Mixer Meter Bridge

[Rob Strand]

I'm going to recheck for more non working LED's in the problem rows.  Maybe something happened since I checked them last time.

Actually if you are testing with the full signal level going in a bad or open LED would have the same symptoms.  I was thinking there was always a current down the diodes.   With no signal you might find the current source voltages are good then when you gradually turn up the level a few leds come on, then you hit one of the faulty LEDs and it all falls in a heap.

[AS74]

Some progress after finding non working LED's.  Still 3 rows no good plus the strange T31/T32 row.

You can see in the 2nd pic the LED's light up when the oscillator is turned down to drop the top LED.

[Rob Strand]

Some progress after finding non working LED's. 

Good progress indeed.  Not far off now.

You can see in the 2nd pic the LED's light up when the oscillator is turned down to drop the top LED.

I suspect the problem is one or more of the LEDs in the top set of 5 has a higher than normal voltage drop.  Possibly because the LED is faulty.

Measure the voltage across a few LEDs on a working channel.   Take measurement on both green and red LEDs as the voltage across the green is a little higher than red.   Then, with the oscillator set to one below top and measure the voltage across the LEDs on the top bank of bad channel.  The drop across the top LED cannot be measured and if you measure it in the dim state you will get a low voltage because of the low current.  Now set the oscillator to maximum and measure the  voltage across all the red LEDs - the reason for doing this is to compare to voltage drop across the top red to the others as that LED might be the problem and this test will give you an idea if voltage across the top LED is higher than the others.

You might expect to voltages of about 120mV to 130mV (for non-dimmed LEDs) but you could see higher even for good LEDs due to the high current.   The thing is when you get too many LEDs around 150mV to 160mV the circuit might not work.   The exact point is in the details but it will be around that point.   So it's possible if you have a few high voltage drop LEDs thethat could stop it working.

Measuring the voltage across LEDs is a good way find bad ones.  Just keep in mind the green LEDs have a little higher drop than the red ones,  say10% higher.

Still 3 rows no good plus the strange T31/T32 row.

For the bank which has the dim top bank of LEDs and the off bottom bank of LEDs, I'd be looking for open circuit somewhere along the collector of "T30" and through the LEDs.  You might be able to dial the oscillator level right down so only the lowest LED is on and it could start working.  Then gradually increase the level until the bank goes off then there's probably a break or faultly LED around that point.
[FYI, LEDs in backwards will look open]

The two banks with the single LED is looking more like a short.  Either an unintentional short on the PCB or shorted LEDs.  Maybe recheck for shorted diodes.  I suppose another possibility is the CMOS switch (the corresponding IC6 through IC10 for the channel) could be damaged.  Even simpler, the wires for those channels might have come off.

[AS74]

More progress! 
The strange row was fixed by swapping out the top LED.  It was reading a higher voltage drop.
The dim, half working row had a broken trace off of the collector.



For the last 2 rows Seems to be a problem on the group board not letting the oscillator get through.
Probably an oxydised switch I'm thinking.  Changed enough of them already.

Thanks for guidance Rob!  I'm feeling pretty bloody good to have it this far along.

[Rob Strand]

More progress!

Thanks for guidance Rob!  I'm feeling pretty bloody good to have it this far along.

Way to go!  You did well, it's was looking pretty beaten-up in that first post. Now back to new.

[AS74]

Got it!  It was a switch as I thought. 




Thanks again Rob.