Vintage Synth Repair

hi folks, i've never had to replace a transformer, but im about to
come into a synth that needs the power transformer replaced. reading
the schems, i know the PSU puts out +15v and -15v. not sure what the
x-former is putting out, as im not in possession of the synth yet.(+24
and -24?)

any recommendations for internal power x-formers? google search just
turns up EXTERNAL x-formers. thanks for any leads.

Hi,

get hold of the synth first, or attach the required schematic part if 
possible, it will help things. When the power transformer fails, there 
often is a problem somewhere else.

If it outputs +15v and -15v, the transformer could be a 18V-0V-18V, as 
diodes will rise the voltage up to 1.414*AC voltage.

There are many different types of transformers, different sizes for each 
of them, and different ways to connect them.

Transformers: 
http://www.mouser.com/search/Refine.aspx?Ne=254453+1447464&N=254453+1323038&Ns=P_SField

Antoine

r_j_d_2.phila a \ufffdcrit :

> hi folks, i've never had to replace a transformer, but im about to
> come into a synth that needs the power transformer replaced. reading
> the schems, i know the PSU puts out +15v and -15v. not sure what the
> x-former is putting out, as im not in possession of the synth yet.(+24
> and -24?)
>
> any recommendations for internal power x-formers? google search just
> turns up EXTERNAL x-formers. thanks for any leads.  
>

thanks much, antoine. good to know mouser has them. the synth is an
arp 2600. the current owner i am purchasing from had the synth looked
over by phil cirocco, and he said the transformer is causing voltage
drift over time, so assuming the part is reasonable, im gonna start
with that and then see if the problem still exists. 


--- In vintagesynthrepair@yahoogroups.com, Antoine Deschênes
<antdes45@...> wrote:
>
> Hi,
> 
> get hold of the synth first, or attach the required schematic part if 
> possible, it will help things. When the power transformer fails, there 
> often is a problem somewhere else.
> 
> If it outputs +15v and -15v, the transformer could be a 18V-0V-18V, as 
> diodes will rise the voltage up to 1.414*AC voltage.
> 
> There are many different types of transformers, different sizes for
each 
> of them, and different ways to connect them.
> 
> Transformers: 
>
http://www.mouser.com/search/Refine.aspx?Ne=254453+1447464&N=254453+1323038&Ns=P_SField

> 
> Antoine
> 
> r_j_d_2.phila a écrit :
> > hi folks, i've never had to replace a transformer, but im about to
> > come into a synth that needs the power transformer replaced. reading
> > the schems, i know the PSU puts out +15v and -15v. not sure what the
> > x-former is putting out, as im not in possession of the synth yet.(+24
> > and -24?)
> >
> > any recommendations for internal power x-formers? google search just
> > turns up EXTERNAL x-formers. thanks for any leads.  
> >
>

Line voltage ( which drifts up n down) isnt perfect.
Most devices should still work from line in AC from 105-130 VAC
I would say that the regulator circuitry after the transformer is the culprit.
Unless the transformer voltage is WAY low I would suspect the voltage regulators.
If you're just driving circuits that supply +/- 15, there are many transformers that will work.
You just need to find one that PHYSICALLY fits and can supply the current and
More current available is better than not enough.

I'm Scott Frye and I approved this message.

Scott  Frye
AudioFixation
Consumer / Pro Audio Repair
Southern VT 

"Resistance is futile but.....
Impedance is much more complex"

thanks scott-lets just say that for safety's sake, i wanted to replace
both the transformer AND the voltage regulators-can you help me read
this schematic?(id rather just do it now while i've got the unit
disassembled). i cant figure out the secondary voltage of the
transformer, or the regulator values.the labeling on the actual parts
is obscured-im putting the ARP 2600 service manual in the files
section-thanks alot guys! 



--- In vintagesynthrepair@yahoogroups.com, scott frye
<painintheamp@...> wrote:
>
> Line voltage ( which drifts up n down) isnt perfect.
> Most devices should still work from line in AC from 105-130 VAC
> I would say that the regulator circuitry after the transformer is
the culprit.
> Unless the transformer voltage is WAY low I would suspect the
voltage regulators.
> If you're just driving circuits that supply +/- 15, there are many
transformers that will work.
> You just need to find one that PHYSICALLY fits and can supply the
current and

> More current available is better than not enough.
> 
> I'm Scott Frye and I approved this message.
> 
> Scott  Frye
> AudioFixation
> Consumer / Pro Audio Repair
> Southern VT 
> 
> "Resistance is futile but.....
> Impedance is much more complex"
>

