help with yamaha ca-610

[caseyaberhart]

hello all,

i have a problem with my yamaha ca-610 (not ca-610II), the right channels pop and crackle loudly at minimal volume, all switches have been cleaned, protection relay is fine. i have noticed that the right hand channel transistor pair are hot while the left ones are stone cold. i was wondering if anyone would care to share a schematic with me or might know what the problem is. thank you

[EchoWars]

Bias might be set way too high, or the bias transistor is toast. Or it could be oscillating for some reason.

I don't have a manual for that one, and I certainly wouldn't operate it as it is.

[caseyaberhart]

i don't know a lot about amplifiers, just RF circuits mainly, how would i logically locate the bias transistor or adjust the bias. i have just realised that both the right diff pair are getting hot, would it be safe to assume that the power trannies are okay as long as i don't operate the amp in it's current condition? i posted in vintage audio weeks ago and received some advice about power supply problems, if this was the case wouldn't both channels be popping and cracking. this amp was working fine for a year since i bought it, if that info helps. any help is much appreciated

[Paul C]

(OK all you pros, this is the Reader's Digest version for novices)

Look at the picture attached. The transistors on the heatsink are the PNP/NPN pair of output transistors of one channel. See the small transistor between them? That is the bias transistor. It is mounted on the same heat sink so that it is at the same temperature (appx) as the output transistors, and this compensates for slight voltage change in the bias as the temp rises.

That bias transistor may bolt on as in the photo, or have a little loop of metal and a gob of heat sink compound, or simply have the leads bent, leaning the transistor over to touch the heat sink. If you replace it make sure to use more heat sink compound. And bias must be adjusted.

See the large resistors? Those are usually .47 ohm 5 w. (For an amp made for 2-4 ohms, they may be .33 ohms) These may be combined into one dual resistor with three leads.

Somewhere in the vicinity there should be a small pot. That adjusts bias.

To measure put the probes of your voltmeter to the leads of either of those large resistors. Adjust the bias pot to whatever the specified voltage across the resistor. If you don't know, measure the voltage across one of the resistors on the good side (with the cool transistors) and use that voltage. If you still don't know, I'll toss out a figure of 50 mv.

(If a dual resistor, three leads, measure from the middle to either of the outside leads)

It doesn't matter which of the two resistors you measure... both will be the same within a few mv. And when you set bias, it will be difficult to get an exact figure, but get close.

This is down in the mv range. Be very careful doing this. Use the voltmeter tips that clamp on to the test points. If you slip and short something with the probe of the voltmeter you can really toast something serious. I speak from experience. Experience means I, uh, made a serious error like that already.

[caseyaberhart]

thank you Paul C! you may have saved this amplifier from being stripped for parts, i have printed your instructions and will follow them closely tomorrow. Thanks

[merrylander]

If the CA-610 is similar to the CA-810 there will be a 0,47 ohm 5 watt resistor on legs that you should connect the meter across and the 810 setting is 20mV. Given that the 610 is of lower wattage the resistor could well be 3 watt. In the 810 it is a proper bitch to get meter leads inside and it is simpler to just tack solder short wire stubs on the solder side of the PCB.

Rob

[caseyaberhart]

thank you merrylander, that will be useful advice when i do attempt to fix this behemoth of an amp

[EchoWars]

That might take care of the heat issue, but there is another issue as well (pop and crackle, to quote).

I'm betting that -

1. Bias currently reads way too high, and
2. It will not adjust, or at least it won't adjust to any realistically usable value (to be certain, bias needs to be measured from 'emitter-to-emitter', or across both emitter resistors to verify the issue)

EW - who's perfectly happy to be wrong if it means you again have tunes

[Paul C]

It could also be a bad solder joint, or other bad component in there. Look for discolored parts, darkened circuit board. And just because the part is gets hot does not mean it is necessarily bad. But your outputs should be about the same temp on both sides. At normal listening levels you should be able to hold your finger on them without burning, just warm. (This is for Class AB amps... Pure Class A gets HOT).

While you have it all apart, it would not hurt to shoot a bunch of new parts in there.

[caseyaberhart]

Paul C: While you have it all apart, it would not hurt to shoot a bunch of new parts in there: do you mean replacing all the caps? what about resistors? relay? and pots? what do you normally replace?

[caseyaberhart]

i adjusted the bias on the right to the same as the left channel (around 52mV), when i did the popping and crackling stopped, now there is a white noise in the right channel like a mild static noise. it also pops lightly occasionaly, is this a power supply problem?

[merrylander]

I would lower the bias on both channels, IMHO 52mV is too high. You could email Yamaha (Parts@yamaha.com) and ask for a copy of the SM page that shows the adjustments, or even ask if a copy of the SM is available. I know that the 810 is 20mV and my experience with Yamaha says the 610 would be the same, frex, the CR1020 and CR-2020 both have the same bias value.

Rob

[caseyaberhart]

i will email yamaha and ask them... now thinking... why didn't i do that in the first place? do you think they will be helpful?

[EchoWars]

No.

Their techs have better things to do than answer the questions of nOOb DIY'ers, especially on a 25 year-old piece of gear that was likely built and sold before most of them were born.

Not my attitude, but it'll likely be theirs.

[Paul C]

For clarification, my post above with the photo is NOT your amp. It was the only such photo I had on my computer, and was used for demonstration purposes. I was speaking of amps in general. And while specifics among amps will vary, Class AB circuit topology is pretty similar among most amps.

Paul C: While you have it all apart, it would not hurt to shoot a bunch of new parts in there: do you mean replacing all the caps? what about resistors? relay? and pots? what do you normally replace?

Caps and resistors associated with the power amp section on that or both channels. Have you changed the output transistors, their drivers, and the bias transistor?

A capacitor can leak through and be crackly. A resistor may get intermittant, opening and closing the circuit, generating noise, allowing voltage to swing around. I'd change all those little parts on that channel's power amp, maybe a dozen parts. And those transistors. But that's just me.

i adjusted the bias on the right to the same as the left channel (around 52mV), when i did the popping and crackling stopped, now there is a white noise in the right channel like a mild static noise. it also pops lightly occasionaly, is this a power supply problem?

If it were the power supply it would affect both channels, wouldn't it? Unless you have a "dual mono" amp (separate power supplies on each channel, both channels complete, self contained, independant units), both channels receive power from the same power supply.

Have you tried wiggling parts and seeing if you can trigger the noise?

Now that you have readjusted the Bias, are the transistors on that channel still getting hot? Or are they now the same temperature as the other channel?

[caseyaberhart]

when i re-adjusted the bias, the transistors are no longer heating up. i noticed touching a insulated screwdriver to top of the metal can of a few of the caps produces a crack in the channel... i assume it would be trying to find a path through the screwdriver and this is not reproducing the problem, i have wiggled and tapped every component and can't reproduce the popping and cracking, other than i mentioned. i think yamaha just moved my email into their deleted items folder without reading it, considering the subject had the words "CA-610".

it seems strange that it only pops and cracks lightly now, personally i feel it's a component failure in that section, i'll start ordering new components on payday, when i have some cash.

thanks all

[Paul C]

Do the pops and crackle come from both speakers, or just the problem channel?

Have you looked at the bottom of the circuit board? examined solder joints? Looked for damaged traces?

Something caused the bias to go way out on that channel... a bad resistor, a transistor, something.