Technics St-S7 Dim Display

[Ian Ramphal]

So in 1977 I was gifted a ST-S7 tuner. Worked perfectly until 2008 when the display panel started to fade away. That's 30 years of daily use. Opened it up and looked inside and threw it away as I had no clue what I was looking at.

Now, 18 years later I wanted another and it has the same issue. Google search revealed an age related problem with dimming display resulting from the "voltage pump" that raises the display voltage to -32V (or could be +32V) for the display to light up.

I have the original ST-S7 Service manual which does not indicate the voltages/pins to check, plus an online download schematic.

Google tells me it's the voltage pump, driven by a couple of Caps and Diodes (that tend to fail) to raise and supply the +/-32V to for the FL Drive to power the display. But, I'm a novice.

Can anyone advise me as to what pins, etc. I should be probing, or what caps I should be replacing?

I assume that the area in question is whatever is powering the "FL Drive".

HELP PLEASE!

 

[Bob@FM]

HELP PLEASE!

The fluorescent display has power on pins 1 and 35, and the schematic only shows 2 readings - there is 3.2 VAC across pins 1 and 35, and pin 35 is -22V DC.
This is accomplished via 2 different transformer windings - see power supply below -
power transformer windings 7 and 9, plus 10, 11 are both used here. The 10, 11 white wirers supply the 3.2V AC, and the components as follows - look them up in the manual - I would first check D706, D707, and C919. Then C702, and R702 and R731. Last D701, D702.

 

[Ian Ramphal]

Many, Many thanks. Will open her up this weekend and follow your advice and reply with results. Best Regards, Ian

 

[wfdudley]

Many, Many thanks. Will open her up this weekend and follow your advice and reply with results. Best Regards, Ian

My fluorescent display died and I am making a replacement display using LED 7-segment displays. Waiting for the first boards to arrive from PCBWay right now.