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2SA725/ 726 - Transistor Question

My significant other has the uncanny ability to hit the call button to the shop intercom at the most inopportune times causing loss of hair and words not to be used on Sundays. Good for getting pulse rate up to zooming levels without hitting the gym too.
 
The only time I have trouble justifying my philosophy of getting those particular transistors out of a circuit (due to the work of getting TO the pesky things versus the possibility of future problems, (and my reputation <grin>)) is when I look at their penetration into the SPEC-1 preamp, and a lack of reported problems with them in that model.
One of my future projects is to do that (total preemptive replacement) to one of my three spec-1's gathering dust waiting for "freshening up" and compare.

hat tip to Lee and blhagstrom for putting to words the things I had in my mind that I was unable to articulate, and agree with wholeheartedly.
 
instead of KSA992, would you agree that 2SA970 is a good substitute to 2SA726? 2SA726 in my 1280, in the flat amplifier - had some probs with them; one channel has slightly more volume than the other. im asking as i have some of them that i can put to good use!
 
markthefixer said:
In the forum, in the past, it has been extensively noted the necessity of certain pre-emptive replacements.
especially the 2sa725, 2sa726, 2sc1451 and all their complements.

Example: sometime a while back someone decided to do a recap - JUST recap, ignoring the parts list that called for those transistors to be replaced, and then came back bitching about how NOTHING changed to fix the sound problems that prompted the recap.
I told him to follow the parts list and do the recommended transistors.
He did.
And he was very happy the next time he was on with how great the improvement in the sound was.

Those transistors were made as they were straightening out the technology, and while not EVERY batch showed faults THEN, the faults have had 30 to 40 years of aging to do their dirty work.

NOW, the replacements are the product of much more mature and controlled technology, with DECADES of feedback experience with regards to aging faults so they are a far superior part.

Impulse noise from breakdowns (static and pops) are the LAST symptoms to occur and doesn't mean that the transistor was perfect until the popping or noise started, the prior symptoms and problems just weren't enough to bring the circuit down.
Click to expand...
Yes! A retired radio tech friend of mine told me, "transistors now days are light years ahead in technology then they were back in the 70's"

I've also seen some of these old transistors test OK with a DVOM but sound terrible in service.

Bob
 
cbusuttil said:
instead of KSA992, would you agree that 2SA970 is a good substitute to 2SA726? 2SA726 in my 1280, in the flat amplifier - had some probs with them; one channel has slightly more volume than the other. im asking as i have some of them that i can put to good use!
Click to expand...

After some investigation, I can find no objections to that substitution. I MAY have even seen some of them in some Later NSA amps, they have smaller than to-92 packages (heat!!) but the 1280 doesn't dissipate a lot of heat there.
 
Still though, you will be able to hear the difference between the 970 and the 992 when used in the flat amp. Both safe to use but a matter of choice to a trained ear.
 
I wrote a reply in another thread re these same transistors in the phono stage of an sx850. I just bought a few of the 992s from Mouser and replaced the two on the phono board, which had become quite noisy. The 1845s were backordered, so they will come later. For now, I can enjoy the receiver in the garage. It sounds great. Thanks to those of you who provide so much great info and expert insight.
 
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Is there smth new?

It's hard to find replacement due to high hfe and ic parameters.
There are many transistors with higher voltages, frequences, but one of them having lower hfe, others having low ic.

The KSA992 having less ic (0.05A vs 0.1) and less ft (50MHz vs 100).
The 2SA970 having less Tj (150°C vs 175) but I guess it's not such important. In other parameters it's better and having good graphs.

The 2SA725 is marked as the Q2 in my Pioneer M-22 and having black legs so I wish to replace it (as the others black-legged). I guess it passes high currents through itself so I'm fear to place the KSA922 with x2 less ic.
 
iPhoneSasuke said:
I guess it passes high currents through itself
Click to expand...

I'm not sure why you would conclude this, but judging by the number of posts, it is clear that you are concerned. Perhaps this will help.

In many circuits, it is possible to make a rough estimate of the maximum collector current that the transistor could ever pass (assuming there are no extraordinary fault conditions). For example, Q2 in an M-22 is in a part of circuitry that has a positive power supply with voltage set by a 22V Zener diode, and a negative power supply with voltage given on the schematic as -36V. The difference is 58V. The schematic also shows that any current passing through Q2 (in the direction toward the negative power supply) must also pass through an 18K resistor. Therefore the maximum current through Q2 is about 3mA, regardless of the operating conditions of Q2 or the other transistors in that part of the circuit. Typical current would be even less. (I think the schematic indicates what the typical current is, but legibility is poor in the copy that I have. I think it says 1.2mA, which is about what I would expect.)

