Picked up a B2 .... feels like I'm gambling

dino74

Active Member
It was in his closet for three years and he said both channels were working then. He had it setup for a demo but sound only came out of the right. I took a gamble and bought it for $170. Came home, hooked it up, neither channel works now .... Another project

Power switch lever missing
Missing buttons
All four pots freely rotate (Already ordered)
I see some resoldering work

Anyone know why the heat sinks are painted blue?
IMG_0001.jpeg
 
This baby needs a lot more than what you mention. Really. Those V-FET's need to be protected. IF they are still alive. I would not power it up again if I were you ;)

Take a look around on AK, there are several rebuild threads.

In my opinion it needs at least a full recap, new trimpots, new relays, cleaned switches and the power supply needs a rebuild. But first, trace the defects. I would test te V-FET's first to determine whether this unit is worth rebuilding.

The blue is epoxy and original, no mica plates needed. Just paste.

Good luck!
 
Took the VFETs out. Just for reference, Ohm meter shorted is 0.05 Ohms
Also the diode voltages were the same for S to G and D to G for each of the VFETs.
Any thoughts if these are good? Or did this test just prove they might not be bad and I should do more testing?

2SJ26
Left Side
1.06 Ohm
0.57V
1.6 Ohm
0.55V​
Right Side
0.83 Ohm
0.54V
1.08 Ohm
0.53V​

2SK76
Left Side
0.85 Ohm
0.55V
0.88 Ohm
0.53V​
Right Side
1.20 Ohm
0.51V
1.20 Ohm
0.55V​


IMG_0019.jpeg
IMG_0018.jpeg

The boards themselves seem to be falling apart.
IMG_0015.jpeg
 

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Hello Dino74

Anyone know why the heat sinks are painted blue ?
yes , read my previous post on B2 (many months ago)

There is 4 different heatsinks
- champagne with no epoxy insulator (only on B2 Silver model)
- anodized black with no epoxy insulator (on Black B2)
- anodized black with black epoxy insulator (on Black B2)
- anodized black with blue epoxy insulator (on Black B2)
 
This baby needs a lot more than what you mention. oh yes !

Really. Those V-FET's need to be protected. IF they are still alive. I would not power it up again if I were you ;) will say the same

Take a look around on AK, there are several rebuild threads. yes read them all they are interesting

In my opinion it needs at least a full recap, new trimpots, new relays, cleaned switches and the power supply needs a rebuild. But first, trace the defects. I would test te V-FET's first to determine whether this unit is worth rebuilding. ok with that too

The blue is epoxy and original, no mica plates needed. Just paste. ONLY if there is no problem of dammaged blue insulator . If it is the case , you can use micas , nothing else ! (no silkpad)
 
Dino74

Any thoughts if these are good ? Or did this test just prove they might not be bad and I should do more testing ?
your measurements mean nothing , you should use a curve tracer , without it , you can't do anything serious

never saw before this "HQ" stamp !

see my graph :

B2-2SJ26-6205-HS-JFET-Id_Vds.JPG

B2-2SJ26-6205-HS-JFET-Id_Vgs.JPG
Working one

B2-2SJ26-6221-HS-JFET-Id_Vds.JPG

this is about "good V-FET" checked with ohm-meter . Ohm-meter will never tell you the behaviour of these V-FET like a curve tracer do

Working V-FET

2SK76-6434-2-Id_Vgs.JPG
 
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Thanks clinic-audio! I don't have a curve tracer but I'm wondering if I can manually capture the data.
I read up on VFETs and came up with this idea but I really need someone
to tell me if I have a handle on VFET technology or just a ......

EDIT: fixed diagram to be 2SJ76

783a9a0f-fc0f-44cc-b95c-d10b2346e7fc.png

My goal is to reproduce this graph as much as PS1 can supply power. Sorry its blurry, I got this pic from
http://www.amplimos.it/images/2sj26_2sk76_data.jpg

EDIT: Note this for the A version
Screenshot from 2018-08-21 17-09-04.png

The Rload is made from three 50 watt 16 ohm resisters, so it gives me about 5 ohms to 50 ohms to play with.
I'd mess around with various Rloads. If it looks like its behaving, then I'd remove the Rload and set the Max Amps on PS1.

I'm hoping to collect data for each of the Vgs up to Max Amps based on the above graph.
Vgs Max Amps
0 7
-2 4
-4 3.5
-6 1.5
-8 0.5

It seems like I should be with in the limits of the device.

