Awakening the sleeping monster

Top view, showing the main board.

Here's the thing with old electronics: don't just turn it on and see if it works! Components degrade due to heat &/or moisture, vermin get in and eat important bits, corrosion destroys wiring & pc board tracks, etc.

And that goes double if it's been sitting in a hot shed with 40+°C temperatures in subtropical Queensland.

So the first thing to do was take the lid off my receiver and have a look…

Bottom view, showing the PLL board & PSU.

Luckily, it looked fine - a bit dusty, but otherwise OK; nothing obviously leaking, eaten, or rusting. Something that was immediately obvious was the number of low-value low-voltage electrolytics - experience has taught me to always be a bit suspicious of those. A close visual inspection showed everything looked OK - no obvious leaking or bulging - but that's often the case with low value/volts electros, particularly good quality older ones.

To check them, you really need to pull them out & check the capacitance & ESR - however, both the upper (main) and lower (PLL) board are a pain to extract. Given that they all looked OK, I decided to just check the ones I could get at with my ESR meter. After much poking around, reading the circuit to determine where I could get at each one, I managed to check almost all of them - and all were within spec, especially since they were likely never particularly low-ESR to begin with...

WARNING!!! Mains voltage is present inside - in fact, the internal terminals on the power socket & voltage selector switch are both uninsulated and right where your hand falls while measuring. If you don't appreciate how dangerous this is, don't fiddle in there yourself - leave it to someone experienced &/or qualified. If this scares you, leave it to someone experienced &/or qualified. If you think you know what you're doing and this doesn't apply to you, have a damn good look at those bare terminals and think where you'll naturally want to rest your hands before going ahead. 

 

Looking at the circuit of the power supply board, I was a little less sanguine. While the regulator part of it is fine (if a bit archaic - but it is a 30+ year old design!) as far as it goes, only the 11v rail is regulated - the two 13.5v rails are unregulated. This is a bit odd, since between them they feed critical sections like the display controller & memory module (although each has their own local regulator circuit) as well as other bits like the audio amp, etc. I was more concerned when I disconnected J03 & J04 on the PS board & measured the main 13.5v & backup BU13.5v rails at 18.3v…

That's not particularly surprising. As I said the alleged 13.5V is totally unregulated, so will depend quite a bit on the nominal vs actual mains voltage and transformer characteristics. That's also the open-circuit voltage, so you would expect it to quickly drop to its nominal value with a bit of load. To simulate that I stuck a handy 100Ω resistor across the rail to draw ~150mA and measured the 13.5V rail again.

17.5V...

At least the 11V rail was 10.8V.

(An aside: Internet wisdom, such as it is, suggests that running your FRG-7700 off an external 15v supply plugged into the DC connector on the back - which, by the way, mine does not have; it appears to have been an optional extra at least on early sets - results in noticeably better performance. More on that in a later post…)

In all likelyhood it's probably always run that high, and would probably be OK. Still, I don't particularly like the fact that a nominally 13.5V rail is ~30% higher than spec, especially in a 30-year-old device that has spent the last 7 years cooking in a hot shed. It really needs a proper regulated supply. Luckily, the voltage seems high enough that a couple of simple 3-pin regulators should suffice..