Help! I am baffled. I went to bias the Voltage Gain board for the BA-3. I went with the K170/J74 input pairs and I used the alternate FQP3N30 and 3NP20 on the output. The supply is +/- 32 V. What is happening is that I followed the directions for biasing as well as watched a couple of videos. I cannot get to full bias. Both channels are behaving exactly the same. I can get to about 0.3 V on Q3 and about 0.15V on Q4. I have checked and double checked the components. I even had a friend check as well. He only did one channel but all parts were correct. K170/J74 are biased and and I see a little more than 8 mA through them. I didnt do the BOM so I dont know if I made a mistake by using the alternate outputs but not the alternate input pair. It's really disheartening to get this far and this happens.
Did you test your output FETs before you installed them? Specifically test them for what is Vgs for 45mA Ids?
just increase trimpots in JFet drains to 1K
or put (say) 330R resistors in series with them
start again from 0 Ohms at trimpots and you're good
or put (say) 330R resistors in series with them
start again from 0 Ohms at trimpots and you're good
That's not a lot of information to go on. The original Toshibas outputs only needed about 2 volts to bias.
What's the voltage from V+ to either side of gate resistor R8 on Q3?
Then Voltage V- to either side of gate resistor R9 on Q4.
What's the voltage from V+ to either side of gate resistor R8 on Q3?
Then Voltage V- to either side of gate resistor R9 on Q4.
Yes thanks for the help. I figured out the problem and I know it is a DIY site but the store profits and they should really pay attention to detail. This a classic recipe on how to build a product that doesnt work. Take the schematic, use the BOM and affix parts to PCB. Product doesnt work! I can guarantee that others will traverse this road as the 313/2013 outputs are hard to find so most people will do like I did and substitute the alternate. The problem is that there is this little tidbit of information that is not included in BOM or schematic. Why not change the BOM to all 1K trimpots. Then both outputs sets would work OR put a note in the BOM saying "Alternate requires different pot." I really hate to have to rework a plated though hole board. I ran a manufacturing plant for years and believe me someone would get fired if this happened.
don't be so harsh on them ;
first thing they have is more than good will to make things easier for horde of Greedy
Boyz
second thing , team responsible for these kits is not professional in electronics
third thing - Papa can't feed us all with everything
fourth thing ....... for price of few pots , you're having nice chat with bunch of Greedy Boyz
first thing they have is more than good will to make things easier for horde of Greedy
Boyz
second thing , team responsible for these kits is not professional in electronics
third thing - Papa can't feed us all with everything
fourth thing ....... for price of few pots , you're having nice chat with bunch of Greedy Boyz
Yes. I was probably blowing off a bit as I was painstakingly careful and spent many hours troubleshooting and rechecking and redoubting myself to discover this. I had a goal of zero rework for this build and this messed it up. BUT they should just change the pots in the BOM to be 1K and then this problem disappears.
If you had carefully read the build threads you would have seen many have been down this road. Mine didn't need the 1k pots and I have built 3 of these front ends.
Russellc
Russellc
Russell,
Am I missing something. How did you get your circuit to bias the 3N30 and 3P20 pairs using 500 ohm pots?
Am I missing something. How did you get your circuit to bias the 3N30 and 3P20 pairs using 500 ohm pots?
No,No, sorry, I had Toshiba parts. Using resistors like Zenmod suggested would work. Point I tried to make was when the Toshiba parts were getting scarce, many were using the Fairchild ones you did. The one K pots were recommended to many. It was a fairly well covered phenomena.
No biggie, sounds like you have it in hand now. Just carefully use solder wick stuff to soak off the solder. Start at one end (sometimes one end will be loose after wicking, rock it up as heating, slowly go down the line. More wicking if necessary. They are pretty tough boards, just take your time. It is a pain to desolder, not my favorite activity!
It will all be worth it, you will enjoy the preamp. Lots of gain, I like gain with Firstwatt amps!
Russellc
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ok I will jump on the band wagon as well. Just built the Ba-3 as preamp, Using Toshibas. Same thing, cant get to 1V across R10 and R11? What gives?Whats the point of buying a PCB that you stuff and then it doesn't work? Any ideas? Really Lame. Thanks all!
care to post some pictures and measurements ?
and yes , all those people , completing their builds , must be enormously lucky to get broken broken pcbs , so non-broken
........ while you got broken only once
and yes , all those people , completing their builds , must be enormously lucky to get broken broken pcbs , so non-broken
........ while you got broken only once
from what I have read-the only the enormous is the failed bias readings by most builders for this. 3 qualcom engineers and an audio circuit engineer are looking at this. I will post photos; all parts are tested and working . I think it has to d with the pots.
if you can't bias it with 1K pots , problem lies elsewhere , not in value of pots
bring more engineers
bring more engineers
"Don't work hard, work smart!" 🙂if you can't bias it with 1K pots , problem lies elsewhere , not in value of pots
bring more engineers
I can comment. The problem is that you MUST use 1K pots when using FQP3N30 and FQP3P20. If you cannot bias then the problem is elsewhere. Check your output stage to make sure that you have correct transistors and they are oriented correctly and working.
I will say that the customer service at DIY changed the BOM as per my suggestion. My point was of course coming from a manufacturing world you must be able to build a working product from the BOM and gerbers. If you cant then you have a problem just waiting to happen. I reworked my board but it is a really painful process with plated through holes and it definitely is suboptimal and ugly.
I will say that the customer service at DIY changed the BOM as per my suggestion. My point was of course coming from a manufacturing world you must be able to build a working product from the BOM and gerbers. If you cant then you have a problem just waiting to happen. I reworked my board but it is a really painful process with plated through holes and it definitely is suboptimal and ugly.
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