SE Labs Scope Repair

[pwdrive]

If it doesn't trigger on CH1 or CH2 then I can't see anything on that schematic that's common to both channels in a switched trigger sense unless I'm missing something.

Lawrence.

[Geaney]

Could you add some Waveforms to the Diagram?
Use a simple Sin wave input to External.

Alan

Hi Alan, OK will try to set that up at the weekend.

Cheers, John

[Geaney]

If it doesn't trigger on CH1 or CH2 then I can't see anything on that schematic that's common to both channels in a switched trigger sense unless I'm missing something.

Lawrence.

Hi Lawrence,
TR66 is driven by an oscillator when the trigger is set to 'chopped CH1/CH2' so I assume this switches the output of each channel trigger amp alternatively. Each channel amp is pretty much identical. My hypothesis is that since the original big fault was the power supply went bang (with smoke) that one of the rails did something to take out both channels since the same resistors show signs of overheating in each one.

I'll get some more time it at the weekend.

Cheers, John

[Geaney]

Here are some waveforms - the source is the scope's own calibration square wave.

The amplitude of the signals at this point track the settings on the scope front panel so I know I'm looking at signals from the CH1 input.
The image showing the signals on the collectors of TR55 and TR56 looks good and these are tapped off to go to the display where I can see the waveform on the scope screen so long as trigger is in Auto mode. The waveforms at TR59 and TR60 clearly differ so I imagine it is here that my problems start. I can find no signal on the common base connection between TR59 and TR60 but since this is just pulled up to the 17v rail I was not expecting to see anything there.

The investigation continues.

Cheers,

John

[pwdrive]

In the second picture where were your scope probes connected to:

A) Upper trace comes from?.......

B) Lower trace comes from?.......

Lawrence.

[Geaney]

In the second picture where were your scope probes connected to:

A) Upper trace comes from?.......

B) Lower trace comes from?.......

Lawrence.

oops, Hi Lawrence,

A) Upper trace comes from?.......collector of TR60

B) Lower trace comes from?.......collector of TR59

Cheers, John

[Geaney]

Since the 60v rail is a bit high at around 67v I did some checking but can't find anything out of spec. I checked the ac ripple on it - diagram and ripple info images attached. If this looks OK I'll carry on.

Cheers, John

[AlanBeckett]

I din't think I understand the Display John.
Is that really around 3 Volts P-P?

Alan

[Geaney]

I din't think I understand the Display John.
Is that really around 3 Volts P-P?

Alan

Hi Alan, yes that's 3v p-p. I thought it was high given the full wave rectifier nature of the power supply, etc. Maybe there's a dodgy diode in there somewhere - the caps all check out OK though most of them have not been replaced - except the biggest one (900uF).
Cheers,
John

[AlanBeckett]

With all due respect young John 3 Volts P-P on a 60 Volt Supply is bloody awful. Something's wrong.
To save me going back through the Circuits is it meant to be Stabilised?

Alan