Help with 1970s Guyatone Moving Box Flanger - SAD1024

[clarisso11]

delete Q3, C26

Thanks, I will try that next. I checked over the board to see if there are solder bridges, nothing I can see though.

I'm wondering, without D1, would Q1 overheat and fail in this circuit?

[clarisso11]

Connect the LED like this, but remove transistor Q1 for now. The brightness of the LED will indicate a short circuit. As soon as the short circuit is gone, the brightness will drop significantly or not at all.

That's a nice idea, I will try that!

[x-pro]

I'm wondering, without D1, would Q1 overheat and fail in this circuit?

Where does 18v come from and what is the max current there?

[clarisso11]

It’s a Cioks DC7 power supply, max current is 330mA

[g1]

To me, it is evident the schematic is the wrong one for this unit. What happened with D3 is not possible with this schematic. And the supply polarity is backwards.

[x-pro]

It’s a Cioks DC7 power supply, max current is 330mA

With this power supply current margin, the BC237 transistor will easily fail if the load exceeds 100 mA.

[x-pro]

To me, it is evident the schematic is the wrong one for this unit. What happened with D3 is not possible with this schematic. And the supply polarity is backwards.

but the parts on the board are quantitatively the same.

[clarisso11]

To me, it is evident the schematic is the wrong one for this unit. What happened with D3 is not possible with this schematic. And the supply polarity is backwards.

I think there's a chance that the schematic is not 100% accurate, but it seems to match the components and tracks on my board closely. In any case, it seems concerning that the DC rail is shorted to ground.

Setting aside accuracy, if the D1 Zener was removed on that schematic, would that cause Q1 to melt?

[g1]

I think there's a chance that the schematic is not 100% accurate, but it seems to match the components and tracks on my board closely.

From your board picture in the first post, I see TL071, and one 8pin chip that appears to be covered with something.

[g1]

Maybe I misread that TL071, it's hard to see in the photo.
Yes, I think without the zener D1, the pass regulator might burn up.
Also, have you checked that D3 is a regular diode and not a zener? And what is it's polarity compared to the cap in parallel with it?

[x-pro]

Setting aside accuracy, if the D1 Zener was removed on that schematic, would that cause Q1 to melt?

It is not the absence of D1, but the short circuit of the emitter of Q1 that will lead to the transistor damage.
Without D1, this transistor is a simple power filter. 18V came in and almost the same amount went out. In this case, the equivalent capacitance at the emitter output = C23*h21e.

[clarisso11]

Thanks both, that explains it! Working on original bypass switching, but the true bypass took out D1 with the effect off, melting Q1 (and probably other components)

[clarisso11]

Maybe I misread that TL071, it's hard to see in the photo.
Yes, I think without the zener D1, the pass regulator might burn up.
Also, have you checked that D3 is a regular diode and not a zener? And what is it's polarity compared to the cap in parallel with it?

Yes all the IC markings were pretty much gone on this board, I’m not sure why.. The original D3 had signs of overheating, I replaced it with 1N4001, using the same polarity. This was working well without generating excess heat (I checked at the time), until I installed 3pdt true bypass.

[x-pro]

Analogs: 2SC1959, 2SC3331, KSC1845FTA, 2SC1213 with preserving of the pinout.
You can put almost any diode instead of D3. Its task is to protect the circuit in case the polarity of the power supply is incorrectly connected. In this case it must either burn out and cause a short circuit to blow the fuse in the power supply, or withstand the power supply current and also blow its fuse.
The scheme is of course drawn according to the rule - "you can't draw like that".

[x-pro]

did you find what's shorting out the emitter of the transistor?