So I've got this nice instrument for some timenuttery acticities.
The TA520 is a time interval analyzer that can do gapless period width or time delay measurements of signals of more than 10MHz
The unit works nicely as I received it from a seller of used equipment, except when it was operated for more than one or two hours. Then it started to display really strange results:
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282483;image)
And the built-in self test reported various failures:
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282489;image)
Sometimes it's the Multiplexer Board only, sometimes Multiplexer Board and Aquisition board 2, sometimes Measurement board and Multiplexer board. Turning the unit off, letting it cool down fully restored the normal operation, so this is pointing to some thermally induced failure, like a broken bond wire inside a chip package, bad solder joint, ...
This is a view of the Multiplexer board, I started to diagnose here because this board failed always, the other boards only sometimes.
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282495;image)
As there's no service manual of this instrument, the only hint is this block diagram from the user manual:
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282501;image)
So, there's four time-to-voltage converters that are driven by the multiplexer and fractional pulse generators. Additionally, in this list view, one can see every fourth measurement is wrong. This gives some hints, one of the four T/V channels might have failed.
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282507;image)
I started investigating the multiplexer board, there's just some "standard" ECL chips, none of them ran hot nor was susceptible to thermal shocks (I've used freezer spray on them).
Then I've checked the analog output of the T/V converters and found them all operating. After some more thermal testing (heating or cooling the multiplexer and measurement board and watching the results) I've ruled out measurement and multiplexer as the cause of this failure. There was no obvious correlation between cooling or heating these boards and the occurrence of the failure.
Moving on to the next board in the signal chain, this is the acquisition board 2. It has a set of four large chips, most probably multiplexing and storing the ADC results into the acquisition RAM. The RAM is a bunch of four static RAMs near each large chip. I've started measuring and comparing the RAM signals (data, address, control signals) - and there it is: The RAM chips have the CE2 input unused, and the board designer tied this pin to 5V through a 10k pull-up resistor, one resistor for each group of four RAMs.
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282519;image)
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282513;image)
Three of the four groups of RAMs had the CE2 pin at high, with some capacitively coupled noise:
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282525;image)
At the fourth group, the signal looked different:
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282531;image)
Here's the signal at the adjacent pin, A15.
![](https://www.eevblog.com/forum/repair/yokogawa-ta520-time-interval-analyzer/?action=dlattach;attach=1282537;image)
Note the similarity of the waveform, though the amplitude is different. Measuring with a DMM, I found there's a resistance from16k (cold) to 12k (warm) from Pin CE2 to Pin A15 on this group of RAM chips. Now it's quite visible what is the cause for the thermally dependent failure mode: When the instrument is turned on in cold state, the 10k pull-up is enough to keep the level at CE2 just above the "high" input level, and the RAMs are operating. When the unit slowly warms up, this resistance falls to about 12k, the signal level reaches "low" level territory and the RAMs start to act randomly, sometimes storing the acquired values, sometimes not. A perfect match to the symptoms I've observed. A quick test by shorting the pull-up confirmed this.