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Old 26th Mar 2011, 1:12 pm   #1
yestertech
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Default "Ferguson" prototype stereo power meter

Sometime ago I acquired this useful piece of kit and recently decided to check it over and set it up properly.
It has the "Ferguson" sticker attached to the side and the construction looks to be some sort of "in-house" prototype, as the PCBs are unlabeled and there is no mains switch !
It is well made though, and the front is fully engraved, together with the legended meter movements.

The electronics ( 2 identical channels on one PCB ) appears to be a simple dual op-amp based on SN76131. The input from the amp is fed via a rotary selector to either 4 or 8 R load resistors. This is then fed to a dividing ladder on the rear of the pushbuttons, and the resulting signal is fed to the op-amp and diodes, to drive the meter. There is a preset for each channel sitting on the back of the rotary selector, which I'm guessing is the overall sensitivity, and one on the PCB which appears to make no difference ?

Sadly, there is a problem with it. After it has been on a while , the meters start to climb from zero to around 3/4 scale. Left channel first, then after a while, the Right. I have checked the 2 dual rail power supplies ( separate, to avoid the channel earths being combined ) and these seem OK - I did spot a tantulum in the top half of the PCB which I changed in case of noise, but to no avail. No waxies in here, only "allsorts" caps which I could change next, but I'm sceptical. Could the problem be down to the op-amps, as I known there was a thread concerning SN76131 relating to Studer PCBs.
In this application, I would imagine I could replace with something like UA739, or perhaps something more modern ?

Any thoughts gratefully received !

Andy
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Old 26th Mar 2011, 2:07 pm   #2
Jonster
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Default Re: "Ferguson" prototype stereo power meter

I should imagine most pin compatible op amps would work, might be worth putting a socket in and trying?

I would say it probably wasnt made by Ferguson, that's a 'property of' not 'made by' sticker in my opinion.

Interesting and useful bit of kit!
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Old 26th Mar 2011, 2:52 pm   #3
frsimen
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Default Re: "Ferguson" prototype stereo power meter

Andy

The SN76131 seems to be equivalent to the uA739 and the TBA231.

The TBA231 datasheet is available on the internet and shows a fairly basic dual low noise operational amplifier with a single ended output stage.

http://www.bg-electronics.de/datenbl...ise/TBA231.pdf

The TBA231 requires frequency compensation. I don't think you'll find anything modern that is fully pin compatible with this, but if you don't mind adding a small daughter board, something like the NE5532 or a pair of NE5534s will do the job.

Paula
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Old 26th Mar 2011, 9:13 pm   #4
Valvepower
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Default Re: "Ferguson" prototype stereo power meter

Hi,

I remember getting a similar piece of equipment from the Thorn (Ferguson) factory, which from memory was based in Southend and closed some years ago. This was a combined stereo coder and audio measuring test set to test and align stereo radio receivers. This was made by the [Thorn] test engineering department for use on the test lines. I remember it used exactly the same type of meters.

Terry.
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