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IFR 1000A with intermittent 10 MHz Oscillator

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warnergt
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IFR 1000A with intermittent 10 MHz Oscillator

Postby warnergt » Mon Jan 01, 2018 11:10 am

I have an IFR 1000A radio test set that intermittently stops working. When this happens, the PLL lock lights go off. I tracked the problem down to the 10HMz Oven and Osc. unit. I opened the unit up but, inside, it gets a little hairy. The crystal is in a three-transistor circuit which is kept temperature stable within a heater element which is housed in insulation. The circuit does not appear very serviceable. It's in a metal case with four wires (plus two heater wires) coming off of it.

Even when it is working, the 10 MHz signal coming out of this circuit seems very weak.

Any ideas how to approach this?

This is my first post to this forum. I read somewhere else that there is IFR info here. I'm a big fan of IFR 1000 and 1000A test sets. I have a few. It would be good to find others out there who are interested in these as well in order to swap parts and info.

Image

warnergt
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Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby warnergt » Tue Jan 02, 2018 6:52 am

Here is the schematic.

Image

jry
Posts: 344
Joined: Fri Feb 06, 2004 3:14 pm

Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby jry » Tue Jan 02, 2018 7:01 am

for the cost of 10Mhz OCXO's on Ebay I would just go for a replacement ...less than $20.

You can tear into what you have but given the age the material may disintegrate as you try to disassemble and try to repair.

Just get a replacement module and mount in the box. You may have to drill another hole to align with the new adjustment.

warnergt
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Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby warnergt » Tue Jan 02, 2018 12:16 pm

That's a good idea I hadn't considered but it would take a little engineering to get it in there.
After some more experimenting, I've found that the 7404 driver(/inverter) IC is extremely temperature sensitive. Just a drop or two of freeze spray on that component immediately revives the oscillator. I'm not sure if the IC is operating in a critical region because it is being driven so weakly or if, perhaps, the IC is defective and loading down its input to make it look weak.
Since it's a pretty easy change, I will change the 7404 first. If that doesn't resolve the problem, I'll pursue integrating a different OCXO.

fineshot1
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Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby fineshot1 » Wed Jan 03, 2018 6:05 am

jry wrote:for the cost of 10Mhz OCXO's on Ebay I would just go for a replacement ...less than $20.

You can tear into what you have but given the age the material may disintegrate as you try to disassemble and try to repair.

Just get a replacement module and mount in the box. You may have to drill another hole to align with the new adjustment.


+1 on jry idea above. I have purchased a few Vectron or IsoTemp 10Mhz self contained oscillators via ebay and they would serve
well as a replacement. All you would have to do is mount it and provide power as well as get it properly connected up.
fineshot1
NJ USA

jry
Posts: 344
Joined: Fri Feb 06, 2004 3:14 pm

Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby jry » Wed Jan 03, 2018 6:46 am

chances are the IC is bad. You often get fractures of the internal IC especially in the use case you have inside the reference with the temp changes over the 30+ years .

You should see a pretty solid 10Mhz coming into the first buffer in the 7404 on pin 11 since that should be TTL levels.

You can try adjusting the cap to peak the circuit with Q7405 output .

When you buy an updated OXCO a lot of that circuitry is no longer used ...just take the connect the A+ and output to the 7404 pin 11. Some of that will vary depending of the exact OXCO you used so some have sine outputs and may still need the transistor and others may need 5V so a better regulator circuit may be needed.

warnergt
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Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby warnergt » Sat Jan 06, 2018 1:14 pm

Some replacement 7404 ICs arrived today so I replaced the suspect 7404 in my OSC/OVEN circuit. With the new chip, the oscillator was working.

My scope showed the signal still looked kind of weak so I adjusted the variable capacitor and the signal got strong! Much stronger than before.

I put it all together so I could verify that everything worked when it was up to temperature and it appeared to be working well. Then, I checked the frequency. It was out. And the little pot on the front panel couldn't bring it in far enough. Seems odd that the frequency would have changed but it is what it is. So I had to open it back up (the story of my life) to access the variable capacitor at the crystal. That cap was TIGHT! I feared I would break it as I adjusted the frequency. But I was able to dial it in. I put it all back together and all appears good.

I'm waiting for good propagation and low interference conditions to double check the frequency against WWV.

fineshot1
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Joined: Wed Oct 12, 2005 5:13 am

Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby fineshot1 » Tue Jan 16, 2018 1:21 pm

You do not need to use WWV - you can check it against any local NOAA weather freq in your area or
any of your local trunking control channels all of which are timed off GPS timing.
fineshot1
NJ USA

warnergt
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Joined: Mon Jan 01, 2018 11:02 am

Re: IFR 1000A with intermittent 10 MHz Oscillator

Postby warnergt » Wed Jan 17, 2018 5:02 pm

WWV works perfectly because the IFR time base is 10 MHz -- one of the WWV frequencies. I can receive both signals at once, listen to the beat frequency and dial in my oscillator to the exact right frequency.


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