Roland SH2 Power supply strangeness

[FreqFarm]

I am in the process of restoring an SH2 which is in much need of some love.

I have started with the power supply as it makes sense to get that sorted and stable first before sorting out the various other issues.

This is a 220v SH2.

Issues Solved so far:
1) PCB is a different version to any of the ones in the service manual, but not a problem to trace and confirm the connections coming back from the transformer which is where the difference is from the diagrams.
2) Main PSU caps C45, C46 and decoupling caps C51, C52 replaced (these were the original lilac coloured ones) and are working as expected.
3) Output from transformer to PCB was wired wrong, although the power supply was working as expected both prior and after correcting the mistake with the same output voltages at the 15v rails.

Current Issues:
1) Output from 1B4B41 Bridge Rectifier is +35.5v / -34.8
This is 15v higher than I was expecting and 5.5v higher than the 30v max the TA7179 is spec'ed to handle so clearly not a good thing.


Questions:
1) Are these output values within spec?
+15v rail: +14.88
-15v rail: -14.86

2) If I am getting higher than expected DC voltages from the 1B4B41 Bridge Rectifier is this due to the 022H020D transformer outputting a higher than expected AC voltage? And if so this will mean the transformer needs replacing?




Relevant Diagrams:

 

[Khron]

If you're getting too-high voltages, is the wire coming from the fuse going to the red wire of the transformer, or the brown?

If it's going to the brown, you'll get higher secondary voltages, than if it were connected to the red. Especially if you have 230-240V mains where you live.

 

[FreqFarm]

If you're getting too-high voltages, is the wire coming from the fuse going to the red wire of the transformer, or the brown?

If it's going to the brown, you'll get higher secondary voltages, than if it were connected to the red. Especially if you have 230-240V mains where you live.

looks like brown is on the main fuse



The numbering on my PCB does not match the diagram for the pins connected to the transformer output, but everything else is identical. So just ignore the numbers and go by the position of the connector/wires


So in the diagram:
- 10 and 11 connect to the fuses and then to the 1B4B41 bridge rectifier
- Red is on a non-connected pad
- 12 is GND

 

[FreqFarm]

If it's going to the brown, you'll get higher secondary voltages, than if it were connected to the red. Especially if you have 230-240V

We are on 230v+ here in London so yup that is a great suggestion!




I totally missed that on the wiring diagram, there are so many corrections on there I had wrongly assumed that the arrows were a correction not a wiring option.

I will swap the red and brown over and report back

 

[FreqFarm]

after swapping the red/brown on 8 + 9 the voltage output of the bridge rectifier is 32.7 and -32 which is better but still too high for the poor little TA7179 to be happy long term?

I am reading 246v on the mains supply so that would go some way to explaining the higher voltage,

 

[Khron]

I am reading 246v on the mains supply so that would go some way to explaining the higher voltage,

That's almost definitely it, I'd say. Maybe chuck a couple of suitable-current-rated diodes in series with each secondary rail, to bring them into a suitable range?

 

[FreqFarm]

the output is:
+15v rail: +14.88
-15v rail: -14.86

So just below the 15v target and within tolerance I would have thought?

> Maybe chuck a couple of suitable-current-rated diodes in series with each secondary rail,
Do you mean after DC output of the bridge rectifier? Or on the AC after the transformer with some zeners?

It's quite a lot to drop, I am expecting 20v so that's a 12.7v drop on the DC side which would mean a lot of diodes on the AC side to drop the DC enough?

 

[Khron]

Do you mean after DC output of the bridge rectifier?

Yes, but only to drop the rectified & smoothed DC down to below +/-30V, within the input spec of the regulators.

 

[FreqFarm]

Yes, but only to drop the rectified & smoothed DC down to below +/-30V, within the input spec of the regulators.

ahhh, yes that makes sense

I have got fixated on specs from other sources and missed the big picture (again).

thanks for your help, it's been amazing to get a second opinion and the benefit of your experience.

 

[FreqFarm]

so i think i have worked out that it's the transformer that is the problem here.

checking the output of another sh2 the output from the transformer is 22.7v ac, where I am getting 49.8v.

I have ordered a new transformer and will swap it over and see what gives.