The Technics ST-S7 receiver was introduced about December of 1979, and was top-line equipment back then. Due to an unfortunate power problem at my house, my ST-S7 was "ruined", in that the power transformer (primary) was burned out, and worse, the vacuum florescent display was also destroyed (burned out filament).

When I realized that the VFD was dead, I was quite dismayed. Parts have been unavailable for decades, so I can't buy a new one. I looked online for generic replacement VFDs, to no avail. Then I wondered if I could make an LED replacement for the VFD. The service manual is available, with a full schematic, even including voltage readings.

Perusing the schematic showed me that the display was "normal", in that it was multiplexed, with a select line for each digit, and 7 select lines for the 7 segments of the digits. The microprocessor is normal, too, running on 5 volts. This might be do-able.

Next step was to hack in a replacement for the toasted power transformer. The stock transformer has three windings: 30v, 9v, and 3.2v for the VFD filament. We can forget that one. So I cobbled up two transformers from my collection, and with 30v and 9v applied, the unit "works", in that it responds to button presses (and would probably make audio if I hooked it to an amplifier).

Studying the schematic, I see that the digit selects are active low, and the segment drive is active high. I dug out some common cathode 7 segment displays from my parts collection, some experimenting with bread boards later, I have a functioning LED display.

The digit selects run through transistors in a ULN2081, which then pull the cathodes to ground as each digit select goes high. The segment drives connect to the gates of P-channel MOSFETS, which then supply 5V via some (initially 220 ohm) resistors to each segment.

Many hours of fiddling and experimenting later, I figured I had the circuit dialed in. I fired up KiCad, drew the circuit, and then generated PCB artwork. I've uploaded this to JLCPCB and am awaiting the boards now (August 16 2026).

While I wait for shipping from China, it's time to work on the power supply.

Power Supply Repairs and Modifications

The original transformer had three secondary windings, of which we now need two. With the tuner running, I can measure the current required from each different winding. It turns out that the 30v winding only needs to supply 15mA max, and the 9v winding needs to supply 110mA when the unit is "off", and 360mA when it's on.

This means that it's wrong to try to get one replacement transformer to supply both voltages, as the replacement will not have one voltage at a much different output current than another voltage (winding or tap). They'll all be the same, which means that the 30v winding will be grossly overbuilt. This makes the transformer much to big to fit in the tuner case.

So the answer is to use two transformers, one that is good for the 9V and 360mA, and another one that is good for 30v at 15mA.

This worked out well, and some eBay shopping later, I found my two transformers. Fortunately, there's enough empty space in the tuner case to easily hold the two transformers.

One new power supply requirement is for more 5V current, since the LEDs need much more power than the VFD. I measure about 50mA draw from the 5V supply just for the LEDs. I'm afraid to pull this from the stock power supply, because it's not designed for that additional load.

Fortunately, the transformer I found for the 9V AC supply has duplicate windings, and each one is good for more than 400mA. So I built an additional 5V source, consisting of a diode brige, filter cap, and 7805 regulator, that is dedicated for the LED display.

One note: I intentionally skimped on the filter capacitor on the new 5V supply. This "small" cap (47uF) will mean there's a lot of ripple (about 5V peak to peak) in the unregulated DC. I want this, because this lowers the heat that the 7805 will generate, because it'll be dropping a lower average input voltage to the 5V output.

Unrelated Product Recommendation

FAQ

Q. Will you fix mine?

A. No, I'm retired and so not interested in a job.

Disclaimer/Warning

This is just my documentation for my repair. I don't claim that doing this is safe or recommended.

Soldering irons are dangerous, be careful. Oh, and don't eat the solder.

William Dudley
June 27, 2026