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Old 25th February 2006, 07:36 AM   #1
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Join Date: Jun 2005
Location: Ottawa
Default Repair of Yamaha B-2

Hi All,

I've been running my Yamaha B-2 Power amp for
2-3 months with no problem until this week.

After it's been on for a while, the left channel
slowly starts to decrease until cutting out completley.
Toggling the speaker select switch or toggling input
or power will all bring it back, but only for a few
moments before it repeats the behavior.

Now since this thing is 30yrs old or so, the first thing
I thought I'd do is replace all the Elec Caps on all
boards, including the 4 mains, and reflow all solder
on all boards, checking wires and connections.

I was wondering any thoughts on this? Anyone go
through this with a Yamaha B-2 before?

Also, if anyone has replaced the main caps on the B-2,
let me know what you replaced them with.
I'm curious about what ripple current you went with?

Thanks,
AK
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Old 25th February 2006, 09:44 AM   #2
ilimzn is offline ilimzn  Croatia
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I would start looking at the output protection relay, then using deoxit on any switches and pots and also look for dry solder joints (especially in the auxiliary power supplies) then go on with electrolytics there, and only after I was sure all of that was OK, would I go replacing other caps. Although, I would be willing to bet the first step will solve your problem.

I would also be EXTRA CAREFUL with a yamaha B-2. Why? Well, because it is one of the VERY few VFET (NOT VMOS, MOSFET!!!) amps ever made, and an error resulting in frying the output stage would not be fixable, except perhaps by a part unit as a donor. The B-2 uses output transistors that are literally unique - they were only ever used on the B-2 and custom made for it, there are NO spares, in fact there have been no spares for nearly 2 decades. Auxiliary power supplies are very important for this type of amp as without them the output stage becomes a short circuit across the main power supplies. So, be sure you know what you are doing!
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Old 25th February 2006, 04:31 PM   #3
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Thanks for the reply.

And yes I'm aware of the vfets and how hard
they are to come by.
As it happens, I have 3 Yamaha B-2s. I forgot
to mention in the original post that I eliminated the
rest of the system as a problem by replacing
the amp with my "spare" B-2.

The reason I mentioned changing the caps was
since they are at the end of the life span anyways,
and if I have to do some work I may as well do
it in one shot instead of waiting for something else
to fail. Just my way of thinking.

I was mainly concerned with what replacements
people had used for the 4 main caps.

Cheers,
AK
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Old 25th February 2006, 06:04 PM   #4
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Join Date: Mar 2005
Location: California
Why replace everything? I doubt for example the main caps are bad.
"The sound gradually decreases" - this is an unusual problem, and I think probably not a poor connection or contact which would tend to give static type sound and/or complete loss of sound - though cleaning contacts can't hurt. Resoldering lots of solder joints is a PITA and occassionally can cause problems (usually a solder bridge as one gets careless on the 50th solder joint - there were amps that this was the first thing one tried when presented with no obvious fault.)
I would look for bad joints especially around any transistor that runs hot. I'm not real familiar with B2s so can't give any specific advice.

The electrolytic cap that is usually in the feedback to reduce the DC gain to unity (RC from neg diff input to ground) could possibly be going bad - if it opens the sound nearly dissappears (gain of 1x instead of about 20x or whatever) this could be happening graudually as the set warms up, perhaps a little surprising that it will come back but I've seen plenty of wierd things. Worth checking out.

I second the bit about being careful, DO NOT put on a variac or other slow start circuit like a light bulb in series, the outputs have to have their full bias circuitry working otherwise they are dead shorts and will blow up. These and the Sony VFET amps were scary to fix: repair, then turn on and pray, or pray and turn on, if you missed something you would have to start all over. I did not fix that many (a couple Sonys I think one with blown amp, and a Yamaha but not for blown outputs) and was successful first time every time.

IMHO you should try to actually figure out what is wrong, fix it and then do preventitive cleaning of contacts. That way way you know you really fixed it.
Good Luck
Bill
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Old 25th February 2006, 06:14 PM   #5
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Location: California
I forgot to mention that I'm listening to the radio - the tuner is a Fisher FM-90B with original caps, working just fine thank you, age unknown but it has to be 40 plus years.

Replace what needs to replaced first.

Bill

oh, a 90B is a tube tuner, probably had its least some of its tubes replaced when I bought it 20 or so years ago, otherwise it is original, and sounds great.
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Old 1st October 2006, 01:43 PM   #6
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Location: NJ Shore
Would be surprised if the main caps were bad. They are Nichicon's best Gold, which was about as good as it got back then. You might want to add a by-pass cap on each, is all. Unless you definitely see leakage/swelling. You'll have to lift the power supply board to see them. I'll post a few pic's. My B-2 amps have all been recapped (four of them) and used Black Gates throughout. Also, a recurring theme is that the fuses (underside of the power supply board and behind the back plate, which is hinged for easy access) seem to get old and brittle. Symptom is that a channel will drop out because the low voltage power supply starts dropping out. Try changing them. And, a thorough cleaning with Deoxit of all switches and pots is a great help. This includes removing the relay covers and cleaning the contacts. Replace the bias pots with 10-turn Bourns if you want to get fancy. Here are pic's of the fuse locations. Wonderful amps.
Click the image to open in full size.
Click the image to open in full size.
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Old 4th October 2006, 12:43 AM   #7
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Wink T-shooting B-2

Hook-up failing amp to a known good source ie. radio,CD player etc. And a set of speakers. When the channel fades out hit the suspected output fets with Circuit cooler also any driver devices. I'm thinking you have a output that is failing once it heats up.
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