PA-20 Voltage query

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Pav
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PA-20 Voltage query

Post by Pav » Mon Apr 20, 2020 2:01 pm

When Yamaha state the voltage rating of the PA-20 as 17.5 V ac x 2 does that assume a load of some known value ?
I measured my PA-20 at 19.2V with just a meter across the pins.

My reason for doing so was because i wanted to see if I could drop the 17.5 with three 400x rectifier diodes by 2.1v to 15.5 (or as close to 15V ac.
This is for sharing the PA-20 between my Oakley rack and a vintage Maplin synth built in 1985 which currently has an open transformer in the wooden case. I wish to have a "safer" supply on the bench while im refurbishing pcbs out of the case.

I forsee the regulated side of the power supply might run too hot if i just took a straight feed, and now seeing an extra 2 volts is a worry.

Hope we can see the Oakley ordering system back up soonish.
Just completed the 2nd of two Euro Classic VCAs, successfully, but nothing left to build - hence the refurb of the old synth.

Cheers
Pav

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Synthbuilder
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Re: PA-20 Voltage query

Post by Synthbuilder » Tue Apr 21, 2020 2:20 am

Pav wrote:
Mon Apr 20, 2020 2:01 pm
When Yamaha state the voltage rating of the PA-20 as 17.5 V ac x 2 does that assume a load of some known value ?
Yes. The voltage is measured when the transformer is under its maximum load.

However, the input voltage on such a transformer is normally 230V, and we have 240V here in the UK. And it varies by a certain amount too. So take any voltage output of any transformer with a pinch of salt.
I forsee the regulated side of the power supply might run too hot if i just took a straight feed, and now seeing an extra 2 volts is a worry.
It could run warm but it could run cooler. The original transformer could well be producing more than it says on the tin. Have a go at using the PA-20 and see what happens. If it gets too warm you'll usually have plenty of time to establish this. If you can't touch the heatsink for more than ten seconds it's too hot. And if the PA-20 gets warm you're probably taking too much current so something else is required.

I'd not recommend powering both the Maplin and the Oakley from the same transformer.

Which Maplin synth? The original ETI ones are probably quite heavy current draws.
Hope we can see the Oakley ordering system back up soonish.
See today's announcement above.

Tony

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Pav
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Re: PA-20 Voltage query

Post by Pav » Tue Apr 21, 2020 8:10 am

This is the 5600S variant of ETI ...build guide says 1979...but I built @85 I would only use PA20 with one synth at a time...and at first on the bench with only the load of a couple of boards. The stated max expected current draw from other boards is 300ma per rail. Unlike modular, it has +/- 14 , +/- 7 and a +5 rail , the latter being ref voltage and a rail max 60 ma. Total current is approx 680 ma. The 15vac goes into full wave rectifier of four diodes and creates +/- 20v rails to power tip35 and equivalent transistors. That 20v Dc would become 22 at 17vac and close to 25 at 19vac. I'll just give it a go.
rgds Pav

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Re: PA-20 Voltage query

Post by Synthbuilder » Wed Apr 22, 2020 2:38 am

Pav wrote:
Tue Apr 21, 2020 8:10 am
Unlike modular, it has +/- 14 , +/- 7 and a +5 rail , the latter being ref voltage and a rail max 60 ma. Total current is approx 680 ma.
The PA-20 is good for up to 520mA per rail. 680mA will make it run rather hot. Probably OK for just testing on the bench though.

One thing to note about those Maplin power supplies is that they have zeners (ZD5, 6, 7) between the lower voltage rail outputs and 0V. It's worth replacing those, or even swapping them for IN5819s, as they tend to age, get warm and eventually fail.

Tony

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Pav
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Re: PA-20 Voltage query

Post by Pav » Wed Apr 22, 2020 3:27 pm

Thanks for the caution on pa20. I have replaced all zeners and diodes with fresh ones, replaced the tantulums with regular caps. Whilst the 2200uf caps look fresh,replaced with quality ones. Do not know if I should replace the rest. There was no specific fault on the psu ...just age, and not that many power ups.
I plan to start a photo record thread in build diy, as I progress in future.
rgds Pav

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eggpie
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Re: PA-20 Voltage query

Post by eggpie » Wed Apr 22, 2020 3:42 pm

Hi Pav.
I sent you a PM last night regarding your 5600s, I think it may be stuck in my outbox here on Muffs!
Please let me know if it came thru to you or not.
Thanks and kind rgds.
Tom.

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Re: PA-20 Voltage query

Post by Pav » Thu Apr 23, 2020 9:25 am

Pm replied to
rgds Pav

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