AF Frequency Meter from Test Equipment Construction. R A Penfold

[Jimbo]

Moderators, if this posting belongs more correctly in the earlier string about ‘Magazine Projects’ then please relocate it.

I have collected many of the books published by Bernard Babani from the prolific writer R A Penfold over the years, and have had some success with building projects from ‘Simple Power Supplies’ and ‘IC 555 Projects’. However, the AF Frequency Meter from ‘Test Equipment Construction’, [BP248], has so far defeated me.

I successfully built the companion AF Generator from the same book, [although there were a number of discrepancies between the circuit diagram, the Veroboard layout and the wiring diagrams], but despite many hours of testing and checking my efforts, I cannot get the AF Frequency Meter to perform.

The difference between a project published in a Magazine and that in a Book is that any errors or refinements in a magazine article can be picked up and corrected with a ‘Mea culpa’ or ‘Points arising’ in the next month’s edition, but with a book [especially one of specialist interest/readership], the chances of a revised edition are limited, [BP248 has been reprinted, -the version on worldradiohistory.com is later dated than mine, but the text is unaltered].

I [foolishly] made the case to house both the Generator and the Frequency Meter before I had completed the electronics bit, intending to have a combined test unit, so I really would like to get it working if possible!

I realise any discussion on this will be of interest only to those who have built the same project, [successfully or otherwise!], but I would be grateful to hear from anyone with experience of it, [especially if there is a publisher’s error that I cannot find], and happy to communicate by PM if that is better.

Thank you
Jimbo M0GJD

[TonyDuell]

I've looked at the circuit diagram (not checked the stripboard layout) and I think it should work. Some bits I don't like, but..


Start with some simple voltage checks :

Are you getting 5V at the output of the regulator?
Are you getting 5V at pin 7 of IC1 and IC2, pins 4 and 8 of IC3?
Is pin 4 of IC1 and IC2 0V? Ditto pin 1 of IC3

What voltage do you measure on pin 3 of IC1? Now pin 2 and pin 6 of that IC.

What voltage do you measure on pin 3 of IC2? And pin 2 of that IC?

Now feed in a signal. Do you have a 'scope? If not, try a crystal earpiece with one lead grounded and the other as the probe. Probe pin 6 of IC1, the signal should be there. Ditto pin 2 of IC2. You should be seeing a square wave version of the input signal at pin 6 of IC2 I think. Check that is getting to pin 2 of IC3. What voltage do you measure on pin 3 -- the output -- of IC3?

[Jimbo]

Good Afternoon Tony,

Thank you for your helpful reply to my enquiry.

I have carried out the voltage and simple signal input checks that you suggested:


Are you getting 5V at the output of the regulator? 5.05V to all points on the positive rail.
Are you getting 5V at pin 7 of IC1 and IC2, pins 4 and 8 of IC3? Yes
Is pin 4 of IC1 and IC2 0V? Ditto pin 1 of IC3 All read zero volts
What voltage do you measure on pin 3 of IC1? 2.40V
Now pin 2 and pin 6 of that IC. Both 2.56V
What voltage do you measure on pin 3 of IC2? 2.55V
And pin 2 of that IC? 2.69V

Now feed in a signal. Do you have a 'scope? I used a 1KHz signal from the AF generator with an amplitude of 50mV peak to peak on the 'scope.
Probe pin 6 of IC1, the signal should be there. The 50mV signal is present.
Ditto pin 2 of IC2. At pin 2 of IC2 the signal is greatly amplified to just under 3V peak to peak, [Mr Penfold's text says that the circuit around TR1 will give a 40dB gain and my figures suggest about 36dB so not far adrift].
You should be seeing a square wave version of the input signal at pin 6 of IC2 I think. Yes, I do but the amplitude is down to about 50mV again.
Check that is getting to pin 2 of IC3. No, it does not. There is some noise around the horizontal trace on the 'scope but not enough to be measurable.
What voltage do you measure on pin 3 -- the output -- of IC3? No reading on the normal 20V range of my meter, perhaps a few millivolts if I go down to the 200mV range, there is nothing for the 'scope to measure either.

I checked all individual components before soldering them into position, and all of the 'DC' conditions appear to be as I would have expected, and I have tried substituting a replacement for IC3, the CMOS version of a 555 timer, as I would have expected the output to be a series of DC pulses that charge up the capacitor around the meter movement part of the circuit.

I would be grateful for any other thoughts you may have.

Jimbo M0GJD

[TonyDuell]

You have noticed that in the stripboard layout in said book that IC3 'points the other way' to IC1 and IC2? This confused me for a short time.

The thing that seems very wrong to me is the output of IC2. This op-amp has positive feedback applied (R7) and the output should be hitting the supply rails. So either IC2 is faulty, or there's an error in the wiring of that stage, or something is pulling the output down. Try disconnecting C3 and look at pin 6 of IC2 again. Make sure pin 6 is not shorted to something else, for example by an incomplete track cut.

[buggies]

I added some tracks in Paint to make it easier.
Should be able to test IC2 without input signal by shunting input resistors with dvm on output pin.
ICs in sockets? Never used FET op-amps so don't know if they are static-sensitive...

[Jimbo]

Thank you Tony and George for your further thoughts, -the track overlay is particularly helpful as it is awkward to keep turning the board over and over with test leads attached!

I had picked up the reversed orientation of IC3, and all of the ICs are in sockets.

I agree that there is something strange going on at the output of IC2. Should the square wave that I see at pin 6 of IC2 be swinging up to supply rail level?

I thought that I took the necessary precautions when handling the static sensitive ICs, but I will see if I have a spare to do a substitution if all other investigations fail.

I am not able to do much on the job today, but I will pick up our correspondence as soon as I can. Thank you again for your help.

Jimbo M0GJD

[TonyDuell]

I would expect the output of IC2 to be swinging pretty close to the supply and ground rails.

[Jimbo]

Would you believe it, -a dud CA3140 operational amplifier. All the more annoying as it had been tested with an op-amp checker, [from another Babani book, this time by E A Parr], and appeared to be OK!

Removing the capacitor link to the 555 timer IC still showed only a tiny signal on the output of the CA3140, but as soon as the op-amp IC was substituted, [without testing it this time in case it was the tester that killed the original!], I had the full rail to rail swing on the square wave output, and with the coupling capacitor back in place, deflections on the meter.

The circuit still needs some 'taming' as there is noise on the input of IC2 even with no input signal applied. [The output pin 6 of IC1 is 'quiet' and so is the base of the transistor but there is a spiky waveform present on the collector and the input of IC2 which then drives the op-amp mad and confuses the timer as well]. I am not sure if it is a question of it needing better supply rail decoupling, better screening of some of the internal wiring or my misguided choice of putting the AF Generator and the Frequency meter in the same box.

However, that is for another day, and at least I have a working project to refine and improve.

I can only thank you again for encouraging me with the analytical approach to fault finding.

Jimbo M0GJD

[buggies]