Hi I need some help to resurrect an old synthesizer that had stopped working possibly due to a failure in its power supply circuit. The parts are diffucilt to locate so I thought if I used equivilent parts I may be able to get it to work. It has 44v AC input and a 12v and 5v + and - DC output.
I will post a pic of the circuit. It has 44v AC input going through a bridge rectifier and then through 2 x 4700mf 25v caps. So the filtered supply to the front of the circuit is 25v + and - with a gound from the transformer.
This then is stepped down to 15v by two regulators. One I cannot find the same part number and is the one that is possibly faulty.It is marked as uPC 14315H. I have tried a 7815 in its place but did not have any output after firing it up.It did get hot so I switched off.
I have made another supply that used similar parts but I am not very good with electronic design so I made this section similar to the first part of the cicuit with an LM 317 for + and an LM337 for the - and a variable pot for both sides to get to 5volts.
When placed back into the syn I had the keyboard light up but no sound and when I checked the voltage I had no 5 v output and extremely hot IC's. I think my design was flawed because on further research the 5v section
has two transistors to boost the current level as opposed to my two 1 amp regulators.
Can anyone please help with an alternate design or subsitute part numbers for the original supply as I am an amateur in respect of electronics and would like to try to get this synth back up and running. Thanks Tom
Absolutely not. Even if you found one that was 5.000V at room temp, they have both a tempco and load regulation effect. The envelope for those would be 3% or more on that type of regulator. Those are what we built in the last place I worked...... I won't mention their name out of fear that if I write National Semiconductor, it might influence somebody to buy product from one of the crappiest semiconductor houses on earth.You can probably use 7805 and 7905 for the +/- rails, IF you pick 2 that are exactly 5.00v and both the same, AND both stay the same when under load. Some brands of 7805 have much better regulation.
There used to be a film type material we bought in sheets that was temp sensitive. If you laid it over a PCB and powered it, the warm one showed up quickly.Some tips I have... ICs in older equipment often run hotter than you'd expect.
This is a controversial one: If there's a short circuit on a 5 V power rail and you can't find it, connect a bench power supply of 5V to the shorted rail and turn up the current and leave it for a while. The chances are if it's an IC, it's dead anyway, so look for hot spots appearing, or even better smoke leading you right to the offending part. If still can't find it, despite feeding 5 A into the PCB, get a can of Freezer spray and cover the PCB in frost, then you'll see some tracks/ICs/capacitors thaw out first, you can literally see the current path as though you're using a thermal imaging camera!
Sure looks like it considering how much money they threw at the 5.000V rail. The 0.1% resistors shown are temperature compensated (since they have the little braces around their schematic symbols). Those suckers are not cheap. And the 2.49V reference is trimpot adjusted for zero tempco and to dial in the 5.000V rail voltage. hand adjusted = cost $$$.The +/- 5v supplies are supposed to be matched very well and precise at 5v. The synths analog VCO/VCA/VCF etc are all voltage controlled and need an exact 5v supply.
This is probably a valuable collectable old synth being butchered.
Thanks for the response.
This is exacty what I don't want to do. The PSU I am using as a subsitute does not use any of the original parts as I have kept that pretty much intact.
When first tested with the psu still connected I had no 15v + or + and - 5v output but I did have - 15v .I checked the voltage at the output of the 14315H but had not output at all.
This led me to think this part may be faulty.I cannot find this part number anywhere (uPC 14315H). Can I use a 7815 in its place ?
Now that I have the PSU out of the synth I will put power back through it and check the voltages again with no load. I will put the 14315 back in and if still no voltage I will try the 7815.
Is there other parts I can use as substitutes for the originals to maintain the design as I cannot find them here in Australia (original parts).
I will replace the caps with new equivilents and will put the original 15v reg back in. ( uPC 14315H ) and post when done.
Thanks for the advice
BTW: both the 5V and -5V rails have fuses which could be blown. Don't know if you have checked those yet.That would be an ideal situation to try but unfortunatley I don't have access to such equipment. The synth stopped working 2 years ago and my son asked could I get it going. I have limited electronic experience and knowledge. My temp psu did power all the LEDS when hooked up but no sound was made so I disconnected it to avoid any further problems.
I will later today connect the original PSU back up with no load and check the voltages so see if it does indeed put out the 15 and 5v respectively. If it does then yes I believe the problem is further inside the synth and I am not sure what to do as I don't have the required test equipment or knowledge to go any further. The synth weighs too much to send away to get repaired, so rather than butcher it I may just put it back into storage and let it gather dust. Thanks again for your help.
Tom
Korgs normally use sockets for all the ICs.
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