Howdy I'm trying to fix an old synth,when i hook my old oscope up to the secondary of the tranny, i get a waveform that appears to be missing parts of the signal. When i zoom in on the peak i t is actually there just very thin with a spi

ke at the be ginning. thanks for any words of advice. Rob

 

If this is at the secondary of the mains transformer, it tells us more about the quality of the mains supply than it does about the synth. The flat tops are normal but the apparent gaps appear to be due to a rapid transition.

 

Howdy I'm trying to fix an old synth,when i hook my old oscope up to the secondary of the tranny, i get a waveform that appears to be missing parts of the signal. When i zoom in on the peak iView attachment 102926 View attachment 102926 View attachment 102926 t is actually there just very thin with a spi

ke at the beView attachment 102926 View attachment 102926 View attachment 102926 View attachment 102926 ginning. thanks for any words of advice. Rob

You can't have gaps in the trace - it just looks that way.

Those bits are where the trace is moving too fast to make an impression on the CRT phosphor.

The waveform isn't quite making it to be a sinewave - part of it is getting awfully close to a square wave.

 

i'll try to get a picture of the zoom. but on the zoom you can see the spike going in the other direction, it is the secondary of the mains transformer. I forgot to mention that if you diconnect the leads from the power board the wave is pretty normal. sorry i'm trying to learn how to post.

 

i'll try to get a picture of the zoom. but on the zoom you can see the spike going in the other direction, it is the secondary of the mains transformer. I forgot to mention that if you diconnect the leads from the power board the wave is pretty normal. sorry i'm trying to learn how to post.

That sort of thing can be caused by a rectifier fault where a diode fails to conduct until some arbitrary threshold is exceeded.

Maybe start with a replacement rectifier. Not worth taking chances with - if it fails short circuit, the transformer could get damaged.

 

There is nothing wrong with that. The flat tops are where the diodes are conducting current to the filter capacitor(s) and the impedance of the transformer winding is using up that tiny bit of voltage during the conduction time of the rectifiers.

Yeah, if your transformer had zero point zero zero zero ohms of impedance, that flat top wouldn't be there, but you can't buy or build a transformer that's perfect.

 

Thanks for the help. Rob

 

The rectifier in this circuit, appears to be marked Gi wlo2. I found the datasheet on this but not a supplier I have never bought a rectifier before other than single diodes and tubes. Can anyone steer me in the right direction?

 

There is nothing wrong with that. The flat tops are where the diodes are conducting current to the filter capacitor(s) and the impedance of the transformer winding is using up that tiny bit of voltage during the conduction time of the rectifiers.

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The TS doesn't seem that concerned about the flat tops; "i get a waveform that appears to be missing parts".

AFAICS: The sharp vertical transition that's too fast for the CRO to "write" the phosphor, still hasn't been explained.