Yammy CA-2010 hummmmm

[DragonMaster]

I've had loads of trouble with this amp so far... (noise, problems with meter calibration, DC in subsonic filter, pop, crackle, dirty selector causing dead inputs, won't turn on, hum, dead bulbs, missing screws, cigarette gunk everywhere, cold solder joints, etc.)

I fixed most of the problems over the last few years, but there's hum coming from the power amp (even if I disconnect the preamp and put a load on the power amp input, it hums in both channels -- the lower the impedance, the more hum there is)

I recently recapped the PSU board on the bottom (only the polarized electrolytics), which fixed problems, but that should be unrelated, since it's not supplying the power amps. However, the hum was intermittent before I recapped it, but it seems to be permanent now...

I doubt the problem is on the power amp boards, since both channels hum the same way.

 

[SoCal Sam]

Until Merrylander checks in...

I had an amp with bad humming and it turned out to be a bad main filter that was rippling. Could also be a grounding problem somewhere or worse case scenario, a tranformer with de-laminating windings.

BTW, my CA-2010 humms but luckily it is barely audible at idle.

 

[avionic]

the hum was intermittent before I recapped it, but it seems to be permanent now...

Hmmmmm..What is common with both amps.:scratch2: Power supply!!! I'm with SoCal either the main filter cap(s) or a broken solder connection/wire on a common ground...:yes:

hum was intermittent

cold solder connection.Did you deoxit the coupler switch on the rear panel? And the speaker select switch?

 

[merrylander]

Good advice so far. If you did replace the main filter caps and found no change then I would go with the ground connection problem. Yamaha went to using wired grounding after having some problems with chassis screw grounding. The sequence of thos wired gtronds is important so as to avoid ground loops.

 

[DragonMaster]

Just to specify, I didn't touch the main PSU (and main filter caps), just the one for the preamp, small relays, startup delay, etc.

Can I bypass the main caps with smaller electrolytics to see if the hum is reduced?

I'll check the wiring as well... I can see that someone screwed with it in the past. However, the service manual is missing a few wires on the wiring picture (the power amp ones IIRC), anyone has a pic?

 

[merrylander]

As long as they are of the correct voltage rating that would give you a clue as to the main caps state.

 

[DragonMaster]

I think I only have some old 220uF 200V caps to test it with, and they might not be in great shape. I think that the ESR measured fine on the Yamaha caps, but I don't remember if I actually tested them or not... All I know is that I haven't found a single bad cap since I got my ESR meter...

 

[merrylander]

That pretty well matches my results since I built the Blue ESR meter.

 

[DragonMaster]

The main caps test 0.00 ohm :scratch2:

However, I found that by removing the fuses that connect to the white transformer wires on the power supply board, the hum disappears, and the amp still works, but the protection circuit becomes a little unstable.

So... looking at the service manual... those fuses power the B1 line, which feeds the power amp driver transistors! Looks like I'll throw away those new Elna Cerafines and put back the good ol' '70s Nichicon caps!

Aren't Cerafines said to be the next best thing after Black Gates, and 70s Japanese caps to be a small step over counterfeit caps found in computers? :nono: Now let me be even more skeptical about those claimed oh so awesome component upgrades. I bought those caps mainly because that's about the only thing I can buy locally other than decades old surplus, and it's not really more expensive.

Now that I think about it, the hum started when I half-recapped the PSU, and it was intermittent before I found a cold solder joint on one of the Cerafines that is connected directly to the B1 circuit - that's what happens when you're too lazy to switch your soldering iron tips after doing some surface mount stuff... It looks like the amp would work better without those caps. :scratch2: I don't have my soldering iron with me to test it though...

 

[DragonMaster]

OK, I put back the old caps, there's less hum. I now hear more hum coming directly from the amplifier than the speakers. If I add the Cerafine in parallel, hum becomes easy to hear, so it looks like something is wrong somewhere...

:scratch2: The PSU isn't working properly, that could probably explain why I always disliked this amp's sound. Now if only my 'scope's trigger section worked...

I've just compared voltage between the service manual and schematic, and something is wrong with the B1+ output voltage transistor. I get 79V on collector and emitter, and base has 76V, while it should be : Vc ~77V, Vb ~72.5V, Ve ~71.5V. The negative line has voltages a bit on the high side, but they're much closer to the real thing (Vc-Ve=~5V, etc).


