# VARIAC for troubleshooting ?



## greco (Jul 15, 2007)

My friend (Hamstrung in this forum...Thanks again) gave me the electronics (i.e, complete chassis...well, I think it is complete) from a Yamaha B50-115 amplifier, as it was not working (when he bought it) and he was primarily interested in the cab. So far, I have had no luck finding a schematic for it on the internet.

I am trying to check through it to see if, with my limited electronics knowledge, I can determine what might be the problem. 

I don't want to check it at 115 VAC (that is just too scary for me...at this point ) so I wondered about using my VARIAC at about 10 VAC or so. Can I check anything much after the transformer with any degree of reliability at this low voltage?

If not, any other suggestions?...BTW..."Give up now" will be considerd as a reasonble suggestion...LOL

If I can't get it working, I intend to use it for parts.

Thanks

Dave


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## nonreverb (Sep 19, 2006)

Hey Dave,

If memory serves me correctly, the B50 is a solid state amp. A variac is one good place to start. However there are a couple more things to get/do before the variac stage. First would be the check the fuse. Then check the conductivity of the output transistors with a digital meter. If you see a dead short across the pins, then the variac won't be necessary till they're replaced. Another device which is indispensable when troubleshooting SS equipment is a 60 watt light bulb in series with the Variac.
This will limit the current from the 120AC side to about half an amp and usually will give you enough warning that somethings still wrong before the output blows.
Hope this helps:smile:


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## greco (Jul 15, 2007)

Thanks nonreverb...appreciate your help and taking the time to teach me.

Yes, it is an SS amp...should have stated that in my initial post

One fuse was blown. This was one *inside* the chassis and in series with the one in a black fuseholder. A friend of mine who builds amps said that manufactures sometimes do this as a secondary precaution for those that like to stuff the external fuseholders with tin foil, etc. I replaced that fuse.

The first problem (for me) might be identifying the output transistors  LOL ... I think I could sort that out, but I have no schem or layout diagram. Still, I'll have a look.

Is the light bulb in series acting as a 60 watt power resistor (basically)?

QUOTE _"and usually will give you enough warning that somethings still wrong before the output blows."_ END OF QUOTE 

... Could you explain this a bit more please. I want to be sure I understand how this works. Sorry, I'm sure it is very basic.

Thanks again

Dave


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## fretboard (May 31, 2006)

http://mysite.verizon.net/rtellason/manuals.html

This guy says he's got a service manual for the Yamaha in question - might be worth trying to track him down and see if it's got a schematic as well, or if it's a little more basic than that (this is your power switch, this is the cord you plug into the wall, etc...) 

If you read the stuff he's got above the list, it implies that the Yamaha stuff tends to have schematics inside the manuals - although none of the actual Yamaha stuff he lists has "schematic" specifically listed.

Hope it helps. Good luck.


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## nonreverb (Sep 19, 2006)

greco said:


> Thanks nonreverb...appreciate your help and taking the time to teach me.
> 
> Yes, it is an SS amp...should have stated that in my initial post
> 
> ...


The power transistors are going to be the biggest transistors in the amp. They're mounted on a large heat sinc. Usually located at the rear end of the inside of the chassis.
The light is acting as a current limiter. Because a 60 watt bulb requires 1/2 amp @120VAC to fully function, it prevents an over current situation (ie. a SS amp with leaky output transistors. Be warned though, you can still blow the amp this way as well. The bulb is there to indicate that the current draw is not normal. ie. after replacing bad components, if you apply 30 VAC with the Variac to the amp with no input or load and the bulb is glowing at about 1/2 power, you'll know you still have a problem.
SS amps are far more finicky than tube amps and the danger of grenading the entire output section is very real if the circuit is not thoroughly checked for faults during the repair process.


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## greco (Jul 15, 2007)

Thanks *fretboard*...I sent him an email. 

Will see what happens.

