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Old 24th May 2020, 8:52 pm   #121
Radio Wrangler
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Default Re: HP8640B Signal Generator

+6.54dBm on the analyser?

DAvid
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Old 24th May 2020, 10:57 pm   #122
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Default Re: HP8640B Signal Generator

Possibly David... but Jeremy was definitely correct.
The range switch shaft doesn't have a flat so the knob can be set anywhere and inadvertently in the wrong place it seems. It may be that the owner had tuned to a frequency in the overlap area in the 2-4MHz range say 4.2Mhz and thinking it was the 4-8MHz range ended up with the knob one place right?

Re the 6.54dBm. I suspect the meter on the HP6840B can be set to exactly 0dBm regardless of the true output. I haven't calibrated it yet. There are 3, 4 or 5 pots that need setting up.
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Old 25th May 2020, 1:47 pm   #123
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Default Re: HP8640B Signal Generator

I tackled the setting up of the ALC loop and found everything worked as the book suggested except I was getting 3dB more output than that the documentation gave ie about +17dBm max instead of the recorded +13.2dBm fsd in the +10 setting.
The home brew detector test point reads about -2 volts so I'd guess this magnitude is too low forcing the output to go high by 3dB.
That extra 3dB RF output is constant from 200MHz down to about 12MHz, +2.5dB high at 400MHz and rises to +6.5dB at 5MHz, +8.8dB at 3MHz, +8.5dB at 1.5MHz and +6.7dB at 750KHz.
Harmonics are a bit high indicating the whole loop is running at too high an RF output.
Presumably I could either increase the detector output to deliver a greater feedback voltage to lessen the RF level by 3dB or alter the reference level to give the same results.
From the measurements, the detector output is dropping off below 5MHz resulting in too high an RF level so upping the 33pF coupling capacitor in the home brew detector to at least one order higher?
AM works OK insofar as setting it up on the meter. That has to be done prior to setting up the RF output versus meter readings.

On the other hand I could just add a 3dB pad in the output lead and pretend everything's fine.

I used a spectrum analyser rather than the power meter suggested in the set-up procedure. Because of the out of spec harmonics a power meter would have been a bit misleading because it would have summed the total power making the error seem even worse.

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Old 27th May 2020, 6:21 pm   #124
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Default Re: HP8640B Signal Generator

Hmm. I read that as the output hybrid being overdriven ... not a good idea being as how originals are borderline unobtainium. Or have I misunderstood?
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Old 28th May 2020, 11:02 am   #125
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Default Re: HP8640B Signal Generator

Latest results are pretty good Guy
I'm using a few chip sm parts in the homebrew detector and with the output set to 0dBm on the meter I see +0,8dBm on the spectrum analyser, give or take over the whole frequency range.

I guess when the detector output was too low (when the coupling capacitor was 33pF) the ALC loop was bumping up the RF amplifier to compensate.. hence too much RF output. The capacitor is now two 10nF chips in parallel. I used these when I found a small discrete 15nF worked OK'ish except for 500MHz.
A different detector diode (better than the 1N5711) might nudge up the output of the detector because the RF level is only 10dBm which is only circa 0.7 volts.
The detector output pot is set at max output so a little more voltage might help.

I'll be looking at the modulation aspects next to see how those function.
cheers
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Old 28th May 2020, 6:23 pm   #126
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Default Re: HP8640B Signal Generator

Good stuff!
Have the harmonic levels reduced?
Guy
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Old 28th May 2020, 6:52 pm   #127
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Default Re: HP8640B Signal Generator

Quote:
Originally Posted by allan View Post
(snipped)
I haven't confirmed all the modulation bits and bobs work yet, but I did check wbfm on a broadcast receiver in passing and it worked OK. In fact when I tuned off a local FM broadcast to check it, a voice in Spanish came up... must be the weather, or maybe global warming?
Allan
My post of that same day refers:

https://www.vintage-radio.net/forum/...d.php?t=166970

The 'sporadic E' season is upon us

Guy
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Old 28th May 2020, 10:49 pm   #128
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Default Re: HP8640B Signal Generator

