Toasted 400 still toast
- Thread starter roccus
- Start date
Yep I agree bias seems ok but my concern is the cause for the DCP contacts opening up and the voltage going up on the offset on the right channel when I apply 120V. That is what we need to find out here.... the offset voltage did not go up on the right channel when 120V was applied before the control board was installed it stayed down at .0001V with just the backplane boards installed... am I explaining this so you understand ok???
Hi Ray
You can measure the +/-15V regulators when you bring it up on the variac using the 3 test points provided on the control board. Negative DVM lead on the black ground test point and then the positive DVM lead on the red +15V test point followed by putting it on the blue -15V test point.
Be very careful when you do this to ensure you do not short anything out with a probe slip.
That will answer your question.
OK thanks Ray, that one is now checked off the debug list.
Next I recommend you separate the protection circuit from the main amplifier section FOR THE RIGHT CHANNEL ONLY since that is the one acting up. It is easy to do. With power off and bulk caps drained, locate diodes D9R and D10R. The are horizontally placed right above transistors Q6R and Q9R. Carefully heat up and lift the (most accessible) end of each of these 2 diodes. This will disable the SOA protection circuit.
After doing this, run the same test that you have been with the variac and report results.
Next I recommend you separate the protection circuit from the main amplifier section FOR THE RIGHT CHANNEL ONLY since that is the one acting up. It is easy to do. With power off and bulk caps drained, locate diodes D9R and D10R. The are horizontally placed right above transistors Q6R and Q9R. Carefully heat up and lift the (most accessible) end of each of these 2 diodes. This will disable the SOA protection circuit.
After doing this, run the same test that you have been with the variac and report results.
OK so here we go.... D9 and 10 lifted on right channel
First at 115V DCP contacts are closed
Ground (TP3) to Output .0001V
Ground (TP3) to Bias + .3432V
Ground (TP3) to Bias - .3422V
Ground (TP3) to B+78.60V
Ground (TP3) to B- 78.53V
Control board bias .3512V
Off set 0V
This is with ground from DMM to Output
Bias + .3371V
Bias - .3360V
Now 120V DCP contacts appear to have stayed closed
Ground (TP3) to Output .6617V
Ground (TP3) to Bias + .3325V
Ground (TP3) to Bias - .3544V
Ground (TP3) to B+ 82.66V
Ground (TP3) to B- 82.54V
Control board bias .3598V (this is real jumpy from .35xx to a bit over .47xx)
Off set .3648V (just a mention off set on the left is still 0V)
This is with ground from DMM to Output
Bias + .3547V
Bias - .3531V
My summery at 120V... with D9 and 10 diodes lifted looks like the offset voltage dropped but still a bit high and the DCP contacts appear to be staying closed
First at 115V DCP contacts are closed
Ground (TP3) to Output .0001V
Ground (TP3) to Bias + .3432V
Ground (TP3) to Bias - .3422V
Ground (TP3) to B+78.60V
Ground (TP3) to B- 78.53V
Control board bias .3512V
Off set 0V
This is with ground from DMM to Output
Bias + .3371V
Bias - .3360V
Now 120V DCP contacts appear to have stayed closed
Ground (TP3) to Output .6617V
Ground (TP3) to Bias + .3325V
Ground (TP3) to Bias - .3544V
Ground (TP3) to B+ 82.66V
Ground (TP3) to B- 82.54V
Control board bias .3598V (this is real jumpy from .35xx to a bit over .47xx)
Off set .3648V (just a mention off set on the left is still 0V)
This is with ground from DMM to Output
Bias + .3547V
Bias - .3531V
My summery at 120V... with D9 and 10 diodes lifted looks like the offset voltage dropped but still a bit high and the DCP contacts appear to be staying closed
OK you are zeroing in on it Ray. You are reporting the offset without a polarity sign. Is this positive? Please confirm
And you are reporting essentially 2 different offset readings. They should never be different.
The first offset you report is:
Ground (TP3) to Output .6617V
The second offset you report is:
Off set .3648V (just a mention off set on the left is still 0V)
This is with ground from DMM to Output
Ground is ground (within a millivolt or so), so I am not sure how you are getting multiple offset readings.
Another question: Have you disconnected the output wire from the backplane board to the DCP connector?
And you are reporting essentially 2 different offset readings. They should never be different.
The first offset you report is:
Ground (TP3) to Output .6617V
The second offset you report is:
Off set .3648V (just a mention off set on the left is still 0V)
This is with ground from DMM to Output
Ground is ground (within a millivolt or so), so I am not sure how you are getting multiple offset readings.
Another question: Have you disconnected the output wire from the backplane board to the DCP connector?
Agree very important we get on the same page here...
yes positive on the polarity of the offset and output I will put these in in future posts
where I give output voltage that is from the ground on the backplane to the output test pin on the backplane I am getting offset from the speaker terminals, not a clue why different was hoping you would have an idea it seems ok on the left side
I will disconnect the wire from the DCP and send new readings.... also I don't see much sense at this point to keep doing the readings at 115V since they seem stable there, the problems begin at 120V so unless you need the 115V readings I will drop them....
yes positive on the polarity of the offset and output I will put these in in future posts
where I give output voltage that is from the ground on the backplane to the output test pin on the backplane I am getting offset from the speaker terminals, not a clue why different was hoping you would have an idea it seems ok on the left side
I will disconnect the wire from the DCP and send new readings.... also I don't see much sense at this point to keep doing the readings at 115V since they seem stable there, the problems begin at 120V so unless you need the 115V readings I will drop them....
Wow Joe I disconnected the wire from the DCP and bias went way up so I checked it again on the control board and it was way up I turned the trim pot all the way down and voltage is still way high at +1.389 Left channel is still good.... I do still have D9 and 10 on right channel lifted up
I guess I found the ground problem.... before I disconnected the wire on the DCP from the right channel I snugged the mount nut down a bit more looks like that did it, the nuts I have have those star lock washers attached to them and they are a bit larger than the pads on the board so a bit of snug I guess gave better contact?
I guess I found the ground problem.... before I disconnected the wire on the DCP from the right channel I snugged the mount nut down a bit more looks like that did it, the nuts I have have those star lock washers attached to them and they are a bit larger than the pads on the board so a bit of snug I guess gave better contact?
Also Ray, have you triple checked the backside of the right backplane board for any long leads that have not been clipped off close? You only have 1/8" total from the backside of the backplane board to the chassis so all leads must be trimmed to no more than 1/16" from the backside of the backplane board in order to ensure proper clearance. This is very important.
Here is full set of numbers 120V D9 and 10 lifted Bias trim turned all the way down on control board
DCP contacts appear to have stayed closed
Ground (TP3) to Output +1.079V
Ground (TP3) to Bias + +.2489
Ground (TP3) to Bias - +.7230V
Ground (TP3) to B+ +82.66V
Ground (TP3) to B- -82.54V
Control board bias +1.0834V
Off set +1.3904 I took this from ground on back plane to the #2 out pad
This is with ground from DMM to Output
Bias + +.2515V
Bias - -.2494V
DCP contacts appear to have stayed closed
Ground (TP3) to Output +1.079V
Ground (TP3) to Bias + +.2489
Ground (TP3) to Bias - +.7230V
Ground (TP3) to B+ +82.66V
Ground (TP3) to B- -82.54V
Control board bias +1.0834V
Off set +1.3904 I took this from ground on back plane to the #2 out pad
This is with ground from DMM to Output
Bias + +.2515V
Bias - -.2494V