Have not had any success with efforts to find culprit(s) in my checkout of A-75 board bias problem (see http://www.diyaudio.com/forums/showthread.php?s=&threadid=4829)
So I am wondering if any forum member can point me to a general set of procedures for circuit board checkout, such as signal tracing or methods. (Besides my A-75 bias problems, I have a 13 year old Leach channel that never worked either. I wound up building a whole new board for the subwoofer amp that went into. I am sure I could find other applications for such procedures.)
Any help would be appreciated. I have to get this A-75 rig running because my ZenV4 boards are already on order and my finance manager will want to see a success before authorizing any further parts purchases!!! She is beginning to doubt my electronic genius status. HELP!
So I am wondering if any forum member can point me to a general set of procedures for circuit board checkout, such as signal tracing or methods. (Besides my A-75 bias problems, I have a 13 year old Leach channel that never worked either. I wound up building a whole new board for the subwoofer amp that went into. I am sure I could find other applications for such procedures.)
Any help would be appreciated. I have to get this A-75 rig running because my ZenV4 boards are already on order and my finance manager will want to see a success before authorizing any further parts purchases!!! She is beginning to doubt my electronic genius status. HELP!
Zapped
I have built the A75 and I am willing to take some static resistance measurements for you to compare.
With no power, measure gate to source resistance, then gate to drain of biasing mosfet. When you report these figures I'll compare them to my A75 and we will go from there.
If you would prefer we could correspond by e-mail, but then no one else on the forum will benefit.
BDP
I have built the A75 and I am willing to take some static resistance measurements for you to compare.
With no power, measure gate to source resistance, then gate to drain of biasing mosfet. When you report these figures I'll compare them to my A75 and we will go from there.
If you would prefer we could correspond by e-mail, but then no one else on the forum will benefit.
BDP
Thank you BDP. I will take those measurements as soon as I can get to it, and post it on the forum. I like the idea of exchanging the info. over the forum so that everyone can benefit. The time difference compared to e-mail is unimportant, since I have been collecting parts/machining the chassis/building the output stages/stuffing the boards/making 1 million trips to the store/ testing/and generally fiddling around with this project for about 9 years anyway (with some admittedly very long stretches of downtime in between)!
Again, thanks for the offer to help, and I'll be back with data soon.
Larry Wright
Seattle area
Again, thanks for the offer to help, and I'll be back with data soon.
Larry Wright
Seattle area
Hello BDP: Last night I took the two static resistance measurements you suggested on each channel. Here are the results. These measurements were taken with the front end board out of the amplifier, sitting on an insulated surface.
Channel A Q11 Gate-Source R = 4.55 K ohms
Gate-Drain R = 2.72 K ohms
Channel B Q11 Gate-Source R = 5.28 K ohms
Gate-Drain R = 2.72 K ohms
You be the judge.
Thank you for your help!
Larry Wright
Seattle area
Channel A Q11 Gate-Source R = 4.55 K ohms
Gate-Drain R = 2.72 K ohms
Channel B Q11 Gate-Source R = 5.28 K ohms
Gate-Drain R = 2.72 K ohms
You be the judge.
Thank you for your help!
Larry Wright
Seattle area
Zapped
Here are my measurements. They are made with P3 at its operating point and R 80 is 3.32K. Ohms gate to source 1.56K, Ohms gate to drain 1.83K. Ohms source to drain 1.37K. Check the jumpers between R27 and R29 on PCB. Also check R24 and R23 they are in parallel with the drain to source so I think you should be measuring less than 2K.
BDP
Vancouver WA
Here are my measurements. They are made with P3 at its operating point and R 80 is 3.32K. Ohms gate to source 1.56K, Ohms gate to drain 1.83K. Ohms source to drain 1.37K. Check the jumpers between R27 and R29 on PCB. Also check R24 and R23 they are in parallel with the drain to source so I think you should be measuring less than 2K.
BDP
Vancouver WA
Thank you BDP. I will go back to the board tomorrow and check again, based on your posting.
In the middle of the night it struck me that your properly biased and working amp would have a value for the bias pot that is somewhere between the 5K of mine at CCW and 0, with likely value somewhere in between. Since I'm checking at no power, I will fiddle a little with the pot setting and see what it does to the resistance readings we are looking at.
Also will double check the jumpers you mentioned.
Again, many thanks for your generous offer to help.
Larry Wright
Seattle Area🙂
In the middle of the night it struck me that your properly biased and working amp would have a value for the bias pot that is somewhere between the 5K of mine at CCW and 0, with likely value somewhere in between. Since I'm checking at no power, I will fiddle a little with the pot setting and see what it does to the resistance readings we are looking at.
Also will double check the jumpers you mentioned.
Again, many thanks for your generous offer to help.
Larry Wright
Seattle Area🙂
BDP: I will need the assistance of our Greek-speaking friends here, because I cannot say "Eureka" or "I have found it!", I must say instead whatever the Greek phrase is for
"You have found it!"
Last night I looked anew at the circuit board with the fresh eye provided by your suggestions and found two empty spots for the jumpers you mentioned, lurking beneath a couple of bent over silver micas. 😱
Seriously, but for your suggestions, I probably could have looked at that board another hundred times and not seen the clear omissions. As my father would say: "If it had been a snake, it would have bitten you".
Again, my thanks for your help. I will install the J's, reassemble and retest. I am energized for the next phase!
Also, I want to reiterate my thanks to the other forum members who offered suggestions, including Joe Berry, PassFan, and Mr. Pass himself.
😀
"You have found it!"
Last night I looked anew at the circuit board with the fresh eye provided by your suggestions and found two empty spots for the jumpers you mentioned, lurking beneath a couple of bent over silver micas. 😱
Seriously, but for your suggestions, I probably could have looked at that board another hundred times and not seen the clear omissions. As my father would say: "If it had been a snake, it would have bitten you".
Again, my thanks for your help. I will install the J's, reassemble and retest. I am energized for the next phase!
Also, I want to reiterate my thanks to the other forum members who offered suggestions, including Joe Berry, PassFan, and Mr. Pass himself.
😀
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