Fender Power Chorus

[jurb...@gmail.com]

No chorus. The rate pot has been changed, and it appears successful but messy. It is a slightly different pot but seems right enough I would think. It is a 100K. On it I have a negative DC voltage, varies at the wiper.

I do not know if the pot was bad, maybe the last guy put in the wrong one. Or maybe it is the right one and he erroneously replaced it but it didn't fix it. I have no way of knowing this.

It is pretty sure that the LFO (oscillator) is not working. When you adjust the depth pot there is a change in the sound which indicates the variable filters, VCAs whatever are working.

One thing would be nice is a print. I have one but it is poor quality.

https://dl.dropboxusercontent.com/u/29948706/fender_power_chorus.rar

A better one would help, in fact probably ANY print of that section in an amp using the same chorus circuit.

Thanks. I am not as adept at reverse engineering as I used to be. Blind in one eye and can't see out the other. In my heyday some would joke that I could do this with my eyes closed. And now... LOL

[N_Cook]

Is this a "digital" pot the wiper feeds an ADC ?

[Phil Allison]

jurb...@gmail.com wrote:

> No chorus.

** The chorus effect ( similar to Leslie) is created in the air around the amp by having two channels and two speakers, one with no effect applied and the other with a varying time delay.

If one channel or speaker is dead, you will hear no effect.

Same scheme used in the many Roland JC120 amps.

> It is pretty sure that the LFO (oscillator) is not working. When you adjust the depth pot there is a change in the sound which indicates the variable filters, VCAs whatever are working.

** Pretty sure it is a bucket brigade IC that does the job.


.... Phil

[mrob...@att.net]

Phil Allison <palli...@gmail.com> wrote:
> ** Pretty sure it is a bucket brigade IC that does the job.

Page 4 of the PDF schematic that jurb posted does indeed show a
Panasonic bucket-brigade chip and its associated clock driver. (U13 and
U18... just left of center, under the "CHORUS BIAS ADJUST" pot.) The
parts list says these are MN3007 and MN3101 respectively. Datasheets
are available online at the usual suspects.

The only reason I know about these is that about 10-15 years ago, I
built an "echo box" using chips from that series, out of plans from a
book. If I remember right, the clock driver gives you a couple of
clocks with a certain phase relationship (quadrature or 180?), which you
then feed to the bucket-brigade chip itself. In that series, I think
there was just one clock driver chip, and various bucket-brigade chips
available with different numbers of buckets. At the time, Digi-Key
carried them, but I'm pretty sure they are now NLA new.

The schematic shows what the clocks are supposed to look like. If they
aren't there, you might be able to fake them with a function generator
to see if the bucket-brigade chip itself is working.

Matt Roberds

[jurb...@gmail.com]

>"Is this a "digital" pot the wiper feeds an ADC ? "

No. See other post I am about to make.

[jurb...@gmail.com]

>"If one channel or speaker is dead, you will hear no effect."

Both channels work. Also, I am not the first intruder on the unit. The replacement pot is kinda rigged in there, bent so that the nob indexing would match. I thought that might be the problem but it is not.

[jurb...@gmail.com]

>"Page 4 of the PDF schematic that jurb posted does indeed show a ..."

Now I feel like a total asshole. (shutup Phil, you'll get a less cheap shot than this) One of those PDFs that do not automatically page down.

Well now that I have seen page four (lol) I see where to concentrate my search. It's obvious we got an OP AMP oscillator and a waveshaper. I see it is not DC controlled, and it uses a negative supply so the negative DC is pretty much expected.

When I varied the depth control I heard the sound change, that indicates the BBD and all that is working fine, it is just this little oscillator. Whaddya think, the cap ? I'll find out Monday.

Thanks folks. I'll let you know.

[jurb...@gmail.com]

Well I got that fow frequenct oscillator running. The last guy in it changed the rate pot and used a different type. The original made a ground conection to some of the circuitry. However I still have no chorus.

There are no clock pulses coming out of the MN3101. The voltages seem alright, so I figure it is either the IC or the 22 pF cap in the oscillator circuit. I'll just change them both so as not to have too much time in there.

