B2 tests to verify it’s safe to install VFETS?

[MiamiBoy]

I'll keep this short and to the point.

I have a B2 that I have not powered on. I removed all the VFETs and they all tested good. Now I need some pointers on what tests I should perform to verify that it is safe to install the VFETs and power the B2 on.

I did search in a number of the B2 threads here but could not find anything specific. If there is something you could point me to or give me some specific instructions it would be greatly appreciated.

Thanks in advance.
Henry

 

[clinic-audio]

Hello Henry

I have a B2 that I have not powered on. I removed all the VFETs and they all tested good.

can you explain "they all tested good " ? I know what to do, but I ask this question for other members

 

[MiamiBoy]

Hello clinic-audio

I have a Word file that I cut and pasted from some posts that Echowars made a long time ago on how to test VFETs. I think it is from one of the many B2 refurb threads here. I have uploaded a copy of that Word file in pdf format if anyone is interested. It is what I use to refer to when I test mine.

Also included a jpg drawing of the VFET with G,S,D indicated.

 

[clinic-audio]

Hello MiamyBoy

1) I know Echowars and it is a reputable member of AK , and this test is fine for shorted V-FET but will never tell you if your D.U.T. work normally or not (D.U.T. = device under test )
2) only a curve tracer will give you 100% sure answers

here is something that your ohm meter will never tell you :

Bad V-FET 2SJ26







What a good 2SJ26 V-FET should give :




I hope that it is clear for you

I already post same message months ago

 

[MiamiBoy]

I need some pointers on what tests I should perform to verify that it is safe to install the VFETs and power the B2 on.

If there is something ANYONE could point me to or give me some specific instructions it would be greatly appreciated.

 

[rottalpha]

I have a B2 that I have not powered on. I removed all the VFETs and they all tested good.

Hi MiamiBoy, long time no talk. You left the dark side of tubes to come back to the sublime world of the V-fet

to summarize my understanding:

• you had a working B-2 with known working v-fets,
• you pulled them in order to update the unit,
• but you have not mixed the pairs, right?
• all v-fets pass the multimeter test as per EW instructions (or Sony instructions)

If all above are true, then your V-fets that were known to be good are most likely still good and you have no concerns about that, correct?

Now I need some pointers on what tests I should perform to verify that it is safe to install the VFETs and power the B2 on.

Clinic, Vintage and others might have more insigthfull advice here, but at the minimum, you would need to make sure that:

• you can power up the PSU without the driver boards or the v-fets installed
• once you do so, make sure that you can adjust the +/- 85v supply and that the 56v rail is also correct (i.e. watch the input primary of your unit...if 100V Japan unit, use a step-down transformer) note that the unregulated rails will be a few volts higher without any load.
• introduce the driver boards (without bringing in the v-fets yet) and make sure nothing weierd happens...readjust PSU if necessary
• let us know once you reach this stage

In short, you will find that the advice is scarce because the stakes are high, which is a segway in to the following disclaimer:
everything you do is at your own risk....but you already know that

Bad V-FET 2SJ26

Hi Patrice, from your curve tracer test, how can you tell that the rest of the DUTs do not have issues?

 

[MiamiBoy]

Hi MiamiBoy, long time no talk. You left the dark side of tubes to come back to the sublime world of the V-fet

Hey Pete. Hope you are doing well. Thank you for your detailed response. While I did go over to the "dark" side, I do have a working B2 that is as good as any tube amp I have or have heard. I previously refurbished that B2 and it is now in my main listening system. I cherish it and thus my caution / paranoia about taking any chances with this new one..

The back story on this B2 is I picked it up for a steal from a local antique shop that also sells vintage (very old) tube audio. The owner of this local shop could not give me any history on the B2. So the first thing I did was pull the VFETs to verify they measured OK and they did. I never powered it up. That is where I am now.

I am clear on how to adjust the +/- 85v power supply as that is part of the documented B2 adjustment steps but I am not clear on how to verify that the "56v rail is also correct". Can you refer to the specific points in the schematic where I should take this measurement?

 

[rottalpha]

While I did go over to the "dark" side, I do have a working B2 that is as good as any tube amp I have or have heard.

Ha Ha, no surprise there Henry!
The B in the B-2 might as well be the same as the B in a Bee, 'cause the sound it makes is honey for the ears

first thing I did was pull the VFETs to verify they measured OK and they did. I never powered it up

OK, but you did not mix them up, right?

