Console Oscillator Issues

[man-bot]

Hi there,

While performing some repairs on my console I ended up haveing a stray lead land on the monitor board of my Soundcraft Series Two console.  Ultimately it blew R42 where the main voltage as well as R62 and T15 in the oscillator circuit.

All was repaired and the oscillator works again albeit messed up.  I am getting essentially a square wave now and the frequencies are about 1/2 of what the range should be (i.e 50hZ instead of 100 hZ).

I can't seem to track down what else may be blown causing this issue... any thoughts?  Schematic attached.

Thanks

Mike

 

[joaquins]

Did you replaced the transistor with the same model, apron the same gain? R62 was the same value as in schematic? The freq is being set by C27, R56, R57, C25 and the pot, so look for those values to be ok. Also TH1 tº makes it to be sin rather than square if everything else is fine, maybe look at it.

JS

 

[man-bot]

Thanks!  I'll take a look and report back.

Mike

 

[man-bot]

Ok - I have the frequency range back to where it should be… the big issue is the wave form.

The oscillator works with an ITT RA53 glass envelope thermistor - which is unobtainium… wondering if there is a work around?

I'll try a different 5k glass bead thermistor I can get locally and see what happens, but something tells me I may be rooked.

Mike

 

[PRR]

This is a very clever, and thrifty, design.

The TM1 is a large part of it.

It "seems like" an H-P-like lamp would work, but that goes the wrong way, and also takes more power than this thing has.

OK, just to get "tone" you can try replacing TM1 with a fixed resistor. My guess is 270 ohms down to 220 Ohms. Try 220 Ohms fixed plus 100 Ohms variable.

Turn too low, oscillation quits. Turn too high, oscillation goes way too large and clips horribly. (As you know). There is a setting where oscillation is barely-enough and clipping is not huge. Now turn the FREQ knob, also vary the supply voltage (cold turn-on may show a problem until everything is stable). Typically oscillation will quit at some conditions. Turn-up a little more until it starts reliably. Now it will clip at most other FREQs or small changes in supply voltage. It may also be way too big, though you can make changes at OSC.LEVEL. (Stick 22K or 47K in series with that pot if WAY too big.)

This may be good enough for Tone and Slate. It will be terrible for frequency response checks or looking for significant distortion.

You could use the Wein Bridge (FREQ-set) parts with another stronger opamp and an H-P style lamp feedback. However this turns out to be a small power amp, so supply drain may be an issue, and oscillator leakage into supply rails may be an issue (but I see the osc can be killed when not in use).

While un-authentic, *perhaps* the best path is to abandon this oscillator and wire a jack on the back for an external oscillator. (However the on/off/slate switch won't kill that, so leakage is an issue.)

 

[man-bot]

Thanks PRR.

It is really only an issue as I align my tape machines with the oscillator... looks like I'll have to use a hendheld for that going forward.

If I can get a stable 1k tone I'll be happy... probably the best I can hope for now!

Mike

 

[ricardo]

I think your RA53 is open circuit.

IIRC, I found some non glass enclosed bead Thermistors that sorta behaved similarly.  You want something that drastically drops in resistance when the voltage goes up.  (The glass envelope is to keep the device at a constant temperature).  Something that used to be common in television sets.

The beads weren't as low THD as RA53 but did give a nice sounding Sine.  The output voltage was different too.

But I was still in high school so it was a looo.oong time ago.

 

[man-bot]

Thanks Ricardo - I'll do some hunting and see what I come up with.

For the time being I threw a 220r resistor in and the waveform is still clipped, but much better - and it seems to works through a pretty useable range of frequencies (nothing really in the 100hz area but thats ok, but above seems fine - somewhat clipped - but fine).  I'll keep experimenting with values and see if I can find something that works for me for the time being,

Mike

 

[daniu]

If you take apart one of those laptop chargers,

you will find a thermistor by the main capacitor,  on the PCB you might see "TRxx".

You might find a laptop charger at a local good will store,  e.g. Salvation Army, for 2~3 dollars.

If you are using a  tungsten light bulb,  4w120v  or  1W48V  should do.