IFR-1200S Off Frequency

[Diogynes]

I have an IFR-1200S that is consistently off frequency by -7KHz across the entire range.

Here are the symptoms:

- To generate a signal on (for example) 150.0000 MHz, I have to set the unit to 149.9930 MHz. If the unit is set to 150.0000 MHz, it transmits on 150.0070 MHz.

- The reference output is at 10 MHz. I double checked it using the calibration procedure and it is on frequency.

- It behaves the same with an external 10MHz reference applied.

- I am unable to zero the frequency meter in the calibration step unless I tune the unit to 7 KHz below the calibration frequency.

- The unit is in lock throughout the entire frequency range.

- It is always off frequency by the same -7KHz, from 10 MHz to 470 MHz. I don't have anything to compare it to above that range.

- I am able to tune the unit to generate and receive on the correct frequency by moving the GEN/Lock control to its midpoint.

I plan to send it to KG Electronics for repair after Kurt gets back from vacation, but it bothers the heck out of me that I can't figure out what's wrong. Does anyone have any ideas?

Thanks!

[Jim202]

I have an IFR-1200S that is consistently off frequency by -7KHz across the entire range.

Here are the symptoms:

- To generate a signal on (for example) 150.0000 MHz, I have to set the unit to 149.9930 MHz. If the unit is set to 150.0000 MHz, it transmits on 150.0070 MHz.

- The reference output is at 10 MHz. I double checked it using the calibration procedure and it is on frequency.

- It behaves the same with an external 10MHz reference applied.

- I am unable to zero the frequency meter in the calibration step unless I tune the unit to 7 KHz below the calibration frequency.

- The unit is in lock throughout the entire frequency range.

- It is always off frequency by the same -7KHz, from 10 MHz to 470 MHz. I don't have anything to compare it to above that range.

- I am able to tune the unit to generate and receive on the correct frequency by moving the GEN/Lock control to its midpoint.

I plan to send it to KG Electronics for repair after Kurt gets back from vacation, but it bothers the heck out of me that I can't figure out what's wrong. Does anyone have any ideas?

Thanks!

I am going to throw this out for consideration. There is a small hole on the front panel just to the right of the BNC connector that is labeled DUPLEX. You will see a small shaft with a screwdriver slot. this is the reference adjustment to set the internal time base on frequency. I have used a TV station in the past to set the service monitor on frequency. problem is most TV stations have an offset frequency. so you need to find out what the offset is before you go cranking on any frequency adjustment.

Jim

[Andy Corbin]

I have an IFR-1200S that is consistently off frequency by -7KHz across the entire range.

Here are the symptoms:

- To generate a signal on (for example) 150.0000 MHz, I have to set the unit to 149.9930 MHz. If the unit is set to 150.0000 MHz, it transmits on 150.0070 MHz.

- The reference output is at 10 MHz. I double checked it using the calibration procedure and it is on frequency.

- It behaves the same with an external 10MHz reference applied.

- I am unable to zero the frequency meter in the calibration step unless I tune the unit to 7 KHz below the calibration frequency.

- The unit is in lock throughout the entire frequency range.

- It is always off frequency by the same -7KHz, from 10 MHz to 470 MHz. I don't have anything to compare it to above that range.

- I am able to tune the unit to generate and receive on the correct frequency by moving the GEN/Lock control to its midpoint.

I plan to send it to KG Electronics for repair after Kurt gets back from vacation, but it bothers the heck out of me that I can't figure out what's wrong. Does anyone have any ideas?

Thanks!

I am going to throw this out for consideration. There is a small hole on the front panel just to the right of the BNC connector that is labeled DUPLEX. You will see a small shaft with a screwdriver slot. this is the reference adjustment to set the internal time base on frequency. I have used a TV station in the past to set the service monitor on frequency. problem is most TV stations have an offset frequency. so you need to find out what the offset is before you go cranking on any frequency adjustment.

Jim

I doubt the front panel frequency adjustment will have any affect on it. If you ran the box off an external 10 mhz reference and still getting the 7khz error, then there is something else going on. You could always turn the front panel adjustment say 1/4 of a turn and see what happens and if nothing does, simply return it to the original position. If the front panel adjustment does fix the problem, you may have an issue with the 10mhz reference input on the back.

Andy

[Jim202]

I have not read the manual on the IFR-1200S for many years, but think there is something you have to do to use the external 10 MHz. input. Don't think it is automatic. If this is the case, then connecting an external 10 MHz. signal will not change the time base of the internal oscillator.

Jim

[Diogynes]

Thanks for the replies.

