Faulty Yamaha CA-800

[eldarvanyar]

Hi guys I have seen a Yamaha CA-800 for sale on ebay with the following fault




"PLEASE BE AWARE : This amplifier will only function correctly when in 'CLASS A' mode, and all filters are turned off."

Does this look like an easy fix or should I stay clear?

Thanks

 

[djk]

It means it needs a bunch of switches that are no longer available.

The class A switch may fail in such a way that will cause the transformer to burn up.

I like this amplifier, but I would not buy one unless it was virtually free, and then I would replace all the electrolytics, and remove the class A switches.

 

[spind]

Does this look like an easy fix or should I stay clear?

Thanks

You may want to search a bit over at AudioKarma. 'Merrylander' has found Omron microswitches at Mouser I believe that can replace the Class A switches. There is a big Yamaha following over at AK.

 

[djk]

That takes care of one problem, and leaves all the others.

All the switches and pots are unique to the old Yamaha gear.

 

[ilimzn]

It's unlikely the switches are bad, probably just dirty. That being said, there are caveats.

djk is correct about the power transformer, I've seen two amps with burned ones. The reason however is not the switch section that switches the power supply voltage, but the one that switches the bias.
It has the classic mistake of using a Vbe multiplier as a bias servo, but with the adjustment in the C-B leg of the transistor. If the trim-pot fails, bias will become excessive, and the output stage will fail. Fortunately, this one does not happen often at all. The real problem is that the mode switch selects between two sets of pots for class A or AB respectively. If that switch becomes intermittent, neithe pot is selected and the amp has 'infinite' bias. A truly unexpectedly stupid design mistake, which is BTW not too difficult to correct, also making the wiring simpler. The one combination where the transformer burns is when the amp ends up working at class AB power supply voltages, but with class A bias. This is 1A and in most cases the output transistors will survive long enough for the power transformer to overheat and fail due to 200W of power being drawn from a 300VA approx. transformer feeding 36000uF of compbined filter caps. The rectifier diodes will also overheat at which point one tends to fail and burn up the mains fuse if it goes short. if not, the transformer gets severe DC imbalance and saturates, and burns up. Or, power stages reach a temperature at which secondary breakdown causes failure and the fuses melt. Since the transformer, rectifiers and one channel heatsink are very close to the PSU caps, these tend to get cooked as well. All that being said, whit the proper modification, it is completely reliable.

The amp is also prone to developing a noise problem - it uses the old style Japanese plastic BJTs in the small signal stages, that tend to become leaky. The last unit I worked on was so niosy that when i replaced the transistors, it became so much quieter i thought it wasn't working any more. A bunch of new 2SC2240 will cure that problem. Beware, there are a few component marking errors on the preamp boards, be sure to check how the original parts were inserted before replacement.

In other words, if the description is correct, you might have caught one that is worthy of restoring, with appropriate reliability fixes made. If this is done, you will get a very nice amp but keep in mind that it requires knowledge and skill, and won't be cheap.

 

[BobtheBuilderMLG]

It's unlikely the switches are bad, probably just dirty. That being said, there are caveats.

djk is correct about the power transformer, I've seen two amps with burned ones. The reason however is not the switch section that switches the power supply voltage, but the one that switches the bias.
It has the classic mistake of using a Vbe multiplier as a bias servo, but with the adjustment in the C-B leg of the transistor. If the trim-pot fails, bias will become excessive, and the output stage will fail. Fortunately, this one does not happen often at all. The real problem is that the mode switch selects between two sets of pots for class A or AB respectively. If that switch becomes intermittent, neithe pot is selected and the amp has 'infinite' bias. A truly unexpectedly stupid design mistake, which is BTW not too difficult to correct, also making the wiring simpler. The one combination where the transformer burns is when the amp ends up working at class AB power supply voltages, but with class A bias. This is 1A and in most cases the output transistors will survive long enough for the power transformer to overheat and fail due to 200W of power being drawn from a 300VA approx. transformer feeding 36000uF of compbined filter caps. The rectifier diodes will also overheat at which point one tends to fail and burn up the mains fuse if it goes short. if not, the transformer gets severe DC imbalance and saturates, and burns up. Or, power stages reach a temperature at which secondary breakdown causes failure and the fuses melt. Since the transformer, rectifiers and one channel heatsink are very close to the PSU caps, these tend to get cooked as well. All that being said, whit the proper modification, it is completely reliable.

The amp is also prone to developing a noise problem - it uses the old style Japanese plastic BJTs in the small signal stages, that tend to become leaky. The last unit I worked on was so niosy that when i replaced the transistors, it became so much quieter i thought it wasn't working any more. A bunch of new 2SC2240 will cure that problem. Beware, there are a few component marking errors on the preamp boards, be sure to check how the original parts were inserted before replacement.

In other words, if the description is correct, you might have caught one that is worthy of restoring, with appropriate reliability fixes made. If this is done, you will get a very nice amp but keep in mind that it requires knowledge and skill, and won't be cheap.

Hi, I know this post is old but can you tell me how the Class A switch mod works? I've looked through a few forums but haven't found the mod yet.

I'm working on a CA 800 that has a blown power PNP. Everything else so far seems to be okay. I'm replacing all electro caps and going to replace many of the signal transistors.