Testing varistor

[Prattacaster]

I have a single STV-3H varistor in my SS amp. Transistors that are before and after it in the circuit are blown(which may not mean anything but I thought I would include anyway). The varistor reads open in one direction and roughly 11megaohms in the typical "voltage drop" direction. These varistors seem impossible to find, what is a suitable replacement? I've heard a string of common 1n4148's in series might work, but I need to be 100% positive.

[Jazz P Bass]

The STV-3H is a string of (3) diodes in one package.
They are typically used in the bias string, mounted to the heat sink (this is important), to sense output transistor temperature & adjust the bias accordingly.
It can be tested as any other diode.
Yours sounds like it tests fine.
A meter with "Diode Check" would indicate the FW drop.
The datasheet indicates a forward voltage of 1.6 to 1.8 V.
Yes you can use 2 or 3 diodes to accomplish the same thing.
UF4004 is a fast diode that would work.
I have seen where others have used 1N4148's.

[R.G.]

I'd use a string of 1N4000 series (4004...4007) diodes for this instead of a 1N4148. The 4148 is only good to 100ma, the 4000 series to 1A.

SS amps often have a chain of destruction on output stage failure. It is **possible** that one of the diodes inside here opened up and that all by itself could have destroyed the amp, as this lets the output devices all conduct simultaneously. Other things are possible too.

Not knowing the amp's schematic I can't advise on how the replacements should be mounted. Some amps need the diodes mounted on the output heat sink to prevent thermal runaway, some don't.

[Jazz P Bass]

So yeah, that does beg the question "what amplifier are we talking about?".
Here are the specifications on the SVT-3H
[STV3H
Forward Reference Diode - VF 1.60V-1.80V at 7.0mA,Tstg -60 to 100C.
Sanken Electric
Diodes
I(FSM) Max.(A) Pk.Fwd.Sur.Cur.=10
V(FM) Max.(V) Forward Voltage=1.8
@I(FM) (A) (Test Condition)=7.0m
I(RM) Max.(A) Reverse Current=10u
@V(R) (V)(Test Condition)=50
Semiconductor Material=Silicon
Status=Discontinued
Package=TO-92]

[Prattacaster]

Big thanks to both R.G. and Jazz P Bass. The amp is a Yamaha G100. On more thing, the varistor tested OL either way, I think this is because the diode drop function on my DMM only reads up to .999v drop.The Varistor was mounted on the output transistor heatsink, so I am going to mount the diode string there as well. Here is a schematic. I've been working on this amp, and all but one transistor on the output board blew. I dont want to take the chance of a bad varistor when I'm spending $80 dollars on transistors and caps. Here is a schematic: The varistor is the diode symbol on the right side(also marked T5 51). Yamaha :: Yamaha_G100_series_I2.jpg picture by Prattacaster - Photobucket.
What is the simpliest/cost effective method of mounting the diode string?

[R.G.]

Thanks for the pointer to the schemo. It helps.

You can bring that up by replacing the diode string with piece of wire. What it does is set the idling current for minimum crossover distortion.

When everything else works, put it in and test for idle current and lack of crossover distortion.

*** Make and use a light bulb limiter if you have the AC wiring skills to do it safely.*** This will let you bring it up without taking the chance that a mistake will cause all that new silicon to vanish in a few milliseconds.

[R.G.]

Forgot to mention: if it were me, I'd replace the diodes with a Vbe multiplier, using a single transistor to replace the string of diodes. The existing adjustment pot can be rewired to let you dial in any bias voltage you like. And you can use a four-diode string as a sudden-death preventer in case the Vbe multiplier goes on the fritz.

... and you could add current limiters and V-I protection circuitry with small bit of perfboard.

I guess I need not to get started. 8-)

[Mark Black]

And just to be clear (and educational for future reference) that is not a varistor...

Metal Oxide Varistor

[Prattacaster]

Thanks Mark Black. It seems like the STV-3H is actually a diode? Its funny that yamaha gave it a diode symbol on the scheme but called it a varistor on the parts list. Maybe it got lost in translation.
R.G. - I am repairing this amp for a guy at work, this is my first SS repair, I normally only repair tube amps, anyway, the guy is immediately going to trade it for a smaller amp, so, I just need it functioning properly for the lowest cost, I appreciate the idea of using the Vbe multiplier, if it were my amp I certainly would. I think I will do the least time consuming repair and install a string of maybe 1n4004 diodes. It looks like 3 in series would be good but not perfect?
You say I should wire a jumper in place of the diode string when I first power up?
The Service manual says I need to adjust the waveform, center voltage, and idle current, how do i do this without an oscillator?

[Jazz P Bass]

Thanks Mark Black. It seems like the STV-3H is actually a diode? Its funny that yamaha gave it a diode symbol on the scheme but called it a varistor on the parts list. Maybe it got lost in translation.

It is a string of diodes. beleive it.


The Service manual says I need to adjust the waveform, center voltage, and idle current, how do i do this without an oscillator?

You need a sine wave input to set the waveform.
That is not all that critical.
The bias voltage (idle current setting) can be checked with no signal.

[Prattacaster]

SO for the waveform and center voltage adjustment should I just set their respective trimmers in the center and go from there? I suspect if it is clipping or sounding bad I could adjust for best sound correct?
For the bias adjustment...How much current is optimum, Is that something a gather from the output transistor datasheet, or from the driver datasheet, and on this amp where do I measure the current, these arent question I have with tube amps, I'm a newbie to SS. The schematic is my the 2nd post. Thanks guys.

[Jazz P Bass]

My service manual shows 3 adjustment pots.
Step #1: VR3: Waveform Adjustment: Set Symetrical roundness at top & bottom of waveform.
(requires a signal generator & a scope)
(or a good ear)
Step #2: VR2: Center Voltage Adjustment: 30Vdc @ +CO terminal
Step #3: VR1: idle adjustment: 10mv/ no signal. Across TP terminals

[Prattacaster]

Hey jazz P , big thanks!