I am not clear on how to verify that the "56v rail is also correct".

do you have a high resolution schematic, or an original schematic?

For the left channel you have Red (+56v) Blue (0v) White(-56v) wires going to the vfet assy.
For the right channel you have Red (+56v) Brown (0v) White (-56v) wires going to the vfet assy.



again, this is an unregulated supply and it will most likely be a higher value since you will have no load and the mains here in the US run hot (120v+ vs the 117v of the primaries) A few volts higher is not an issue...just make sure it is not some crazy value.

 

[rottalpha]

you test here...between red and white you should have 112V

 

[rottalpha]

oh, one more observation is that if you rebuild your PSU, you should move the trimmers to the back otherwise is a PITA to adjust the 85v supply

 

[clinic-audio]

Hum

your picture is strange Pete
the leg of he capacitor on the right is closer to the ground and +85V

The capacitor on the right is used what for ?? shadows in pictures ?

 

[rottalpha]

Hum

your picture is strange Pete
the leg of he capacitor on the right is closer to the ground and +85V

The capacitor on the right is used what for ?? shadows in pictures ?


View attachment 1434594

just a shadow from a crappy phone photo

this is my bench PSU for testing B-2 driver boards

 

[clinic-audio]

 

[MiamiBoy]

Pulled VFETs out and tossed them in a jar....j/k. First thing I always do is mark the VFETs with a number to know their order going back in.
Measurements looked good. I got right at +/- 84.5 on both measurements.
The measurement between the Red & White were exactly the same on both sides: 124v.

Is there more I should test before putting the little puppies back in?

 

[rottalpha]

Pulled VFETs out and tossed them in a jar....j/k. First thing I always do is mark the VFETs with a number to know their order going back in.
Measurements looked good. I got right at +/- 84.5 on both measurements.
The measurement between the Red & White were exactly the same on both sides: 124v.

Is there more I should test before putting the little puppies back in?

well, yeah, there is. Did you do anything to the driver boards since you pulled them out?
You would have to follow the manual instructions for the DC offset adjustment and the Bias adjustment.

....and is your unit a 100V or 117V. The 124v seems a bit high....that implies you have +/-62v rather than +/-56v. I do not have a unit on the bench, but this seems to be on the high side, especially if your filter caps are rated at 63v. This will go down once the v-fets are back in, but still...

I am hoping that someone else here has a unit on the bench and can verrify that voltage...

 

[MiamiBoy]

My B2 says 117v on the back.

 

[rottalpha]

My B2 says 117v on the back.

let's see if anyone has a B-2 on the bench. If not I will pull one in over the weekend.
In the mean time, can you let us know what have you changed in the amplifier?? recap, any semis? any work you did to the driver boards?

Can you adjust the DC balance to zero without the v-fets in ? Look at the manual for the adjustment procedures.

 

[MiamiBoy]

let's see if anyone has a B-2 on the bench. If not I will pull one in over the weekend.
In the mean time, can you let us know what have you changed in the amplifier?? recap, any semis? any work you did to the driver boards?

Can you adjust the DC balance to zero without the v-fets in ? Look at the manual for the adjustment procedures.

Pete I have a another B2 (B2.1), the first one I bought, that I need to open up because it has recently been dropping its left channel until it fully warms up and then plays fine as long as it is hot. So I suspect there is a cold solder I need to find and fix. When I open it up I can take some measurements without the VFETs in and see if the measurements match between B2.1 and B2.2. So lets put this thread on hold until I have a chance to do that and then I will come back and report my findings.

 

[rottalpha]

Pete I have a another B2 (B2.1), the first one I bought, that I need to open up because it has recently been dropping its left channel until it fully warms up and then plays fine as long as it is hot. So I suspect there is a cold solder I need to find and fix. When I open it up I can take some measurements without the VFETs in and see if the measurements match between B2.1 and B2.2. So lets put this thread on hold until I have a chance to do that and then I will come back and report my findings.

OK, make sure to clean those selector switches...they are notorious for failing. I know your first B-2 has new pots and new relays, right?

 

[Vint Age]

Look at the manual for the adjustment procedures

Unfortunately the procedure in the manual requires the V-FET's to be installed for the adjustments.

.Can you adjust the DC balance to zero without the v-fets in ?

Yes you can but it's somewhat dangerous.