Going in blind , Sansui BA-1000 V-fet restoration , wish me luck. Fixed !!

[Nashou66]

I did another test today with the diodes . Since I found that broken trace on one of the voltage rails to cap . I thought maybe this might be a problem .

I was able to get a small adjustment reading across the emitter resistors on both channels , the same amount. From .3 mV to 1 mV . Not much but it might be something positive.

Video showing these results .

@Vint Age . What ya think? @Sparkplug Anyone else?

lol

Athanasios

 

[Sparkplug]

I did another test today with the diodes . Since I found that broken trace on one of the voltage rails to cap . I thought maybe this might be a problem .

I was able to get a small adjustment reading across the emitter resistors on both channels , the same amount. From .3 mV to 1 mV . Not much but it might be something positive.

Video showing these results .

@Vint Age . What ya think? @Sparkplug Anyone else?

lol

Athanasios

Nice setup there Nash … cozy and comfortable.

I don’t think there’s anything wrong with the unit except maybe for the age related deterioration here and there, like the one you found. In the absence of an SM, it’s hard to say what’s right or wrong. You’re definitely on the right track though and your perseverance will pay dividends sooner or later.

I‘m checking the auctions for an SM knowing full well that they usually take kind of info to their graves with them rather than share it over here.

 

[Nashou66]

Nice setup there Nash … cozy and comfortable.

I don’t think there’s anything wrong with the unit except maybe for the age related deterioration here and there, like the one you found. In the absence of an SM, it’s hard to say what’s right or wrong. You’re definitely on the right track though and your perseverance will pay dividends sooner or later.

I‘m checking the auctions for an SM knowing full well that they usually take kind of info to their graves with them rather than share it over here.

Thanks Sparky, did you look back to the other page ? I think I finished the Drive stage
Schematic with input and I added the V-fets .

Have a looksie

Nashou

 

[Vint Age]

What ya think?

I think you're doing great detective work Nashou! If it were mine I probably would have given it a try by now ... but I know how you feel! Even after the most diligent and thorough effort, hitting the power switch for the first time is always nerve racking, especially when you know that you'll likely never find the unobtainium needed if something goes wrong. It's even more nerve racking when that piece of gear belongs to someone else. I've been lucky so far but I've had a few close calls that just add to the excitement of that first moment.

All of that being said, ONLY YOU CAN DECIDE WHEN YOU'RE COMFORTABLE ENOUGH WITH THINGS TO THROW THAT SWITCH FOR THE FIRST TIME but if you do:

I would first set VR03/VR04 fully CCW before powering (as stated in the procedure)
Meter all of the GS's for sufficient pinch off voltage
Meter both channel outputs for 0Vdc or close to it

All of which will likely take a few seconds.

I'd also keep a close third eye on the protection LED ...

 

[Sparkplug]

Did you throw that switch yet? No pressure or anything …

 

[Nashou66]

@Vint Age @Sparkplug , I might have another plan . I have some 2SK76's and one 2SJ 26 from a Yamaha Organ . I do not think these were used in the amp section but were used in the power supply. I think I might try those first . Not exact in data sheet specs but they might work .

Although my confidence is much higher now with the new voltages I am getting across both channels on all V-fet socket pins,ie they all match closely as before there were a couple that did not .

Maybe ?

 

[Sparkplug]

@Vint Age @Sparkplug , I might have another plan . I have some 2SK76's and one 2SJ 26 from a Yamaha Organ . I do not think these were used in the amp section but were used in the power supply. I think I might try those first . Not exact in data sheet specs but they might work .

Although my confidence is much higher now with the new voltages I am getting across both channels on all V-fet socket pins,ie they all match closely as before there were a couple that did not .

Maybe ?

These are similar but not equivalent, you could try it but it won’t mean much. Maybe it’ll just show that the amp in general works? I guess why not if you don’t need these just in case they die.

 

[Nashou66]

These are similar but not equivalent, you could try it but it won’t mean much. Maybe it’ll just show that the amp in general works? I guess why not if you don’t need these just in case they die.

