Heath 4552 (aka pintek 251, elenco s1325) arc and audible pop, HV???

[DavidCG]

Hello,
so here is another pintek/elenco/heath oscilloscope repair.
I've repaired broken solder joints, tested the capacitors for leaks (all ok), no sings of burned components, and measured the power supply outputs (+/- 12v, 144v, 5v. I don't have a HV probe so I can't measure that) the 144v was a bit high but I did the measurement with no load, directly from the linear regulators so I guess this is ok.

ok so i turned on the unit with the current limiter and i immediately heard a "POP" . I turned the unit off, waited 30 seconds (because the popping scared me) and turned it back on, heard the popping again. I repeated this 3 more times and could see a small ARC right on the HV section of the top pcb (I recorded it with my cell phone). The weird thing is that after that I was able to leave the unit on without hearing that pop again (No trace, intense dot more like a blot).

My suspicions are:
-preavious repair: Not by me, there is a 100v zener diode in the hv section replaced by two zeners (two different values in orther to reach the 100v...i guess, like i said im not making any measurement until i figure out how to do it safely)
 -negative HV: the scope uses -2.1kv, there is a potentiometer to set up the value of this negative HV. Im not going to mess with this until i make a plan to make the measurement safetly.
- large capacitors in this HV section?: Both flashing i taked on video happens were two large (poly?) capacitors are. I suspect of this because of the fact that ther was no more pops/flashing after i turn on and off the unit 5+ times
- pcb arcing: I seems the flashings/arc occurs on the trace side of the pcb, not the component side. So that would mean this scope sucks like no one? . anyone knows if ROSIN prevents this kind of arcing when the rosin itself solidifies?.

Im going to visual inspect the pcb.
..then i have to repair horizontal or maybe trigger section because of the No trace 

i ask for your help/suggestions/warnings.

thanks!

[wasedadoc]

The flashovers may be caused by the HV generator producing more than 2.1 kV.  There is a feedback circuit which should regulate that.  Check the chain of four resistors from the 2.1 kV to the +ve input of the 741.

[DavidCG]

The flashovers may be caused by the HV generator producing more than 2.1 kV.  There is a feedback circuit which should regulate that.  Check the chain of four resistors from the 2.1 kV to the +ve input of the 741.

thanks!, Yes, one resistor (R958 - 1.5M) in the feedback circuit was "open", i guess the reactance of the parallel capacitor was making HV to be on the sky (i dont know how to calculate that). Im preparing to replace that component, already check all of the other components in this flyback circuit with another osciloscope component tester and a DMM to be sure.

p.s.
I know that isolated flyback topology works with a chopper switch, in this case a BJT transistor D880. How is PWM (i guess) generated with that opamp and the xfmr feedback Coil? armstrong oscilator? (i want to use my other oscilloscope to test the pulses. the flyback use +-12v but to do the test i will use +-3v from another PSU)

[wasedadoc]

The opamp is not part of the oscillator. The transistor is in a blocking oscillator circuit with positive feedback from the 1.45 ohm winding to its base. The opamp adjusts the dc voltage at the transistor base and that changes the on/off time ratio.

You may find that it will not oscillate when powered from only 3 Volts.

Ensure the replacement resistor is rated for more than the voltage across it. If the four are all 1.5 Mohm, then about 500 Volts across each.

[DavidCG]

The opamp is not part of the oscillator. The transistor is in a blocking oscillator circuit with positive feedback from the 1.45 ohm winding to its base. The opamp adjusts the dc voltage at the transistor base and that changes the on/off time ratio.

You may find that it will not oscillate when powered from only 3 Volts.

Ensure the replacement resistor is rated for more than the voltage across it. If the four are all 1.5 Mohm, then about 500 Volts across each.

thank you. You were right. Here is a picture of the chopping on the Collector of the transistor

The Good news,
i was able to repair the HV section, now all of the voltages, +12, - 12, 5, 145 and 2Kv are correct!

the bad news:
-at first, no trace, not even a dot on screen. I measured transistors and diodes on the Vertical and Horizontal Final Amp. LOTS OF DEAD TRANSISTORS (15 more or less), also all the logic ICs do not work, i haved to take it from the sockets, then make a circuit on the protoboard just to test them (74LS74, CD4011, 74S00), i guess they broke because of the HV arcs and flashings. I replaced all of this faulty components. Now i just have a dot, it moves with the horizontal position potentiometer, it does Not move anything at all with the vertical position pot, but the dot is in the middle of the Y axis, so i guess the vertical final amp its ok (if not, it would be a little more up or down in the y axis right? )

I tested the vertical attenuators, on both channels, by removing the pcb, they seem OK. The sweep generator is in this pcb too IMG_3468.JPG , its single opamp is completaly dead, i replace it, but i dont know how to test this sweep section to confirm tha now all is ok

ok, so now here are my problems:
i dont know how to test or debug the Sweep generator and the trigger circuit , i dont know how it works and if i make measurments of the Q or Q̅ pins i will not know what it would mean   (if anyone can give tips on this i would apreciate it a lot, i dont even now how to google on this with out lost my self)
i guess i need to test the vertical preamp (because the attenuators are ok) and confirm that the Final Vertical Amp its ok too. how i do this without risking the circuit and my other scope(the one im using to make the measurments)?? i have to use x10 probe in orther to not affect the Device under test?

[wasedadoc]

The sweep sawtooth should be generated at the junction of R714 and R716.  It is buffered by the FET (Q703) and the NPN (Q704).  The emitter of Q704 is where I would look for it.  Increasing positive ramp at a rate dependent on the timebase range switch and the variable control. Faster negative going part of the sawtooth.

You will not see the sawtooth on any pins of the op-amp.  Its function is to keep the emitter of Q702 at the same voltage as the wiper of the time/div variable control.  That keeps an adjustable constant voltage across whichever of the timing resistors in RN701 is selected.  Depending on that resistor value, a constant current flows from the +12 Volt supply through the resistor and through Q702.  (Negligible current goes into the inverting input of the CMOS op-amp.)  Q701 is off and D702 is reverse biased so the current goes through R714 and charges the timing capacitor - either C719+C705+VC704 or those plus C706.  A constant current into a capacitor gives a linearly increasing voltage.

Subsequently a pulse coming in to the base of Q701 turns it on and through R714 discharges the capacitor relatively quickly causing the fast negative going portion of the sawtooth.  When the pulse ends, Q701 goes off and the charging part of the cycle begins again.