Replacing The STV-3H and -4H Diodes

[Oilmaster]

In theory that could work, but then 3 surface mount 1N4148 in series would do as well; there is no added value to use 3 diode-connected transistors compared to that

The thing is to have a heat transfer from heatsink (or much better: directly at the output transmitter) without a heat resistance path.
Soldering 3 devices together and then encapsulating them in a non-conducting way unto a sort of holder does mean a (significant) heat resistive path, which translates into a temperature sensing delay, which translates into a swinging bias voltage across the output devices.

Depending on the amp circuit and the way it is used (power demanded), this could result into a situation where the thermal runaway of the output transistors is faster than the bias temperature sensor (i.e. the 3 encapsulated devices) can compensate (which means....smoke). And when the power demand is over, the cool down response is delayed in the same fashion up to a point that not enough bias voltage may be supplied.

This is why a flat-faced bias transistor directly mounted under the mounting screw of one of the output device (of the associated channel) is the most proper and most common execution.
Also because only one PN junction needs to warm up / cool down, hence speeding up the compensation process with a factor equal to the bias spreader multiplication factor.
In case of 3 devices in series, in a thermal resistive path, the speed is reduced by the slowness of each PN junction to follow the temperature.

 

[Ray Brack]

It does work, and the temperature curve more closely matches the transistors. I'm sure that is how the STV diodes are made. The idea has been used since OP Amp ICs have been around. The Multiplier circuit works fine also, but there is the resistor current to worry about. As for temperature gradient problems, the heat sink should be large enough to stop most of that. The ones I made with the solder lugs work fine, no problems at all. This photo is of the first ones I made, TO-220 size. Thought one was bad, kept blowing out transistors in one channel, so made the solder lug ones. Turned out to be bad connection from front to back of board. This is FET, but it takes STV-3 type of diode to compensate. If you really want to get rid of some temperature problems, replace that Mica insulator with the Anodized Aluminum ones.

 

[Oilmaster]

It looks like you have a project going that is a bit out of context of this thread.
It's better to first fully tell/explain what your thing is about and then ask the the relevant question.

 

[Scott Muni]

Xraytony just made a vid about repairing STV's. He used a SMD and a home made mount.

 

[cferry1]

Xraytony just made a vid about repairing STV's. He used a SMD and a home made mount.

WONDERFUL ! This deserves credit !
Much credit should also be given to a early invisionary on this topic , member " pa_200".
I PM messaged him once and it was delightful informative.
One of my posts here was Oct. 04, 2017.

 

[SteveAndBell]

Great thread and I've just ordered a handful of CMXSTB400s to experiment with... however I'm yet to start work on a Pioneer Receiver containing a couple of original working STVs and I'm wondering if there's any DIY methods AKers use to try to prevent damage to the originals? Maybe I could carefully solder new flexible wires onto the leads of the original STVs in situ right up near the case and then carefully/neatly cover that in epoxy then unscrew/remove/handle the devices only once cured. Anyone done anything similar to proactively avoid damage to the original devices?

 

[elnaldo]

1st time using one transistor and 2 resistors (BC550 + 3K3 and 1K ohm) to replace a SV04 in a Sony TA-1630 amplifier.

Until now, I used to replace it with 3x 1N4148

 

[elnaldo]

Great thread and I've just ordered a handful of CMXSTB400s to experiment with... however I'm yet to start work on a Pioneer Receiver containing a couple of original working STVs and I'm wondering if there's any DIY methods AKers use to try to prevent damage to the originals? Maybe I could carefully solder new flexible wires onto the leads of the original STVs in situ right up near the case and then carefully/neatly cover that in epoxy then unscrew/remove/handle the devices only once cured. Anyone done anything similar to proactively avoid damage to the original devices?

The only thing I've done is to treat them with care. But adding some epoxy near the diode body is not a bad idea... Soldering, cutting , and so, in place, could damage them I wouldn't handle them too much "in situ". From your ideas, I'd add some 10 min epoxy to the original leads before handling the wires.

 

[SteveAndBell]

Soldering, cutting , and so, in place, could damage them I wouldn't handle them too much "in situ".

Cheers @elnaldo. So the only weak point is where the leads enter the casing... the leads themselves are OK?

