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Sunday, January 31, 2021

Repairing My Maplin Audio Waveform Generator

I picked this generator at the Kempton Park rally in London many years ago.  I use it quite a bit, not only for circuit testing but also as an easy way to get my homebrew BITX rigs to send a signal so that I can adjust my antenna.  I just plug this thing into the mic jack, crank in a small amount of 1 kHz audio, and I am ready to minimize SWR. 

It never gave me any real problems until last week.  I opened it up and examined the circuitry for the first time.  Lots of mystery chips in there. Fortunately they are all socketed.  Thank you Maplin. 

Even without really knowing how each of the ICs work, it was easy to troubleshoot. See the schematic in the article that begins on page 21 in this .pdf: 

My Rigol scope showed a good signal going into and coming out of IC8, the LF351 op-amp.  From there the signal goes to IC5 a 4066BE.  But nothing was coming out of IC5.  Thus IC5 was my lead suspect.  I put in a quick order to Digikey.  A couple of days and a few bucks later the chips (as always, I ordered several) were on my doorstep.  Within a few minutes IC5 was replaced and the generator was as good as new. 

One annoying problem with this generator is that it has no automatic shut-off circuitry.  So if I forget to turn it off, I soon have to buy two 9V batteries. Tony G4WIF suggested a simple timer circuit.  I may do that soon.  Or, given that we are already on the IC dark side with this project, there is this: https://www.youtube.com/watch?v=C4C2H_3BU3o&feature=youtu.be 

I recently find myself replacing a lot of chips in various rigs and devices in my shack.  It seems that chips go bad more frequently than discrete transistors or even tubes.  But maybe I'm just using older gear with older, more fragile chips.   What do you folks think? 


  1. I haven't worked on chip reliability for over 25 years but my first thought is not the chips but those sockets. Next time you might try re-seating the chip in the socket first.

  2. You got me thinking. So I opened it up and put the old chip in there. It didn't work. I then hit the leads on the old chip with some DeOxIt D5. Still, the old chip didn't work. The new chip worked right away. So it was definitely the old chip that was bad. Thanks and 73 Bill

  3. It was worth a try :)

    BTW, I built an 8038 based audio function gen from the 1st issue of Ham Radio Horizons magaine: https://worldradiohistory.com/Archive-DX/Ham-Radio-Horizons/Ham-Radio-Horizons-03-1977.pdf

    And I put it into a Heathkit Q-multiplier box - didn't need a 4066 because the multi-ganged 4 position rotary switch was already there :)

  4. It is curious why would IC5 fail - it looks like the CD4066BE has a max supply rating of 20 volts, which is good. But I notice that some 4066 parts on Digi-key have a max supply rating of 18V - which could be an issue if the two 9V batteries are new and are running higher than 9V. Does the old IC5 part marking indicate that it is the CD4066BE part?

    1. David -

      That's an excellent point.

      This is the winter season, static damage time. In my experience 4000 series CMOS was more prone to latch-up at the highest supply voltage. An over-volt on the supply pins could trigger the parasitic SCR structure and cause failure.

      But my experience is from the stone age (20th century) so later model parts may not have these problems.

      Chuck, WB9KZY

    2. The part that failed was a Motorola MC140668CR. Does that provide any clues?
      It could have been winter season static discharge -- the output plug goes directly to pin 11 on IC5. I have killed LCDs here with static discharge.

    3. The on-Semi sheet (heir to Motorola) says it's an 18 volt part: https://www.onsemi.com/pub/Collateral/MC14066B-D.PDF
      and two fresh 9V batteries will probably be above that. It shouldn't be a problem but ? Designers love to skate at the limits :)


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