much better , but no values
as I wasn't involved in finalizing of sch and pcb , I'm not having enough clues
as I wasn't involved in finalizing of sch and pcb , I'm not having enough clues
ok , ref. to attached schm , give voltages:
- across R8 group and give me resistance of R8 group (measure while amp is off)
- voltage across R7 group , same as above - for resistance
- voltage across R16 (or R17)
-voltage across R18 (or R19)
-output voltage
Attachments
Zen, the schematic is not exact. R7,R8 only one in each position R7 is variable set to 1k reads 56 ohms in circuit R8 is fixed 1k. R27 is a jumper and a variable set to 68k. Voltage across R7 is 4.88v., across R8 is 8.5v, across R16 is 350 mv and 355mv across R18. I get a -16.1v at the output.
increase R27 pot to get 0V45 across R16
then decrease R7 pot to get 0 at output
while doing that , both inputs grounded , no load at output
then decrease R7 pot to get 0 at output
while doing that , both inputs grounded , no load at output
Adjusted R27 to get 450mv at R16.when I powered on I had 17volts at output that dropped over 2min to 4v would fluctuate between 4v and -4v but never go to 0 volts. Had to leave for a hour and when I returned I reset R7 to 1k now all I get is 16 to 18 volts at output regardless of how much I adjust R7
Right channel is good only the left is a problem. Jfet's came from a good source. Tested them when I got them. Now what?
remove pcb from heatsink
check all solder joints , resistor values , transistor orientation etc
check all solder joints , resistor values , transistor orientation etc
Hi all, after reading a lot of posts on the Aleph J I jumped in and ordered a set of PCB's at DIYaudio. Now it's sourcing the different components. I managed to buy some of it from a guy, including 8 JFETs (2SJ74). Reading through some other posts, I begin to wonder if I have bought fake...
Probably this question has been posted before but I cannot find it through the search function.
How can I check? To be honest, I am quite unknowledgeable on electronics (have not gone beyond passive filters for loudspeakers years ago...).
Any advise will be very appreciated.
Probably this question has been posted before but I cannot find it through the search function.
How can I check? To be honest, I am quite unknowledgeable on electronics (have not gone beyond passive filters for loudspeakers years ago...).
Any advise will be very appreciated.
Hi VDO,
Welcome and I hope your build goes well.
If your JFETs are from ebay then the chance of them being fake seems quite
high.
There's a recent thread on 2sj74 that may interest you:
http://www.diyaudio.com/forums/pass-labs/241642-mystery-sj74-jfets.html
I recommend reading the Borbeley article in the first post. Measuring/estimating
the Idss and Vp will give you an idea if they are real.
Cheers,
Dennis
Welcome and I hope your build goes well.
If your JFETs are from ebay then the chance of them being fake seems quite
high.
There's a recent thread on 2sj74 that may interest you:
http://www.diyaudio.com/forums/pass-labs/241642-mystery-sj74-jfets.html
I recommend reading the Borbeley article in the first post. Measuring/estimating
the Idss and Vp will give you an idea if they are real.
Cheers,
Dennis
Hi VDO;this is one fine amp so I hope your build goes well.
Should your jfets prove to be fakes genuine ones are available from spencer on this forum.Here's the link to his site.
FET Audio | Hi-End Audio Projects
Ha_ sells well matched mosfets if you need these as well.
Should your jfets prove to be fakes genuine ones are available from spencer on this forum.Here's the link to his site.
FET Audio | Hi-End Audio Projects
Ha_ sells well matched mosfets if you need these as well.
Thank you for the swift replies! I bought them from a man who kinda trades i electronics: his dad was an service engineer and he inherited tubes and diversified in buying/selling stuff. His remark that a man in China send him the JFETs got me worried... after I paid him.
Although it was only a few euroos for the JFET's it's more the trouble finding faults when the amp is not working as expected.
I'll try to check my little critters and if I'm not sure, I'll order some guaranteed genuine stuff from this forum.
One other question: if I was to build two monoblocks, I presume half the heatsink size should suffice, I think. Does the same apply for the transformers and can I use 220 VA for each channel.
Again thanks for the help and I'll keep you posted on progress.
Regards, Vranc
Although it was only a few euroos for the JFET's it's more the trouble finding faults when the amp is not working as expected.
I'll try to check my little critters and if I'm not sure, I'll order some guaranteed genuine stuff from this forum.
One other question: if I was to build two monoblocks, I presume half the heatsink size should suffice, I think. Does the same apply for the transformers and can I use 220 VA for each channel.
Again thanks for the help and I'll keep you posted on progress.
Regards, Vranc
Mine uses a single power supply using a 500VA transformer so 220VA per channel should be ok.I think the First Watt version uses a 300VA transformer but I might be wrong.
I used the PS385 heatsinks in a 165mm length on the link per channel.They run at about 45C although I am running with slightly more output bias than normal.
http://www.gdrectifiers.co.uk/uploads/Heatsinks_for_large_isolated_modules.pdf
Similar sinks are also available from Birmingham Aluminium.
I used the PS385 heatsinks in a 165mm length on the link per channel.They run at about 45C although I am running with slightly more output bias than normal.
http://www.gdrectifiers.co.uk/uploads/Heatsinks_for_large_isolated_modules.pdf
Similar sinks are also available from Birmingham Aluminium.
Vranc,
The first watt amps, as far as I know, all use 2x18VAC, 300VA Plitron transformers.
I've purchased jfets from Spencer before and they are genuine.
Hopefully the parts you have on hand are good. You will then want to pick pairs reasonably well matched in Idss.
Cheers,
Dennis
The first watt amps, as far as I know, all use 2x18VAC, 300VA Plitron transformers.
I've purchased jfets from Spencer before and they are genuine.
Hopefully the parts you have on hand are good. You will then want to pick pairs reasonably well matched in Idss.
Cheers,
Dennis
Thanks,
I guess i am not the adventurous type and will order some genuine parts as i will need them matched.
... and order some 225 VA 18VA transformers.
Plan is to realize the amps in the form of two mono's in frosted perspex enclosures because of the WAF involved. It's going to be an interesting spring...
Vranc
I guess i am not the adventurous type and will order some genuine parts as i will need them matched.
... and order some 225 VA 18VA transformers.
Plan is to realize the amps in the form of two mono's in frosted perspex enclosures because of the WAF involved. It's going to be an interesting spring...
Vranc
Pulled the board checked Q2,3 and 4 all were installed right and the junctions test good.inspected and reflowed all solder joints. Verified all resistors. Can the MOSFETS be tested while installed if so what I'm I looking for?
I'm trying to get back on track with my Aleph J project. Need to order a transformer and I recall that 6L6 recommended a 300VA 18V. Antek finally has them in stock. Any input before I order?