2SD2385 + 2SB1556 Transistors

What was the base-to-emitter voltage on the questionable transistors, when the amp is idling? How does it compare to the working parts?

These parts (when genuine) seem to be Darlingstons, not just single transistors. You may have been sold a pair of mislabeled standard transistors instead - this would probably result in over-biasing and a high idle current.

Or, possibly they're genuine parts, but have a higher beta than the ones for which the amp was spec'ed? The beta bin would be a one-character A, B, or C, possibly stamped on the lower left corner of the plastic. If that's the case, they might work OK with some adjustment to the idle/bias settings.

TIP142/TIP147 *might* be acceptable substitutes although their gain seems lower.

--
Dave Platt                                    AE6EO 
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"Dave Platt"

** For sure.

** Nonsense.

Output transistors are typically biased with a voltage that varies with the temperature of the heatsink.

** The fact they have a Vce of 100V as compared to the originals with 140V is far more of a worry.

The BDV65C and BDV64C darlington pair is closer at 120V Vce.

The OP can measure the +/- DC rails see if the total is more than 120V.

.... Phil

The design uses an STK350-030 ic to drive the output transistors. According to the service manual, at idle, the output from the driver ic to each transistor base should be around 1.2 volts. These new ones are causing that voltage to drop to around 0.6 - 0.8v, and this is with the bias adjustment full up.

I tried using one of the new ones with an old complimentary one from the good side and this caused a massive imbalance in the idle bias 1.4v for the good one and 0.6 for the suspect one. Can i assume from these voltages that the replacement's are not darlingtons? 0.6v is right for a single transistor and 1.2v (approximately double) for a darlington?

Regards

--
fruittool

Any external giveaways as yo piracy?

ground surface ? smudged printing or looks as though logo was stamped with a potatoe die?

only carries genuine semiconductor manufacturer's lines. Chuck

It sounds as if these aren't Darlingtons.

Do you have a way to measure the current gain? It should be at least 1000 -- probably higher.

Pull one of the new transistors and check it with an ohm meter. If it tests like a single transistor, it is.

I had this same concern, each rail is 52V. Your suggested parts seem closer but, would i have to change both pairs to keep the response of both channels identical?

N_Cook;2949784 Wrote:

On first examination there's nothing particularly suspect but, after checking what the proper markings would be, it turns out the gain classification (A, B or C) is missing on one and is the letter 'O' on the other. Another giveaway is the LOT number, it should be one number for the last digit of the year and one letter 'A' for January to 'L' for December. Both have three numbers and no letters.

Regards

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fruittool

I assume you're referring to the base-emitter junction.

"fruittool" schreef in bericht news: snipped-for-privacy@diybanter.com...

No doubt you were sent single transistors instead of Darlingtons. The latter have two diode drops Vbe so 1.2V-1.4V instead of 0.6V-0.7V. If you did not find out already you could check using the diode tester of a common unimeter.

petrus bitbyter

"fruittool"

** LOL - err, no.

You really are a babe in the woods.....

.... Phil

"fruittool"

** That will only make the transistors you have get very hot. ** Yep.

Non darlington devices in that amp will not only run hot but deliver much less that full output power into a load.

The fact is you simply have NO driver transistors unless darlingtons are fitted.

.... Phil

"William Sommerwanker"

** Darlington power transistors all have inbuilt diodes from collector to emitter - a very rare thing among regular transistors.

The base emitter junction will also read higher than usual on an ohmmeter, the difference being more obvious with an analogue type on the ohms x1 range than with a DMM on "diode test".

.... Phil

"William Sommerwanker"

** Most transistor checkers will not read correctly with power darlingtons.

... Phil

off ebay is a good clue they're not legit.

Particularly if they ship from China, Hong Kong etc.

Gareth.

"Gareth Magennis"

** The only offer I see on Ebay for the 2SD2385/2SB1556 pair is from a UK dealer called "elmassari" in Surrey.

He has a big range of spares for sale at low prices and some of it must be fakes.

.... Phil

That's the one but the package is labeled 'ELECTRONIC COMPONENTS, SEMICONDUCTORS, PARTS, TRANSISTORS, INVERTERS, TRANSFORMERS | DALBANI'

and 'electronic component suppliers and distributors, Nikko Electronics for semiconductors, transistors, capacitors all major brands of spare and replacement parts'

I found 'CHS - replacement remote controls, instruction manuals, audio & video accessories, electronic components & spare parts'

to have what i need. Anybody know anything about them?

'2Sd2385'

and '2Sb1556'

To me, the pictures look genuine, the site looks a reasonable quality and has a uk landline number unlike the other site with it's 020 number.

What do you guys think?

Thanks, Ant

--
fruittool

Yes, Charles Hyde, I seem to recall, used to sell Tascam and other spares - I bought some from them. They were always ridiculously expensive, but they would definitely not knowingly be dealing with pirated parts.

Gareth.