Vintage Synth Repair

Hi Christian

The 78 series regulators usually have the metal mounting tab connected to
the output terminal, pin number 2.  So if you ground the tab you will short
circuit the output.  As has been suggested the tab must be insulated from
the heat sink with a suitable washer and the securing screw insulated from
the tab with a stepped washer.  You can buy a kit of insulating washers for
this task.

There is an alternative you could use the 78 series ending in FP, L7805FP
for instance.  This is a plastic package and the mounting tab itself is
plastic.  I've just used one of these to repair a Yamaha YDP131.  It is
necessary to use heat sink paste between the tab and the heat sink in order
to improve thermal contact, this is also true if using a metal tab device
and insulating washers.

Regards

Brian G3OYU

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Christian Oncken
Sent: 04 May 2012 8:56
To: vintagesynthrepair@yahoogroups.com
Subject: Re: [vintagesynthrepair] Re: Simmons SDE

 

  

So I swapped out the 7805 and things are a little better, but different.  

 

First of all, I can actually hear the FM generators functioning now in
response to midi signals.  They are very faint and buried under lots of
noise but they are there. 

 

Unfortunately, the knobs seemed to have stopped responding.. the front panel
buttons work as before, but the knobs don't seem to affect the lights and
display in the way they did before and have no effect on the sounds.  Before
I could turn the knobs in certain modes and see the display change along
with them, but no longer.  

 

The new 7805 also gets very hot, though I'm not sure if its as hot as the
first one.  

 

One strange thing I found... The 8535 (7906 equivalent) will throw sparks
from the metal tab when its pushed into contact with the sub-chassis/heat
sink.  Any idea what could be causing this?  Is the metal tab on these
voltage regulator supposed to be connected to the chassis ground, because I
don't believe they were connected before... 

 

 

 

 

 

On Mon, Apr 30, 2012 at 6:46 AM, John Rose <johnhenryrose@...>
wrote:

  

I would be inclined to change the 7805 as there are a quite a cheap
component as this is where the voltage is low, if you use the 1A version as
per the datasheet it should be fine, any electronics shop should have these,
I would also consider picking up a tube of heat transfer compound (the
ordinary white stuff would be fine) to put the regulators back against the
heatsink as they will appreciate this.

 

John Rose

 

(www.astrodevelopments.com)

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Christian Oncken
Sent: 30 April 2012 05:35
To: vintagesynthrepair@yahoogroups.com
Subject: Re: [vintagesynthrepair] Re: Simmons SDE

 

  

Hi Alan, 

 

I found a jumper on the PS board that the carries the 5v to the ribbon cable
header.  I unsoldered this instead to avoid clipping the IC lead...

 

The DVM measured 0.465A... good call on using the other DVM input, I would
have missed that.

 

I should mention again that the front panel digital functions of the box
seem to work fine, the CPU seems intact and working properly.. the buttons,
display, lights and knobs all work like they should, its just that it
doesn't make the right noises... The only thing a little strange about the
controls is that the knobs seem to only respond to about half of their
travel, their range in restricted from full left to about 12 or 1 o'clock,
after that they're full on... they are digitally encoded.  

 

So I replaced the jumper and fired it up again.  Now the encoders seem to
only work on the rights side of their travel, full right to about 12 oclock,
then they digitally "stick" at 12 even as you turn them the rest of the way
to the left.. 

 

As far as heat goes, I'm trying to not stick my fingers in there, but I
touched a wet q tip to the 3 PS ICs... the 7805 is very hot, sizzles the
water.. the other 2 are not.  The main transformer also gets pretty toasty,
(there are actually two transformers in the box.. )

 

I don't think any of the ICs on the main board get hot.  

 

Thanks again.

Christian

 

 

On Sun, Apr 29, 2012 at 4:44 PM, Alan Probandt <alan_probandt@...>
wrote:

  

The 7805 output is low but not low enough that the system shouldn't work.
It may be drawing too much current on the +5Volt.   Got a soldering iron, a
pair is small diagonal cutters, and a multimeter that measures up to two
Amps of current?  Yes, then cut the +5 volt pin on the 7805, bend legs in
different directions, attach the ammeter to the different legs (check if you
have to put the DMM probe into a different hole) and measure the current at
4.85 volts.  My guess that a good system will be between 300 and 800
milliAmps. That's normal for a microprocessor-based 'light' synthesizer of
the mid 1980s.  Are any of the ICs physically hotter than the others?