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IBM 5154 Problems & Electrolytic Caps Question

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    Are the power supply output voltages stable ? Minor voltage fluctuations may not show up on a modern digital multimeter.....
    but would show up on an older analog meter (Simpson/Triplett) or an oscilloscope.


      I'm noticing more weird things. First, the text sometimes starts moving in a wavy motion as shown here. I also looked up the part number on the green "25000V" part of the flyback transformer and found it on eBay here, is this exactly what I need to replace? Or does the "8000V" part need to be replaced too? Finally, I noticed that the pins on the board for the "8000V" transformer also have pins in a circular arrangement (like those be used by most flyback transformers) connected to the pins in the square arrangement. Could a more modern, easily obtainable flyback transformer be connected to these?


        Great idea with the AM radio Chuck(G).

        Well now at least you know what you are seeing is changes in the geometry of the raster due to an electrical discharge altering the EHT, so where is it?

        Wherever it is, it is likely drawing current (intermittently) from the output of the EHT transformer secondary. So this could be anything in the HV tripler, focus control and its wiring to the CRT socket assembly and the CRT's anode cap (externally on the CRT) along the surface from the anode connection to the external aquadag. This can also happen if the CRT's external aquadag is not earthed properly or connected to the correct place. So which is more likely ? Probably the discharge (arc) is happening in air, not in a potted medium and likely after the EHT cable exits the tripler.

        Since the tripler assembly is potted as is the EHT transformer, they are probably less likely to be the culprits (could always be proved wrong).

        I would leave the set off for a while . Put a thin grounded screwdriver blade under the EHT cap, discharge the CRT. Fold back the anode cap a little, reach under with thin long nosed pliers, squeeze the anode clip pronge together to release the anode cap from the CRT bulb. Clean the surface of the CRT all around the anode connection, clean the cap (use CO contact cleaner or similar, nothing ionic like windex). Then coat the inside of the cap and CRT bulb around the anode connection with clear silicone grease and re-fit the cap.

        That will then help to exclude the anode cap/connection area.

        Check the wiring leading to and from the focus control and leading to the CRT's base clean the pcb around the focus connection and check for anything that looks suspicious. However, since the focus is staying good with the fault, likely its somewhere from the EHT output from the tripler, to the CRT anode.

        Check places where the EHT wiring gets anywhere near a metal object on the way to the CRT's anode.

        If everything else gets excluded, it would more likely be the tripler assembly than the EHT transformer.

        (I would still double check the main power supply voltages are stable as a similar effect could occur with unstable voltage to the scan stages if there was some intermittent arcing in the main power supply , but on balance it looks like the EHT system is the likely problem).
        Last edited by Hugo Holden; July 27, 2019, 04:14 PM.


          I finally worked up the courage to check the voltages on the PSU. It looks like all of the lines work (-6.1V for -6.3V, 11.9V for 12V, 20.16V for 21.2V, 55.59 for 56.7V, and 150.3V for 154V). They all seem to be pretty stable. However, two pins also seem to be giving random voltages (one has 2-7V, the other has 15-25V). IMG_0999-2.jpg. The longer I measure them with the multimeter, the more they will decrease. I've also noticed now that the screen will now turn completely green again. The raster is green, and everything has a green tint. The green tint will turn on and off at seemingly random times. You said earlier that no capacitor deals individually with the red and blue colors, so what would cause this?
          Attached Files


            Originally posted by willmurray461 View Post
            I've also noticed now that the screen will now turn completely green again. The raster is green, and everything has a green tint. The green tint will turn on and off at seemingly random times. You said earlier that no capacitor deals individually with the red and blue colors, so what would cause this?
            There will be a DC offset appearing at the cathode of the green CRT's gun (causing the cathode voltage to drop and the green beam to turn on) Less likely a simultaneous fault in the R & B channels increasing the cathode voltages on both there.

            The green channel amplifier is a direct coupled affair, right back to the Green input circuitry. It could be a poor solder connection to any component in the entire green channel, possibly a defective resistor transistor. The thing to do is to monitor voltages at various points along the green channel amplifier and try to find where the problem is.

            (there is a small chance of an intermittent heater to cathode short in the actual CRT that could lower the cathode voltage turning on the green gun, fortunately that is less likely than a problem in the green amplifier chain).


              Could you give me the names of the parts which I would need to check, or perhaps where they are on the board?


                I will look at the schematic when I am home.

                One thing you could do is disconnect the connection to the Green gun cathode of the CRT (at the video amplifier outputs), connect it onto one of the other cathode connections (either the R or B channel video amplifier outputs/ cathode connections) the colors will then be wrong because two of the guns will then be getting the same drive, but if the intermittent green changing effect goes away, you can then be definitely sure that it was an offset voltage coming out of the green channel amplifier, and not a green CRT gun heater cathode defect in the CRT or a bad CRT socket connection to the green gun's cathode. If you don't really get what I am saying here, and are confident to do it, don't attempt it yet.


                  Looking at the schematic, T707 is the green video output transistor, leading to itfrom the input connections, buffer IC 501 handles the green signals (MSB & LSB) these pass via buffer IC 502, buffer IC 503 output pins 6, 8 to T702 & T703 transistor to mix the LSB & MSB green signal into the base circuit of T706, green driver transistor, via T705 to the base of T707 the output transistor. So you would need to measure the outputs (preferably with a scope) of each of these stages to find where the signal has intermittent change. The hottest running parts of this circuit are the output transistor and its emitter load resistor R716, check the solder joins.

                  Possibly though it could be the pots. Mark the position of the adjustment potentiometers RT702 (green lsb) RT701 (green msb) and TR703 (green cutoff) and give them a little wiggle back and forth. Sometimes an intermittent connection of the pot wiper to the track can cause the problem you are seeing, just for example if RT703 went momentarily open circuit, then the base voltage of T706 would shoot up and its collector voltage drop. This would pass via T705 to T707, causing the emitter voltage to drop and the green gun to turn on.