Yep that's my thinking. With a the Yamaha B-2 there is a similar test to make sure that the Bias circuit and more importantly the drive board itself is properly functioning. But instead of diodes you just short the S and G on one V-fet . I tried that with this and with resistors as you would with a BJT transistor but did not get the same results.

That is when I remembered, that some use a diodes instead of resistors. Also the V-fet test instruction diagram, I posted earlier in this thread, show the source to gate is basically like a diode connection. So now my reasoning is that the Yamaha V-fets would more closely represent the overall properties of the correct 2SK70/2SJ20's .

So tonight I will use those Yammy V-fets and see what happens . Hopefully the bias numbers will be more meaningful than the small .3mV to 1mV numbers I get with just diodes. I will follow @Vint Age recommendations of how to monitor the voltages he mentioned earlier today.

This is exciting times

Athanasios

 

[Sparkplug]

Yep that's my thinking. With a the Yamaha B-2 there is a similar test to make sure that the Bias circuit and more importantly the drive board itself is properly functioning. But instead of diodes you just short the S and G on one V-fet . I tried that with this and with resistors as you would with a BJT transistor but did not get the same results.

That is when I remembered, that some use a diodes instead of resistors. Also the V-fet test instruction diagram, I posted earlier in this thread, show the source to gate is basically like a diode connection. So now my reasoning is that the Yamaha V-fets would more closely represent the overall properties of the correct 2SK70/2SJ20's .

So tonight I will use those Yammy V-fets and see what happens . Hopefully the bias numbers will be more meaningful than the small .3mV to 1mV numbers I get with just diodes. I will follow @Vint Age recommendations of how to monitor the voltages he mentioned earlier today.

This is exciting times

Athanasios

Can’t wait to see what you find …

 

[Nashou66]

Can’t wait to see what you find …

Well I decided to first record all voltages at the sockets and then on the headers .
Very different results on some voltages between socket and board header for the V-Fet's Gates. The BL and W wires are the Gates , on my Layout and Schematic I had them as the source . I need to fix that on the latest posted Schematic .

Red is Left, and Green Is Right

I think I'll try to get to adding the Yamaha V-fets later if I have time.

Athanasios

 

[Nashou66]

Ok Some good and, maybe bad news?

I put the Yamaha V-fets in the amp and tried them in both channels .
I was able to adjust the bias to the suggested 50 ish mV's .
The Gate voltages on ALL V-Fets also moved as I adjusted the bias .

Now the Bad news, maybe? I had -18v DC on the Out wires , Red and White, that go to the protection circuit, which of course would stop the amp from coming out of protection. Could it be due these are not the correct V-fet's or maybe they need both V-Fets installed to work properly since this is a bridgeable amp?

But the other bit of good news is the V-fets still tested ok with my Atlas DCA 75 pro.

NOTE: With no V-fets or Diodes in place of the V-fet's the protection comes off.



Athanasios

 

[Sparkplug]

Ok Some good and, maybe bad news?

I put the Yamaha V-fets in the amp and tried them in both channels .
I was able to adjust the bias to the suggested 50 ish mV's .
The Gate voltages on ALL V-Fets also moved as I adjusted the bias .

Now the Bad news, maybe? I had 18v DC on the Out wires , Red and White, that go to the protection circuit, which of course would stop the amp from coming out of protection. Could it be due these are no the correct V-fet's or maybe they need both V-Fets installed to work properly since this is a bridgeable amp?

But the other bit of good news is the V-fets still tested ok with my Atlas DCA 75 pro.

NOTE: With no V-fets or Diodes in place of the V-fet's the protection comes off.



Athanasios

Could very well be, these are not equivalent transistors, you can’t put them in and expect correct behavior. There could also be other issues that need attention, at least the V-FETs are not dead or damaged.

 

[Nashou66]

Could very well be, these are not equivalent transistors, you can’t put them in and expect correct behavior. There could also be other issues that need attention, at least the V-FETs are not dead or damaged.

I agree, as does @Vint Age with his "thumbs up" to your post.

So Since the K76/J26 were not damaged and still tested ok I will put the 2 pair or K70/J20's in and hope for the best. Hopefully that relay will click as id does with no V-fets in the unit.

