Acoustic 470 Indicator Lamp Mystery

[gbono]

http://i284.photobucket.com/albums/ll33/bonosurf/470Lamp.jpg

I am working on a 470 and have run across a simple lamp driver circuit issue which is driving me mad. The B channel lamp will not operate when the switch is put in the B channel mode. I have checked both bulbs, replaced Q211/210, resoldered every possible node.

It looks like the drive transistors get the correct base bias voltages but the collector on Q210 reads 26V instead of 33 in the B channel mode and Q211 collector is at .7V instead of .3V in the B channel mode. I'm stumped by this very simple circuit.

[teemuk]

It looks like the drive transistors get the correct base bias voltages but the collector on Q210 reads 26V instead of 33 in the B channel mode...

Which begs a question: If the collector voltage at this point is somewhat lower, does the transistor Q211 then saturate fully in order to perform the switching adequately?

...and Q211 collector is at .7V instead of .3V in the B channel mode.

As I said. Are you really, really sure that the transistors are biased to full saturation? I would double check that... It seems to me that Q211 won't turn completely on, hence it has a slightly higher collector voltage than what is indicated by the schematic. Yet I find it unlikely that a difference of only few millivolts would cause the lamp at collector circuit not to illuminate...

Anyway, your description seems to indicate that the transistors switch correctly, yet the voltages just do not match exactly. The way I see it, that can point to 3 places:

- The supply: Do you measure the said (approximately) 92V and 33V supply voltages in all switching conditions?
- The lamps: Are they correct type? Do you measure the approximately 14V across them when they are supposed to illuminate?
- The resistors: R251, R252, R253, R254, R255 and R256. Some may have drifted in value, which could explain why voltages are somewhat far off. Do you measure the correct voltage drops across them in all switching conditions? (All those can be derived from the schematic).

I'm pretty sure that one of those three is bound to reveal an error somewhere.

[gbono]

Teemuk,

  Thanks for getting back to me. All resistor values are in specification. R255/256 needed to be desoldered to check - I assume they are in parallel with the BJT junctions.

  The voltage at the node of R252 and R253 is only 26V when the B channel switch is turned on and it returns to 33V when the switch is in the A channel mode.

  The voltage at the node of R254 and the B channel lamp is only 2.9V when the switch is in B mode.

  Lamps are original and correct and function normally with a bench supply.

  The collector of Q211 reads 70mV when the switch is in the B mode. If Q211 was not in saturation, I should see a greater voltage at its collector (i.e. transistor in on state).


   

[teemuk]

The voltage at the node of R254 and the B channel lamp is only 2.9V when the switch is in B mode.

Aha!

If I understood the schematic and your explanation correctly, this node should read at least 10V – 19V instead, depending on the supply voltage and lamp current draw. No wonder the lamp won't light up.

Question is, what causes this?

The way I see it...

Although the indicator circuit's supply rail reads only 26V it's still not a big difference in comparison to 33V and the difference in voltage would only make the bulb burn somewhat dimmer. Also, if the collector of the Q211 indeed switches to nearly zero volts when the transistor is saturated (even few hundred millivolts of difference is not that big deal) it means there still should be about 25V (or higher) voltage drop across the indicator light bulb and its 680-ohm series resistor.

Judging by your description these two clauses are true, so then the options of what is wrong are pretty limited. Either your lamp is faulty / incorrect type and presents almost a short circuit (hence there is much less voltage drop over it than what is indicated by the schematic) or the 680-ohm resistor is not a 680-ohm resistor (hence you have too much voltage drop over it).

Personally, I would suspect the lamp as the voltage sag from 33V to 26V seems to indicate that excessive currents are being drawn when the B-channel indicator is switched on.

[gbono]

Okay I swapped the lamps and the B channel works now and the A channel doesn't. It is the bulb. I had tested the bulb with a bench supply and it worked . I have to assume that it acts differently in the circuit - though both bulbs have the same DC resistance???

These bulbs are grain of wheat style Incandescent Pilot Lamp Lights with 6" of purple wire attached to them. I think I found a source on Google - I'm assuming they are 14V bulbs?

Thanks for the help...............