Dave


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## greco (Jul 15, 2007)

nonreverb said:


> The power transistors are going to be the biggest transistors in the amp. They're mounted on a large heat sinc. Usually located at the rear end of the inside of the chassis.
> 
> The light is acting as a current limiter. Because a 60 watt bulb requires 1/2 amp @120VAC to fully function, it prevents an over current situation (ie. a SS amp with leaky output transistors. Be warned though, you can still blow the amp this way as well. The bulb is there to indicate that the current draw is not normal. ie. after replacing bad components, if you apply 30 VAC with the Variac to the amp with no input or load and the bulb is glowing at about 1/2 power, you'll know you still have a problem.
> SS amps are far more finicky than tube amps and the danger of grenading the entire output section is very real if the circuit is not thoroughly checked for faults during the repair process.


Thanks nonreverb.

The power transistors you describe were the ones I had in mind. They are on a HUGE heat sink. I need to check them first. If they are blown, I might just stop at that point, as I don't really want to to start putting money and time into this amp...especially given that I really don't know what I'm doing and have no schematic to refer to (not that a schematic would necessarily help me with that many of my endless list of "whys" and "hows" related to testing AND troubleshooting AND repair). 

I might take it as far as I can and then have a look at it with my friend who builds and repairs amps. He might also have some parts on hand.

I understand the current limiter concept now (i.e., using a light bulb) ...thanks for explaining this.

QUOTE _"if you apply 30 VAC with the Variac to the amp with no input or load"_ END OF QUOTE 
Just to clarify, in an SS amp it is not essential to have dummy load on the speaker output when the amp is running (without an input) at full power (i.e., 120 VAC).

I was also surprised to see that the recommended fuses were 250 VAC / 2A...I assume that the fuses can (therefore) handle 4A at 120 VAC . Is this a typical approach ? Any specific reason for using 250 VAC fuses in a 120 VAC circuit?

Sorry for all the questions....I'm just very curious by nature and I enjoy learning.

Thanks again.

Dave


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## nonreverb (Sep 19, 2006)

"Just to clarify, in an SS amp it is not essential to have dummy load on the speaker output when the amp is running (without an input) at full power (i.e., 120 VAC)."

Correct, In some cases it's a good way of telling whether the amp is properly repaired. If the current draw is high with no load then you definitely have a problem.

"I was also surprised to see that the recommended fuses were 250 VAC / 2A...I assume that the fuses can (therefore) handle 4A at 120 VAC . Is this a typical approach ? Any specific reason for using 250 VAC fuses in a 120 VAC circuit?"

Interesting question Dave. It's not the voltage so much as the current. Fuses are made from certain metals that are formed to tolerate a specific current through them. The excessive current flow heats up the metal and when the current rating is crossed, it melts. All HT fuses in high power tube amps are 250V rated and they are taking 450 to 500V and yet they fail the same as an identical fuse in a circuit with 30V. Therefore the voltage really isn't a factor.
An interesting experiment would be to see how much voltage it would take to blow a fuse when the current was below its rating...probably fairly high.

"Sorry for all the questions....I'm just very curious by nature and I enjoy learning."

Hope this helps:smile:


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## Wild Bill (May 3, 2006)

nonreverb said:


> "I was also surprised to see that the recommended fuses were 250 VAC / 2A...I assume that the fuses can (therefore) handle 4A at 120 VAC . Is this a typical approach ? Any specific reason for using 250 VAC fuses in a 120 VAC circuit?"
> 
> Interesting question Dave. It's not the voltage so much as the current. Fuses are made from certain metals that are formed to tolerate a specific current through them. The excessive current flow heats up the metal and when the current rating is crossed, it melts. All HT fuses in high power tube amps are 250V rated and they are taking 450 to 500V and yet they fail the same as an identical fuse in a circuit with 30V. Therefore the voltage really isn't a factor.


Yep, what NR said!:smile:

Actually, the voltage rating is just that, a voltage rating. Car fuses are usually rated at 32 volts. If you use them at high AC voltages you can get sparking inside the fuse.

Power in Watts = I^2 x R, or current squared times the resistance of the fuse element. You can use Ohm's Law to re-arrange the math to have voltage as a factor but you'll find that the resistance and the current are much bigger factors, making the actual voltage "mice nuts"!

:food-smiley-004:


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