On Apr 27, 11:19*pm, "Dave M" wrote:
Andrew VK3BFA wrote:
On Apr 26, 11:41 am, "Dave M" wrote:
Scott wrote:
I'm trying to download a copy of the manual, but at the rate it's
downloading, I may have it by Tuesday, April 27th.....2013!

Anyhow...does anyone have a copy of how to set the timebase
oscillator frequency? I measure the signal coming out the back of
the counter (with another counter) at 10.000066 MHz. I adjusted the
"coarse tune" adjustment on the back of the unit and this is as
close as it gets to
10.000000 MHz, so I assume there must be some other adjustment
somewhere...I took off both covers, but any adjustment control is
not blatantly obvious....

Scott
N0EDV

Scott,
I've seen the oven temperature control circuit more likely to cause
this type of problem. The fact that the oscillator is working
eliminates a lot of circuitry.

Before you do any more adjusting, do this:
Check that the oven temperature control circuit is working. Check to
see if the collector of Q1 is in the ballpark of about -21 volts.
That point should start out (cold oven) at around -5 volts or so,
and go toward -21 volts as the oven warms up. If it stays at the
lower voltage, you have a problem with either Q1 or one of the
transistors on the oven control assy A25. That same reasoning goes
if the voltage at Q1 collector starts out and stays at -35 volts or
so.
An extreme cause would be that of the oven heater being open. In
that case, the oven assembly is bad, and will need to be replaced.
You can attempt ot disassemble the oven and repair the heater, but
it's iffy. One of the temp sensor components inside the oven might
be bad as well.
If the oven and the oven control is OK, that means that the
oscillator is out of tune. That could be caused by the crystal
itself, of one of the capacitors inside the oven.

I have an operable oven assembly that we could make a deal on, but
since I recently moved from Florida to Alabama and don't have a shop
area yet, it's still in storage. If you can wait for a couple of
months, I could dig it out for you. You might look on Ebay for an
oven; I've seen them offerred there, but not often.

--
David
dgminala at mediacombb dot net

Excuse me for seeming a Patronising Old ******* here, but I feel some
basic radio theory is in order here, and to respond to the original
question......

Quote.
*I measure the signal coming out the back of the counter
(with another counter) at 10.000066 MHz.

Response.
How accurate is the frequency counter you are using to measure the
10Mhz TXCO? *(and does it have a current calibration certificate?) -
unless it is at least an order of magnitude BETTER than the HP, any
results will be meaningless. *Your comparing the HP ref. signal
to......what?....

A easier way is to zero beat the HP TXCO with WWV. Yes, for the
purists, there are problems with this method, (which are NOT relevant
here) but if you tune for zerobeat by ear as best you can - by then,
the S meter should be pulsating (it can see the beat note, you cant
hear it) and then tune.

Get really hi-tech, use a GPS locked counter.

And I hope you took notes of the original "frequency" and can take it
back to this. Its a well aged, ovenized xtal. It will be pretty bloody
close, unless someone has been into it before you.....

These things were, in their day, laboratory grade instruments. They
still are, but have been superseded as technology advanced. You need
to learn what the display is telling you, and how much is relevant to
what you do. Its unlikely that you will be building something that
needs 10-12 accuracy. (A nominal figure, not going to dig the manual
out to look)

The error you quote is totally insignificant for average amateur (and
most professional ) use.

If the oven is working, it will get warm. Simple.
Plot the frequency change from cold switch on - see how long it takes
to stabilise. Do it again, later, when the ambient room temperature
has changed. If no shift, its OK. And if it is not working......well,
they ARE fascinating instruments, the service manual a joy to read.
(Bout the size of the phone book) They even give Boolean equations for
it. *You could be entranced by 1960's technology - if you so choose.
g

Later model ones had a "Oven"light on the front panel - if it was out
after warm-up, then it was ok. BTW - anyone got a service manual for a
HP5248L counter.......got a gating fault, how similar are they to the
5245L - any HP test gurus out there?

In good faith, and hopefully no insult intended or received.

Andrew VK3BFA.

The 5248L manual is on Agilent's site athttp://cp.literature.agilent.com/litweb/pdf/05248-90001.pdf. I can't say how
similar they are to the 5245L, but the DCUs seem to be the same. *I used to
be a calibration tech many years ago, and while I saw many of both
instruments on my bench, I rarely had to repair them; quite reliable. *I
remember two cases where the counters exhibited the exact problem the OP
has; significant oscillator error. *Both turned out to be problems with the
oven control circuit.

I have to disagree with your statement that the OP's error is insignigicant
for professional use. *That might be true for some hobbiest uses, but for
many amateur radio operators and certainly for professional use, *it's a
very significant error. * The original time base spec for the 5245L is
3x10^-9 per day.
--
David
dgminala at mediacombb dot net

David, than you for the link - will try for it, and hopefully it can
run overnight here. I stand corrected re accuracy - as I said, I
should have dig out the manual, (and, indeed, revise my own theory re
where the decimal point goes) a bit better.
I was hoping someone like you would stick their head up - helps to
talk to someone who has actually worked on them as a job.....thanks
mate, appreciated.

Andrew VK3BFA.