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Need help diagnosing and fixing a Yaesu YC-500 frequency counter

 

Hi all,

As a weekend project I decided to attempt to repair a non-working YC-500 frequency counter "just for fun." The problem I have identified so far is that it does not generate its internal 1 MHz reference. The reference assembly is a small PCB module with what appears to be a straightforward crystal oscillator and divide-by-10 SN7490 IC. Unfortunately, the IC is not outputting anything. The input to the IC is a 10 MHz sine wave, about 1.5 volt p-p amplitude, centered around +1.5V. I am not sure if this is the proper amplitude and offset to pass through the hysteresis points of the 7490 input to trigger it. The +5V supply is clean and running at +5.05V.

The reference board appears to be untouched, other than my having removed both transistors to check them (both are fine) and then replaced them, and having swapped out the 7490 for a known good one. The symptoms have not changed. Any ideas?

I've attached the schematic of the reference oscillator and a screen grab of the input to the 7490.

Thanks,

Stan N6BYU

Yaesu YC-500 Ref Osc.pdf
Yaesu YC-500 Ref Osc.pdf
YC-500.png

 

Idon't think I have worked on TTL for 40 years. As I remember a low is less than maybe 1/2
?Volt and a High is at least 3 1/2 volts. The circuit input should never go negative. Usually a shotkey? input would be used to square up the sine wave. If you can remove the "90 input and inject a clock? signal, that would show if it works, or even just take the input hi to vcc a few times. Those IC's? do go bad.? But I would look at the input signal not being right.
I will look at what you have sent and reply again.
TTYL,
Randy? ?WB9QQA
On Sunday, May 29, 2022, 05:35:04 PM EDT, Stan <swperk@...> wrote:


Hi all,

As a weekend project I decided to attempt to repair a non-working YC-500 frequency counter "just for fun." The problem I have identified so far is that it does not generate its internal 1 MHz reference. The reference assembly is a small PCB module with what appears to be a straightforward crystal oscillator and divide-by-10 SN7490 IC. Unfortunately, the IC is not outputting anything. The input to the IC is a 10 MHz sine wave, about 1.5 volt p-p amplitude, centered around +1.5V. I am not sure if this is the proper amplitude and offset to pass through the hysteresis points of the 7490 input to trigger it. The +5V supply is clean and running at +5.05V.

The reference board appears to be untouched, other than my having removed both transistors to check them (both are fine) and then replaced them, and having swapped out the 7490 for a known good one. The symptoms have not changed. Any ideas?

I've attached the schematic of the reference oscillator and a screen grab of the input to the 7490.

Thanks,

Stan N6BYU
 

Check the transistors emitter. since there is a capacitor. you shouldn't see much of a signal if any.? The bypass could be bad, and that could cause low gain and the output to be incorrect.
73.
RB

On Sunday, May 29, 2022, 05:35:04 PM EDT, Stan <swperk@...> wrote:


Hi all,

As a weekend project I decided to attempt to repair a non-working YC-500 frequency counter "just for fun." The problem I have identified so far is that it does not generate its internal 1 MHz reference. The reference assembly is a small PCB module with what appears to be a straightforward crystal oscillator and divide-by-10 SN7490 IC. Unfortunately, the IC is not outputting anything. The input to the IC is a 10 MHz sine wave, about 1.5 volt p-p amplitude, centered around +1.5V. I am not sure if this is the proper amplitude and offset to pass through the hysteresis points of the 7490 input to trigger it. The +5V supply is clean and running at +5.05V.

The reference board appears to be untouched, other than my having removed both transistors to check them (both are fine) and then replaced them, and having swapped out the 7490 for a known good one. The symptoms have not changed. Any ideas?

I've attached the schematic of the reference oscillator and a screen grab of the input to the 7490.

Thanks,

Stan N6BYU

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