Vintage Synth Repair

Have you looked at the schematic?  If not you are fighting in the dark!  You
need to know precisely what voltages are required.  You make the comment
'from what I can tell', this suggests that you haven't looked at the
schematic.  Correct me if I'm wrong but someone posted a link to the
schematic some time ago and pointed out that you need a 15V +- winding and a
12V +- winding.  This does not tally with you guess that it needs a 36V
centre tapped winding!

 

I haven't looked at the schematic and I no longer have the link so I can't
now check.  However if, as you think, you do only need a 36V minimum centre
tapped then I would suggest a 25V - 0 - 25V secondary.  Most circuits used
something like the 78/79 series voltage regulators, if you check the data
sheets for these you will find that they have a maximum voltage in excess of
30V, speaking from memory absolute maximum is 35V.  Now a 25V winding will
give you an absolute maximum voltage of 25 times root 2 which is 35V so this
will be OK.  I've used a 25V centre tapped transformer to produce a 24V DC
supply many times using the 78 series 24V regulators.

 

Your wacky megohm measurement on the secondary does suggest that the
secondary is open circuit.  However I was under the impression that that
under these conditions the fuse blew so unless I've got that wrong how did
you measure the correct mains voltage at the primary?

 

Post me the link to the schematic and I'll try to help you.  Might I suggest
that you email me off list?

 

Regards

Brian G3OYU

 

From: vintagesynthrepair@yahoogroups.com
[mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of Boddington
Sent: 02 June 2011 12:01
To: vintagesynthrepair@yahoogroups.com
Subject: [vintagesynthrepair] Re: Crumart Trilogy replacement power
transformer

 

  

See it can be rocket science! :)

I'll check the PSU board once it's up and running again for sure. But to be
clear: even IT wasn't connected. Just a cable, switch, fuse, and trafo. The
trafo shows the correct ac voltage on the primary and that wacky megaohm
measurement on the secondary.

From what I can tell, it needs to be center tapped with a minimum of 36V
split down the middle to handle the needs of both sets of voltage outputs.
The reason I inquired is because I didn't want to get a replacement that
would put too much or too little load on it at 500mA. Wouldn't 50V be
overkill? Also - the schematic also hints at 1A after the secondary but the
fuse shows a 500mA limit. See whay I'm asking? :D