KORG SE-500 voltage collapse

[larsove.iff]

Hi!

This is my first post here, and I am hoping that someone in here can push me in the right direction as I am troubleshooting my KORG SE-500 tape delay.
The machine has no delay signal and I believe that has to do with the +side not delivering enough voltage. When I scope pin 7 on A1(4558 op amp) I get a nice reproduction of the sine wave that I send into the input. But it fails to deliver any output on pin 1. And I believe that is because the + side of the circuit I am only getting 5.2 volts in to the 7815-regulator(It is a  new 7815-regulator), and and output of around 2.2v (If I measure pin 8 on any of the 4558 that is all I get). The bridge rectifier (S1VB10) reads 35v when I measure AC current between the two ~ points on the rectifier. It also reads around -35vDC on the negative side, and only +5.2v on the plus side. This leads me to conclude that the bridge rectifier is only working on the negative side. I don't have a new rectifier on hand, but I thought that this KBP02G would be a good replacement? https://www.digikey.no/no/products/detail/diodes-incorporated/KBP02G/248715
Can anyone confirm that this would be a good fit?
I believe I am on the right track, but would like to get some feedback if I am searching for faults in the wrong places. (attaching the schematics)

Thank you in advance!

Cheers,
Lars Ove

[neillnz]

I think your reasoning is spot on.

If the AC is present on the bridge rectifier and so is the - DC but not the + DC then it stands to reason that the bridge diodes are not conducting as they should.

I don't think that the issue is an overload of the + DC as this would cause an issue with the - DC since they are fed from the same transformer secondary winding.

This leaves the question, what has caused the bridge rectifier to fail? Possibly it has been overloaded at some point.  If you had a bench/lab power supply you could use this to supply perhaps 20V to the input of the 7815 and observe the current draw.

You could use a pair of 1N400x diodes temporarily in parallel with the bridge diodes and see if this makes it work.

My guess is the bridge rectifer has simply failed all by itself perhaps partly due to lack of thermal management in the design.

That bridge you have chosen looks to be a good choice, just so long as it physically fits(pin spacing and body size).

   

[larsove.iff]

Thank you for your reply!

This machine is from the 1970 and have seen some touring through out its life time, so I guess failed due to age and, as you mention, lack of thermal management(no heatsink on this one). I must mention that it did work before, but it had a slightly present hum when powered on, even with output volume turned all the way down. The original voltage regulators in these units are rated to 0.5A, and I replaced them with modern ones rated to 1.5A. The capacitors have also been replaced(with the same uF-rating and same or higher voltage rating), and polarity have been double checked.

I unfortunately don`t have a bench/lab PSU to test the current draw on the 7815.
I'll order some new parts and check back!

cheers!
-Larso

[MathWizard]

Do you have any random parts, like resistor's, BJT's , breadboard? And maybe a 24V laptop PSU ?? It wouldn't be that hard to make a current limited emitter-follower if you really wanted to test the 15V rail, or the bridge diodes.