>>the synth is an arp 2600. the current owner i am purchasing from had
the synth looked over by phil cirocco, and he said the transformer is
causing voltage drift over time, so assuming the part is reasonable,
im gonna start with that and then see if the problem still exists.<<

this only makes sense if there is something physically wrong with the
transformer- like corroded windings shorting out or something. in any
case, the regulator circuits (however they are designed) downstream of
this should either be taking care of the variations or dropping out
entirely. these symptoms, in addition to any deterioration in the
transformer, should & would have been picked up by any reputable synth
tech during his "looking over" the synth. being as it's a 2600, this
is no small matter- the thing is practically a valuable antique by
today's market standards. I though I recognised cirocco's name as a
reputable tech- perhaps someone else can confirm this? in any case,
you need to get a second opinion before throwing any money at this or
opening the thing up yourself. something sounds wrong here.

duncan (an actual engineer)

>>thanks scott-lets just say that for safety's sake, i wanted to
replace both the transformer AND the voltage regulators-can you help
me read this schematic?(id rather just do it now while i've got the
unit disassembled). i cant figure out the secondary voltage of the
transformer, or the regulator values.<<

ah-hah. the synth uses +15V & -15V, so the transformer is likely to be
(as someone has already pointed out) something like 18-0-18. 
I would leave this alone unless it's obviously damaged somehow, or if
it's buzzing & you need to quieten the synth down acoustically. you
could replace it with a suitable toroidal. 
look on the case of the arp for guidance as to the number of VA the
transformer should be rated at, but bear in mind that it likely uses
more current on the + rail than on the -.

if you are unable to begin, or are worried about measuring the
secondary voltages of the trafo, find a tech who can. it doesn't need
to be a synth tech, just someone who understands electronics. 

the power supply is college-grade stuff, a simple series regulator
design, & could easily be replaced with off-the-peg 78xx/79xx devices
if the current load is not excessive. other designs using
pass-transistors could still use these devices as reference voltages.
again, this is not beyond the scope of any decent radio/tv/audio tech,
even if they've never seen a synth before. find one, & have him/her
measure the secondary voltages & the regulator outputs on & off load;
you may discover that there's a bad module in the arp causing these
problems. I wish I was nearer & able to help more.

duncan.

thanks alot, duncan. i've already got the unit open. i'm not an EE,
but i've done a good amount of synth/electronics work over the years,
and avoid by any means necessary taking any steps that im not 100%
sure about. just a little backstory, fyi...

-my readings: PSU is putting out +15/-15. i have yet to leave it on
for a few hours to see how the voltage drifts(previous owner said phil
diagnosed a voltage drift after being on for a while.) so it is
working currently, but i want to restore it best as i can before
putting it into my studio for regular use.


-previous owner had synth looked over by phil cirocco-yes, he is
apparently one of the foremost ARP techs in america. also is
apparently MIA-i have sent him an email or two over the past few
years, no reply. common story, i believe he is moving or something.
nevertheless, previous owner was told by phil it needed a transformer
replacement, and would cost $200. transformer APPEARS to be in good
shape, but again, could be a time-delayed problem. seems a little
unlikely to be the x-former, but phil's the pro, im not. 

-as i would like to maintain the unit's health for many years to come,
i'd like to get the PSU in best shape as possible. with the power
transistors being obsolete, i am now considering another approach:
drop an entirely new, linear power-one PSU in as a replacement. the
1.5A unit is only $72, which is relatively very cheap. puts out
+/-15V. i have used power-one linear units several times before on
other projects, and have always found them to be rock solid, easy to
calibrate, and good build quality. is this a horrible idea? here is a
link to the spec sheet:

http://www.power-one.com/cgi-bin/acdc_search.cgi?volts_1=15&volts_2=-15&ref=linear&x=61&y=4&LINEAR=f

the main issue i have now is measurement of current draw, as im not
sure what the + and - sides of the 2600 need. is my line of thinking
way off here? the old PSU is actually very simply laid out; there is
continuity amongst the 2 +15V output pins and the 2 -15V output pins,
as well as common, so i dont see why i couldnt tie each respective
output wire grouping together, and run directly to a new PSU. 

sorry so long here, just trying to be comprehensive. please punch any
holes in my theories if anything sounds too wacky! thanks alot,
guys-really appreciate the help. -rj

 