The external 10MHz reference starts working automatically when an external signal above 5Vp-p is applied. I have a 10MHz source so I built an amp to bring the source level up to the point where the IFR starts using the external signal. It didn't help.

The only way I can get the unit on frequency is to either deliberately enter the desired frequency exactly 7KHz low, or turn the GEN/LOCK knob to about the midpoint, which means I really don't know what frequency the IFR is on without using yet another device as a reference.

[Jim202]

If you don't have a copy of the service manual, you can download it from the following link. It is a little over 40 megs in size, so it will take a while to download all of the 491 pages of the PDF file.

http://www.siliconinvestigations.com/if ... 00-srv.pdf

Good luck with your problems. I own one of the IFR-1200S units and found it to be a real work horse. I lost the high voltage transformer about 15 years ago and it was no longer available back then. So there is no longer a functional scope for me to use. I really miss having a working scope in the box.

Jim

[jry]

Sounds like you have an issue in the low loop.

Could also be an issue loading the correct values into the low loop from the uP

From the service manual info you can easily get the correct low loop frequency for the current RF setting and measure the output form the LOW LOOP module.
The low loop is what varies the tuned frequency at the Khz steps and is around the 80Mhz range IIRC

Normally the cal process has you loop the 10Mhz ref back into the antenna port and set the zero ,10khz and 3khz freq error. If that does not zero than you can also inject an external 10.7 directly into GEN/REC module and see if you can get the zero adjust. If you still cannot adjust with a good external 10.7 signal ( doubt it ) than you need to look at the Digital and 10.7 Gen REC modules and see why.

Assuming the low loop output is off the easiest thing would be to swap low loop modules into a known good unit and verify if it's that module or a data I/O issue to the module.

Luckily the 1200 was a great design and very easy to service. the opened modules can be serviced but laying them on top of an insulator and connected back into the unit.

[Diogynes]

I studied the service manual and jry seems to have nailed the probably causes. I wish I had another unit to swap modules with to show me where to focus. I feel like I'm all over the place in my troubleshooting as I study each circuit after reading the descriptions in section 2.

I checked the 10.7 IF and it is on frequency and I also injected an external 10.7 as jry suggested. As he suspected, it didn't help. In calibration step 4-2-6, I cannot zero the meter unless I set the IFR frequency to 9.9930 MHz instead of 10.0000 in step 1, 10.0030 in step 4, and 9.9960 in step 6. I plan to check the low loop frequencies against the calculated values tonight.

It's funny that I was able to understand and fix a bunch of odd problems with my Wavetek/SSI 3000, but it's a lot less capable unit and was easier to work on. If I have to surrender and send the IFR in, at least I'll end up with a calibrated unit...

Thanks again to all for the replies.

Bob

[kcbooboo]

I haven't looked for it, but does the GEN/LOCK control have a switch or detent at one end that isn't working right, since you can get the unit exactly on freq with the control set at its center. There might also be another internal pot that takes over when the GEN/LOCK control is not in the circuit. I've seen that sort of thing on scope variable sweep or gain controls, where there's a CAL position at one end or the other.

Bob M.

[jry]

i believe you also posted the spectrum analyzer is off as well so it seems pretty certain from everything the low loop output is off frequency.
I would be good to know the exact amount off since that may help determine which divider may be suspect.

I would pull the low loop module and remove it from the can to make some measurements.

There were two major versions ( some sub versions as well ) of that module and most 1200S's have the later FAST LOW Loop .

If you have the later version I would first verify the 72.5Mhz is on frequency. If not check the 6.9v across CR15... it may be low ( had a couple fail this way ) and if so you may need to add a standard diode 1N914 in series to bring the voltage up ( goes the opposite way of the zener ) the .7 volts. Combination of 20 year old zener being low and the crystal needing the full voltage to pull it on frequency.

Read the theory of operation on that module. the low loop generates over a 2MHZ range from 77.3 to 79.3. The HIGH loop skips every other Mhz so for 0.000RF setting the Low Loop should be 77.3., for 1mhz on the RF setting it would be 78.3 and for 2.000 it goes back to 77.3

[Diogynes]

jry,

This unit has the fast low loop. I think you've identified the problem. The 75.2 MHz oscillator is running at 75.193 MHz. I replaced CR15 because the voltage across it was 5.9V and the reference is now at 6.9V. The loop is still off frequency and the voltage at TP3 is at 6.5V. I disconnected R82, the 10K resistor feeding the varactor, at TP3 and input 1 to 7V into the varactor through that resistor. The frequency doesn't seem to vary by much so I suspect the MV209 varactor might be bad. The loop frequency without any voltage into the varactor is right around 75.188 MHz. With 7V applied the frequency rises to 75.193 MHz and will not go higher. I limited the input to the varactor to only what it would see from the 4046. I'll fool with it some more later tonight.


