might have been mixed with the RF output in the diode to produce zero-beat checks- but it doesn't look that way from the circuit.

[Skywave]

I've dug down into my Advance Test Kit documentation archive and have discovered that the D1 signal generator was produced in several variations: D1/D; D1/DE; D1/DNA; D1P1; D1P2.
According to the Advance tech. manuals for all D1 models, they are capable of producing 30% A.M. and a 50:50 square wave.
So my first 'guess', viz. P.M., wasn't that far off the mark, FWIW.

Barrie: you say that the D1D R.F. O/P 'comes straight out'. That appears to be in contradiction to the above. Two possibilities spring to mind:
1. That particular D1D has been modified, or
2. The sq. wave mod. is achieved in a manner different to that of the DIP2. From what I have discovered about Advance in practice and the way they developed their kit (some would say 'evolved'), that seems much more likely to me.

Al.

[Bazz4CQJ]

Well, thanks very much indeed for the replies; how Laurence found that old article I'll never know.

Brief background; I'd owned a D1D for years - very decent instrument but the low end is 9MHz. I was at a rally and saw the D1P and bought it for a few quid, seeing that it went down to 2MHz -rather more useful.

The manual for the D1D is easy to find and it is totally conventional; AM applied via a small transformer on the HT to the RF oscillator.

I've never found a manual for the D1P and initially I thought that the only difference from the D1D was the inclusion of the 2MHz calibrator and the revised frequency coverage. Exactly the same mod transformer is present. However, when I started to use it I found that the output levels appeared to be wrong, so I started to take it apart and that's when I realised that it has that microwave diode stuck on the end of it. I too thought initially that it might be a way of getting 2MHz harmonics up to VHF.

Going back to the Wireless World article, the term "narrow-band receivers" is curious. Nothing on the front panel gives any clue as to whether or not it is an AM or FM unit. I will need to spend some time doing some re-assembly to get to a point where I can do any tests on it. Hours of winter fun !

B

[Skywave]

Originally Posted by Bazz4CQJ

Going back to the Wireless World article, the term "narrow-band receivers" is curious.

Well, not necessarily. Let's examine what has been said - and what we know.

1. Maarten suggests that only a small number of these sig. gens. were produced: I suspect he's correct.
2. You (Barrie) observe that this part of this sig. gen. "looks like a hasty build: not to the usual standard of Advance".
3. ms660 has shown an extract from W.W. which is dated July 1957.

That period of history was just after the end of the Korean War. Eddystone Radio produced the 770R during that time: I believe it was specifically designed with that conflict in mind. It particularly catered for NBFM*: freq. range 19 - 165 MHz. Military comms. commonly used VHF for communications and surveillance during that period of the Cold War (and for many subsequent years) and used various forms of pulse modulation, including radar, of course. So, to me, it seems a strong possibility that yes, only a few of these generators were produced and those under a government contract: details were probably classified. That, and the (probable) small number produced, (and those 'produced' in a rush), could go a long way to explain why tech. info. on this model is so scant - if available at all!

* It covered WBFM, A.M. and CW, too.

Further thought: we have D1/D; D1/DE; D1/DNA; D1P1; D1P2. The /suffixes could be cryptic - possibly 'D' for defence; DE: defence, extended; defence, naval attachment: all wild guesses though. But I note the 'structure': it changes for the DIP1 and DIP2: no / separator. Could the P in 'P1' and 'P2' denote 'prototype', I wonder?

Al.

[Radio1950]

Highly likely to be the AM modulator.
Not uncommon to use a diode for this purpose.
The modulator RF output is always "downward modulated", if you apply the terminology of olde days.
Microwave diode is used so the low diode capacitance does not unduly affect the carrier output amplitude linearity over the frequency range, whether modulated or not.
ATC VORs etc use PIN diode AM modulators at modest RF power levels.
Some VHF AM transmitters for ATC use this type of modulator before a 10 or 50 watt linear amp.
A PIN modulator acts as a variable resistor loading the source impedance of a previous stage, so as to give a varying output.
mike

[Skywave]

Originally Posted by Radio1950

Highly likely to be the AM modulator.

Which is what I thought at one point (viz. post #10). Then I recalled this extract from the OP:

Originally Posted by Bazz4CQJ

In the DIP2, one half [of a 6J5] gives sine-wave modulation - using the same transformer circuitry to give A.M. as [in] the D1D.

Doesn't that rule out this strange cct. for A.M., since otherwise A.M. would be occurring twice? Or is my idea (as in post 10): "A supplementary / additional form of A.M., perhaps to assist mod. depth at V.H.F.?" correct?

Al.

[Bazz4CQJ]

Interesting; the only way I will close some of these questions out will be to trace out the whole circuit and then do some tests when it is re-assembled.

Re the build quality, most of the chassis looks like it was pulled directly off the D1D assembly line (half complete) and then various changes hastily tacked on. May well have been a response to an urgent requirement (from the MoD) for something that would be "quick and adequate" for some need.

I'm glad I saved it; it was lying in a small trailer at the end of the afternoon at the Newbury rally looking very sorry for itself. Quite dusty and looked like it had been rained on at various previous rallies at which it had failed to sell for the asking price of £10 and the guy bit my hand off when I offered him £8. Most critically, I think the VFO tuning seems good. Of course, these Advance Dxx's are built like battleships; with the lid on, a Warsaw Pact tank could have run over the top of one and merely scratched the paint.

B

[Skywave]

Quote, Barrie4CQJ: These Advance Dxx's are built like battleships: with the lid on, a Warsaw Pact tank could have run over the top of one and merely scratched the paint.

Ha! In the light of my earlier comments, very apt!
So you picked it up (pun not intended) for £8: that was a good buy.
However, nothing quite like a good mystery to to keep one occupied during there dark, cold and dingy days, eh?
Keep at it: I'm waiting for whatever you unearth . . . .

Al.

[Bazz4CQJ]

Tracing out the circuit will be a bit demanding; all wires are in Henry Ford's black! However, I've just spent a few minutes Googling 'Advance' and they are still in business in Wrexham making power conditioners http://www.aelgroup.co.uk/products/history.php, so I've mailed them asking for a copy of the manual . Nothing ventured...

B