I recently made my first purchase from the United States, they seem to have a lot more interesting Radio & Television service equipment than can be found in the UK. One thing in particular being Scan coil and LOPT testers. This Heathkit offering seemed ideal, as on the rear it stated it operates on 120V/60Hz -240V/50Hz. So the order was placed and a little over 3-weeks later, the device arrived at my door. Payment for the item, shipping, Customs etc was all sorted in one payment, all very simple.
What I found appealing about this tester is it can not only test out of circuit, but in circuit too. Rather than having to isolate components, allegedly you can test the entire output section for both Valved and silicon stages. Also Perform peak-to-peak drive voltage checks. All a little more sophisticated than the Go-No-Go of my Jabco or a simply ringing with a scope.
I'm reasonably confident as to what I need to do, to get it running on 240V instead of the 120V it was on. However, being a constant doubter, would like to just run it past you guys.
The transformer is currently looped 1 to 3 and 2 to 4 configuring the TX into 120V mode. To configure the TX to run on 240V I remove those loops and just loop the middle two, 2 to 3. Do you agree?
I have the full-service manual and will upload to the data library
Despite wiring as shown, something is amiss. It does power up, the indicator light illuminates and oddly the meter deflects negative. I wondered if it might need D.C. and A.C. calibration due to now operating on 240V. I then noticed what looks like a past burn up on R144 (220R), there should be 15V here, but there's only 500mV. Further checks reveal no 23.5V rails.It looks like I've voltage into the TX but nothing coming out at point (R) & (N). Wonder if the TX is dead.
Hi Chris, it should be relatively easy to obtain a suitable transformer to provide about 24-0-24 volts, assuming this one cannot be repaired... I guess you can check continuity of the windings to see which is open.
But you need to find out what, if anything, caused it to fail. Maybe a short elsewhere?
To understand the black art of electronics is to understand witchcraft. Andrew.
As Andrew suggested, I assume both primary windings are continuous? A transformer with 2x 24V windings will be an off-the-shelf component from the likes of RS/CPC/Farnell.
Found a lot more wrong, two 1N4002, diodes had failed, two 15V zeners also and R144(220R/1W) massively overheating. Turns out an LM311P was shorted. None in stock, some on order, awaiting arrival.
Fed up waiting for the LM311 IC to arrive, I thought to myself, I must have an LM311 somewhere on one of my Stock PCB's. I felt the most likely candidates would be VCR or laserdisc player boads
With that in mind, the first board I located a VLP700 PSU/EHT generator PCB, would you believe it, the only IC on that board was an LM311N, what a stroke of luck. Moreover, it was socketed so an easy extraction.
It was swiftly installed into the IT-5235, again socketed and I powered up, this time the meter did not deflect in a negative direction and R144 was not overheating, result.
I then next went through the DC calibration, thankfully the unit still had its DC volts calibration sticker, so I knew it had to be calibrated to 7.36 on the 10kV scale. Result, it was spot on. Next AC calibration, that should read between 13-17V on the 30kV scale, and it does.
Simple test now, out-of-circuit LOPT test. I chose the LOPT I intend to install in the G6 this coming winter. Rotating the Ring selector, a good LOPT should ring on every setting. Happy days, not only is the Flyback test working, the LOPT is good.
This was the main reason I imported the tester from the states, its a far more sophisticated device/circuit, and as such so is the test. An oscilloscope ring test or the go-no-go of the simple JABCO tester are inconclusive at the best of times.
A brief description
The secondary output of the power supply is rectified to produce the line sync pulse that is used for the Ring operation. The test applies an exciting pulse to the coil under test and detects the number of rings which are of preset amplitude or higher. Set at 25% of the initial ring, which is adjustable from 10% to 50%. A bad reading for less than 9 rings, a good reading for more than 11 rings. Drive voltage levels in deflection circuitry 0 to 30 V P-P and 0 to 300 V P-P.
The line sync pulses from the power supply are shaped by Schmidt triggers to create the short pulse needed to energise (ring) the device under test. A diode provides a clamp to protect the Schmidt triggers from negative-going pulses. Another diode provides a clamp to protect the base of Q101 from excessive negative pulses of more than about -16 volts. Transistors Q101 and QI02 invert and amplify the pulse and provide the needed drive capacity. The output line sync pulse is routed to the coil-impedance matching combination, the buffer, and the pulse accumulator discharger. Diodes block loading of the ring pulses from the coil being tested and, with Q101 and Q102, also limit (quantise) the ring amplitude by not allowing TP1 to go below about -17 volts. This clamps the first negative going ring at that value of -17 volts and thereby standardises the maximum voltage of the first positive ring. This allows the tester to retain calibration despite encountering a wide range of coil characteristics.
An N-channel FET, provides a high impedance input to avoid loading (damping) the ringing of the coil-impedance matching combination. The output of FET goes to the comparator. A diode and resistor combine to protect the gate of FET from overvoltage damage. The comparator receives the line sync pulse and the subsequent ringing of the coil. It then compares the amplitude of this input from the buffer with the reference level. If the input pulse is greater than the reference, then the output of the comparator goes low; if the input pulse is less than the preset value, then the comparator does not produce any output and stays high.
Another IC inverts the negative pulse from the comparator and produces an output with a consistent pulse width. The ring calibrator adjusts that pulse width to calibrate the circuit to the meter scale. A diode provides overvoltage protection for the IC's input. Pulses from the pulse former charge a capacitor, a diode prevents the charge from leaking off until the end of each line sync measurement cycle. At that time, the line sync pulse from the line sync pulse generator turns Q104 on, discharging the cap in preparation for the next line sync measurement cycle. Thus, the peak charge voltage on the cap is only related to the number of ring cycle pulses that reach it during each measurement cycle.
An N-channel FET, QI06, provides high input impedance to avoid loading of the pulse accumlator/discharger in the Ring mode of operation. Again, a diode provides overvoltage protection for the gate of the FET. Q105 acts as a follower to QI06, and supplies the drive current to charge a cap, which holds the peak voltage for the meter driver. The peak detector is used for both the Ring and the Volts P-P modes of operation.
The tester can also provide two voltage ranges (30V p-p & 300V p-p) for measuring the drive voltages to the TV's output stage by selecting the 300V p-p button, connecting the red VOLTS test lead to the output valve grid (on valve TVs sets) or the drive transistors base (on solid-state TVs). It is also an EHT meter ranging from 10kV - 40kV, sadly the HV probe was missing.
13-days later the LM311P I ordered for this project turned up. Oops, me thinks I made a mistake on the order, for number required, a zero where one shouldn't have been. I thought the bill seemed a tad higher on checkout, but as it was bundled in with a load of other bits, ignored it. Hey-ho, looks like I'll be sorted for comparators for the foreseeable future 🤣
13-days later the LM311P I ordered for this project turned up. Oops, me thinks I made a mistake on the order, for number required, a zero where one shouldn't have been. I thought the bill seemed a tad higher on checkout, but as it was bundled in with a load of other bits, ignored it. Hey-ho, looks like I'll be sorted for comparators for the foreseeable future 🤣
You didn't fall for the old trick of not noticing they were supplied in multiples of ten, did you?!
Colleague at work did that: I'll not be short of A5 board-backed envelopes as a result...
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