Someone apparently replaced the 47pf C82 with a 33pf value in the past. Paralleling C82 with values from 22-56pf made no difference in the oscillator frequency. If I parallel the varactor with 22 to 56pf, the frequency goes down to about 75.185 MHz. I pulled the varactor and it shows on my component tester as a diode with an internal capacitance of 50pf, but I'm not sure how reliable that measurement is.

[jry]

You may want to replace Q15 and C69 ( common 2n3905 and 100pf ) and see if you can get the freq above 75.2 with the MV209 varactor out of the circuit first.
The worst case is that the crystal has aged off frequency the effective load capacitance will need to be decreased or the crystal replaced. ( International Crystal ?)

With the loop at 6.9 it says you are too low and its trying to get the oscillator higher.
the MV209 varactor should be around 50pf with no reverse bias and goes to min cap at 7v reverse bias. It could be bad if the oscillator runs above 75.2 with it out of circuit.

Good news is you have isolated the issue to a very simple piece of the 1200S and all the parts are obtainable.

[Diogynes]

I replaced Q15 and C69 with no change. After I removed the shield, I found that someone had been working on it before me. There was a SMD capacitor across the 5.5 pf capacitor at C68 that is not in the schematic. I do not know the value of the SMD part because my tester will not measure low pf values, but removing that cap did not affect the oscillator frequency, amplitude, or start-up. I swapped C69 with an 82pf SMD part and the oscillator did not vary from the current free running frequency of 75.190 MHz. I'm going with jry's suggestion that the crystal has drifted.

Other notes:
- The MV209 had been replaced with an axial device. Since no direct substitute is available, I ordered a couple of close replacements, just in case.
- The LM329 voltage reference had been replaced with a standard Zener. Mouser has the LM329 in stock, so I ordered some.

International Crystal amazingly has the crystal specifications on file, so I ordered one. The cost is $26, which is still a bargain when I think of what I paid 20 years ago for custom crystals.

The good news - the 75.2MHz oscillator offset I see as I play with the capacitance values exactly corresponds to the offset I see on the IFR indicated frequency, so I'm convinced that jry has identified the problem. I sincerely appreciate the help and will post again when I get the crystal in about 3 weeks.

Bob

[Diogynes]

The IFR-1200S is fixed. It turned out to be a bad 18.8 MHz crystal, but the MV209 varactor diode and LM329 voltage reference in the 75.2 MHz reference oscillator were also bad.

In case it helps anyone else:
- International Crystal has the replacement 18.8 MHz crystal in their catalog under the IFR part number 2363-0109-000. They promised three weeks and delivered in less than a week.
- The MV209 varactor is no longer available. An NTE614 works fine as a replacement.
- The LM329 6.9V reference had been replaced in the past by a Zener and was at 5.9V. I ordered a Mouser 926-LM329DZ/NOP8 to replace it.
- Check under the shield. Someone had added SMD capacitors across the originals in mine, apparently in an attempt to compensate for the failing crystal.

A couple of capacitors were apparently changed in a previous service attempt. I changed them back to the values indicated in the schematic and the unit works fine. I can only hope that I didn't inadvertently undo a modification suggested by a service bulletin.

I sincerely appreciate the help I received here on the forum, particularly the suggestions offered by jry.

Bob

[ELF]

Thanks very much for this topic. I had the same problem with my 1200, it seems to have a defective cd4046 u12 I think. My 1200 is running 7-8kc high in output frequency. The voltage at tp3 is near zero. If I connect a variable dc supply to tp3 the output frequency can be varied with 5vdc being near correct. Both inputs to that chip look ok. Chip on order. Thanks for pointing me in the right direction.

[jry]

low loop is bad.

1200 series has two loops ...high and low. High loop is the 2Mhz ( 1.3 to 2.3 Ghz IIRC ) granularity part.

Low loop covers the under 2Mhz part of the frequency synthesis.


there are some alignments on the low loop and the crystal used as part of the loop has been known to drift out of VCO range as well with age. I often have to "trim up " the VCO voltage range on the board to get it in spec.
Luckily this all relatively low frequency stuff and the board is very accessible.

There are some caps on the both the high and low loop cards that need to be replaced as wellnow that they are over 20 years old ( actually closer to 30 now )

[RFguy]

Just an FYI for those that want to find someone to fix a IFR1200

Contact Kurt at KG Electronics
https://kgelectronics.com/

He has repaired several for us over the years. He does a great job at a reasonable price.

I believe he only works on 500's and 1200's (not 1000's, 1600's or 120's)