Vintage Synth Repair

Hi everyone .

I am working on a Jupiter 6 PSU and I am having problems with the 5v dc.
It seems that when the psu is disconnected from the synth then everything is fine and all readings are correct.
However, when I connect the PSU to the synth I lose the 5 volts, they drop to about 0.7v.
+-15v stays intact and I get the somewhat classic blinking light on the LFO 2 board.

It seems that whatever card I plug in I lose the 5v so it seems like a PSU problem rather then something downline.

Any ideas to help me get this synth up and running?

Thanks 
Arnar

Sounds like a bad 5V regulator or perhaps a cold joint on one of the pins? Does the DC look ok on an o-scope when unloaded?

  ----- Original Message -----
  From: plat@...
  To: vintagesynthrepair@yahoogroups.com
  Sent: Saturday, December 21, 2013 6:03 AM
  Subject: [vintagesynthrepair] Jupiter 6 PSU.



  Hi everyone .

  I am working on a Jupiter 6 PSU and I am having problems with the 5v dc.
  It seems that when the psu is disconnected from the synth then everything is fine and all readings are correct.
  However, when I connect the PSU to the synth I lose the 5 volts, they drop to about 0.7v.
  +-15v stays intact and I get the somewhat classic blinking light on the LFO 2 board.

  It seems that whatever card I plug in I lose the 5v so it seems like a PSU problem rather then something downline.

  Any ideas to help me get this synth up and running?

  Thanks
  Arnar

°Hi. I did that and all the boards kill the 5v.
Thats why I am assuming it has got something to do with the PSU?

Thanks
Arnar

----- Upprunaleg skilaboð -----
Frá: "Scott Frye" <painintheamp@...>
Til: vintagesynthrepair@yahoogroups.com

Sent: Sunnudagur, 22. Desember, 2013 17:21:30
Efni: [vintagesynthrepair] Re: Jupiter 6 PSU.










Find out how many board assbys the +5 volts goes to ( PROBABLY all digital)
Disconnect each board assby , one at a time and see which one kills the +5 volts
Once you narrow it down to a certain PC board, visually inspect for discoloring
Pull any socketed parts and try powering up again. That should get you somewhere





--
Scott Frye

Audio-fixation.net

Vermont

Hi Scott.

Yes, it seems more discreet.

There are no large 3 terminal regulators, however there is a D1406 Transistor , a thyristor 5P05M and 2 ic's ( IC1:Nec C3423C / IC2:Mitsubishi M5218L ).
IC2 seems like it is for the 10v Ref but all the others belong to the 5v circuit , the +/- 15v is looking solid.

Just found and ordered the D1406 and the M5218L , have not been able to find the NecC3423C.
I replaced the thyristor 5P05M (was missing) with a BT151.

I do have a scope but I am not sure how to measure DC ripple , I have read up on that but could not get it to work?

I will check for loose solder joints and replace the caps.

Thanks
Arnar







----- Upprunaleg skilaboð -----
Frá: "Scott Frye" <painintheamp@...>
Til: vintagesynthrepair@yahoogroups.com

Sent: Þriðjudagur, 24. Desember, 2013 13:49:32
Efni: [vintagesynthrepair] Re: Jupiter 6 PSU.







Yes, I would say it's the PSU. I dont have a diagram for it here but , in a rig with THAT much digital, the +5volts is either a LARGE three terminal regulator ( in a T0-3 case) , or something more discreet. Also, weak solder joints are a problem when a circuit asks for current. So, replace all electrolytic caps in the +5 supply, resolder any connections that appear "grayish" or weak and find the PSU diagram


--
Scott Frye

Audio-fixation.net

Vermont

The PSU on the JP6 is a "discreet" linear design with no 3 terminal regulators used. IC1 (UPC3423C) is an overvoltage detection IC that is connected to SCR1. This combination is configure to act as a "crowbar" effectively killing the 5v output if there is a regulation malfunction. This is used on the Digital 5v supply but not on the analog 5v (pin 3 of the PSU board). Is this voltage correct?

You say that with no load the D5v on pins 1 and 2 of the PSU board is good but it drops to 0.7v when the

JP's boards are plugged in. This indicates a couple of scenarios, it could be that IC1 on the PSU is malfunctioning or that the caps associated with D5v are "boarderline" (looking more like resistors in circuit than caps) or there is a fault on one of the boards attached to D5v (usually a capacitor across the power line on one of the boards).

Test the PSU by connecting a high wattage (couple of watts - a small 5 - 12v lamp will do well) load on the D5v output and look at the voltage across it and see what it does. If it stays at 5v with this load you can say the PSU is good. Next connect the other boards one at a time to the PSU, checking D5v each time till you

find the board that causes the PSU to shut down. Fault find the offending board, repair and you should be back in business.

You mention that SCR1 was removed from the PSU before you got to look at it. I'd say someone has had problems before and removed the PSU protection to get it working without actually rectifying the cause of the fault. This sometimes can work (in the short term) but WILL cause major problems further down the track.

The ripple is just the AC measurement on the line. The power supply rectifies the incoming AC to DC of course then drops the voltage.

You look at the power on the lines with the o-scope set to AC to see how well the rectifiers and capacitors are doing their job. The peak-to-peak value should be less than 15% of the DC value, hopefully a lot less. Like a 5 VDC line will probably have around 2 to 3 mV of ripple (that is, ± 1 to 1.5) in my experience. Others with more experience can probably give you a better numbers.

A quick and dirty method is to just check it with an meter set to AC, but you really need the scope to tell the tale and make sure you don't have one leg of the rectifier gone bad or something. and the ripple image is symmetric and, not cocked off to one side.

  ----- Original Message -----
  From: plat@...
  To: vintagesynthrepair@yahoogroups.com
  Sent: Thursday, December 26, 2013 5:48 PM
  Subject: Re: [vintagesynthrepair] Re: Jupiter 6 PSU.



  Hi Scott.

  Yes, it seems more discreet.

  There are no large 3 terminal regulators, however there is a D1406 Transistor , a thyristor 5P05M and 2 ic's ( IC1:Nec C3423C / IC2:Mitsubishi M5218L ).
  IC2 seems like it is for the 10v Ref but all the others belong to the 5v circuit , the +/- 15v is looking solid.

  Just found and ordered the D1406 and the M5218L , have not been able to find the NecC3423C.
  I replaced the thyristor 5P05M (was missing) with a BT151.

  I do have a scope but I am not sure how to measure DC ripple , I have read up on that but could not get it to work?

  I will check for loose solder joints and replace the caps.

  Thanks
  Arnar

  ----- Upprunaleg skilaboð -----
  Frá: "Scott Frye" <painintheamp@...>
  Til: vintagesynthrepair@yahoogroups.com
  Sent: Þriðjudagur, 24. Desember, 2013 13:49:32
  Efni: [vintagesynthrepair] Re: Jupiter 6 PSU.

  Yes, I would say it's the PSU. I dont have a diagram for it here but , in a rig with THAT much digital, the +5volts is either a LARGE three terminal regulator ( in a T0-3 case) , or something more discreet. Also, weak solder joints are a problem when a circuit asks for current. So, replace all electrolytic caps in the +5 supply, resolder any connections that appear "grayish" or weak and find the PSU diagram

  --
  Scott Frye

  Audio-fixation.net

  Vermont