EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: AERVBlog on February 28, 2024, 08:13:54 pm
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Hello everybody. This is my first post here and I hope the brain trust can help me. I have a bunch of Dorrough Loudness meters to fix and for the most part I have but I've run across a problem that is just driving me crazy. The negative 9 volts for these is produced by a JRC7662D IC. When they work the output voltage is around -9.05V when they don't work I get -0.7V. After a bunch of fiddling around I discovered that sometimes if I lift pin7 the darn thing will work again but not always. Pin 7 isn't connected to anything. I did a video on badly made boards for these meters but in the current bunch that is not the problem.
The thing only takes 2 external parts in this configuration, a 10uF electrolytic and a 100uF electrolytic. The 10uF goes across pins 2 and 4 and the 100uF goes from pin 5, output, to ground. Comparing one working unit to a non-working unit the only thing I can see is that pins 2 and 4 are missing the square of about 4.4KHz on the one that doesn't work. The things are very flakey and will work sometimes and not others. Like as I was writing this I checked for voltages and the unit I have been fussing with for over an hour suddenly started working!
Has anyone had any experience with either this 7662D or the Dorrough part #1761042-6 usually paired with a model 10A-D display board
I have included a part of the schematic as an attachment
Thanks for any help you can give.
Dave
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Not used the 7662, but have worked on items using the 7660, they also use a 10uF cap, which gets replaced due to it's age (30+ years old) & electrolyte leakage problem, occasionally the IC is dead too. New ones are still made by Microchip or Analog Devices under part numbers TC7662 or ICL7662 (or in my case TC7660 or ICL7660).
David
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Thank you. At least I won't have to rely on maybe real from Ebay. I have ordered a bunch of Low Z caps, as low as I can get them anyway. I had to get surface mount bu there is room. We will see if that helps.
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Mike Dorrough was supposed to be the rocket scientist of audio gear. Sounds like he cut corners in the production part of his devices. We used those panel mount 'Loudness Meters' retro fitted into all of our Pacific Recorders BMX boards. I actually never had one fail even after 20 years or more.
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For the most part they did a really good job and they were cool looking to boot. I have actually only found a few badly built boards and think that could happen to anybody. Since I have seen so many with the same problem I have to conclude it must be design. Not bad just starting to show weak spots, maybe the need for special caps. Don't know but will see.