Hi Tom,
I would still go for the probes as the most likely source of the non linear square waves, since they are from a different scope and they may not match particularly well. If I am correct in my identification, your other scope is a Gould OS 250, which is only a 10 MHz scope, so the probes for that may not be very well matched to the OX 8050.
If you don't mind the wait, those **broken link removed** seem reasonably good although you might have a bit of a delay while they ship from China.
Certainly for the price they make a good spare, for when the inevitable happens and you break a probe. They claim to be good for 100 MHz so they should work fine on your 8050. But reading the small print, it is likely that 100 MHz is only available on x 10 mode (better response on x 10 is usually the case, unless you spend $100+ on a probe), the x 1 response is probably lower, perhaps as low as 6MHz. Spec. of the probes I linked is below.
Code:
Module: P6100
Attenuation ratio: 1:10
Input resistance:1M¸/10M¸
Input Resistance: 1M:10M
Bandwith: DC-6MHz, DC-100MHz
Input Capacitance: 1X:85pF-120pF, 10X:18.5pF-22.5pF
Compensation Range: 10pF-30pF
Working Voltage:1X:<300VDC+AC Vp-p, 10X:<600VDC+AC Vp-p
Definitely good enough for most things, and certainly good enough for audio work and low end microcontrollers. The response doesn't drop off a cliff at 6MHz on x 1, but becomes progressively poorer and using the x 10 attenuation mode with them on your OX 8050 should still allows you to fill the screen with a +5v signal at 60MHz. In the unlikely event that one of those probes also shows a problem, then we need to look at the set up of the front end.
That would be a learning experience for both of us, since we don't have a service manual. Don't go fiddling with the trimmers on the front end until we are certain we have a problem though. It can be a bit of a nightmare getting things working accurately again after someone has been for a random twiddle.
There is a good
video of Dave taking us through adjusting a Tektronix 222v 50 MHz scope. If you haven't seen the
EEV blog, bookmark it, subscribe and join the forum. Lots of good stuff there.
If we do end up having to go down that route, I suggest we would need to take a few pictures of the board to reverse engineer the layout and function of the necessary trim pots and caps.
Scope probes are a little misunderstood, their response is always a bit of a compromise due to the way they work. If you are up for a bit of a 'scope probe electronics lesson you will find this fascinating.
http://www.dfad.com.au/links/THE SECRET WORLD OF PROBES OCt09.pdf
Of particular interest is Figure 3 - the diagram shows the multiple square waves obtained by trimming a scope probe, overlaid one on top of the other. You can see the effect of over and under compensation (the rounded shoulder you are seeing versus the spike and dip of over compensation). Your dual Analog/Digital scope will give you an insight in to the value and limitations of both types of scope. If you ever see another one of those OX 8050's going cheap let me know, it looks like a very useful tool.
Self taught is probably how most of us learned the majority of what we know. I've been learning electronics all my life, and am still discovering things. The first rule of electronic and electrical engineering is safety first, so if you do go prodding around on those headers I mentioned (J18,J19 etc) be aware that there are some high voltages near there (particularly next to J18/19) so proceed with caution. Also worth noting is that I am based in Scotland, so expect some delays in my replies.
EDIT: Just checked the probes on my Tektronix TDS 1012B (100 MHz - 1GS/s) in the office, they are also only good for full bandwidth (200 MHz) on x 10 and 6 MHz on x 1