Thing I found out though is that my time is limited when the PC board is not mounted. The outputs are not getting enough heat sink and it will get HOT. If I wind up working on alot of amps built like that I might have to figure something out. Maybe a fan or something. I got standard threaded hole on the transistors', mmounting plate o maybe I can just screw a spare heat sink to that.

If necessary. Might not be because there really isn't that much there to troubleshoot. One of my main problems was my PDF reader not autoscrolling to the next page. I have to select the next page up at the top. Don't say to update my reader, it is new enough and aggravating enough. What I really want is version 4 but it won't read shit anymore. One PC at work has a really newer version in which I can't even find the hand tool. Using the scrollbars is worth shit on a schematic. then they have this propensity for downloading updates for Chinese or Mandarin or Korean fonts. Sorry, maybe I am just a dumb yankee but I do not read Chinese. Or Sanskrit or Farsi or anyting but English. If I had it to do over again I would have taken as many languages as possible when I was young.

Enough rant. The chip and cap will be ordered tomorrow and that is that. I got this thing in a basket but that isn't how it is going out.

Oh Matt, about faking the thing out, I think it needs the two clock phases. I don't have a generator tht can do that so I would have to go through some crap to accomplish it.

Now that I think of it, maybe I should build something just to invert a signal for such circumstances. Simple OP AMP should do it. there may be other things I could use it for, like that breadboard down in my basement. I might actually build something again...

[jurb...@gmail.com]

>"When I varied the depth control I heard the sound change"

That was obviously NOT due to the BBD working. the way it is connected though to the BBD bias coming out of that other OP AMP seems like it could allow some effect, but not the desired one.

If the BBD was being clocked and the only problem was the low frequenct oscillator not running I think the effect of the depth control would be much more pronounced.

So really, that indicates that the guy who changed the pot either made an error or broke the thing somehow. Of course that's the kind of shit I get all the time. I get a nice unit with all the original screws and parts, with the original problem only, I just don't know what to do with myself.

[mrob...@att.net]

jurb...@gmail.com wrote:
> There are no clock pulses coming out of the MN3101. The voltages seem
> alright, so I figure it is either the IC or the 22 pF cap in the
> oscillator circuit.

Apparently the diode and 5.1 K resistor are part of the MN3101
oscillator as well. In general it looks like they are using U16/U17/U11
to apply some additional signals to the MN3101 oscillator. (If I
remember right, in the one I built, the MN3101 just oscillated on its
own... there was an RC network on pins 5/6/7, but that wasn't connected
to anything else.)

> Thing I found out though is that my time is limited when the PC board
> is not mounted. The outputs are not getting enough heat sink and it
> will get HOT.

How about a few of those heat sinks that use a spring clip - either flat
metal or a wire bail - to hold the heat sink to the transistor? Not for
running the thing outside of the case at 200 W for hours on end, but
just for running at low power for 10 or 15 minutes while
troubleshooting.

> Don't say to update my reader, it is new enough and aggravating
> enough.

If you don't like Adobe Reader (or Acrobat Reader or whatever Adobe is
calling it this week), I have had reasonable luck with Foxit Reader.
Seems to be less bloated than the Adobe offerings.

> The chip and cap will be ordered tomorrow and that is that.

I think the MN3101s are surplus/Ebay only now, but there still seem to
be some around. It's CMOS, so take your shoes off.

> Oh Matt, about faking the thing out, I think it needs the two clock
> phases.

I don't remember the details, but I'm pretty sure that it does need
both phases.

A quick Google shows that some people have been kicking around the idea
of using some 4000-series CMOS chips instead of the MN3101. Some of
them are worried that it'll sound different:
http://www.diystompboxes.com/smfforum/index.php?topic=72447.0

Matt Roberds

[Gareth Magennis]

Peavey Classic Chorus 212 uses those IC's.

http://elektrotanya.com/peavey_classic_chorus_212_sch.pdf/download.html


Gareth.



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