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Strange. Found this supposed graph for the 2sk76 but according to the spec sheets, IDSS should be 5 when VDS=10V and VGS=0.
The A version graph above looks that way but the graph below looks like IDSS would around be ~3 when VDS=10V and VGS=0.

2SK76_graph.jpg
 
I'm hoping this weekend I'll have access to two power supplies that have remote (RS232) control so I can write a script set the Vgs/Vds voltages and read the Ids. Then generate a plot curve from the values.

I took some measurements and this is what I got.

Overall the 2SK76 seem good.

Looks like the 2SJ26 are having problems.
LT=Left Side Top
LB=Left Side Bottom
RT=Right Side Top
RB= Right Side Bottom
Numbers are for Ids

Code:
Vds   Vgs           LT       LB        RT     RB
 -5     0       -2.70     -1.91    -3.94   -2.90
-10     0       -3.78     -3.09    -5.40   -4.07
-10     2       -2.01     -1.46    -4.05   -2.54
 
I made some graphs. I could use some opinions if these are any good.
Edit: Vgs stepping is 1V

Left side 2SJ26/2SK76
left_side.png


Left side 2SJ26
left_side_26.png


Left side 2SK76
left_side_76.png

Right side 2SJ26/2SK76
right_side.png

Right side 2SJ26
right_side_26.png

Right side 2SK76
right_side_76.png
 
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neither channel works now .... Another project

if the unit did not go into protection, it was most likely the selector switches that were the culprits....relays too, but my money is on the switches.

I have a couple of these beauties here that have same symptoms...and in perfect physical condition otherwise.


I made some graphs. I could use some opinions if these are any good.

obviously your left side looks a little off, but you really do need a curve tracer. Maybe @ilimzn would chime in with more insight.


that said, if I were in your shoes, I would do a basic maintenance that includes caps, relays, trim pots, clean the selector switches. Get everything back together set the bias to factory spec, then then check distortion and abnormal heat on the channel that had the V-fets replaced..assuming it is the left channel based on your graphs.


It is not like you have a choice to replace the v-fets anyways, so it it what it is...keeping my fingers crossed for ya


if you think it is too much hassle, I can take over your problem ;)
 
Any update on this? I hadn’t seen the original post until now but I agree with rottalpha that it was probably the selector switches (one channel out when auditioning and both out after purchase, at a good price). I don’t have much experience with B-2’s but that’s what I have to deal with when I bring mine into rotation for a listen.

Cheers,
John
 
No real updates, the A-1 restore is my highest priority because I want to use it at work with a turntable. After this, the B-2X/C-2X or MX-1K is next. In the meantime, I'm trying to get a sense if the VFETs are in a good enough state to justify the B-2 restore. I also entertaining the dumbass idea making a circuit using audio op-amps to modify the gate voltages for each VFET so that they all have the same output curve but I have to be really careful about how the B and B2 lines powers up. I love the simple look of the B-2 and want one but for now its more a waiting game.
 
I also entertaining the dumbass idea making a circuit using audio op-amps to modify the gate voltages for each VFET so that they all have the same output curve
well, that is sort of how the B-1 works (sans opamps)...I say sort of because B-1 does not use complementary V-FETs, but it does have an additional bias "balance" adjustment to smooth-out some of the differences beween the individual K-77s.

That balance setting is adjusted after everything else is adjusted, so that the lowest THD is attained.
Without the balance circuit and with possiblly unmatched V-FETs, THD will be out of wack comparing to the "good" channel.
That is why I suggested you to get everything back into the box, measure THD on the "good" channel then see if you really have an issue on the "questionable" channel. You might or might not, but before going out of the way to mess with the bias circuit, I would make sure there is an issue to begin with.
 
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dino, rottalpha, you guys really know about how these babies work and what to do with them. :thumbsup::thumbsup:

Me, I know that if I start something I probably won’t finish. I’ve got so many unfinished projects (of all kinds) and honey do lists that I know better than touch something that sounds good to my old ears. So far I’ve been lucky and nothing has self destructed. I guess I’m just a collector at heart.

Cheers,
John
 
Me, I know that if I start something I probably won’t finish. I’ve got so many unfinished projects (of all kinds) and honey do lists that I know better than touch something that sounds good to my old ears.
yeah, somehow this sounds so familiar...particularly the honey-do part..
 
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