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PG502 freq too low

Szabolcs Szigeti
 

Hi,


I just got a PG502 pulse generator. After I fixed a bad solder joint on one of the pots, everything seems to be good, , except for the 250MHz setting. It only goes up to 200MHz. All other ranges are spot on, i checked all steps in the SM for adjustments.
According to the service manual, there is no internal adjustment for this frequency range, it's implemented as a separate multivibrator.
Any advice on where I should be looking?


Thanks,


Szabolcs

Dan Rae
 

On 5/30/2016 12:46 PM, szigszabolcs@... [TekScopes] wrote:
Hi,

I just got a PG502 pulse generator. After I fixed a bad solder joint on one of the pots, everything seems to be good, , except for the 250MHz setting. It only goes up to 200MHz. All other ranges are spot on, i checked all steps in the SM for adjustments.
According to the service manual, there is no internal adjustment for this frequency range, it's implemented as a separate multivibrator.
Any advice on where I should be looking?
I've just been working on one of those, on and off for about a month. Check the value of the series R: R149 18 Ohms; I found the similar one for the other ranges, R162 had gone up in value from a nominal 51 Ohm to about 250 Ohms.

My problems are now with the darned cam operated switches which don't make contact reliably...

Dan

 

The specifications say <= 4 nanoseconds. 5 nanoseconds is 200 MHz so
how much faster should it go?

The circuit built around U150A is the fast period generator. I would
check the values of the resistors around it and swap U150 for U180 or
U260 to see if that changes anything.

The ECL logic runs on +5 volts so I would take a careful look at the
+5 volt supply. Check for level and noise when operating. Check that
bulk decoupling capacitor C650 is good. If C650 is bad, then the +5
volt regulator might behave oddly allowing the +5 volt supply to
droop.

On 30 May 2016 12:46:26 -0700, you wrote:

Hi,

I just got a PG502 pulse generator. After I fixed a bad solder joint on one of the pots, everything seems to be good, , except for the 250MHz setting. It only goes up to 200MHz. All other ranges are spot on, i checked all steps in the SM for adjustments.
According to the service manual, there is no internal adjustment for this frequency range, it's implemented as a separate multivibrator.
Any advice on where I should be looking?

Thanks,

Szabolcs

 

Make sure the pulse width is not longer than the time between pulses.
If it is, the pulse rate will divide by 2.

HankC, WA1HOS, Boston

Dan Rae
 

And further to my earlier suggestion to check the value of the 18 Ohm
resistor in series with the variable pot (these are small carbon
resistors which can drift high with age) I see that in mine at least
there /is/ an adjustment for the <4nS range which is /not/ mentioned in
my manual. It's a small ceramic trimmer (10 pF?) just to the rear of the
variable pot and presumably replaces the fixed cap in my manual. My
serial is B033902.

Mine goes up to 270 MHz so I'm not about to play with this but if you
find this trimmer in yours it should get you up to spec.

But with so many suggestions that have been offered that's it from me :^)

Dan

Szabolcs Szigeti
 

Hi,


Thanks everyone with the suggestions, It was indeed a resistor issue.

Dan, you were spot on. The 18 ohm resistor was about 40ohms, I replaced that. And yes, the trimmer cap is there, I was able to get up around 280 MHz, so it more than meets the specs now.


Thanks all!


Szabolcs