Philips PM5135 func. generator : FIXED !

[odium]

Hi folks,

Since i can't  find service manual for this device anywhere,
i kindly ask you for any guidance to try find the problem.

 Problem description :

The issue is that there's only signal at the bnc output when the potentiometer is
at the upper limit of the range (200 mark).

In other words :

for each of the multiplier sub-ranges (x0.01 - x1 - x100 - x 10k)
i have output signal only when the pot reaches around the 200 mark,
the rest of the range the output remains KO (no waveform, no ac voltage, no dc voltage)
When the potentiometer sits at the 200 mark, the signals are perfect (2Hz - 200Hz - 20Khz - 2Mhz as per sub-range selected switch)
and all controls responds correctly (freq. offset, dc offset , sweep function, amplitude, attenuation, etc)

Please give me any ideas a what stage or part to look for faults !

thank you in advance for your time !

PS: beautiful picture depicting the problem attached 

[M0HZH]

Poor contact in the potentiometer ?

[wn1fju]

As the previous post suggested, I would put an ohmmeter across the tuning dial pot and carefully measure.  You might want to disconnect the pot first.

Also, when I absolutely cannot find a service manual, I often look at similar products.  For instance, the PM5132 service manual is available on KO4BB's manual site and looks somewhat similar.  At least it might give you some of the theory of the design.

[odium]

thank you guys for replying ….

if the pot was dirty I expect the problem to be intermittent and randomly appearing along the whole range,
but in this case , the output remains absolutely dead, then suddenly bam! it comes alive near the 200 mark
and this behavior is consistent ! maybe the pot winding is open near one of the ends ...

the model PM5132 mentioned seems to be a bit older (early 80s) compared to this one (early 90s), at first glance I don't see matching ICs or board topology.

Anyway, as you suggest I will check pot values, try cleaning it … will be hard though, because the pot is fully sealed with some sort of resin
against the aluminium front plate !

thanks again and stay tuned ! 

[picburner]

the pot is fully sealed with some sort of resin against the aluminium front plate !

Could you take some internal instrument's pics? Someone could help you better by seeing what you see.

[odium]

 Hi all,

I did the measurements and indeed it is the pot that it's faulty ! it goes open around the 2K position (10K total value).
silly me I was thinking about some obscure current source problem 

data :

manufacturer : COLVERN LTD ENGLAND
ref. model : CP.16/001/15S
value 10K  linear - very fine stepping
rotation : 1 turn
power rating ??
tolerance ??

Since this brand and model seems discontinued, I took it off the front panel
to try some desperate repairs, but as stated in previous post its phenolic body
is fully sealed (no metal brackets that could get me in) (see pictures)

Next, I will try some Dremel penetration 
then after de massacre order some kind of replacement 


Thanks & cheers !

[George Edmonds]

Hi

These potentiometers are of a standard form manufactured by a number of suppliers, they are allegedly hermetically sealed, but can be replace by any standard 10K carbon or wire wound one.  I may even have one in my spares stock.

George G6HIG   

[George Edmonds]

HI

Plessey also manufactured identical potentiometers, see eBay UK item 312187958193

George G6HIG

[odium]

HI

Plessey also manufactured identical potentiometers, see eBay UK item 312187958193

George G6HIG

Thanks for suggestion, I need to check physical dimensions … im not at the workshop right now.

cya

[odium]

hi guys,

this is just to let you know that the device is fixed and back in business
thanks to a compatible potentiometer that I had in my spares bin.
part is Bourns model 3851A-282-103BL. It surely needs a little
calibration but will do fine for now.

thanks to all of you that helped or got interested !

PS: winna' winna' chicken dinna' ! 

cya !