Advance J1-A audio sig gen puzzle. - UK Vintage Radio Repair and Restoration Discussion Forum

avocollector · 22nd Jun 2019, 11:18 am

I recently won one of these audio sig gens at auction and when it got here it appeared in great shape. Downloaded the manual and it appeared nothing had been changed internally (in fact it still has what appear to be original wd valves with the broad arrow). So cautiously fired it up to reform caps etc. All seemed OK so put that valves back in and started her up for real.

It seemed to have very little output on band 1 (15-150hz) but there appeared a couple of volts on bands 2 and 3 (up to 50kc). I decided to checjk the obvious ie the valves and while I wait for others to get here (6X5, 6SN7 GT, 6J5) I did have a spare 6V6 GT so I popped that in and turned it on. Yay all seemed well with output on lowest band now. So I wired a 680 resistor across the 600 ohm output terminals and put a DMM across it to check the output voltage as against the figures marked on the output pot. At most I seem to get about 3v ac when it should be 5 or higher and this does NOT increase when you turn the output pot up. BUT when I hooked up my scope and turned the volts/div down to the lowest (5v) the voltage shows up as correct AND does increase as you turn up the pot by the correct amount.

So I checked the DMM again and it still showed low, used the freq measurement on the DMM and that showed correct to within about 10hz, then used the DMM to check mains ac voltage - that showed a correct 235v. All these measurements were made on autoranging. Even more peculiar is that the DMM shows this low value when the output is is also hooked up to the scope and showing the correct voltage by scope divisions.

So quite puzzled by this - either I'm forgetting something elementary, the low range ac on my DMM is up the wop or something else is happening.

Any ideas

orbanp1 · 22nd Jun 2019, 11:54 am

Do check the specs of your DMM in the AC mode!
Some of them works only to quite low frequencies!

Regards, Peter

govjohn · 23rd Jun 2019, 11:50 pm

Are you confusing peak reading on 'scope with RMS reading from meter?
John.

avocollector · 25th Jun 2019, 11:48 am

Quote:

Originally Posted by orbanp1

Do check the specs of your DMM in the AC mode!
Some of them works only to quite low frequencies!

Should be OK as I took measurements at 50 hz - same as the mains voltage which the DMM showed correctly.

Quote:

Originally Posted by govjohn

Are you confusing peak reading on 'scope with RMS reading from meter?

Suspect you might be right although what I'm doing on the scope is counting divisions between bottom and top of the sine wave being displayed - so when it's on the 5 volts per division and it shows 5 divisions between the trough and peak of a sinewave = 5 x5 =25 volts - this accords with the supposed voltage output into 600 ohms according to the markings. The DMM's are not RMS meters so should show ac peak surely

tried it again tonight and got the same result with a different DMM.

ms660 · 25th Jun 2019, 12:43 pm

Quote:

Originally Posted by avocollector

Suspect you might be right although what I'm doing on the scope is counting divisions between bottom and top of the sine wave being displayed - so when it's on the 5 volts per division and it shows 5 divisions between the trough and peak of a sinewave = 5 x5 =25 volts - this accords with the supposed voltage output into 600 ohms according to the markings. The DMM's are not RMS meters so should show ac peak surely

tried it again tonight and got the same result with a different DMM.

25 volts RMS should display as approx. 70 volts peak to peak on the 'scope taking the 'scope probe and cal setting into consideration.

If the meter isn't a true RMS meter it will be of the type that does a fudge to display the RMS voltage based on a pure sine wave input.

Lawrence.

Chris55000 · 25th Jun 2019, 9:10 pm

Hi!

If you're using the DMM on a low a.c. range (10V or 25V) it often presents a much lower impedance across it's input terminals than you might expect, particularly if it's a cheaper one, and a surprisingly large percentage of your generator's output is dropped across it's internal source impedance with the dmm connected, and there is often very noticeable non–linearity of the meter's rectifier/a.c. converter at low a.c. ranges causing the displayed reading to be much lower than expected when the source impedance is non–trivial – most inexpensive dmms are only specified as accurate with power frequency sine wave a.c
from a low impedance source, whereas an oscilloscope is designed to be a constant 1M nominal over it's specified vertical bandwidth with a nominal (uncompensated without a ×10 probe) parallel capacitance of 22-40p, and the 'scope's amplifier is designed to be linear over the full range of input (10mV to 50/100V pk–pk depending on it's max V/div coefficent) signal applied to it!

Chris Williams

avocollector · 27th Jun 2019, 8:16 pm

Thanks for all the thoughts folks - one thing I'd assumed is that the voltage markings on the sig gen were what you would expect to see on say an analogue avo of the period (50's) - however if they are peak to peak it could explain some of it.

ms660 · 29th Jun 2019, 11:38 am

The voltage output given in the sig gen's specs is not the peak to peak voltage, it's the RMS voltage.

The 'scope shows peak to peak, to find RMS from peak to peak multiply the peak to peak voltage by 0.3535.

Lawrence.