The harmonic levels are lower Guy.
I ordered some new diodes from Farnell. It was going to be RS but my main order was double the price of Farnell so I ordered SMS7630-079LF.
I'm thinking I might try two in a doubler cct if one doesn't provide enough voltage. They have a rating of 0.3pF/0.7nH.
A couple of lift repairs tomorrow that have to take precedence.
Sporadic E or tropo. I remember the days of black & white Band I watching bullfights from Spain and even a signal from South Africa up in Liverpool.
Allan G3PIY
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Old 29th May 2020, 11:01 am   #129
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Default Re: HP8640B Signal Generator

(Sporadic E - ionisation layer reflection, generally May - October in UK; multi-hop distances can be several hundreds of miles & sometimes thousands ... Crystal Palace Band 1 sound has been received in Australia before now;

Tropospheric propagation generally associated with slow-moving / collapsing areas of high atmospheric pressure - temperature layer inversion can further enhance these effects)


Back on topic!


Sounds like you've got the '8640B eating out of your hand now What make/model analyser do you use there?


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Old 29th May 2020, 4:01 pm   #130
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Default Re: HP8640B Signal Generator

Hello Guy
I invested in a Rigol DSA815TG a few years ago. I regularly checked the price of the CRT SAs but delayed buying one because of VAT and duty, having been caught out by an earlier delivery of spares for a lift (I did get a refund).
The newer Rigol appeared on the scene so I looked for a UK supplier and found they had a UK distributor from whom I got a decent deal. I'm very pleased with it and you only live once.
Today I first checked the 8640B wth the 1N5711, then substituted a new SMS7630-079LF. Not easy as I couldn't really see it! I eventually managed to see the cathode band with difficulty. I kept dropping it but got it soldered OK, turned on and no meter reading. The Test Point read +3 volts so I unsoldered it.. rechecked the band and refitted it. Still no luck so repeated a third time and failed again. Maybe the thing is printed back-to-front so soldered it with the cathode band towards the amp input.. and it worked. The meter reads 0dBm and so does the signal, and it's within 0.5dB from 450KHz to 550MHz.

I checked up on the SMS7630-079F and of course it's marked with an anode band! Very strange. Size is 28x48 thou with legs 6 thou each.
RF box lid now screwed down in place......
Allan
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Old 31st May 2020, 9:09 am   #131
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Default Re: HP8640B Signal Generator

I should really finish off this thread.
My 8640B is now working pretty well. All the switches and things seem to do what their labels say and a test on 120MHz gives a nice clean CW signal.
Not sure about the level of harmonics but it does meet the >30dB down figure.
I took some pictures yesterday and a decent camera lens will be here on Tuesday to replace my manual focus example.
http://www.radiomuseum.co.uk/HP%20Test.html
I'd be interested to hear how these compare with proper non barn-find examples.
Picture here is 2nd harmonic with attenuator set to 0dBm.
I should also raise the possibility that some damaged RF hybrid modules may work as amplifiers with a blown-up detector (and therefore usable). It would depend on exactly how much RF had been fed into the output socket.
Allan
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Old 31st May 2020, 11:09 am   #132
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Default Re: HP8640B Signal Generator

If you're really testing a receiver, say a new design, one thing you have to do is to tune it to a chosen frequency and then sweep a strong input from DC to daylight and verufy that only the wanted response is strong, and that all spurious responses are suppressed within spec.

So your receiver will operate with signals down to the -100dBm and lower range. A big bad interferer might be -20dBm and you might have to get spurious responses to out of band signals down by 60 or 80dB. So you dial up -20dBm on the sig gen, get the 'phones on and slowly screw the frequency up. You'll find images, you'll find spurii due to synthesiser ref freq sidebands and all that sort of stuff. So you have to check that the strength of these responses is less than that to a -80 or -100 dBm signal. But the big ones you hit are tha harmonics of your sig gen, when the generator tunes across an integer submultiple of the frequency the receiver is sat on.

Oh rats! With only 30dB harmonic suppression in the sig gen the response you get is more like that from a -50dBm signal. 50dB more than you're listening for.

Now you can't just say 'Oh, it's only the sig gen' and go on, ignoring it. How do you know your receiver front end doesn't have a harmonic generation problem all of its own?

So you have to build a low pass filter to remove the harmonics down to a low enough level to prove the receiver. If the receiver tunes over much range, that filter expands to a group of them.

Yes, sig gen harmonics are very poor compared to what is sometimes needed.