Back to cooking Breakfast , its slow though today, snowing at the moment here in Blasdell NY.

Athanasios

 

[Nashou66]

Well I first put the left channel V-fets in and got no relay . Was able to dial in the 52mV bias but still had -18 volts on R75 and R73. These are the resistors connected to the two white wires that go through the protection circuit onto the relay . Also the gate voltages went to about 12 volts at 55mV Bias.

I then added the other two V-fet's and still the same results. No relay click, adjustable Bias and -18 volts on Resistors R76,74 that the red wires go to.

With out a schematic that has proper voltages marked I think Im at a stand still.

Good news is the V-fets tested ok on the DCA75. So I will be using the Yamaha V-fets for testing from now on as they gave me the exact same results.

R619 looked a bit cooked, it is a 120 ohm but it reads 132 ohm, There was a break in a trace to the cap that is on the same trace so that might have been why it looks a little cooked . I think it's a 1 watt , I need to get some if I can't find any in my stash.

With all outputs removed I get the relay click.

So that it for now , not sure where to look next.

Athanasios

 

[Sparkplug]

Well I first put the left channel V-fets in and got no relay . Was able to dial in the 52mV bias but still had 18 volts on R75 and R73. These are the resistors connected to the two white wires that go through the protection circuit onto the relay . Also the gate voltages went to about 12 volts at 55mV Bias.


I then added the other two V-fet's and still the same results. No relay click, adjustable Bias and 18 volts on Resistors R76,74 that the red wires go to.

With out a schematic that has proper voltages marked I think Im at a stand still.

Good news is the V-fets tested ok on the DCA75. So I will be using the Yamaha V-fets for testing from now on as they gave me the exact same results.

R619 looked a bit cooked, it is a 120 ohm but it reads 132 ohm, There was a break in a trace to the cap that is on the same trace so that might have been why it looks a little cooked . I think it's a 1 watt , I need to get some if I can't find any in my stash.

With all outputs removed I get the relay click.

So that it for now , not sure where to look next.

Athanasios

Not too bad, at least there weren’t any casualties.

Months ago there was one of these on auction here and I got ready to bring it home, but then quit when I realized I couldn’t get any technical info on it. Seems like I need to double down on my efforts to find at least a schematic. LOL

 

[Nashou66]

Not too bad, at least there weren’t any casualties.

Months ago there was one of these on auction here and I got ready to bring it home, but then quit when I realized I couldn’t get any technical info on it. Seems like I need to double down on my efforts to find at least a schematic. LOL

I sent a message to @ocerrr who "I think" has one. He has not been on since Dec 24th.

I need to find out where that -18 volts is coming from. I took a break and cant remember if it was + or - 18.

I'll use the 2SJ26 and 2SK76 for probing around . I wonder if I might have put a tranny in wrong or another part etc. I also wonder about the SMD replacement I used for the 2sa798 . I wonder it that could be it.

EDIT: it is -18.xx volts on the speaker outputs before the relay.

 

[Nashou66]

Adding the protection circuit to the schematic . I also found a few mistakes I made.
I had the header labeled wrong for pins 2,3,4,5. I also have the Red1 and Red2 tied together which should not have been. And a few other small changes.

So here is a little more .

Athanasios

 

[Nashou66]

And a little more ......

 

[Nashou66]

I'm doing a little more today on the schematic but decided to try something to see where the -18 volts is coming from. I thought that maybe it might be getting injected from the protection circuit, so I removed the two red wires that have the -18 VDC from that location. The protection relay clicked so that told me there was probably -18 VDC on the red wire location at the amp end, yep, it's there. So I now know to look for it on that end. Now the side with no V-fets installed does not have this issue, the left channel, most likely due to know output devices installed.

Next step I plan is to remove the dual transistor ( SMD adaptor) and re install the 2SA798's . I have a feeling it might be that.

Weird thing is even with the -18 VDC I can still adjust the bias to 52mV's .

If the 2SA798's dont work then I might shelve it till a schematic comes available with voltages. I could add some to mine, but with it not working, not sure how many will be correct.


Athanasios

 

[Nashou66]

Some more, these parts are not easy. Also the program isn't the easiest to make changes with.