Monday, July 10, 2017

Transistor coming off the rails

Bench Press

With the help of the borrowed bench power-supply I manged to narrow down the -26V rail issue to board G3. Without that board the total current was less than 200mA, but with it the power-supply would switch the voltage down instantly. The current was probably lower than normal because the clock signals generated on G3 were off , which in turn meant that the other boards were not active.

Bench Power-Supply On Loan

The next step was to isolate which part of G3 was causing the problems. I started by lifting resistors to the -15V and -8.2V regulators and Master Clock Oscillator but the problem remained. Then I realised that the transistor on the -15V regulator was also connected to the -26V and I would need to remove that too.

Once that was out the problem stopped immediately. So now I had a prime suspect.

Checking the resistance between collector-emitter showed it was blown. Practically short circuit between the two pins. As I don't have a replacement to hand the next step was to connect the other side of the bench supply to the -15V rail and verify that there were no more problems there.

On doing this I was relieved to see that the unobtainium parts - the YM25400 octave dividers - were still running. However, I'm not out of the woods yet. The Master Clock Oscillator on G3 not looking good, to put it mildly.

The effect of this on the dividers that it feeds varies but it's possible that the bench power-supply is the problem here. I will reserve further judgement and investigation till I have the -15V supply back in place.
Perhaps I'll just piggy-back off the G4 -15V supply and see what that does. My HW engineer colleague agrees and points out that the bench power-supply is switched-mode and this could be an issue.

Transistor Sourcing

The 2SA509 "Silicon PNP Epitaxial Transistor for audio power (1W) amplifier applications".  As this is a regulator though it's not used in that way on the SS30.
A voltage divider is used in place of the more usual zener diode to set the voltage at the base. The divider gives -9.5V. which means a difference of 16.5V base to collector. A voltage of -26V here would switch the transistor fully on, it being most negative with respect to the emitter.  Whilst if it was gnd/0V the base and emitter would be at the same potential no current would flow. Therefore the voltage  of -9.5V at the base results in approx -15V at the emitter.

It's possible to get the original parts - which are Toshiba - but these are pricey. There are equivalents which are cheaper but these all seem to be in the US and cost a ridiculous amount in P&P. I have found an eBay seller with the right part at a reasonable price, but not much information. I've contacted them to check that the pin-out is the same.  

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