B1 Amp Precautions and Recommendations

basil13

Active Member
A customer brought in the C1/B1/UC1 combo for me to repair the C1 and go over the B1. I have never worked on these before, but repaired the C1.

The B1 is working, but does run hot. After researching the B1 I understand there are a number of parts with no substitutes.

I am a technician with experience on various vintage pieces but not a B1. Additionally, since the B1 is valuable with unobtainium parts, the various warnings in the service manual and in the forums, it makes me hesitant to work on it.

How difficult are these beasts to work on? What should I be cautious with or doing? What would be a good and low risk check on it's health?

Thanks.
 
I would venture to say that if you were able to fix the C-1, you should do fine restoring the B-1.

What was wrong with the C-1?
 
The B1 is working, but does run hot.
The B-1 does run hot. There's no getting around that due to the higher 2SK77 bias setting and all of the PS regulation and protection circuitry.

As a bare minimum it would be a good thing to replace the electrolytic caps in it; lots of them are located near hot components and by now are likely suffering from that.

The -200V supply is absolutely critical to keeping the amp alive and as such there's a big cap mounted to the chassis rear that's worth changing after its 40 years of service.

Another weak point in the B-1 can be the dual main caps which are difficult to replace since they're no longer made.

If you're plugging the circuit boards in and out, before doing so always make sure to discharge the main caps and the -200V cap although that 1 bleeds away fairly quickly.

Also, don't rely no the service manual or the schematic because they're full of errors!

The list goes on and on. You'll find lots of useful info here if you do a search.

Restoring a B-1 is truly a labour of love which can take lots of time depending on its condition but it's well worth it once done. Above all be careful in there because as you know 1 little mistake can be very costly and good luck!
 
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I would venture to say that if you were able to fix the C-1, you should do fine restoring the B-1.

What was wrong with the C-1?

The power supply was acting up. Kind of a strange issue. When powered on, it would hum through the speakers. This would start low and increase in amplitude for about 10 seconds. Then there was a pop and everything worked fine.

After some troubleshooting I found the -25V chain had 4 defective transistors. Additionally, because of the problem it was loading down the +25V line. Neither would adjust to spec. I could watch the 25 volt supply start at zero volts on power up and increase in voltage as the hum got louder. When it reached 25 volts, the pop would occur and things were fine. Although the -25V was too high and wouldn't adjust.

I couldn't believe the thing was still working after the power on sequence and pop. The power supply is well built, I never seen that many transistors in a power supply!!

If you're plugging the circuit boards in and out, before doing so always make sure to discharge the main caps and the -200V cap although that 1 bleeds away fairly quickly.

Thank you for the response! If I go forward I was planning on plugging the boards in and out. Looking at the service manual it shows using a circuit board extender and states to discharge the power circuitry before insert/removal. It would be easy to check the caps on the power boards with an extender but since I don't have one should I be OK with just discharging the main filters and -200 supply? I assume the caps on the power supply boards bleed off quickly?
 
Hello
The big cap are the problem in B1 .
To preserve your VFET it is highly recommended to change these bad capacitors ! it is silly to use two cap together in the same can (+ and - 100V together ! ) in case of trouble you can dammage the VFET !

B1-condos-RR.JPG

B1-VFET-RR.jpg
 
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Yes that should be fine.
Hello
The big cap are the problem in B1 .
To preserve your VFET it is highly recommended to change these bad capacitors ! it is silly to use two cap together in the same can (+ and - 100V together ! ) in case of trouble you can dammage the VFET !

View attachment 918216

View attachment 918217

Thanks for the responses. I see both of you have made boards to accommodate the new filter caps. Do either one of you have these available for sale?
 
Thanks for the responses. I see both of you have made boards to accommodate the new filter caps. Do either one of you have these available for sale ?

Yes , of course ! board with specific capacitors . Send me a PM. Ask also vint Age for his one
 
Ask also vint Age for his one
I'm out of them at the moment and I have to order at least 5 at a time. So far I think there are 8 of them installed in B-1's out there and since I'm not expecting to see more B-1's come my way for repair/restore I'm not sure that it makes sense for me to build more of them.
 
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I'm out of them at the moment and I have to order at least 5 at a time. So far I think there are 8 of them installed in B-1's out there and since I'm not expecting to see more B-1's come my way for repair/restore I'm not sure that it makes sense for me to build more of them.

I tried to PM you but it looks like y.ou have PMs disabled. Are the 5 you are ordering spoken for? Would you be willing to do a sixth and how much would it cost? You can PM me if you prefer.

Thanks
Bob
 
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