Anothe failing of modern sig gens is broadband noise floor. The 8640, 2008 and all that dort of thing have cascaded wideband amplifiers and a wideband output match... resulting in a wideband noise contribution. This gets really nasty when you use it as an LO and the signal is wideband, or there are many signals you get a noise X noise mix dropping the whole lot into your receiver bandwidth. Nasty. THe matest sig gens with I/Q fncy modulation have to go to rather low levels for modulation, then lots of gain.... so lots more broadband noise.

THe old Marconi TF144H was fairly low on broadband noise, being a single valve power oscillator with attenuation. The HP606/608 had tuned buffer and output stages tuned with extra coils in the turret and extra sections on the tuning capacitors.. Their broadband noise was amongst the best.

So it isn't all just phase noise.

David
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Old 31st May 2020, 12:59 pm   #133
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Default Re: HP8640B Signal Generator

You're quite right, I can't disagree David.
My main use of a signal generator is simple alignment of commercial superhet receivers, complete with all their failings, so harmonic output doesn't usually worry me.
My Wavetek 2407 and Marconi 2008 have harmonics and usually these are invisible in practice. In fact I've never taken the trouble to measure them.
The HP is of course a loser from the start, generating square waves to produce its outputs. Although I suspect the even harmonics arise from square waves that are asymmetric. In the repair manual it does cover mods to its filters I guess to deal with component ageing but it would be an academic exercise to deal with this and time is quite valuable.
Nice to have would be a tunable band pass filter running say 50KHz to a gig with an out of band response 100dB down. Maybe chuck in an adjustable bandwidth as well.
Having said all this, the last receiver I looked at was a Marconi 730 a TRF design... so simple I found it incredibly boring. What I really like is a challenge, and the 8640B certainly supplied enough..
I can't immediately think of the next project unless it's a little box to extend the 8640B down to zero hertz... like the TF2008.
Allan G3PIY
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Old 4th Jun 2020, 1:06 pm   #134
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Default Re: HP8640B Signal Generator

Allan,

Before this thread is closed, I just want to compliment you on what you have recorded here in getting your 8640B back to full working order. I am amazed at the detail you have provided, both here and on your website. I wish I had the patience to do the same. If and when I have to open up my 8640Bs, it will be of huge assistance to me and any others who have to do so.

Many thanks

Ian
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Old 4th Jun 2020, 3:54 pm   #135
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Default Re: HP8640B Signal Generator

Thanks Ian.
I put the HP in the workshop ready for use but nothing seems to be inviting me to use it so I carried out an experiment.
I used my spectrum analyser tracking generator set to 100Hz span and 0dBm output and measured the maximum frequency the HP frequency counter could handle. It could just manage 715MHz feeding the tracking generator into the counter input.
I finally found my spare RF output transistor for my Wavetek 2407 which starting giving trouble (which is why I bought the HP) so I'll fit that and carry out some further tests. The Wavetek covers 10KHz to 550MHz and uses push buttons so doesn't have complicated gears and mechanical contrivances.
Whether the new transistor (SRFT3034) will fix it is anyone's guess. It's similar in shape to the original MRF839F, but it's possible the fault is a dodgy tantalum capacitor...
I might start another thread...
73 Allan G3PIY
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Old 23rd Nov 2020, 4:31 pm   #136
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Default Re: HP8640B Signal Generator

Quote:
Originally Posted by factory View Post
HP used UNC grub screws for the knobs, below are the commonly found sizes & appropriate Allen keys, the larger size is sometimes found on the older valve based test equipment.

#8-32, 5/64"
#6-32, 1/16"
#4-40, 0.05" (50 thou)

David
Hello again David and many thanks for answer. Than I have to find some allen wrenches and get started! By the way, I had the problem with the new replacement sorted out as I found the diagram! Now I am only waiting for ordered parts! LA9CHA Thorvald
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Old 23rd Nov 2020, 8:19 pm   #137
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Default Re: HP8640B Signal Generator

If you have a lot of US made equipment it would probably be wise to buy a few of each of the smaller sized Allen keys, as they can damage the grub screws if the hex is worn.

Rather frustratingly I've lost the bag of small sized Allen keys I have somewhere in the shed.

David
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Old 23rd Nov 2020, 10:06 pm   #138
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Default Re: HP8640B Signal Generator

Thorvald/David

I got mine from Model Fixings:
https://www.modelfixings.co.uk/tools.htm#allen

Best wishes
Guy
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Last edited by Nymrod121; 23rd Nov 2020 at 10:08 pm. Reason: clarification
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