[sdiy] Flanger???

Gene Stopp gene at ixiacom.com
Fri Jun 28 18:58:43 CEST 2002


Hope you guys don't mind the AC inverter thread... homebuilt synthesis
includes homebuilt FX, right? And it's very analog, right? This project is
really like a backwards power supply. Juice flowing in one direction is old
hat for me; juice flowing the other way is a new experience. This is really
becoming "Gene's troubleshooting class for the day" but I think that
troubleshooting is very much a part of DIY. It's good to bounce ideas off of
others of like mind as my subjective brain tries to wrap itself around the
objective reality of wires and silicon.

So I find another big transformer in my transformer drawer. I was wondering
what I would ever do with it. It is very large, maybe fifteen pounds, made
from what appears to be coat-hanger wire wrapped around a big core with a
bunch of taps sticking out (the ends of the wires bent into loops). They all
look the same. No part number to google search with... but I see the
tell-tale four taps together, with pairs jumpered together and two cut-off
wires sticking out. Ah ha! The primary, with 110/220 taps configured for 110
operation. All I have to do now is plug it into the mains, and poke around
the other taps with my DVM to find out what the secondaries are.

(Politically correct disclaimer - in no way do I advocate that anybody do
like I do and just plug big strange transformers into the mains like this. I
have years of reflexes at the ready to avoid sparks, flames, and flying bits
of exploding components. Not only that I know where the circuit breakers

I did this, and my bench power managed to stay up, although a nice
"Bum-mmmmmmm" noise was made as the voltage hit it. So far so good. Six
taps, two center-tapped secondaries. I verified this by beeping them out
with an ohmmeter. 21 VCT, and 17.5 VCT. I presume this was for a +5 and
+/-12 supply at one time long ago (communications equipment - TTL plus EIA
cable drivers) back in the days before switchers.

I wired this into my LFO/amp setup, and no luck! I tried the power amp
standalone, and one channel was dead. Blown output fuse - changed it, now
that channel is pegged at +40v. Ooops, must have fried the amp. I didn't
kill the other transformer after all. Why did I come to the conculsions that
I came to?

As I had mentioned my scope was monitoring the amp outputs on two channels
during the test. When I thought I had fried the transformer, I thought the
amp was OK because I saw two triangle waves out of phase. Had I looked
closer, I would have probably seen two triangle waves IN phase - out of one
amp channel to the scope, and through the transformer to the other scope
channel, to the blown fuse on the amp to nowhere.

So I picked up another homebuilt amp of the same design (yes I have another
- there was a time when I was not married!) and set it up the same as
before. It all worked again, but this time the power trannies were not too
happy. I could tell they were ramping up to some higher temp and I wasn't
going to wait around to see how hot they would get. Obviously this new
transformer sucks too much juice for this setup. I dug the old one out of
the trash, and everything was working again.

Next time - the use of sine waves. Hopefully this will get rid of those
nasty harmonics, as long as I avoid clipping.

Well enough for now... the experienced ones on the list are probably
chuckling at my mishaps, and the newbies maybe learning something!

Best Regards,

- Gene

-----Original Message-----
From: Grant Richter [mailto:grichter at asapnet.net]
Sent: Thursday, June 27, 2002 11:24 AM
To: Gene Stopp; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Flanger???

The triangle has a discontinuity at the switch over point. This is going to
cause a voltage spike in the inductor as it tries to continue the current
flow. You would be a lot safer using a sine wave, this should prevent
unexpected insulation failure.

> I'm fairly sure that the amp isn't running DC thru the transformer...
> outputs look symmetric around zero on the scope, power trannys aren't hot,
> pretty sure the inputs are AC coupled... OK yeah I haven't actually
> *measured* anything, that would be cheating, wouldn't it?

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