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Hello All,
After building a Buf II with single Placid I ordered up an IVY III with Placid BP along with the recommended 15V+15V (30VA) Power Transformer.
I built up the Placid BP and IVY III boards and have just tried to power up the Placid BP board. Running through the steps in the manual I adjusted the resistance of all VRs and then applied the power from the transformers. LEDs came on and there was no smoke. I set the output voltage on both rails via VR3 and VR4. I was then able to adjust VR1, such that R1 was .25V. However, VR2 seemed not to be working as I couldn't adjust the voltage across R2. At that point I noticed that the heatsinks on the R1 side were very, very hot to the touch. The heatsinks on R2 side were normal - room temperature. I quickly shut down the power to the transformer and unwired it from the Placid BP board.
I then measured the voltage the transformer was putting out, which was 18V not the 15V that it is marked as. Living in the US I wired the transformer up in parallel (Blue/Violet & Grey/Brown). Should it be putting out 18V or nearer 15V?
I saw another post indicating that it is possible to cause a short when measuring across R1 and R2 that fries the associated VR. Not sure that this was the cause as I was careful when placing my DMM probes - they have little hook mini-clips on the ends - but cannot rule that out as the cause.
At this point measuring the resistance across VR1 seems to work OK, but VR2 is dead measuring a constant 1 for resistance.
Are there any troubleshooting steps that anyone can suggest to determine if it is just VR2 that is fried ?
Thanks
ianp
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It is not really possible to "fry" VR2 unless you swapped a couple transistors. Or completely shorted the main CCS transistor.
If you can't adjust it to ~200R then it is likely dead from one cause or another.
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I'm going to look at the board in detail again to see if I can spot anything, but let me ask if 18V is OK for the transformer and it is expected that the heatsinks get very hot on what appears to be a correctly adjusted side?
thanks
ianp
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Sure this is ok and normal, especially if you don't have a load connected yet. This is because the shunt transistors now have to dissipate the whole current, instead of the load taking most of it.
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I was lucky as I had the exact part in my spares box. I replaced VR2 and everything else went fine. ianp Edited by user Saturday, May 22, 2010 3:52:27 PM(UTC)
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Rank: Administration
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Joined: 10/24/2006(UTC)
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Location: Nashville, TN
Thanks: 25 times
Was thanked: 89 time(s) in 83 post(s)
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Occasionally if you turn the adjustment screw too far it can destroy the pot. I am glad you had a replacement.
Cheers!
Russ
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