Need Advice on Transistor Replacement - Page 1
Forum section: electronics / beginners

author: Grimmer04
author_rank: Newbie
timestamp: February 07, 2025, 11:10:12 pm
content: Working on repair of my vintage cassette player. I found that the two power transistors 2SA885-S and complementary 2SAC1846-R were open base to emitter. I believe these are part of the power supply circuitry. Anyway, I got lucky and found another pair in a parts doner cassette player. However, they do have 1 difference. The last letter (which I believe delineates the forward current transfer ratio). The doner parts are 2SA885-R and 2SAC1846-8. Does this matter much? Can I use these or do I need to find a modern equivalent?Link to the datasheet -https://www.alldatasheet.com/datasheet-pdf/pdf/107604/PANASONIC/2SA885.htmlThanks,

author: benj38
author_rank: Frequent Contributor
timestamp: February 07, 2025, 11:38:16 pm
content: Well, a lower beta may or may not be a problem, depending on the exact circuit design. However, the chance of that being the case is not high in my opinion.I guess you have 3 options:Just put the parts you have in and see what happensFind (or trace) the circuit diagram, and post it here so we can analyze itLook for a more exact replacementIf I were you, I would start with option #1, but I like to live dangerously

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: February 08, 2025, 03:37:33 pm
content: I think you have to make the replacement. If the cassette player plays, there should not be a problem.

author: Grimmer04
author_rank: Newbie
timestamp: February 08, 2025, 05:59:17 pm
content: I'll probably go option #1 but because I am new at this, I would like to understand more of what these transistors do in the circuit. They are part of the power supply for sure as they come right after rectification. I understand how they would work in an amplifier circuit but don't understand what they do in a power circuit. Attached is the schematic any info would be appreciated.Thanks,

author: Grandchuck
author_rank: Super Contributor
timestamp: February 08, 2025, 06:54:55 pm
content: They regulate by changing resistance as needed to keep the out voltage constant. They are called series pass transistors.

author: floobydust
author_rank: Super Contributor
timestamp: February 08, 2025, 07:11:53 pm
content: For replacement parts, their package (TO-126) is not popular and you are fairly limited for replacements especially Japanese semi's nowadays.What heatsinking do these parts have? Or are they just in the open air?Consider easy to get, non-Japan parts such as BD135, BD139 and PNP complement BD136, BD140 (with -16 suffix for the higher hFE class). Or 2SD882 (PNP 2SB772) is popular and easy to get as well.What I will do is put in larger TO-220 sized parts (if the originals are in open air) despite their pinout being flipped, so the part has to be put in backwards. The tab alone is a decent heatsink and they run much cooler than if the originals were in free air.It's a bit odd that both transistors failed open EB junction.The circuit is not all easy either because it is using only one zener diode, to save money, yet it regulates two rails.Do check zener D102 is not also dead (open-circuit). You should get at least a diode-test reading OK on it in the forward direction. I think D102 is MAZ1220/1N5251B/BZX55C22 etc. 22V 500mW.Each transistor is part of a voltage regulator circuit. They take the constant zener diode voltage presented at their base, and output that (minus 0.7V VBE) on their emitter at higher current than a zener alone can cope with.For the +ve rail, Q102 gets a nice steady 11.5V on its base (from D102) and the transistor outputs 10.8V at the emitter, even though the input voltage moves around 13.1V unregulated DC coming in.So the circuit seems to be regulating that both rails are always about 21.3V apart (+10.8V and -10.5V shown on schematic). The 22V zener is a ballpark rating.

author: benj38
author_rank: Frequent Contributor
timestamp: February 08, 2025, 07:24:09 pm
content: To add a bit to @Grandchuck's answer: you have two simple Zener-transistor voltage regulators. Q102 is the pass transistor for the positive voltage regulator, and Q103 for the negative voltage regulator.Quote from: Grimmer04 on February 08, 2025, 05:59:17 pmThese pass transistors simply act as DC emitter follower amplifiers, providing current gain. Another way to think about it is that the transistorbuffersthe Zener shunt regulator. The emitter follower has a high input impedance and a low output impedance, and thus the Zener shunt regulator sees a very light current load.Most importantly, the lower hFE of the replacement transistors you have will make no practical difference in this case.

author: benj38
author_rank: Frequent Contributor
timestamp: February 08, 2025, 07:31:47 pm
content: @Grimmer04, I would just comment that when I wrote "simple Zener-transistor voltage regulators" in my previous post I was trying to make you see the basic overall design.As @floobydust wrote, the two regulators share a single Zener, so the situation is a bit more interesting, but the basic idea behind the transistors roles remains.

author: Grimmer04
author_rank: Newbie
timestamp: February 08, 2025, 10:26:39 pm
content: Thank you for the explanation and advise. Checking that zener diode now. The caps were also leaking a bit, they all test with high ESR so they are getting replaced as well.That zener is open. Back to Digi-Key LOL