STV-4H AND STV-3H diodes array

[ostie01]

Hi, On my schematic, in the bias section, there are those two diodes array. The STV-4H are represented by 4 diodes and the STV-3H is represented by 3 diodes. Is it really the case, if yes, what kind of diodes could have approx. the same temp variation. Or could it be replace with another thermistor having a PTC (positive temperature coefficient) that could be glue to the heatsink. If I replace them with standard diode like 4148 or 914, do I have to put 4 and 3 of them in serie. Thanks for any advice.

[EchoWars]

What happened to the originals?

Look around...you can find the STV-3H diodes fairly easily. The STV-4H's are a totally different story.

[ostie01]

Hi, thanks for the reply, well this amp is near 30 years old so you can imagine. The wires connected to those diodes are single solid wire so as soon as you touch it, they almost break everytime. They are from the bias section in a Pioneer Spec 2. I have trouble adjusting the bias as per the service manual (30mV) . Fully clockwise the pot give me only 15mV to 20mV max. at TP. I have succes restoring them by grinding off some material with a dremel to get leads long enough to put a very small connector and glue it in place with epoxy.Still trying with the bias, they made so many errors when they print the circuit board, I must check for polarity every time a put back a component .

[nunes]

Hello guy I want Pioneer SPEC-2 Power Amp Owner’s Manual/Schematic for I fixed it. Can some guy shown me for Pioneer SPEC-2 Power Amp ’s Schematic ?

[unclejed613]

1N914's or 1N4148's stacked and put together in thermally conductive epoxy should be a good replacement. for the STV-3 use 3 914's in series, for the STV-4, use 4 of them. lay them out flat, side by side on a flattened STV-3 clip, and cover them with thermal epoxy and let it set. the 4 junction stack should take care of your current being on the shy side, since it gives you another 0.7V diode drop across your driver bases for bias. or you can do what Peavey and other manufacturers have done, and goop the diodes with heat sink grease and somehow set them in physical contact with the heatsink (the best being to use a screw and transistor spring clip, just don't overtighten and crush the glass diode bodies). Peavey actually runs the diodes through holes in the heatsink, and gooped with HS grease. but if you want to get as close as possible to original, making your own diode stacks in epoxy is probably the best.

[saj888]

I have used 4148 coated in epoxy for years. Those amps are still functioning.

[saj888]

I have used 4148 coated in epoxy for years. Those amps are still functioning.
Does anyone have a spec 4 schematic?
Thanks.

[saj888]

I will be posting the spec 2 and others after the 5th of feb.
Presently I need a spec 2 schematic.
Thanks.

[akshay]

Hi, I have a pioneer MA22 which seems to have the stv-4h. I had replaced it with 4 1N4148 but without any epoxy. I had checked the forward voltage of each diode with a forward current of 2mA and found the closest pairs so that I got a total of 2.35 forward voltage drop when all 4 are connected in series. But the problem that I face is that the bias voltage starts fluctuating after some time after turn on. It is a class A amp and the heatsinks do heat a whole lot. (around 80 degrees, cant touch it after 15mins). Any leads on the fluctuating bias voltage. Thanks in advance.

Regards.

[unclejed613]

did you use epoxy? if so, do you know if it's thermally conductive?
check the soldering on the diode leads (which you can't do once they're epoxied).