Heath 4552 (aka pintek 251, elenco s1325) arc and audible pop, HV??? - Page 1
Forum section: electronics / repair

author: DavidCG
author_rank: Newbie
timestamp: August 25, 2022, 09:49:21 am
content: Hello,so here is another pintek/elenco/heath oscilloscope repair.I've repairedbroken solder joints, tested thecapacitorsfor leaks (all ok),nosings ofburned 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 thelinear regulatorsso I guess this isok.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 asmall ARCright 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 tracei ask for your help/suggestions/warnings.thanks!

author: wasedadoc
author_rank: Super Contributor
timestamp: August 25, 2022, 07:04:40 pm
content: 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.

author: DavidCG
author_rank: Newbie
timestamp: August 30, 2022, 06:50:28 am
content: Quote from: wasedadoc on August 25, 2022, 07:04:40 pmthanks!, 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 transistorD880. 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)

author: wasedadoc
author_rank: Super Contributor
timestamp: August 30, 2022, 07:13:20 am
content: 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.

author: DavidCG
author_rank: Newbie
timestamp: September 24, 2022, 02:35:00 pm
content: Quote from: wasedadoc on August 30, 2022, 07:13:20 amthank you. You were right. Here is a picture of the chopping on the Collector of the transistorThe 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 logicICs 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 thehorizontal position potentiometer, itdoes Not moveanything at all withthe 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 tooIMG_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 okok, 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?

author: wasedadoc
author_rank: Super Contributor
timestamp: October 04, 2022, 01:56:38 pm
content: 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.