DSE Q1260 (SAMPO SS 2020) oscilloscope - no vertical

[blaing23]

Hi all, have just acquired this analogue scope which was working before sale,  but now it's not...

Originally sold by Dick Smith Electronics, and appears to be a SAMPO SE 2020 (manual here: http://www.eserviceinfo.com/downloadsm/26699/sampo%20/%20voltcraft_SSI-2325.html)

Since I'm new to using an oscilloscope tried a couple of things to check is was working while learning what the controls did, but found it didn't match what I expected

• 1.5v battery, channel 1 DC, volt/div 0.5, showed not change in trace ie didnt move further up the screen. Can move trace up and down the screen via the y-pos knob. Adjusting time base moved line to slow dot etc, intensity, focus, beam find etc all working as expected.
• Tried component test, 0ohm/short should have a single vertical line, but only changes the 5 graticule wide horizontal line to about 1/5th graticule wide, but no vertical movement - should be a single vertical line according to manual.
• Changed supplied probe (x10) with new x1.
• Tried connecting to CAL, but no square wave displayed (same flat line) - changed to AC.
• Channel 2 has same behaviour as Channel 1. INV appears to work - shifts the horizontal line from center to a couple of graticule lower.

also had a read thru of the tektronix troubleshooting doc, and think fault is in vertical section, however that's where my understanding stops.

Any help with how to get this working again would be appreciated...

TIA & Rgds

[tautech]

Welcome to the forum.

So you've worked through the instructions on P12 and can't get the 4 div 1 KHz square waveform ?
Check again all controls are set exactly as listed. Tell us what you see and where it is on the display.

If your DMM does frequency check the probe Cal output is ~1 KHz.
Check all but the EHT voltages from the PSU unless you have a 2KV capable DMM. All the LV supplies should be close to stated values. (P31)
Check ripple (using AC DMM) is under 50 mV on LV rails and under 100 mV on the 260 and 140 V rails.

If all looks OK then we start by following the signal path with just the probe Cal as a source where I suspect you might find the dual FET's in the input stages are blown (P25), not uncommon from an overload event.
To fully trace the signal path you'll need another scope and some knowledge as to where to probe and know what signal to expect.

[blaing23]

Thanks for the reply, and the welcome...

Yes, that's correct re P12 in manual and no vertical change. Checked CAL via DMM and frequency about 0.997 on 2K Hz setting, so approx. 1K Hz as expected.

Will check voltages and ripple (having now seen eevblog #594 "how to measure power supply ripple and noise") over weekend and report back.

To clarify, are the FETs you refer to, marked as T101 and T201 on P25?

TIA & Rgds

[tautech]

To clarify, are the FETs you refer to, marked as T101 and T201 on P25?

Yep, that's them but I wouldn't attempt replacement offhand until checking the signal path does cease there.
Sort of hard without another scope but with the least attenuation setting your DMM might be up to it.

[blaing23]

This issue is now resolved thankfully.

On closer physical inspection the trace rotation pot (VR588) had been repair on one leg previously. Additionally both the left and right legs had separated from the body of the pot. The orginally repaired leg also had lifted the track. Replaced with a smaller (physically) 5K pot and soldered new wires to complete the broken tracks.

Now on to the new Telequipment D61A with no trace, just a dot that doesn't move...stay tuned for a new thread.

Thanks for assistance.

[tautech]

Well done and good detective work. 

Now on to the new Telequipment D61A with no trace, just a dot that doesn't move...stay tuned for a new thread.

Is that the one I saw on Trademe..........I used to have a D83.

[blaing23]

Yes, that's the one - now I've got one working should be in better shape to sort the second - it was cheap enough so if it doesn't work out I can always reuse the case with a raspberry pi and LCD screen (or maybe use the tube etc as a pong game :-))