Am new at trying to diagnose circuits, but trying to troubleshoot an amateur radio that won't switch bands. Normally, at start-up, you could hear all the relays reset, but that no longer happens. MFR manual says to check all the voltages at the IOC, which are all fine except for pins 9 and 10, which are clk in and clk out from a 4.0 MHz 3-pin ceramic resonator. Mfr lists those voltages as normally being 0.2V and 0.15V respectively, but on this board, they are approx 0.5V and are always the same value in both pins. The voltage varies slightly over time, and never really holds steady.

Am I right in thinking the ceramic resonator is the problem? Thx for any input.

 

Schematics are the language of electronics. The description fails to convey the actual circuit.
Additionally, we would require the resonator’s data sheet or at least manufacturer and part number.
Last but not least, what device drives the resonator?

 

You will need to supply a lot more information for anyone to be able to help you. We don't even know what radio you are talking about. Posting the schematic would be a good first step. What is the IOC ?
Les.

 

Schematic attached, though not sure if i got all the area of interest.

The only description of the resonator is "4.000MHz resonator, ceramic resonator w/caps, 0.2% tolerance."

IOC is PIC16C72.

 

Hi,
What are you measuring the voltage with?
Good luck........

 

You need an oscilloscope and a 10X probe to determine if the resonator is working. A multimeter will not read it correctly.

 

You need an oscilloscope and a 10X probe to determine if the resonator is working. A multimeter will not read it correctly.

You're right, I was measuring the voltages with a DMM, and didn't think to question it. The manual says the voltage readings can all be made with a DMM, which I guess is not correct.

I have an old analog scope, though I've only used it from time to time for basic things. Guess I'll need to get it out. Thanks.

 

Yes, your DMM is sampling and averaging your signal at frquency well below two times the oscillation frquency. I'm not sure it correlates to much of anything.

 

If you don't her the relays operating on turn-on, I would check the power supplies, before going any farther.

 

One thing that seems strange is the PIC is being powered with 6 volts. The maximum voltage rating of the PIC is 5.5 volts.
The loading that the DMM puts on the oscillator pins may be stopping the oscillator. This is why Bob suggest using a x10 probe with the oscilloscop as a x1 probe will put more capacitive loading on the oscillator. As ceramic resonators are so cheap you could just try changing it. (If you have a 4Mhz crystal in stock you couls use that together with two capacitors betweem 15 and20pF.
Les. Note the frequency of this resonator does not control the transmit or receive frequencies.
Les.

 

Thanks everyone. So i got out my old Tektronix 465, which i think has a bandwidth of 100 MHz, but couldn't get any sort of signal from pins 9 and 10 which go to the ceramic resonator. The probe is set to 10x.

The scope accurately receives signal from an old RF generator that I have, but nothing from the ceramic resonator. I've only ever had experience with the scope for doing simple things, so there might be something about the set up that i'm getting wrong. I'm grounding to a nearby ground clip on the same board.

 

The Tektronix 465 should be fine for testing the oscillator in the PIC. I think the most likely cause of the problem is the ceramic resonator. If itis not that then I think finding a programmed PIC will be very dificult.
Les.

 

One thing that seems strange is the PIC is being powered with 6 volts. The maximum voltage rating of the PIC is 5.5 volts.

Which might have damaged it.

 

Noted. Thx for the help.

 

I had one of those crystal bad as one of the internal capacitor was leaky.
May have to remove it to verify.
Look on the web should be able to find the values to use if you using just 4mhz crystal itself