Ideally you would use a multimeter that have a capacitor test function. As capacitors age their capacity drops - In Rapid Start (including SRS) circuits this will show up as unreliable starting or reduced light output
The SRS circuit is intended to only start the T12 (fatty) lamps (for 5ft that's 65W), however, an SRS ballast in good condition may be able to start T8 lamps, although this is not guaanteed to always work
Yes it’s the thin tube that has trouble starting, I will check my multimeter has a capacitor testing setting, I know very little about fluorescent lights, I have never come across old instant start units, I will try and find another 5tt fat tube, I saw a lot of them at the library yesterday