--- In vintagesynthrepair@yahoogroups.com, "duncan" <ferrograph@...>
wrote:

>
> >>the synth is an arp 2600. the current owner i am purchasing from had
> the synth looked over by phil cirocco, and he said the transformer is
> causing voltage drift over time, so assuming the part is reasonable,
> im gonna start with that and then see if the problem still exists.<<
> 
> this only makes sense if there is something physically wrong with the
> transformer- like corroded windings shorting out or something. in any
> case, the regulator circuits (however they are designed) downstream of
> this should either be taking care of the variations or dropping out
> entirely. these symptoms, in addition to any deterioration in the
> transformer, should & would have been picked up by any reputable synth
> tech during his "looking over" the synth. being as it's a 2600, this
> is no small matter- the thing is practically a valuable antique by
> today's market standards. I though I recognised cirocco's name as a
> reputable tech- perhaps someone else can confirm this? in any case,
> you need to get a second opinion before throwing any money at this or
> opening the thing up yourself. something sounds wrong here.
> 
> duncan (an actual engineer)
>

>>previous owner was told by phil it needed a transformer
> replacement, and would cost $200.<<

$200 for a transformer?? a tad excessive, even with the price of
copper being what it is... besides, as we have said earlier, if you're
getting good 15V rails most of the time & the thing doesn't look or
smell bad, it's more likely to be something in the regulation that's
going bad. check for bad solder joints, especially around the large
power components- the rectifier diodes & the pass-transistors.
see the posts about the juno 106? same thing, more or less.

yes, & let the thing drone for a few hours (2600s are supposed to be
good at this!) into a guitar tuner, see if it drifts any. of course,
this could all be component breakdown elsewhere in the thing & nothing
whatever to do with the power supply. 

e.g. I had a korg monopoly that would not stay in tune- this turned
out to be the way the thermistors were glued to the oscillator chips;
the glue went bad & the thing started drifting. the 2600 is full of
such design quirks, as I recall. mostly in epoxy-potted blocks, right?
:-/ 

>>i am now considering another approach:
> drop an entirely new, linear power-one PSU in as a replacement. the
> 1.5A unit is only $72, which is relatively very cheap. puts out
> +/-15V. i have used power-one linear units several times before on
> other projects, and have always found them to be rock solid, easy to
> calibrate, and good build quality. is this a horrible idea?<<

not really so horrible, & it would mean you could use the arp off any
line voltage/frequency. but it wouldn't be stock any more, & that
would affect resale value. personally, I don't care about such things-
to me it just indicates that someone did some restorative work so that
the instrument had a longer active life. I hate this "museum-grade"
nonsense... :-)  

but maybe you can measure the current draw on the existing power
supply first. 
the first "gotcha" is likely to be the -15V which, from many
commercial computer grade SMPSU modules is likely to be capable of
only a few hundred mA, compared to a couple of amps at 5V or 15V
positive. your average analogue synth has more or less symmetrical
current draw....
 
the second is going to be radiated HF noise from the power supply. put
it in a faraday cage (you may have seen this approach inside, say, an
emu sampler) & fit ferrite rings on the output cables.

third, it may just affect the sound a little bit... depends how
"golden-eared" you are, but you'd be replacing an analogue power
supply with a digital one & this will to a small degree affect the
behaviour of the synth, sonically.

let us know how you get on- that's what this list is for! :-)

duncan.

yea, $200 for x-former seemed a little high, but have you seen what
the CMS modules cost? $1300 for a modular VCO. so, im not surprised.
their SMALLEST full system is 7k.

duncan, this other stuff is GREAT news-thanks. ESPECIALLY the
potential of difference in sound w/ digital PSU. resale is not an
issue, as i wont be selling this anytime soon, but i am one of those
kooks who is a firm believer in  the early ARPS being totally distinct
beasts, sonically. if THAT'S the case, then i might as well just try
to source the power transistors, and maybe voltage regulators, and
keep the old PSU. i hate this idea of putting off a potential known
issue(voltage drift), rather just nip it in the bud, but maybe i will
try to get it to behave as such by leaving it on for a few hours.
thanks guys!!


