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Sony Vfet Illustrated build guide
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Old 15th June 2017, 06:03 PM   #201
PJN is offline PJN  United States
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Hi Guys,

Thanks for all of your advice. For this build I did measure every component before installation, and all of the board measurements were in alignment with Nelsons specs. When I reflowed the board I used new flux and solder, then cleaned the board with a tooth brush and IPA followed with some windex, I used a strong magnifying glass to make sure that there were no solder bridges. The pics I posted don't really show the bottom very well, I'll try posting some better close ups.

When I initially powered up the board I remember that it was more finickey then the other one and was harder to set up, and after reflowing the board the values had changed on the voltages. I'm hoping that this is an indication of having fixed a cold/bad solder joint. My problem now is that I have to reset all of the pots correctly with all of the components installed.

My path forward will be to follow wined's advice on the adjustments and see where I land. I'll post my results when I'm done.

Paul
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Old 15th June 2017, 07:24 PM   #202
Rodeodave is offline Rodeodave  Austria
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Sony Vfet Illustrated build guide
Alright, my money is on one or more damaged TL431 regs. How do their reference voltages measure (ref-pin to gnd)?
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Old 15th June 2017, 08:26 PM   #203
PJN is offline PJN  United States
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I'll check them out tonite when I get home
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Old 17th June 2017, 02:25 AM   #204
nycavsr2000 is offline nycavsr2000  United States
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Sony Vfet Illustrated build guide
Default Cascode feedback in the front end circuit?

In the VFET Amplifier Part 2 thread (here and here), Rodeodave was kind enough to show a P2P sub circuit that replaces the resistors that bias the cascode front end transistors (Q3 and Q4) in the DIYAudio VFET kit with the TL431 shunt regulator circuit. However, there is an absence of the cascode feedback circuit (seen in the VFET Amplifier Part 2 pdf as parts R29 and R30). Is there any reason or reasons for not implementing cascode feedback in the DIYAudio VFET kit design? And if there isn't, can one still use a 10K resistor between the drain of the P MOSFET (Q5) and the drain of the N JFET (Q1); and similarly a 10K resistor between the drains of Q6 and Q2?

Thank you!

Anand.
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Old 17th June 2017, 06:14 AM   #205
Rodeodave is offline Rodeodave  Austria
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A short answer to your question: http://www.diyaudio.com/forums/pass-...ml#post4972521
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Old 17th June 2017, 03:50 PM   #206
PJN is offline PJN  United States
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I finally got around to taking some more pics and measuring the TL431's ref legs to gnd.

bad channel Q11 = 2.48 v, good channel Q11 = 2.48 v
bad channel Q12 = -10.32 v, good channel Q12 = -8.40 v
bad channel Q13 = 2.47 v, good channel Q13 = 2.48 v
bad channel Q14 = -21.32 v, good channel Q14 = -21.22 v

I took these measurements after I had biased up the front end, it seemed to bias up more easily and stably this time after reflowing, but that might be because I have done it several time now. The readings between the two boards are pretty close with the exception of Q12, and I'm not sure if it is significant.
the pictures were the best that I could do with the camera that I have the specks that look like dust are artifacts of poor lighting and high compression. I looked carefully with a magnifying glass and went between all close legs with a fine blade and didn't find any bridges. I'll wait until I hear back on the opinion of the experts here about Q12, before I attempt to bias up the back end and give it a listen.

I appreciate all your help, thanks,

Paul
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Old 17th June 2017, 04:42 PM   #207
Dennis Hui is offline Dennis Hui  Canada
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Hi Paul,

Perhaps others can confirm this, but I think your Q12 ref value to ground is dependent on
your setting of P2.

Assuming P2 is at roughly mid point (~2.5K), I believe the ref pin value should be around -10V.

Cheers,
Dennis
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Old 17th June 2017, 04:48 PM   #208
PJN is offline PJN  United States
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We'll that is some positive news, thanks Dennis
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Old 20th June 2017, 01:09 AM   #209
PJN is offline PJN  United States
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Hello All,
my bad channel saga continues. I biased up the back end to 100 mV across R32 by shorting T18 to gnd and adjusting P1 & P2, so I got 100 mV with ~ 5mV DC offset on the output. I Gave things a listen tonight and I'm still getting the same problem. At very low volumes the right channel is fine but as soon as I raise the volume a bit the channel fades out to silence, if I lower the volume again the channel reappears.

So now I've got to assume that I have either a bad board, or a bad component somewhere on the board. A bad board is a long shot, so I'm betting on a bad component. Since the VFETs are not replaceable and were measured by Nelson I'm going to assume that they're fine. I already replaced the JFETs and still have the problem so I'm assuming that they are fine. I carefully measured every resistor before and after installation and compared the in values to the good board and they all agree so I'm assuming that they are good, and I measured and compared the TL431 regulators with the good board with which they agreed, so that leaves the mosfets and transistors as potential culprits. Does anyone have an idea which ones could cause such symptoms. Otherwise I'm out of ideas and I'm thinking just to replace them all.

Thanks,

Paul
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Old 20th June 2017, 02:07 AM   #210
wdecho is offline wdecho  United States
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Sony Vfet Illustrated build guide
With one good board you should be able to find the problem in the bad board. Raise the volume to where is fades out and check the voltages at all points between the good board and bad board. There is going to be a big difference somewhere. Just checking resistance without being powered up probably is not going to tell you where the bad part or circuit is since it only starts showing up when you increase the volume. It sounds like the input signal is being shorted to ground when the volume is turned up, possible.
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