Hi Daniel,
Well, I am about 6000 miles away in the Suffolk market town of Sudbury,
United Kingdom! However, you are very close to Jed and I am sure he will
make you a nice cup of coffee if you decide to drop in! ;-)
Back to your MG issue. I am happy with all the waveforms you have reported
back, but one thing concerns me. IC13 pin 7. You say it is at 4.8v. This
means the MG is oscillating almost flat out! Between this pin and ground you
should see the following.
With the MG frequency set to 0 you should have about 4.95v. As you increase
this pot clockwise you will find the voltage drops to 0v when you are at the
12 o¹clock position. As you increase around to 10 then the voltage will ramp
up to about +4.95v. Can we verify that this is the case with your board? If
it is stuck at +4.8v then we can have a look at both the multiplexor and
buffering stage for faults.
On a working board MG frequency set to 10 give about 50-60Hz at CN6-1. I
just measured it!
Cheers,
Andy
On 11/09/2010 03:12, "Daniel A" <apophis93@yahoo.com> wrote:
>
>
>
>
>
> Andy,
>
> I am in Portland OR. If you saw the last post which I deleted please ignore
> had bad readings. In fact the very first post you responded to had bad
> readings I apologize I just realized I was reading the IC pins incorrectly and
> have now corrected myself in these readings... boy it has been a few years
> since I have worked on a project.
>
> Well I ordered a few replacement parts from a company in Canada and Farnell. I
> installed a new LM13700 at IC 20 and 21. This is because I was still working
> with the salvage LM13600s and wanted to rule them out. Next I got a hold of
> some stock new A798s for Q3 as you had mentioned a possible short at that
> point. So Q3 is now new stock.
>
> I am still getting the 50Hz and it is still 3.8 peak to peak (I have my scope
> set at "1" for the Volt/DIV and I am getting an amplitude of -1.9 to 1.9 or
> 3.8
> pp)
>
> V2 and the panel have no effect on the voltage.
>
> Did a new test of points as before:
>
> IC13 pin 7 = 4.8v
> IC16 pin 7 = A square wave 15v to -15v 10msec
> IC19 pin 12 = A saw wave form at 4v to 4.5v
> IC21 pin 12 = 50Hz 3.8vpp
> Q1 Source +15v Drain is a 50Hz 12vpp
>
> I am still at a loss and trying to track this problem down. I may have to coax
> Jed over if he can spare the time ;)
>
> - Daniel
>
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