--- In vintagesynthrepair@yahoogroups.com, "duncan" <ferrograph@...>
wrote:

>
> >>previous owner was told by phil it needed a transformer
> > replacement, and would cost $200.<<
> 
> $200 for a transformer?? a tad excessive, even with the price of
> copper being what it is... besides, as we have said earlier, if you're
> getting good 15V rails most of the time & the thing doesn't look or
> smell bad, it's more likely to be something in the regulation that's
> going bad. check for bad solder joints, especially around the large
> power components- the rectifier diodes & the pass-transistors.
> see the posts about the juno 106? same thing, more or less.
> 
> yes, & let the thing drone for a few hours (2600s are supposed to be
> good at this!) into a guitar tuner, see if it drifts any. of course,
> this could all be component breakdown elsewhere in the thing & nothing
> whatever to do with the power supply. 
> 
> e.g. I had a korg monopoly that would not stay in tune- this turned
> out to be the way the thermistors were glued to the oscillator chips;
> the glue went bad & the thing started drifting. the 2600 is full of
> such design quirks, as I recall. mostly in epoxy-potted blocks, right?
> :-/ 
> 
> >>i am now considering another approach:
> > drop an entirely new, linear power-one PSU in as a replacement. the
> > 1.5A unit is only $72, which is relatively very cheap. puts out
> > +/-15V. i have used power-one linear units several times before on
> > other projects, and have always found them to be rock solid, easy to
> > calibrate, and good build quality. is this a horrible idea?<<
> 
> not really so horrible, & it would mean you could use the arp off any
> line voltage/frequency. but it wouldn't be stock any more, & that
> would affect resale value. personally, I don't care about such things-
> to me it just indicates that someone did some restorative work so that
> the instrument had a longer active life. I hate this "museum-grade"
> nonsense... :-)  
> 
> but maybe you can measure the current draw on the existing power
> supply first. 
> the first "gotcha" is likely to be the -15V which, from many
> commercial computer grade SMPSU modules is likely to be capable of
> only a few hundred mA, compared to a couple of amps at 5V or 15V
> positive. your average analogue synth has more or less symmetrical
> current draw....
>  
> the second is going to be radiated HF noise from the power supply. put
> it in a faraday cage (you may have seen this approach inside, say, an
> emu sampler) & fit ferrite rings on the output cables.
> 
> third, it may just affect the sound a little bit... depends how
> "golden-eared" you are, but you'd be replacing an analogue power
> supply with a digital one & this will to a small degree affect the
> behaviour of the synth, sonically.
> 
> let us know how you get on- that's what this list is for! :-)
> 
> duncan.
>

ok, new plan-i left the unit on for an hour, and the + rail seems
solid, but -15 drifted down to -12.5. both sides of x-former were in
the 23VAC range, so i have a new plan:

1-stick with old PSU
2-replace + and - power transistors.
3-socket and replace voltage regulators while im in there.

any other long term preventative maintenance? can i expect a
transformer to last another 10-20 years, or should i bother with it
now? parts cost isnt an issue, its well worth the investment.

thanks for the help, folks.

On Thursday 18 September 2008 11:00, r_j_d_2.phila wrote:
> ok, new plan-i left the unit on for an hour, and the + rail seems
> solid, but -15 drifted down to -12.5. both sides of x-former were in
> the 23VAC range, so i have a new plan:
>
> 1-stick with old PSU
> 2-replace + and - power transistors.
> 3-socket and replace voltage regulators while im in there.

How about before you blame the PS you make sure that there isn't something of 
an excessive load pulling it down?  Could be that,  or the PS,  and you're 
assuming there without anything I'm seeing posted to indicate one way or the 
other...

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin

Greetings to all, just joined the group a while back and have had a
ball reading the posts, I'd like to add a humble comment over this arp
2600 power supply problem, me I suggest to try to test the power
supply isolated from the rest of the electronics, that is; with the
synthesizer power cord detached from the wall, lift the +15 volts and
 -15 volt off the power supply board going the rest of the synth. then
attach a 15 ohm (15 to 20 watt) power resistor to the +15 v (and
ground)  and another to the -15 volt  and ground also. then measure
with a meter the voltage delivered by the regulator, this way you can
isolate  what side is failing but at this point it is clear the -15
volts is the culprit. I would replace all transistors and the ua748
opamp, the -15 volt is just an inverted 'copy' off the +15 v side, be
advised that the lm723 is known to fail, not frequently but it does
fail doing strange things. in a nut shell, the 'real' regulator is the
+15v supply, the -15V is a voltage inverted version of the other and
also incorporates current limiting via q3 and r15, now judging by the
vlue of this resistor (1ohm) the current gets hard limited to about
600 mA  at the -15V side, R6 does the same on the +15V side. The LM723
is known to have better transient response ( read better regulation)
than a monolithic regulator like the LM7815 for the same load needs.
But this chip is getting harder to get lately. FUTURLEC has them but I
have no idea how they operate their bussines ( read prompt or late
delivery, quality etc) see
http://www.futurlec.com/ICLinearRegulators.shtml.

