Sony / Tektronix 326 Oscilloscope
This little 1970's scope is newer than some of my other test equipment, but still has some age on it. It only has one IC that I can see, a UA723 voltage regulator in a can, but approximately 123 Sony transistors. It's a 10MHZ dual channel scope with a 300ns delay line built in. It has a Ni-Cd battery pack and can also run from external 12 VDC as well as 120VAC. The inputs for the scope are on the left side.
I picked it up at a hamfest for not much money due to its condition, but it did have a trace on screen when I bought it. The trace lasted until about the time I got home and then faded away. It was there just long enough to convince me to buy it. After opening it up and seeing how butchered the power supply was, I'm amazed it ever had a trace at all.
I picked it up at a hamfest for not much money due to its condition, but it did have a trace on screen when I bought it. The trace lasted until about the time I got home and then faded away. It was there just long enough to convince me to buy it. After opening it up and seeing how butchered the power supply was, I'm amazed it ever had a trace at all.
This is how it looked when it came home.
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Another view of the front panel and the crud covering it.
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The NiCad batteries were covered in potassium carbonate, but cleaned up OK and held a charge. The batteries lasted a few months until I forgot to turn off the unit one day and ran them completely down. A couple of the 1.5AH C-cells in the pack died so I replaced all of them with new 1.5AH sub-C cells.
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At least the battery compartment was nice and clean. The old C-cells were a tight fit, but after replacing them with new, smaller sub-C cells the pack slides in and out easily.
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The scope with the top and power supply covers removed
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Removing circuit boards for cleaning and repairs.
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The power supply has been removed to make repairs. Five transistors in the 5-volt regulator were shorted or open. The regulator transistors are located on the vertical amp board. Also 2 transistors in the power supply that control the battery low circuit were bad.
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The traces to the two current driver transistors for the flyback power supply were badly damaged (on middle PCB) and had to be repaired. In this photo the traces are being pieced back together with new copper and epoxied.
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Gold plated machined IC pins were inserted in the transistor lead holes. Most all the transistors in this scope plug into socket pins on the printed circuit boards. Not sure if the current driver transistors were in sockets originally but there was little choice due to the damaged traces.
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The original current driver transistors are very hard to find so I experimented with substitutes. A single SK9366 worked well in the circuit so I used it instead of the original paralleled transistors because it was too difficult to mount the original TO-5 cans on the damaged board and still screw their slip-on heatsinks to the chassis.
The red loop of wire in the photo is where you're supposed to connect a Tektronix oscilloscope current probe to set the flyback transformer current. But the current probe costs more than the scope so I need to come up with an alternative way of setting it. Initially I set it to where the unregulated power supply voltages were at their nominal values, until I can come up with something more accurate. |
It's working now and initially calibrated to the point of being usable, but still needs more in-depth work and calibration to truly call it done. Just need to find enough time to put it back on the bench.
This ol' scope has had a rough life. I've been using it off and on over the last year without issue, but now a new problem has popped up. Both the primary windings in the charger unit's power transformer are open. But no worries, the windings went before the fuse, so it's just fine. The hundred dollar part will save the 50 cent part every time. I suspect that from all the power supply issues this scope has had before I got it that probably one of the two primary windings was already open, and its been running on one lung, so to speak. And now the second has given out, maybe from the demands of the new battery pack.
Fortunately I found a new replacement transformer. Tektronix didn't mark a single voltage on the schematic for the charger so I had to experiment a bit to see what voltages I needed from the two secondary windings. The closest I could come in a generic transformer, that would fit in the same space, was a toroid with dual primaries and dual 12-volt secondaries. This is not a 24VCT (center tapped) secondary, but two separate 12 volt windings. It's an Avell Lindberg Y236102 from Parts Express and worked in circuit without needing any modifications. Just had to reset the charging current.
Fortunately I found a new replacement transformer. Tektronix didn't mark a single voltage on the schematic for the charger so I had to experiment a bit to see what voltages I needed from the two secondary windings. The closest I could come in a generic transformer, that would fit in the same space, was a toroid with dual primaries and dual 12-volt secondaries. This is not a 24VCT (center tapped) secondary, but two separate 12 volt windings. It's an Avell Lindberg Y236102 from Parts Express and worked in circuit without needing any modifications. Just had to reset the charging current.
Page created 9/25/2019
Last edited on 3/21/2020
Last edited on 3/21/2020
