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EH Polyphase transformer dead

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Re: EH Polyphase transformer dead

PostPosted: Thu Jan 10, 2019 4:49 pm
by Folderol
Many modern caps are far smaller than ones even a few years old, so don't be surprised by the size of a replacement.

Apart from the LFO are you getting correct Envelope behaviour?

Another possible miscreant could be the 1u cap above the envelope sensitivity control. If that's gone leaky it could screw up the DC levels. Again, a small 'dry' type might be better.

Also check the 220u one alongside the {indecipherable} Trim control. There are some seriously screwy DC paths in this circuit, so anything that could upset it is worth replacing.

Re: EH Polyphase transformer dead

PostPosted: Fri Jan 11, 2019 5:55 am
by Adam Inglis
Thanks for drawing my attention to that trim control.
Someone on a stomp box forum explained this area as setting the bias voltage for the LM324 chip, which is responsible for the envelope follower CV.
You can see this clearer on a re-drawn schematic (from Morocotopo on DIYstompboxes) with my notes added,
EH was very keen for this voltage to remain fairly steady - note the thermistor in parallel with the trim.
When I finalised the new power supply, I duly adjusted the trim as suggested by the schematic, to read 6.7 volts. Today, I checked it again. It was down at 6.5. As soon as I trimmed it over 6.7 the LFO came on. So as it turns out, that level is critical to the operation of the LFO. A tenth of a volt less, and the LFO stops oscillating! You can actually hear this if you have a signal going in when you turn the power on. The voltage takes about 15 seconds or more to come up to level. As it does so you hear the LFO come on. I assume that the need for a reduced supply voltage just for that chip is somehow linked to all this...!

Re: EH Polyphase transformer dead

PostPosted: Fri Jan 11, 2019 6:47 pm
by Folderol
It is indeed a very strange circuit. I rather suspect something in there is temperature sensitive, and that's (at least partly) the reason for the thermistor.