So, there is no reason to be concerned with KSA992's Ic rating in this circuit. It also seems mentioning that the KSA992 datasheet indicates a typical fT of 100MHz, but in the case of Q2 I don't think fT is of any importance at all.
 
chazix said:
Q2 in an M-22 is in a part of circuitry that has a positive power supply with voltage set by a 22V Zener diode, and a negative power supply with voltage given on the schematic as -36V. The difference is 58V. The schematic also shows that any current passing through Q2 (in the direction toward the negative power supply) must also pass through an 18K resistor. Therefore the maximum current through Q2 is about 3mA, regardless of the operating conditions of Q2 or the other transistors in that part of the circuit. Typical current would be even less
Click to expand...
Hi, chazix! Thank you very much for helping. I'm not an engineer (your cap) and it's hard to cover all other information and to compilate it to something useful. You done it well. I've sould use the Ohm law to calculate it by myself. Well, now it's clear that 50 mA ic usually is more than enough and making searching range wider. Could you, please, explain one more thing? Let's compare the hfe graphs of the KSA922 and KSA1381 (for example): we're know that our operating range is about 1.2mA; if we'll compare the hfe graphs, we'll se that 992 having much more high hfe but on the edge of 1mA it's beginning to decrease; on other side the 1381 having much less hfe but it absolutely stable in all possible range - so what is more important, the absolute hfe or its stability?

One more question. Lets suppose that we're replaced first transistors by which having high hfe (400+). Is at acceptable to use further by the circuit the transistors with low hfe (~150)? It's clear that complementary transistors are should be matched by hfe but what during to junction to other cascades? Shall I match by hfe only the complementary transistors or to each other in the amp?

Please look also at 2SA1163 datasheet (SMD). Do you have any guesses why does his complementary 2SC2713-BL could sound better than the classic 1814 which is complementary to the classic 992? Hfe parameter is about equal.
 

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@iPhoneSasuke, I am sorry, but I don't think I understand your questions. And I think they are probably beyond my ability to answer, if I did understand them. If you want to pursue your questions, I think it might be best to start a new topic in the Solid State sub-forum.

Some of your concerns seem to be for circuits other than the Pioneer M-22 Q2 circuit, but if Q2 is the main interest, I will add some comments about how I think Q2 operates. It might make it more simple to select a replacement. First, I should say that I am assuming that Q2 is the part with that label on the full schematic in the M-22 service manual that is available on elektrotanya.

In that manual's circuit description, in the section about Overheating Detection Circuit, the part labeled Q2 in the full schematic is instead called Q9. (A serious editing failure, in my opinion!) According to the circuit description, in normal operation the transistor operates as a constant current source for the power amp input stage, and that is what I thought when I first looked at the full schematic. But as I study the schematic, and the voltages indicated on the schematic, I don't think Q2 is a constant current source, in the usual sense. I think it is essentially a switch, and I think in normal operation, Q2 operates in "saturation". That is, its base current is sufficient to make the collector essentially a short circuit to the emitter - the collector-to-emitter voltage will be as small as possible.

In normal operation, I think the base current in Q2 is determined by about 21V across a 33K resistor (R6, if I am reading the schematic correctly). So, about 0.6mA. I think this value is constant during normal operation.

If I am right, the main requirement for Q2 is that it be able to saturate when its base current is about 0.6mA, and when its collector current is about 1.2mA, with these values being nearly constant in normal operation. I don't think Q2 can ever see a collector voltage more than about -22V relative to the other terminals. I don't think fT matters at all. I am not sure whether there can be any importance in its being a "low noise" part (but my guess is that noise rating is not important for a transistor operating in saturation).

These are not stringent requirements :). I think it would be very difficult to find a currently-produced PNP transistor that would NOT fulfill the requirements for Q2.

I apologize to you, and all of AK, for my talking like more of an expert than I actually am. But it was fun :).
 
I know this is an old thread, but I stumbled on it & have an experience to share. I've always taken the advice of the more experienced techs here & replaced the "known bad" transistors when I find them. My recent experience confirms why.
I sold a kenwood amp (completely restored) to a gentleman that contacted me about a month later saying it had issues with a noisy/bad channel. I told him to bring it back & I'd fix it. Long story shortened, it turns out the problem was in his Pioneer TX-9500.....right channel was just crackling static. I traced it down to a bad 2sa725 in the output section....replaced both chan's with ksa992's & all was good. I checked the suspected unit with a cheapy tester & it tested as a "jfet"......weird. I did it a couple times just to confirm. Later on I looked at it again to compare it with the other one that was still working. I straightened the leads on both & ......voila, they both tested good. So just the act of moving the legs around cleared it's issue. I can only imagine how many techs were driven crazy over the years with intermittent issues.
 
I think these run hot as well--I have seen a lot of 2SA725/6 and 2SC1312/13 that have the letters worn off, I am assuming that's due to heat.
 
They don't run hot, it's just the printing wearing off with age, dust. Must have had silver plated the leads for them to go black. All are tin plated these days. Mine are still running fine in my Sx-950.
They laser print today, never wears off just a pain to inspect, the onsemi parts, ksa992 etc, the JCC assembly line, the font is so small, need strong light at the right angle, proper reflection.
 
I have seen the effect of what eastpoint calls running hot. Sometimes one 725 has the letters faded off, while the other channels 725 in the same spot does not have letters faded off. It really does look heat caused, but I wonder now if the heat was from a neighboring component, and the 725 (or whatever 70s transistor) is just susceptible to heat damage.
 
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