After the B1 winding rectifier, there are about 3 resistors, 3 caps and a diode other than the transistor itself. Fortunately, that's not many components to check. Last minute check... The diode on B1+ checks shorted in-circuit, while it doesn't on B1-. Now remains... Any common replacement for a 1S1555? And, why has it failed?

Last minute edit: 1N4148 seems like a cross-reference for the 1S1555. I'll try to replace the diode and see if the hum problem is fixed, or if it fails again.

 

[DragonMaster]

OK, so I replaced the diode, and the voltage is now the same on both lines. There's still a hum problem, but after I read ESP's Capacitor Multiplier page, it looks like the voltage difference between the circuit input and output is simply too low to prevent ripple. It looks like the fix could be as simple as replacing a resistors.

http://sound.westhost.com/project15.htm

 

[ecluser]

It would be preferable to replace the caps C423 and C424 for larger caps.

 

[DragonMaster]

I'll try that, but so far it looks like it's a voltage problem, since it's still not within specs. (Vc-Ve is under the absolute minimum stated by Rod Elliot, and smaller that the schematic states) I'll check if some resistors drifted if larger caps don't solve the problem.

 

[packrat]

Are there any of those tiny 'fuse resistors' in that circuit? Merrylander has often said that those need replacing far more often than the caps in the Yamaha amps he works on. They apparently drift away from their spec'd values.

 

[ecluser]

The important point in this circuit is the ripple voltages at the base and the collector of the transistor. With Vce ~5V, you should have no problem.

There is much less ripple at the base than the collector, because there is a second filtering stage. But the output voltage will never be better than the base voltage.

If the instantaneous collector voltage becomes lower than the base voltage, there will be a loss of regulation. A scope connected to +B1 would show a horizontal line (at 70V or so) with negative ripple.

If one of the diodes D412~415 is open, you will have much more ripple on the collector side. Check the diodes and their solder joints.

If you replace caps C415 and C416 (not sure about the part number since the schematic I have is not very sharp, the 220uF/100V after the rectifiers) by larger caps, you will reduce the ripple on the collector side.

This said, you understand that if you replace the 2.7K resistors by a larger value, the base voltage for TR414 will be lower, and at some point you will have no ripple at the output, at the expense of a lower output voltage.

 

[DragonMaster]

I think I should have used my 'scope from the beginning

First, I placed back the old caps as the ripple was less audible. Then, I found a dead diode causing ripple on B1+. I've just checked with a 'scope, and B1- base had tons of ripple. Every resistors, transistors and diodes measured fine, the only thing left was the 47µF base filter. I placed back the Cerafines and everything works perfectly. Turns out that one of the old UCC is dead. In fact, I just saw it's about to blow from the bottom of the can.

If you replace caps C415 and C416 (not sure about the part number since the schematic I have is not very sharp, the 220uF/100V after the rectifiers) by larger caps, you will reduce the ripple on the collector side.

Fortunately, I didn't need to do that. The Cerafines are larger than the original caps, and 220µF is the largest cap I can place unless I drill holes in the bottom cover.

Now, I still need to find what I can use to replace a VD1212 / HV46R diode on the tone control board...
Ah, looks like two 1N4148 (again) will do the job! http://www.audiokarma.org/forums/showthread.php?t=101598

Looks like I should always keep some 1N4148 in stock...

 

[SoCal Sam]

So, is the hmmmmm gone?

 

[DragonMaster]

It's totally gone!

I still have the crackle to solve on the right channel, which appears and disappears when I touch one of the VD1212 diodes, so I'll replace them with 2x1N4148 in series.

There's also DC voltage accumulating on the left channel when I turn on the subsonic filter, which is released when I turn it off, which causes a loud pop and triggers the protection circuit. That might be a fuse resistor case, because it started to appear when I rebuilt all the solder joints on the tone control board, not knowing there are fuse resistors (I just saw them in the super-clear service manual PDF I bought recently)

 

[rockin1150]

One thing at a time, man you have patience though, I envy that.

 

[DragonMaster]

Oh, I'm used to that, everything I buy used in "working" condition requires tons of repairs. I usually only get the appliances to get basic operations to work at first, and fix them one by one when I've got time, which takes years. I bought this amp 3 years ago, and it's far from perfect. In fact, none of my vintage stuff works properly.

I forgot I still have to fix the meters, buy a few screws and find someone with a CNC to make top cover screw brackets for on one side of the amp. (They're missing)