Vintage Synth Repair

Look for the obvious, transistors accidentally swapped that have different pin outs/ins, or an old part that had a non-standard pin  pin configuration because it was a very early design.  I ran into this with a tapco reverb, I had no idea that early op-amps had 

Non-standard pins.  Also an ebay cap I put in was NOS and turned out to be bad.

Lorne

From: vintagesynthrepair@yahoogroups.com [mailto:vintagesynthrepair@yahoogroups.com] On Behalf Of eidorian@...
Sent: Sunday, August 11, 2013 11:48 PM
To: vintagesynthrepair@yahoogroups.com
Subject: Re: [vintagesynthrepair] Re: CS-70m power supply issue, -15V rail is dead

 

  

Hi Hugh,

Thanks for your email and sorry for the slow response. I've been doing 
too much other non-electronics stuff lately :-(

At the moment the +15V rail is measuring ~ +17V and the -15V rail 
"exponentially" rises from ~ -12V or so to ~ +0.8V when power is applied 
then stays there (it gets to 0V in about 3-5 seconds, then slowly 
creeps up to +0.8V over the next 30 seconds or so). The 1K resistor in 
the +15V circuit is very hot to touch. I've replaced or tested OK 
pretty-much every component on the board now.

I agree that there is probably just one common problem causing the 
issues with both the +15V and -15V rails. Next step I think is trying 
to isolate what's causing the 1K resistor to carry all the current 
rather than it flowing through the darlington transistor circuit like it 
should. I'm hoping once that's fixed the +15V will self-heal due to 
some unusual condition being removed. Sorry if that sounds vague but 
I'm not in front of the schematic at the moment!

Also I'm about to go on holidays for two weeks in a couple of days' 
time, so unfortunately it'll be a while before I get back to this one.

I'm in Australia, so a long way from most people :-)

Cheers,
A.

On 2013-08-07 03:42, Hugh Vartanian wrote:
> Hi Adrian, I have been offline for a bit.  Can you re-cap (! ) where
> the circuit is at today?   Is your post of 8/1 the latest?   I 
> confess
> to being a little amused to hear of +15v issues when debugging 
> -15v!   They are surely dependent on each other, as we all know. 
> Anyhow, please confirm the current (! ) situation voltage readings,
> etc. & we'll figure this out.  Where do you live, btw?  My basement
> always has room for one more thing to work on.  Qualified for 
> hoarders
> show a long time ago!  <smile>
> -Hugh
> On Aug 7, 2013 5:16 AM, "cpt.zilog" <cpt.zilog@yahoo.fr <mailto:cpt.zilog%40yahoo.fr> > wrote:
>
>>  
>>
>> > > I will have to recap my cs-70m's power supply (still working
>> > > fortunately), because it's all original and it takes some time 
>> for it
>> > > to warm-up (too long actually), and the volume pot is noisy when 
>> you
>> > > turn it, but it's not a dirty pot because noise almost 
>> disappears when
>> > > the synth is warmed-up.
>> > >
>> >
>> > Interesting - maybe it is some sort of oxidization that is
>> > temperature-dependent? Sounds like you should break out /your/ 
>> scope
>> > and measure the power rails at that point ;-)
>>
>> I don't think so ;-)
>> As I said, it works quite nicely already, why should I bother NOT 
>> changing the capacitors and thus investigating elsewhere ? :-)
>> The CS had the majority of his life in south France where the 
>> climate is nicely dry, helping avoiding storage oxidization.
>>
>> > > I want to install the kenton midi kit that I have for it, but 
>> will do
>> > > once everything -or at least the psu- be properly recapped...
>> > >
>> >
>> > I had a look at that MIDI kit - looks like it will cost me around
>> > AUD$600 to buy, which is a lot, especially factoring in that I 
>> have a
>> > whole pile of gear I'd ideally like to get converter kits for. I'd 
>> be
>> > interested in hearing what others who have one installed have to 
>> say (or
>> > if anyone has one they'd like to sell me for less than that much.)
>>
>> Well I paid that kind of money for it. It is expensive that's for 
>> sure, but there isn't any alternative. Plus it's such a limited 
>> production that the people who buy it, do install it (quite logical 
>> indeed). Kenton used to sell this kit back in their heydays then stop 
>> producing it. They made a new production run thanks to a group buy 
>> initiated buy Analog Sweden. It allowed for me to be able to put my 
>> hands on one of these kits. What they sell on their website should be 
>> what's left of this group buy (me think). So finding one second hand 
>> out in the wild would be very hard if not impossible to say the least.
>>
>> > > You just don't want to open those beasties to often ;)
>> > >
>> >
>> > In general I agree, but I have to say the CS-70m power supply has 
>> been
>> > pretty convenient to work on. The leads from the transformer are 
>> long
>> > enough that I've been able to leave it connected to the "DC" board 
>> with
>> > that board sitting outside the case, nice! I'm not so impressed 
>> with
>> > the exposed incoming AC supply lines on the rear panel switch PCB
>> > though, best keep fingers away from there :-
>> >
>> > Cheers,
>> > A.
>> >
>>
>> Will keep my fingers away....thanks for the tip ;-)
>>
>> Vincent.
>
> 
>
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