Back in the bowels of time I acquired a NAP90 board and a CD3 trafo with the dubious intention of building a Nait3 with the remains of a 92R.
Roll forward 5 years and my break from the craft is over
Having never powered up the NAP board before I had no idea if it worked.
It didn't. Magic smoke poured from the left channel and tracks around the rectifier fried.
There were two obviously fried resistors. R10 from the soa circuit and the 100R from the emittor of TR10.
Turns out the nasty looking output trannies were both continuous across all legs and therefore shorted the 35 - 0 - 35 rails :eek
So, stripped them out along with the soa circuit on both channels. Cleaned everything up.
Fitted four new BD911's to replace the BD743A's.
Before trying again I built a light current current limiter to reduce damage if something is still amiss.
You can see it in background here.
Worked a treat, no bangs or smoke.
Rails measured at 35 - 0 - 35. Let's check the bias ...
Here's the right channel - 3.9mV across both 0.22R resistors.
But here is the troubled Left - 1.487 Volts!
Clearly something is not right on that channel.
Any ideas what to check first?
Many hours of removing and testing the transistors and it was that one!
Also, not sure you have found this but some comments re transformer as you are using a CD3 transformer you never know.
http://www.pinkfishmedia.net/forum/showthread.php?t=71710
Also earlier you mention the transformer is 35 0 35. That's rectified DC right? as if AC would be too high surely.
At this stage I would probably go over the board while live.
insulate off the probe except for the very point so that it is very difficult to short on anything
Neil's comments on that thread are correct whilst my original thoughts were not. The power amplifier section of the Nait 3 is the same as the NAP90/3. The transformer used in the NAP90/3 and the Nait3(R) is usually labelled as 'NAP90'.
Although shown differently in the circuit diagram the transformer has one continuous secondary which is centre-tapped and measures at around 28 - 0 - 28v AC. This full secondary winding is full wave regulated with the positive half providing the raw DC for the LM317 and the negative half powering the logo display. The transformer also has taps at 22 - 0 - 22v AC and it is these taps that are full wave bridge rectified to produce rails of +/- 29v DC for the amplifier circuits.
If the two channels have identical voltage readings under static conditions but one is running hot the oscillation must be a prime suspect.
I cannot see insulators nor do your retaining blocks appear to have bolts