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Can the secondary voltage of power supply transformer be determined by the double 25volt caps?

kellys_eye

Jun 25, 2010
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Only to the extent that the voltage can be 'anything' UNDER 25V (with appropriate safety margins this could be 'anything' under 20V).

Are there any other component parts in the power supply? If not then the actual regulation is done in the keyboard itself. What the designers try to do is limit the dissipation in the regulation circuitry and they design it to work with as near to the required output voltage as possible i.e. if the regulated output is 12V then the input is likely to be around 15-18V. If it's a 5V regulated output the input would be around 9V.

Then again, how many capacitors are being used? If there are TWO then the transformer may have a double secondary (or centre-tapped secondary) and the supply may deliver plus AND minus 18V (or 12V). If you can't find a manual then post some pics (under 300kb in size) of the PSU section and we might have a better idea.

Although it's economically 'wasteful' to use capacitors rated at a higher voltage than is actually required this may be as a result of using existing stock or a 'deal' for quantity purchase. HOWEVER I'd be surprised if the DC voltage wasn't around 18V in your case - though don't quote me on it if you get it wrong and release the magic smoke........
 

kellys_eye

Jun 25, 2010
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....after a fruitless search I couldn't find the schematic for the KR350 but other types of similar keyboards use transformers with MULTIPLE secondaries so I would doubt whether yours has a single secondary either - which makes posting pics that much more important.
 

musicman1101

Jun 23, 2022
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Thanks kellys-eye and bertus for the help! I spent enough time searching the net too. This is my first posting and I realize I should have spent more time posting more info I will post picture or two of of the ps board when I get home and other pertain-ate info that my help. there is a three terminal jack on the power supply board so I guessing its a center tap. and I believe there is + and - rails and regulators for each. Pictures coming later! THANKS AGAIN!
 

musicman1101

Jun 23, 2022
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I also learned tonight that D1 is a bridge-rectifier and IC1 and 2 are audio outs that operate with a typical voltage 13volts, one plus and one minus with a minimum of 6volts and max of 15volts. IC3 and IC4 are + and - 9v regulators and IC5 is a 5v regulator I can't remember if it was + or - .
 

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kellys_eye

Jun 25, 2010
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IC's 3+4 look to be the main supply line regulators potentially from the 78** and 79** series regulators (or the Japanese equivalent range). The input is indeed a spit-secondary AC transformer output (12-0-12 most likely - or perhaps 15-0-15 less likely).

The 'raw' DC from the bridge rectifier/smoothing capacitors feeds the audio amp devices (IC's 1&2) whilst the regulated (plus/minus) rails from IC's 3&4 supply the main audio processing circuitry. IC5 may well be another regulator - this time down to +5V for any logic devices.

Given the audio IC's can withstand up to +/-18V DC and the regulator devices will deliver a lower voltage (possibly +/-12 or 9V) you could safely use a 12-0-12 secondary transformer (centre-pin of the three-pin connector goes to the 0 connection) to power up the circuit. The transformer should be able to deliver at least 1A (I'd use a 2A secondary). Even a 9-0-9 transformer would get it going....

IIRC I've heard of 9-0-9 being quite common for keyboards - maybe others can chip in on this???

Since the posts-regulation devices are well heatsinked I reckon they will supply 12V or LESS at a maximum of 1A to the rest of the board. The higher the secondary voltage of the transformer the more the dissipation in the two regulators hence the possibility that a 9-0-9 transformer may have originally been used.

If you can identify the two regulators IC3&4 it would indicate whether 9-0-9 or 12-0-12 is the actual transformer to be used.
 

musicman1101

Jun 23, 2022
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I really appreciate all the help and effort from you! These forums are great. I looked them up and they were both 9 volt regulators one plus and one minus. Yes I think a 12v trans would be good for the audios and not stress the 9 volt regulators too much. I have quite a few CT transformers laying around with no voltages marked on them, however they are not going to read 12 volts with no load, Correct?. Should I put them on a load when I am checking for output voltages? or can I expect and certain no load voltage. Also I thought if I have to load test them to find the correct output voltage, I should be able to I put the load only on the outside tappings and ignore the center 0 volt pin? correct? and what could I use for a load other than a load resistor, like maybe a old car sealed beam headlight or two? Just a thought.
 

Martaine2005

May 12, 2015
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I should be able to I put the load only on the outside tappings and ignore the center 0 volt pin? correct?
Correct, 9-0-9 will have 18V. Not sure of your load. Expect 15+ volts. Possibly 17Volts at the bridge rectifier.


Martin
 

musicman1101

Jun 23, 2022
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Correct, 9-0-9 will have 18V. Not sure of your load. Expect 15+ volts. Possibly 17Volts at the bridge rectifier.


Martin
The keyboard states that it uses 35 watts at 117 vac. Can an estimate be made as to the lowest current requirements iu would need? Kellys_ eye suggested 1 amp and better yet 2 Amps. Using a much larger one say 5 or 10 amps should be ok, as long as my rectifier voltage is close to 12 -15 volts?
 

kellys_eye

Jun 25, 2010
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35W @ 117VAC also means 35W @ whatever the unit uses so assuming a 9-0-9 transformer (total 18V) this would be at 2A (36W but near enough).

Larger is ok but only if your pockets are deep enough........
 

musicman1101

Jun 23, 2022
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I have -15.52v and +15.01 on my first two filter caps and -9.09v and +8.89 coming out of my 9 volt regulators. Could this difference be caused by a leaky cap on my + rail? I seems to me that the +9 volt regulator should be putting +9 volts while the input to it is +15.01volts? I don't have a scope to check ripple. I tried to read the ripple with my meter on 200Mv scale but could not determine if it was different from the negative rail.
 
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