Stax SRM-007tA troubleshoot

[clarus7]

I have a 007tA that lost the -350V. Looks like the 150ohm filter resistor between the two HV caps is open. There also seem to be something loading the bias voltage. The voltage before the 4.7M resistor in series with the output socket measure 300V after the resistor but 400V before the resistor with nothing connected.

Searching thru the forum seems like there is a 007t schematic available somewhere but all the links in previous posts are no longer valid.  It would be most helpful if a schematic is available. Would like to find out what causes the 150ohm resistor to open before replacing it.

thanks.

[spritzer]

The schematic I have is direct from Stax and I can't share it but a burned up resistor in the HV- line points to a short somewhere such as a shorted tube or a transistor.  This would be shown somewhere as there should be burn marks somewhere. 

[clarus7]

Since I couldn't see anything that is burnt (even the opened resistor does not look burnt), I replaced the resistor and power up the unit. Everything seems to be fine. Able to adjust the tube balance (TVR3, TVR4 to within 1mV), and balance (TVR1) and offset (TVR2) to less than 1V.  Looks like the amplifier section is running normal so far.

The unit seems to run hot. Checking the tube heater, which is running high at 7.7V DC. Tube spec should be 6.3V. I don't think there is anyway of adjusting it though, or is there?

HV are running just under +/- 360V.  

The remaining problem is the bias. Only getting 300V at the headphone socket, 400V at the output of the doubler, which should be 580V. Any suggestion?

[spritzer]

Wrong primary wiring on the transformer?  That would point to the high voltages... 

[clarus7]

The AC filament winding reads 13.3VAC, which is much closer for the old design where they use AC heating (which require 12.6VAC). In the 007tA, they put a bridge rectifier there and use DC heating. With the conversion and diodes drop, it becomes 15.5VDC which power the four tubes in a 2x2 series-parallel fashion. The rectifier-cap circuit looks questionable to me,  it works but I don't see why anyone would do it that way. It has asymmetrical caps connected in series/parallel fashion with the mid point of the caps floating.

The primary is correct. It is a 100VAC only version and cannot be strapped otherwise. Witness the HV voltage that was reported (360V), which is very close to the 350V target.

I am still baffled by the bias voltage with no load connected to it. Any suggestion?