Yamaha B-1 Problem

[aged one]

Greetings

I was given a Yamaha B-1 by a friend.....but it has a problem.

When power is applied, the overload protection led lights and a relay in the unit activates (with or without i/p or speaker connections).

Does anyone have the schematic for this unit......or can you suggest what the problem could be?

From previous posts I have read, I have formed the view that the devices in this amplifier are obsolete.

[djk]

Those were unbelievably hard to work on, even with the manual, factory spares, and factory tech support.

I don't think the outputs are available anymore, they were very, very expensive even when avaiable.

The outputs are a non-standard case size, and only one pair per channel were used.

Tough call, but I would not fix it.

Unlike the B2, the B1 would be hard to salvage for a new design.

[Kenn]

If a power FET is blown, you've got your self a very heavy door stop. Otherwise
the unit is probably fixable.

The protection circuit engages for a variety of reasons, which include:

- Excessive DC offset voltage on one or both outputs. If you're feeling brave
you can try measuring the voltage between the E and O terminals on each
driver board. It should be < +/-0.5 volts. Be careful, there are very high voltages
in this amp. The output devices are supplied with +/- 85 volts, and -200 volts.
Voltages that can easily KILL. You also don't want to be the one to blow
perfectly good B-1 FETs, which is easy to do if you short something out.

- Loss of a supply voltage.The protection circuits will disconnect the speakers
if a supply voltage fails. On the circuit board with the -200 volt adjustment pot
there are three test terminals. The voltage between the -200V and E termnal
should be 200 +/- 0.5 volts, and between the +40V and E terminal +40.5 +/-1
volt. The 40v supply is referenced to the 200v supply, so adjusting the 200v
supply will automatically adjust the 40v supply. On the circuit board with the
+12v and +/-25v adjustment pots there are three test terminals. The voltage
between speaker ground and the +12v terminal should be +12 +/-0.2 volts.
The voltage between speaker ground and the -25 terminal should be -25
+/-0.2 volts. The voltage between speaker ground and the +25v terminal
should be +25 +/-0.2 volts. Measuring the +/-85 volts supplied to the power
FETs is harder, and requires that you pull all four FET heatsink assemblies
out and insert jumpers into the appropriate places. The voltages should be
+/-85 volts +/-10 volts.

Any accidental shorts during these measurements would be bad. I would
recommend you power the unit down, leave it turned off for ~15 minutes,
attach your voltmeter to the appropriate test terminals with clip leads (making
sure nothing can short out), then turn the amp on to make your measurement.

- An open thermal fuse. The thermal fuse will open if the amp gets too hot,
or if the fuse is defective. There are two fuses, one for each channel.

I have a Tektronix curve tracer that can test B-1 power FETS if you really want
them tested. If you're going to give up on the unit, I would be interested in
making you an offer for it. If you have any questions, please ask!

[EchoWars]

On the circuit board with the -200 volt adjustment pot

Good Lord! 200V in a SS amp? What for??

[Kenn]

The -200 volts is used to drive the power FETs off. These
FETs are fully on when the gate voltage is 0 volts. A large
negative gate voltage is required to pinch off the current
flow through the FET.

That's why the B-1 has so much protection circuitry around
the output stage. If the gate voltage to both output FETs
happens to drop to zero, you end up with a dead-short
across the +/- 85 volt supplies. That can result in a blown
FET. The B-1 has loss-of-voltage protection, over-current
protection, and thermal protection in an attempt to avoid
blowing FETs.

You should check out the voltages in the matching C-1 preamp.
If I recall correctly, the solid state phono input stage runs on
something like +210 volts (not sure about that number). The
phono stage consists of a linear amplifier, followed by a 100%
passive RIAA equalization circuit. No feedback tricks here!
It's a great sounding phono stage, but the bad news is the
bias current tends to climb up as the amp ages, resulting in
blown transistors. If you have a C-1, it's a good idea to get
the bias current of the phono stage checked/adjusted every
4-5 years. It's easy to do yourself if you're comfortable with
voltmeters and screwdrivers.