Switching Transistor In Audio Pre-Amp

[jordankersten]

Hello everyone! I have an old Fender Rhodes electric piano I am rebuilding and I have a question. It uses a stereo pan circuit which pans the signal back and forth between the left and right amplifiers. It does this with a simple oscillator circuit that drives two 2N3053 transistors (now replaced with NTE 128's) that switch two light bulbs back and forth which then control each channel via a photo-cell. When the light bulbs get switched OFF, you get a bit of a pop each time. This results in a slight popping or thumping that pans back and forth when the light bulbs turn off. I put 2 68uf capacitors between the collectors and emitters of the 2n3053's and this helped a lot. The popping is hardly noticeable. I am wondering if A) is that safe to put those caps there, and B) if so, can I put in a higher uf cap to completely rid of the pop. I am wondering if this is safe for those transistors. Also, should I be using polarized or non-polarized electrolytics Thanks everyone!

P.S. Here is a schematic of the entire pre-amplifier with the vibrato circuit: **broken link removed**

 

[FvM]

The fix should be O.K. as long as the 2N3053 driver transistors don't get too hot. They are operated more in linear range now and dissipation can be increased.

But I don't exactly understand what's the nature of the pop interferences. Does it occur also when no tone is played?

 

[jordankersten]

Also, would polarized or non-polarized be more appropriate?

Added after 1 minutes:

The popping is present whether or not there is a signal present. Also, if you take the bulbs out and use another light source, no popping. Would it be safe to put in caps higher than 68uf?

Added after 2 minutes:

Ah yes, and if anyone has a better solution like putting the caps in a different place or using a different type of transistor, I am open. Thanks!

Added after 33 minutes:

I really like the results of having a 100uf cap between the collector and emitter of those 2n3053's. They are now NTE128's. The popping is gone. Which leaves me with wondering if it is safe. before I run the piano too much, do any of you see any reason those 100uf caps would cause a problem? The transistors are not running hot. Also, I have polarized electrolytic caps in there. Would there be a reason why I'd want to use non polarized caps?

 

[FvM]

Popping without signal means, that either a DC voltage is present at the LDR, e. g. due to a leak current of the driving decoupling capacitor or that the bulb inrush current causes a interference because of a bad designed circuit, either a supply voltage drop or a ground interference. Additional decoupling may help.

What source you mean with use another light source? If it works O.K. with LEDs, (may be because of lower current consumption, see above) just use them.

I think, you can use polarized caps in your circuit modification without any problem.

 

[jordankersten]

What I mean by "another light source" is I took the covers off and shine a flashlight back and forth. What capacitor is responsible for decoupling? I rebuilt the entire pre-amp, so all parts are new and know to be good.

Added after 5 minutes:

By the way, I thought that was a direct link to the schematic. To get to the schematic, click on "schematics" and it is Fig 11-8 "Pre-amp for 80 Watt suitcase and super satellite."

 

[FvM]

That's what I would regard a direct link.
**broken link removed**
Of course I reviewed the schematic before giving any comments on it. The incandescent lamps supply is decoupled by 150R/50 uF each. But there are different pathes for possible interferences. I'm sure that it's basically possible to identify and eliminate them, but rarely from a distance.

For the general understanding, it's important to know if the observed effect is an original design flaw, also found with other devices of the same series or if it's caused by a degradation or defect of some part. I assume ther former from your report.