Sinewave · 30th Jun 2019, 7:00 pm

The Dmm could well be 'averaging' if it's not rms.

avocollector · 10th Jul 2019, 6:12 pm

I replaced the entire valve set a day or so ago with either NOS or modern Russian manufacture - and all seems to have worked well with figures measured by scope and DMM making good sense after 5 min warmup. Thanks to everybody all the suggestions/ideas. My thought is the output was so weak that it would collapse under the DMM loading, but not the scope although articles told me they should present a similar loading of about 10meg on the circuit.

One thing that does puzzle me about the instrument is that there does not appear to be any ventilation slots or holes in the casing. You would think with 4 octal base 6v valves in there it would get quite warm if run for any length of time and some ventilation would be needed.

22nd Jun 2019, 11:18 am	#1
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Advance J1-A audio sig gen puzzle. I recently won one of these audio sig gens at auction and when it got here it appeared in great shape. Downloaded the manual and it appeared nothing had been changed internally (in fact it still has what appear to be original wd valves with the broad arrow). So cautiously fired it up to reform caps etc. All seemed OK so put that valves back in and started her up for real. It seemed to have very little output on band 1 (15-150hz) but there appeared a couple of volts on bands 2 and 3 (up to 50kc). I decided to checjk the obvious ie the valves and while I wait for others to get here (6X5, 6SN7 GT, 6J5) I did have a spare 6V6 GT so I popped that in and turned it on. Yay all seemed well with output on lowest band now. So I wired a 680 resistor across the 600 ohm output terminals and put a DMM across it to check the output voltage as against the figures marked on the output pot. At most I seem to get about 3v ac when it should be 5 or higher and this does NOT increase when you turn the output pot up. BUT when I hooked up my scope and turned the volts/div down to the lowest (5v) the voltage shows up as correct AND does increase as you turn up the pot by the correct amount. So I checked the DMM again and it still showed low, used the freq measurement on the DMM and that showed correct to within about 10hz, then used the DMM to check mains ac voltage - that showed a correct 235v. All these measurements were made on autoranging. Even more peculiar is that the DMM shows this low value when the output is is also hooked up to the scope and showing the correct voltage by scope divisions. So quite puzzled by this - either I'm forgetting something elementary, the low range ac on my DMM is up the wop or something else is happening. Any ideas

22nd Jun 2019, 11:54 am	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Advance J1-A audio sig gen puzzle. Do check the specs of your DMM in the AC mode! Some of them works only to quite low frequencies! Regards, Peter

23rd Jun 2019, 11:50 pm	#3
govjohn Pentode Join Date: May 2011 Location: Cardigan, Ceredigion, UK. Posts: 100	Re: Advance J1-A audio sig gen puzzle. Are you confusing peak reading on 'scope with RMS reading from meter? John.

25th Jun 2019, 9:10 pm	#6
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,868	Re: Advance J1-A audio sig gen puzzle. Hi! If you're using the DMM on a low a.c. range (10V or 25V) it often presents a much lower impedance across it's input terminals than you might expect, particularly if it's a cheaper one, and a surprisingly large percentage of your generator's output is dropped across it's internal source impedance with the dmm connected, and there is often very noticeable non–linearity of the meter's rectifier/a.c. converter at low a.c. ranges causing the displayed reading to be much lower than expected when the source impedance is non–trivial – most inexpensive dmms are only specified as accurate with power frequency sine wave a.c from a low impedance source, whereas an oscilloscope is designed to be a constant 1M nominal over it's specified vertical bandwidth with a nominal (uncompensated without a ×10 probe) parallel capacitance of 22-40p, and the 'scope's amplifier is designed to be linear over the full range of input (10mV to 50/100V pk–pk depending on it's max V/div coefficent) signal applied to it! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 25th Jun 2019 at 9:18 pm.

27th Jun 2019, 8:16 pm	#7
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Re: Advance J1-A audio sig gen puzzle. Thanks for all the thoughts folks - one thing I'd assumed is that the voltage markings on the sig gen were what you would expect to see on say an analogue avo of the period (50's) - however if they are peak to peak it could explain some of it.

29th Jun 2019, 11:38 am	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Advance J1-A audio sig gen puzzle. The voltage output given in the sig gen's specs is not the peak to peak voltage, it's the RMS voltage. The 'scope shows peak to peak, to find RMS from peak to peak multiply the peak to peak voltage by 0.3535. Lawrence.

30th Jun 2019, 7:00 pm	#9
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	Re: Advance J1-A audio sig gen puzzle. The Dmm could well be 'averaging' if it's not rms. __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

10th Jul 2019, 6:12 pm	#10
avocollector Heptode Join Date: Jul 2010 Location: Wellington, New Zealand. Posts: 653	Re: Advance J1-A audio sig gen puzzle. I replaced the entire valve set a day or so ago with either NOS or modern Russian manufacture - and all seems to have worked well with figures measured by scope and DMM making good sense after 5 min warmup. Thanks to everybody all the suggestions/ideas. My thought is the output was so weak that it would collapse under the DMM loading, but not the scope although articles told me they should present a similar loading of about 10meg on the circuit. One thing that does puzzle me about the instrument is that there does not appear to be any ventilation slots or holes in the casing. You would think with 4 octal base 6v valves in there it would get quite warm if run for any length of time and some ventilation would be needed.