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?....
Very accurate. It is our Agilent power meter/freq counter (to 26 GHz)
and has been cal'd.
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.....
Yes, I did note the original freq.
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.
Well, I'm wanting to measure down to about 10 Hz "accuracy"...in other
words I am measuring 189.XXXXX MHz. This is the XTAL freq. for my
microwave LO that is multipled by 6 to get to 1136 MHz and that is
multiplied by 9 to get to 10224 MHz, for a total multiplication factor
of 54.
If the oven is working, it will get warm. Simple.
Now THAT'S something I need to check. It should be working...the unit
is only a bit over 40 years old!
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.
None taken. I work on microwave equipment at work all day long and can
get a bit worked up about "being on frequency"
Thanks for the tips...the manual did finally download, so I'll dig into
it soon (the good news is I have two of these, so maybe I'll check the
other one and see how close it is. I just bought the second one for its
3-12 GHz mixer).
Scott
N0EDV
Andrew VK3BFA.