Vintage Synth Repair

The info was in the link I put to the data sheet in an earlier post, but for
reference these are the 2 links you need
http://www.onsemi.com/pub_link/Collateral/MC7800-D.PDF for the 78XX parts
and http://www.onsemi.com/pub_link/Collateral/MC7900-D.PDF for the 79XX
parts, ON is the new name for what was Motorola discretes, just checking the
tab on the 78XX is connected to ground so that is probably OK onto the
chassis unless the design was for a floating ground (can be done to reduce
noise), but the tab on the 79XX is connected to the input supply so would
need to be isolated with a pad and washer.

 

You could also check the ripple on the unregulated side of the voltage
regulator as this would give an indication if there is a problem capacitor,
this can be done by measuring the input voltage using an AC range on the
meter either 1V or 100mV would give an idea, I would expect a reading of
less than 100mV AC anything higher would suggest a capacitor is at fault if
the reading is more than 2V I would go for the electrolytic less than that
it may still be the electrolytic but could be a smaller value cap instead,
there are also a couple of ceramic de-coupling caps often used either side
of the regulator for taking out high frequency noise these sometimes suffer
mechanical damage so may be worth a visual inspection they are going to be
between 10nF and 0.22uF in value.

 

Hope this is of some help.

 

John Rose

 

(www.astrodevelopments.com)

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Christian Oncken
Sent: 09 May 2012 14:41
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Re: Simmons SDE

 

  

So it turns out, looking around for info on those ICs, the 7805 and
7806 parts have the the metal tab connected to ground, the 7906 has it
connected to -6v... Its strange because I don't remember removing any
insulators from it when I disassembled it. I remember the parts being
stuck to the heatsink with some sort of glue, but that was a long time
ago so I might be mistaken.

Jammie, specifically I'm measuring the wire on the ribbon cable that
goes to the rest of the synth that I've traced back to the output leg
of the 7806. If should be +6v but its more like 16v. Unfortunately I
am unable to find a service manual or schematics for this box.

So the next thing I'm thinking is the caps. There are a total of 6
caps in the PS, three 2200 uF 16v electrolytics... these were
obviously blown out when I opened it up and I replaced them. They
connect the input legs of the ICs to ground. There are also 3 dipped
tantalum caps connecting the output legs to ground, the value is hard
to read because they're pretty close in there with the other parts..
I'll bend some things around and look closer tonight.

I'm not sure quite how to test the caps. Looking for shorts with my
DMM and testing them in circuit with the power off, the large
electrolytics give me a steadily rising resistance in one direction
only. The tantalums seem to have resistances of 5k, 7k, and 10k.

Hugh, I really appreciate the invite. I'm in Saint Louis, MO. I'll
try to take some pics but I'm not sure how good my cellphone is at
taking macro shots.

Thank you all.
Christian