 

[elnaldo]

Cheers @elnaldo. So the only weak point is where the leads enter the casing... the leads themselves are OK?

They usually break flush just at the diode body when you move them while working on the amp, giving little o no chance of soldering a new lead. I think if you care of that junction, the rest of the part is OK. And even if the lead breaks at other points you could solder a wire or new lead.

 

[rcs16]

EVerytiime I see a stv diode I get the glue out to secure the leads at the body
If you are going to use a bjt as your sense element, use a 100nF mlcc across the base/collector

 

[Ingy76]

Eerste keer gebruik van een transistor en 2 weerstanden (BC550 + 3K3 en 1K ohm) om een SV04 in een Sony TA-1630 versterker te vervangen.

Tot nu toe verving ik het door 3x 1N4148
View attachment 2449218 View attachment 2449219

Thank you for this information and photo's.
Can you explain how you attached/secured the BC550 to the connector (at the output transistor)?
Is there also thermal compound between the connector and BC550?

 

[elnaldo]

Thank you for this information and photo's.
Can you explain how you attached/secured the BC550 to the connector (at the output transistor)?
Is there also thermal compound between the connector and BC550?

The transistor is hold in place by bending the connector metal tabs, and I've added a bit of "super glue" (cyanoacrylate). It has some thermal compound between the transistor and the metal connector.

 

[elnaldo]

EVerytiime I see a stv diode I get the glue out to secure the leads at the body
If you are going to use a bjt as your sense element, use a 100nF mlcc across the base/collector

what happens if I don't? I didn't add that capacitor to the network, just copied the circuit suggested by Sony.

 

[ring]

moderator edit: links to eBay auctions not allowed as per AK rules

Has anybody tried these? I wonder how they measure in action.

(sorry for violation)

The item being sold is described as STV3H-Y / STV3H / STV-3HY" and has the correct physical form for easy installation.

 

[Scott Muni]

moderator edit: links to eBay auctions not allowed as per AK rules

Has anybody tried these? I wonder how they measure in action.

No, but I used these on my 9090db. Looked to be legit & pricey, measured perfect.


moderator edit: links to eBay auctions not allowed as per AK rules

 

[pac1085]

I wanted to add some data to this thread. The Marantz 4230 uses a diode package the manual says is "SV-3X". I think a lot of people confuse it with the STV-3 Diodes and refer others to this thread. I have used 2x 1n4148 in its place when they break.

I have one here and tested it on the curve tracer (tektronix 575, but cross-referenced the 2mA, 5mA and 10mA readings with my Peak Zen50) so there is some data to compare it to the chart in the first post:

500ua - 1.25v
1mA - 1.29v
2mA - 1.33v
5mA - 1.39v
10mA - 1.43v
20mA - 1.49v

Here is a picture:

 

[KKnight]

Great thread that has been going on for a loooong time.
What I dont get is why someone(s) like old Delta9Electronics does making
replacements for the STK-0050, will just make and sell a decent replacement
for these STV-3H and -4H Diodes like all of the homebrew examples show
here for over a decade.

 

[rcs16]

Got a chuckle out of that one. Probably because old Delta9Electronics has more important things to do with their time
considered it but it’s such a low volume item and low on the priorities. stv diodes are reliable but they fail usually because someone mishandles them and break the wires or mounting ears. So it’s a back burner design in the queue at the moment. It’s more of a mechanical/fab issue than a electrical design issue. Just like other stuff in the design queue, big ecap adapters, relay adapters, you name it, a long list of ideas that need work.
In the new awh-048 design ( power amp for sx-1250) it uses a dual Vbe multiplier design to replace the stv’s

 

[JoseHH]

^^ exactly. I agree with rcs16.

I have seen STV-4 diodes that apparently have shorted junctions internally. I know this because they had a forward voltage that was only 1.2V at 7mA instead of the expected 2.1V. If this failure happens in an amplifier it will mess up the bias current for sure.

I think it would be best to use smd diodes to replace these. There are many SOT23 packages that contain 2 signal diodes in series. Two packages mounted close togeter in a small addaptor board and some thermal glue could do a very good substitute in my opinion.