TS-505D/U Vacuum Tube Volt Meter (VTVM)
The TS-505D/U is a very rugged and accurate voltmeter and ohmmeter.
I bought this particular meter used and was fortunate to find it in rather good cosmetic condition. The only problem that came with it was with the AC scale. When I tried to calibrate the AC function, I ran out of potentiometer range before I had the voltage reading I needed.
The vacuum tubes in this meter are biased with carbon composition resistors and I've found that carbon composition resistors change value with age, usually going up in resistance. A few resistors had gone out of spec. in this meter and I replaced them with carbon film resistors. This didn't help much so I replaced all the remaining carbon composition resistors with carbon film. After doing so, I could calibrage the AC voltage function but the calibration adjustment was very close to the end of the potentiometer range. I suspect that there is still something else wrong in the meter but since I was able to successfully calibrate all functions, I decided to leave well enough alone.