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HI Teksperts
I have the chance of getting my hands on a TDS684A with a discoloured screen and suspect that it mat be suffering from the degradation of the 'gel' between the CRT and LCD 'shutter'?
I have discovered that there is a modification to replace the CRT and shutter with a backlight LCD panel.
Has anyone any experience doing this successfully and any pointers to the technique.
Secondly whilst scouring the web for this repair information, I also noticed that quite a few of these series of 'scopes can be 'hacked' to unleash the full power of the instrument.
i.e. faster and deeper memory etc. (throttled back at the factory)
Are there hacks for the TDS684A in a similar vain?
Finally the TDS684a is sometimes listed as a 500MHz machine and in other places a 1GHz instrument.
So....
What frequency will the instrument faithfully reproduce on screen please?
Thanks in anticipation
Dave

The TDS684A is definitely a 1GHz scope, running at 5G Samples/sec.


Here's info on replacing the CRT with an LCD:

(It says TDS744, but all TDS500, 600, and 700 series scopes share the same colour CRT setup)

Or you can use a SimmConn Labs screen if you don't want to DIY.



And for unlocking options, follow this guide I wrote:



Also, being an 'A' model, be prepared to replace ALL the SMD electrolytic capacitors in the scope. The are practically a guaranteed failure point, and can cause a lot of damage to the PCB's when they leak.
Also check the PSU for Rifa caps, I expect there will be 4x 2200pF caps there, and replace C17 while you are at it..



Hope that helps!
Jared

On Wed, Feb 26, 2025 at 10:02 AM, Dave B wrote:

Has anyone any experience doing this successfully and any pointers to the technique.

According to both the manual and catalog from that time the TDS684A
doesn't have any options that can be unlocked. Also the full 1GHz bandwidth
applies only to attenuator settings 10 mV/div and higher.

About LCD, here's how I did it:

/H?kan

Great reply from Jared, covers pretty much everything. I would add, unlocking 1GHz means you probably need to re-do parts of the calibration. Also I question the actual usefulness of that extra bandwidth, unless you know you'll be using active probes ($$) or 50-ohm input etc... I've had my 744A for a few years and never felt the need to unlock the 1G; depends what you work on obviously.

HI and thanks for all the links and great advice.
I will archive the information as when I went to look at the instrument is was seriously corroded by the leaky capacitors and I decided it was not for me.
However I will now renew my search armed with your collective knowledge :-)
Thanks once again
Dave