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Old 28th Jan 2009, 8:29 pm   #1
Studio263
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Default "glitchy" PM5129 solved

Regular forum users my have seen my request for a circuit diagram for my Philips PM5129 function generator to help solve the problem of highly annoying output glitches, duty cycle changes and random jumps in frequency.

Well, I never did find a diagram but that shouldn't stop one should it? Having worked out how it works (basically from what I can make out it's a symetrical current source that charges a capacitor to form a triangle wave of the desired frequency, this is then converted into a sine or square wave for the other output functions. A sceond oscillator and sine converter can be set to modulate the current sources to form an inbuilt FM modulator). I located the master oscillator. This is very complicated, with 5 ICs and over 20 transistors. I changed the lot but it made no difference, nor did anything else I tried.

I was struck however in an otherwise well designed machine by the total lack of decoupling on any of the supply lines. In my job as a designer I sprinkle decoupling capacitors around like confetti, so does everyone else I know. It seemed it could do no harm to add some so I fitted 100nF capacitors to the supply pins of all the op-amps and logic ICs in and around the oscillator. The result was perfect stability again, and much relief. I used proper Philips ones (the "orange drop" sort) so that it still looks "factory" inside.

Why the extra capacitors were needed after all these years (the unit was made in the mid 70s) is a mystery but I see a lot of this sort of thing, there are a few cases on the Beocentral Workshop Notebook where minor changes have been needed to correct faults that have arisen after a number of years of trouble free use. Oh well...

I still wouldn't mind seeing the circuit if anyone has it!
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Old 28th Jan 2009, 8:44 pm   #2
georgesgiralt
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Default Re: "glitchy" PM5129 solved

Hi !
In the early 80's Service Sa which was the French maintainer of Philips devices used to sell (quite cheap) the service data for their products. At that time I bought from them documentation for a long gone radio cassette player.
You should try to look after them ....
Just my 2ยข
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Old 30th Jan 2009, 1:56 pm   #3
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Default Re: "glitchy" PM5129 solved

Quote:
Originally Posted by Studio263 View Post
Why the extra capacitors were needed after all these years (the unit was made in the mid 70s) is a mystery but I see a lot of this sort of thing, there are a few cases on the Beocentral Workshop Notebook where minor changes have been needed to correct faults that have arisen after a number of years of trouble free use. Oh well...
Curious and worth remembering.

I wonder if the smoothing caps in the PSU are up to scratch?

Pete.
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Old 30th Jan 2009, 6:07 pm   #4
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Default Re: "glitchy" PM5129 solved

I had considered mentioning this - good caps with low ESR act as reasonable decouplers at a lot of frequencies, as they age, the ESR goes up, and then they arent so good at decoupling.....
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Old 30th Jan 2009, 9:08 pm   #5
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Default Re: "glitchy" PM5129 solved

It raises an interesting question.

How do semiconductors age?

If there are subtle changes in the structure this could increase the gain and/or the frequency cut-off. This could then causes the problems seen.

Such performance changes would not affect radio circuitry but with a demanding design such as a function generator, it is just possible. I have never seen any detailed data on long-term ageing and would ask if any has been undertaken. Usually such tests have been heat/damp accelerated ones looking - and then only looking for minimum performance figures so any "enhancements" may not be recorded!

R

Mods - this may raise a new topic so please move if it takes off! Thanks.
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Old 31st Jan 2009, 1:32 pm   #6
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Default Re: "glitchy" PM5129 solved

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Originally Posted by AVO_VCM163 View Post
It raises an interesting question.

How do semiconductors age?

If there are subtle changes in the structure this could increase the gain and/or the frequency cut-off. This could then causes the problems seen.
There are definitely ageing effects in semiconductors which degrade performance rather than cause catastrophic failure.

http://books.google.co.uk/books?id=6...sult#PPT319,M1

I get the impression they are more important in devices with small feature size (e.g. dRAM) than discretes or op amps. Obviously, these effects would be more pronounced if the device was run outside its design spec, and this could happen through sloppy design or a fault condition.

I'm assuming the function generator in question is 20 years old or more and doesn't use high density circuits and the problems are more likely down to the well-known age-related problems of carbon composition resistors and electrolytics.

Pete.
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Old 6th Feb 2009, 1:39 pm   #7
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Default Re: "glitchy" PM5129 solved

Quote:
I'm assuming the function generator in question is 20 years old or more and doesn't use high density circuits and the problems are more likely down to the well-known age-related problems of carbon composition resistors and electrolytics.
All the semiconductors are quite conventional, MC1456 / MC1558 op amps (both quite 741-like), 74 series logic, BC547B / BC557B transistors, the odd MOSFET (BF245). The resistors are metal film types of course and every one I tested was absolutely spot on. Electrolytics were notable by there absense, there are a few in the power supply and that's all, again this is not uncommon in professional / industrial equipment. I think it was made in 1976, quite new for something found in a home electronics workshop!
The circut "building blocks" are not familiar if you are mainly used to radios, the stages look more like the internal circuit diagrams of ICs, eg many more transistors than you would have though necessary, lots of balanced circuitry, current sinks and current mirrors everywhere etc etc. This is clearly done in the interests of stability and resistance to drift which (when it is working!) is very good.
It's been back at work as the signal source for calibrating cassette recorders, no glitches yet.
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Old 6th Feb 2009, 1:55 pm   #8
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Default Re: "glitchy" PM5129 solved

Some thoughts...

It was a marginal design in the first place. Even the best designers make mistakes Very minor ageing tipped it over the edge.

Power supply electrolytics as suggested by Sean. Far more likely than my first suggestion. Probably worth changing the lot.

In fact these 2 ideas might be one and the same. It was probably marginal design to omit local decoupling and ageing electrolytics showed it up.

None of the semiconductors of that vintage are likely to fail except that there will always be the odd one as they get very old. The BF245 is actually a JFET.
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