SE Labs Scope Repair

[Geaney]

Here's a shot of the circuit area I'm working on. I found the R240 and R244 were badly burned so I replaced them. The replacements have not burned so I'm assuming that the conditions that caused the original excessive current draw are no longer present. The most likely reason is the original fault was the power supply going bang complete with smoke, etc. This could have caused a surge on the +60v supply. The +60v supply is now stable and spot on the right value. It could of course be something entirely different.

A few quick voltage check show +60v at test point 35, +36v at the junction of R240 and R244, +17v at the junction of R240 and the cathode of ID26, and +7v on it's anode. The anode of ID27 appears to be zero.

The output (still missing) leaves on test point 48 and goes from there to the timebase module. Both channels have the same pair of resistors burned so whatever happened took out both channel trigger amps.

Cheers, John

[pwdrive]

1TR 59?

Lawrence.

[Geaney]

I've been in the weeds with this project. Looks like someone previously did a lot of work on the channel amp modules and made a right mess. I've been finding dry solder joints, shorted wires, lifted pcb tracks, etc. I think I have it at a stage now where it is relatively stable with most of the nasty work sorted out. I can hopefully get back to finding the original fault now. Just to keep me motivated I lost a few days because of discrepancies between the manual and the actual circuit - nothing like putting the scope probe on the wrong test point and chasing ones tail trying to trace the signal flow.

Ho Hum, at least I went down with the flu a few days ago so I've had to take a break from it

Cheers, John

[Geaney]

Well that bout of flu turned out to be two weeks of banging on Death's door asking to be let in followed by another few weeks of endless coughing. Glad to be finally over it and I certainly hope none of you guys are unfortunate enough to catch it. My flu jab was set for 3rd Nov but the real thing caught me first this time.

Anyway, back to the scope. I was pretty sure I'd narrowed it down but having now replaced all the relevant components in that part of the circuit - still no trigger output. I've managed to scan the documentation so I've attached the latest summary - showing the components I've replaced and the static DC voltages at the various nodes in that part of the circuit.

Originally resistors IR240 and IR244 clearly showed signs of overheating so I replaced them with 0.1% tolerance half watt versions. The replacements get hot too and based on the voltage drops I've measured they are sinking more than 500mW so, not good.

I have good signal up to the transistors ITR55 and 56. It then splits off and continues via ITR57 and 58 and from there off to the display - that part is fine. It should also go via ITR59 and 60 to the trigger amp but there is no signal at either emitter.

As I've said before, my knowledge of the world of three legged fuses is somewhat limited so I'm open to all reasonable suggestions as to next steps.

In the attached diagram, all the comps in red have been replaced (plus ITR61). The two resistors shown in pink (241, 218) are in very difficult locations so would require some major dismantling to get at. They measure fine so I've left them in for now.

Cheers, John

[Refugee]

Are you getting any signal at the collectors of 1TR38 and 1TR39?
The hot resistors will be caused by that 60V rail being a bit high.

[Geaney]

Are you getting any signal at the collectors of 1TR38 and 1TR39?
The hot resistors will be caused by that 60V rail being a bit high.

Yes that branch is working fine so I get a trace on the display but it only triggers on 'auto', not when I select the channel input itself. I'll check the 60v rail and see if I can tweak it down a bit.

Thanks.

Cheers, John

[Refugee]

It looks like the trigger signal comes in through 1TR66 and that there is only a little bit of biasing adjusted with the trigger zero preset coming from the Y amp power supply. All the rest looks like it runs from 12 volts.

[pwdrive]

Is the loss of trigger just on one channel or both?

Lawrence.

[Geaney]

Is the loss of trigger just on one channel or both?

Lawrence.

Same fault on both channels.........

[AlanBeckett]

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

Alan