166 VDC on a LF353N Dual JFET?

[SyncTronX]

Hi,

I"ve got a DAC that never seemed to work quite right.
But it seems it's taken a turn for the worse.

To that end, I've opened her up and tried to figure out
what is going wrong.

The DAC is the Curcio CD12V. It seems Mr. Curcio and
the Sonic Frontiers guy were designing them at the same
time.

The Curcio DAC uses 2 x 12AT7 tubes, which always one
channel was not sounding right.

There are three boards inside the DAC including: PS board, digital
input board, and the processor output board.

It uses three pair of opamps on the processor output board:

Central Process section w/ LF351N & LF353N

DR & DL channels each channel paired w/ LF351N & LF353N.

It is these DR/DL channel LF353N Dual JFETS that have
from 155.4 VDC to 170 VDC on every pin.

They weren't designed that way surely?

I don't have a schematic and I'm trying to meausre
and test in the DAC.

Thanks,

 

[Ken Newton]

Are you certain those op-amps are in the signal path, and are not part of the power supply? If I correctly recall, Curcio has used such op-amps as the error amplifier in a floating high-voltage regulator scheme for tube stages. My guess is, what you have been probing are the tube stage high voltage regulators. Generally speaking, when troubleshooting, the first thing I do is a visual inspection for burnt or over heated parts, or anything else suspicious looking. The second thing to is to verify that all D.C. supply voltages are correct and without significant A.C. ripple.

In addition, and this may be obvious, but, if you haven't already done so, try swapping the tubes between the two channels to see if the bad sound moves to the opposite channel, thus indicating a suspect tube.

 

[SyncTronX]

Hi Ken,

Thanks for your reply. No, I didn't know that is how he
used the opAmps. They don't look burnt though.

What I did notice was the TIP50 HV supply on the output processor
board does shows signs of thermal overheating on the heat sink and
board thermal damage where there are a couple of power resistors.

The same side of the board has the problem, through different
amps, pre amps etc., and tube swaps and tube replacements.

There must have been a problem before it left the factory because
the problem side of the board looks to have gone through some sort
of rework after the board had been intalled.

There are some soldering issues on the two processing boards,
that is the digital board and the output processing board.

Some would be classified as defects and others would be close
or process indicators according to the J Standards, IPC, etc.

Solder fillets a little too large, some look to be solder bridges to adjacent
terminals or traces or soldering to turrets, tabs, etc.,

I'm not here to bash Joe or his folks, just trying to get this going.
What I observe is just that.

There are two side that designate the output board, DL and DR.
It is the DL side of the board that has had the problems.
the strange thing is the 12AT7 sockts, the DL side is different on the heaters than the DR. Measured with and without tubes in place.

They measurements are here:

The tube socket pins differ from the side to side
measured w/o tube, then with tube VDC:
..........DR................DL
...no tube...tube.....tube...notube
1 165.5....112.........112....165
2 21.8.....8.4.............8...21.99
3 -6.4......7.8..............8...-6.3
4 12.0...12.0..............0.....0
5 12.0...12.0..............0.....0
6 164.8..109...........101...164.5
7 15.7....10.2..........10.24..15.9
8 0..........11.4.........11.3....0
9 0...........6.1...........6.0....0

Trying to back track and find the problem source.
I guess time to lift the board and then it goes on
the slippery slope, replace all the 20 year old 'lytics
and other parts that need replacing upgrading.

 

[Ken Newton]

FYI - Curcio has a schematic of their CD12V openly posted on their website. This should greatly assist your troubleshooting effort. Hyperlink follows:

http://www.curcioaudio.com/cddacsch.gif