Hi Bart,
First, I hope that you are working on the transformer/power supply alone, and that it is not connected to your AKSA amp boards at this point. If so, I'd suggest that you disconnect the power from the AKSA amp and from the power supply board. If the power supply board was energized, you want to drain the power supply caps to avoid nasty sparks and shocks while you work (drain with a 15K 1/2 watt resistor across cap + to cap - using aligator clip leads, and watch the voltage on the cap decrease with your voltmeter... when the voltage is low, leave drain wires attached for a minute or two anyway, cap "soakage" can restore the voltage to levels high enough to cause sparks, small shocks, and maybe damage transistors, so be sure that the voltage on your caps is in the millivolt range before you work on your amp).
For the AKSA 100, if I recall correctly, the rails should be:
+ Rail = +50v
SPG (earth) = 0
- Rail = -50v
For my 100w AKSA sub amp, I measured about +/- 51.5V, which is a bit high but marginally OK. Since my Plitron transformers are epoxy potted and taking a few turns off the transformer would have been extremely messy, I've left things as they are...
So, the fact that you are measuring 105v looks suspiciously like:
2 x 52.5 = 105, i.e. perhaps you are measuring across the two secondaries, not between each secondary output and the center tap.
I am not a transformer weenie, so proceed slowly and carefully. I am offering this advice so that you can get started working on the problem, but you might want to wait until a true expert weighs in on this.
On my Plitron xfmr, looking at the wiring diagram that came with the xfmr, I wired for "dual secondaries" and "center tapped"... for this I connected brown and black together to make one lead to the mains and connected white and orange together to make the other lead for the mains.
Yellow and blue were connected to make the 0 volt center tap, this will go to SPG (earth). Then the single red wire goes to one AC lug on the AKSA power supply board and the single gray wire goes to the other AC lug on the AKSA power supply board. On my particular transformer, red and gray are each 33 volts AC relative to yellow+blue (which we will connect to SPG = earth). If the AKSA power supply board is built correctly, you should see something like +50v DC at the AKSA power supply + terminal and -50v DC at the -terminal.
Pardon all the detail about my particular transformer (which you almost certainly don't have)...
and for sure, the wire colors on your transformer will be different (look for directions supplied with the transformer or check the manufacturers website)... the point is that these transformers are made to accomodate different line voltages (115, 220v, etc.) so you have to be sure that you have the transformer set up correctly.
When I first set up my transformer I wasn't sure that I'd done it right, so this is what I did (I am just telling you what I did,
not recommending that you do it, OK?). You will have live mains voltages here and extreme care and caution is necessary. I would not do this if I felt that my concentration was likely to wander or if I was feeling a bit clumsy. AKSA building is about sweet music, thrills and chills, not about dead audiophiles, so pay attention.
With the transformer sitting in the middle of my very clean and unclutterd workbench, I got a spare power cord (left from an appliance long ago thrown away) and (with the AC cord
not connected to the mains) attached the cord to the AC inputs to the transformer (this would be the brown/black connection and the orange/white connection in my case)... I made the connections using the plastic twist cones (I don't know the real name) that electricians use to make mains connections in junction boxes. I then hooked up my voltmeter (set to AC, range good for 200VAC) using clip leads to the transformer outputs (in my case this would be transformer red connected to voltmeter red, and transformer yellow/blue connected to voltmeter black).
Turn on your voltmeter, and make sure that everything is mechanically stable and electrically OK (no screwdrivers on your workbench to short out your transformer, nothing that could tip over onto your workbench and kill the cat when you plug things in)... then, sitting on an insulated stool with my feet off the concrete floor I plugged in the transformer and read AC voltage = 33v (excellent!!), then I unplugged the transformer, connected voltmeter red to transformer gray (the other transformer output lead) and tested that one (also 33VAC, also excellent). At this point, I disassembled my Rube Goldberg test setup and connected the transformer to my AKSA power supply board.
An added saftey margin can be had by building a current-limiting box as suggested by John Curl on AA:
http://www.audioasylum.com/audio/tweaks/messages/29312.htmlI built one of these and I wouldn't be without it. If your new circut has a really big problem, when you plug it in the light glows (tells you that there is trouble) and the lightbulb filament serves to limit the current rushing into your new circuit (limits damage... if you are fast and lucky enough you might get the thing unplugged before parts are damaged or the smoke is too thick to see through
)
I hope this helps, and be careful dammit!
Peter