 Anyhow, if by testing via the 'load resistors' indicates solid
regulator behaviour, then is time to look for the trouble deeper into
the synth.   
 last, if the uA/LM748 is nowhere to be found, try using a lm741
instead, there are some out there, (at least here in my parts bin)
other single 8 pin opamp's should work provided you desolder C5 just
in case.




--- In vintagesynthrepair@yahoogroups.com, "Roy J.
Tellason<rtellason@...> wrote:
>
> On Thursday 18 September 2008 11:00, r_j_d_2.phila wrote:
> > ok, new plan-i left the unit on for an hour, and the + rail seems
> > solid, but -15 drifted down to -12.5. both sides of x-former were in
> > the 23VAC range, so i have a new plan:
> >
> > 1-stick with old PSU
> > 2-replace + and - power transistors.
> > 3-socket and replace voltage regulators while im in there.
> 
> How about before you blame the PS you make sure that there isn't
something of 
> an excessive load pulling it down?  Could be that,  or the PS,  and
you're 
> assuming there without anything I'm seeing posted to indicate one
way or the 
> other...


   totally agree here... before I forget... check those 1ohmm
resistors, resolder and replace if ncecssary.

Mariano

OOOPSS.... sorry, didn't pay enough attention to the schematic and
made an ass out of myself.... the UA748 opamp used there is a 14 pin
version....  even harder to get... seems this is gonna be a bit harder
to replace.

Mariano

--- In vintagesynthrepair@yahoogroups.com, "syntherman" 
<syntherman@...> wrote

>  last, if the uA/LM748 is nowhere to be found, try using a lm741
> instead, there are some out there, (at least here in my parts bin)
> other single 8 pin opamp's should work provided you desolder C5 just
> in case.
> 
>

thanks mariano. actually, the ua723/ua748's are still available and
cheap, check mouser. the mje1100/1090's are harder to find/expensive,
but still out there. 

needless to say, i ordered all of the above today, and will drop in
when they all arrive. i'll check back in a week or so when i have more
news. as far as any problems downstream pulling down the - rail, im
just gonna get the PSU running properly and see where it takes me. i
guess the obvious thing is just to disconnect the +/-/com leads to
synth and then test; i'll probably do that when all the parts come in.

thanks for the help, guys. hoping to have a fully functional 2600 in a
week or so!



--- In vintagesynthrepair@yahoogroups.com, "syntherman"
<syntherman@...> wrote:

>
> 
> OOOPSS.... sorry, didn't pay enough attention to the schematic and
> made an ass out of myself.... the UA748 opamp used there is a 14 pin
> version....  even harder to get... seems this is gonna be a bit harder
> to replace.
> 
> Mariano
> 
> --- In vintagesynthrepair@yahoogroups.com, "syntherman" 
> <syntherman@> wrote
>  
> >  last, if the uA/LM748 is nowhere to be found, try using a lm741
> > instead, there are some out there, (at least here in my parts bin)
> > other single 8 pin opamp's should work provided you desolder C5 just
> > in case.
> > 
> >
>

>>How about before you blame the PS you make sure that there isn't
something of an excessive load pulling it down?  Could be that,  or
the PS,  and you're assuming there without anything I'm seeing posted
to indicate one way or the other...<<

I know it's a different kettle of fish altogether, but I recently had
a fault on a studer b67 deck where the -12V was apparently being
pulled down to about -9V by some rogue component on one of the audio
cards; the symptoms went away with all the audio cards pulled, leading
me to exonerate the power supply & spend a small fortune on tantalum
capacitors. 
I anticipated replacing capacitors on all of the boards, & began by
putting them back in one at a time to begin fault-finding. but on a
hunch, I tried a dummy load on the power supply first (actually just a
small light bulb like the one in the deck's meters) & discovered that
the regulator was fine off load but couldn't deliver anything more
than 20mA. changed it... deck now fine, but I'm down about £50 on
tantalums I don't need. :-(

the offending component was an LM320, a series regulator that looks
like a power transistor. the way series regulators fail never ceases
to surprise me- I had a 7812 in a pro-1 fail into short circuit, so
that it passed about 19V to the board & fried the vcf chip.

hth-
duncan.