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Yamaha CA-800 - keeps frying output trannies

S

sherrema

New Member
Hi all,

I just bought some new ON Semi output transistor pairs for my Yamaha CA-800. I put them in, along with the new fuses, and proceeded to cook them all. I noticed I was letting the smoke out of two resistors too - R619 - a 680 ohm and R624 - a 10 ohm. Should I just replace those two resistors (along with the output pair and fuses), or do you think I have a problem with something else further upstream?

Thanks,
Scott
 
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I'd say there's another problem somewhere. Something is causing WAY too much current to go through them. The problem lies probably somewhere in the biasing circuit. Hmm...
 
Do you have access to a scope to monitor the output signal.
If your amp is oscillating at high frequency you will not hear it, but it may dissipate much more power than you can imagine... Bad substitute may make the amp to oscillate.
Check the DC voltage between the two emmiter from the power transistor in each channel, it must be very low (about 20mv). Better to have underbiassed than overbiassed amp at this stage of restoration.
 
I'm not that experienced with Yammies but if all you did was swap the output devices without checking the rest of the transistors on the driver boards, then you've likely got a failure further upstream on the power amp board. Since they're direct coupled, a failure anywhere back through the power amp will be reflected all the way down stream.

Was this the same failure mode as what originally happened? If so, then that would tend to confirm the issue.

Cheers,

David
 
Thanks all for your replies. I inherited this amp with this problem so I'm "assuming" this is/was the original problem. I don't currently own a scope - only a good Fluke DVOM. I'm sorta' guessing there is another problem "upstream" too - I was hoping someone would maybe have a gut feel for whether or not I'm looking for bad caps, transistors, resistors, diodes, etc. . . I guess I'll start desoldering. . .
 
I've never worked on that one, but I would imagine that if you have bad outputs, you probably have bad drivers, and bias resistors, (maybe even a few other parts) too. You probably don't need an o'scope, but you do need to know how to use your DVM properly, and a schematic would make your life a whole lot easier. Do a search for 'dim bulb tester', one is an absolute must when testing your amplifier repairs.
Or send it to Merrylander, the resident Yamaha expert.
 
Thanks. Yeah, I know Merrylander can do the job but I'm trying to do it on my own and save a few bucks (yes, I know that's not always a good idea!). I have my degree in electronics but it's been a looonnnggg time since I was a bench tech and I know I don't have all the right tools and my chops are pretty rusty!!
 
I wonder if there's a short in the Class A switch (common). There are a few Merrylander threads about this with photos of the switch. The amp adjustments could be wildly out of whack too.
 
Tube Dude gave you good advice as did wajobu, you have something bad upstream of the outputs and those two microswitches need to be tested to ensure that they are functionning correctly. The one I have here had ons switch so cooked that it always was in the Class A position. That totally unbalances the B+ supplies.
 
Hi Scott,

Thanks for the CA-800 schematic. From what I see in this schematic, I agree with other members concerning the "class A / class B" switch.

This 6PDT switch may be a major cause of failure in this amp.

There is a regulated +50V/0/-50V power supply (from the upper winding in the schematic). The differential input stage (I am speaking about the power amp only), the voltage gain and bias stage, and the current protection stage are feeded by this regulated power supply. Part (2 poles for the left channel, and 2 poles for the right channel) of the A/B mode switch changes the bias point in the final stage. The current in this part of the switch is probably safe for the switch.

The supply voltage for the final stages (driver and power transistors) is not the same in class A mode and in class B mode. In B mode, the supply voltage is +44V/0/-44V but in class A mode the supply voltage is +17V/0/-17V

There is 2 poles in the A/B mode switch that change the connection between the multi tap outputs from the power transformer to the main rectifier bridge. As you know, the surge current to the filter caps can be very large at turn On.

If the switch position changes from A to B mode when the power is ON, the high surge current to the filter caps may well weld one or the two poles of this switch in the B mode position.

It is easy to imagine that you may be in trouble if the voltages to the power transistors are +44/0/-44, or +44/0/-17, or +17/0/-44 Volts rather than the +17/0/-17 Volts expected from the designer when the bias current is high for class A mode operation.

You see, if +B is at +44V (rather than +17V), there will be a very high current, much higher than expected, across TR608 and TR610. Then, R624 will fume (what you get). The current across R631 will be high enough to switch TR607 On, and part of this high current will flow by TR607, D601, R619 and TR604 to the -50V regulated PS. R619 will fume, as you get...

To check this, remove the 3A fuses (all 4) on the power board. Put the mode switch in class A position. Turn the amp On, and measure the voltages at the main PS (not the regulated +50V/-50V, this one is not affected by the A/B mode switch). You should measure +17V and -17V. If any one of the voltages at these points is close to +44V or -44V, this switch is welded in B position. I suspect you have +44V/0/-17V

Does it make sense to you?

Tell us what you get. Good luck!
 
Thanks for this precision. May be possible because I have only the schematic to check . But there must be a total of 6 switches and not only 2. You must change the power supply (2 single pole switches needed for this) at the same time as you change the bias current (2 single pole switches in each channel).

From the schematic, there is 6 switches linked together. It may be any combination of switches (one 6PDT, two 3PDT, three DPDT, six SPDT, two SPDT + one 4PDT......).

My point is that the most probable switch to fail is the one (or the two as you seem to know better than me...) between the power transformer and the main rectifier bridge.

It's a good thing that it is made with many SPDT, because it is cheaper to fix ! Is there two SPDT + one 4PDT, or two SPDT + two DPDT, or six SPDT? Just for my information, please...

Cheers!
 
Its two omron micro switches each SPDT..
Click to expand...
:thmbsp:These two switches are all thats used to switch "Normal/ClassA" ...$2.59 each from Mouser.Reccommed rather than the 10A switches that you get the 15A's they are the same physical size with beefier contacts.
 
I understand from your previous post that the switches for the main supply was two SPDT

But, for the bias, around R615, VR603 and VR604, what it is?
 
Its a single little slide switch consisting of 4 spdt. ot two dpdt..the major current bearing switches are the omrons.
 
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