I understand your point. I know have better equipment and time and I will apply this advice from now on. Thank you
Scott A436 transistor replacement
- Thread starter Pasagile
- Start date
ohm470k
Active Member
Being as you have a stereo amp, you can just verify the value of each resistor by comparing it with it's brother on the other channel. You don't even have to remove them, just measure in circuit. However, you must make sure that all bad transistors, caps, etc have been replaced to get correct values though. If you have component tester, you can check those odd measuring trannies if you want. At that point though, since they are out, you could just replace them. If you are like many of us, you buy more than you need of a component to save money and to have available in the future.
Won't measuring in circuit would give me some strange mix of all components in the circuit ?
I was thinking about replacing everything indeed. It's just that it brings more questions to me, such as finding a good equivalent for the diff pair, and having to hfe match it or not...
I also need to find candidates to replace these ma-150.
ohm470k
Active Member
That's the thing, a stereo amp/receiver should have 2 identical circuits, one for the left channel and one for the right. Assuming both channels are functional, then when measuring a resistor on one channel, it should have a match on the other channel that should measure the same. You're right in that the value may or may not be the value of only the resistor itself. It will either be that or a combination of whatever is in parallel with it. Still, they should match. If they don't, then one of the channels has an issue either with that resistor something parallel to it. That something could be a cap, diode, transistor, etc. At least that allows you to do many measurements without having to remove every component. It will help to narrow it down to a specific area.
This is assuming that you have one working channel and one not. Though, it can still be useful if both are bad. It will still give you areas to look at. I find this useful when I don't have a schematic for something, regardless if it is an audio device or not. If I have a known good device or PCB, I can compare my bad one to a known good one and then have a place to start.
^^ this, you're just looking to see if its the same as the working channel, not so much what the actual resistor value is. The other trick with resistors is that anything across them will make the measurement read low. If it reads high, it means the resistor is off value or open. I don't believe I've ever had a resistor fail by going low in value, only high / open.
Thank you both. I'll check that once I replace the components. I'll visit a local specialized shop to see what replacements I can find for transistors and diodes (ma150 according to manual; I read 1N4148 as replacement in another thread, but I haven't learned about diodes yet).
If its just a small signal silicon diode, a 1N4148 is a pretty common sub for many of them. A lot of circuits used not-special diodes just because they needed the voltage drop of a silicon junction, which is pretty much the same in any silicon device. They don't see any significant current or voltage. Make sure you check the specs though, not looking at the schematic but fair guess if you meet or exceed the current and PIV numbers it won't matter beyond that.
Where it can get semi-specialized is diodes in the bias circuit that are attached to the heat sink for thermal tracking. Sometimes the 1N4148 or a chain of them will do, sometimes there are better subs.
Where it can get semi-specialized is diodes in the bias circuit that are attached to the heat sink for thermal tracking. Sometimes the 1N4148 or a chain of them will do, sometimes there are better subs.
Okay, so I went through the transistors listed on the SM, the ones actually mounted on the board, and I'll order some replacement from shops or online if they don't have what I need. I've been through a lot of audiokarma, diyaudio, etc, threads for this but I'd like some confirmation if possible...
The list:
- Q801/Q802 - diff pair / LTP (passed the diode test but will still order backups): 2SC1000 BL on SM and board. replacement should be 2SC1815 from what I read, Vbe and hfe matched.
- Q803 - I don't know what this guy does... SM says 2SA777 or 2SA682, found FMPS A56 (isn't it odd?). Kind of clueless about how to replace this one. alltransistors equivalent finder yielded 2N5401.
- Q804 - Vbe multiplier / bias. SM says 2SC9630 or 2SC372, found 2SC9013. Will replace by 2SC372 as I know it is available in my city.
- Q805 and Q808: part of some kind of security system. SM says 2SC372 and 2SA495 compl. pair, I'd take these.
- Q806 and Q809 - complementary drivers in Darlington config: SM says [2SC1567 or 2SC1382] and [2SA764 or 2SA682]. Found FMPS A56 and FMPS A06. Would the 2N5401 and 2N5551 complementary pair work ? Hfe match ? Vbe match ?
- Q807 and Q810 - outputs: SM says [2SC1567 and 2SC793] or [2SC1382 and 2SC1667]. I'm not sure what's on the amp, uploading a photo for posterity. I gathered several candidates for replacement in other threads: 2MJ21194G from the usual replacements thread (https://audiokarma.org/forums/threads/transistor-substitutions.688515/), also read tesla KD503, 2SD188, 2N4915... also, Vbe matched if I remember correctly ?
I really can't wait to finish this job and listen to this amp... But this is an important step so I'm using all my patience to make it correctly!!
The list:
- Q801/Q802 - diff pair / LTP (passed the diode test but will still order backups): 2SC1000 BL on SM and board. replacement should be 2SC1815 from what I read, Vbe and hfe matched.
- Q803 - I don't know what this guy does... SM says 2SA777 or 2SA682, found FMPS A56 (isn't it odd?). Kind of clueless about how to replace this one. alltransistors equivalent finder yielded 2N5401.
- Q804 - Vbe multiplier / bias. SM says 2SC9630 or 2SC372, found 2SC9013. Will replace by 2SC372 as I know it is available in my city.
- Q805 and Q808: part of some kind of security system. SM says 2SC372 and 2SA495 compl. pair, I'd take these.
- Q806 and Q809 - complementary drivers in Darlington config: SM says [2SC1567 or 2SC1382] and [2SA764 or 2SA682]. Found FMPS A56 and FMPS A06. Would the 2N5401 and 2N5551 complementary pair work ? Hfe match ? Vbe match ?
- Q807 and Q810 - outputs: SM says [2SC1567 and 2SC793] or [2SC1382 and 2SC1667]. I'm not sure what's on the amp, uploading a photo for posterity. I gathered several candidates for replacement in other threads: 2MJ21194G from the usual replacements thread (https://audiokarma.org/forums/threads/transistor-substitutions.688515/), also read tesla KD503, 2SD188, 2N4915... also, Vbe matched if I remember correctly ?
I really can't wait to finish this job and listen to this amp... But this is an important step so I'm using all my patience to make it correctly!!
ohm470k
Active Member
Typical value for the MJ21194 was not given in the Onsemi datasheet but the minimum was given as 4MHz. Typical value for the 2SD371 is 8MHz. I think it will be ok. There were about 11 transistors with the same package, same or better Vcb and Vce when I searched Mouser. Do that search and you can view comparisons and make your decision, but I think it will be fine.
You could even go with something like the MJ15015G.
You could even go with something like the MJ15015G.
Alright thank you ! I must have misremembered something I read.
I'd like to have some opinions on the choice of 2N5401 and 2N5551 for the drivers, so I'll wait a bit before ordering.
ohm470k
Active Member
KSC2690AYS and KSA1220A would be better choices. I don't like the lower collector power dissipation of the 2n5401 and 2N5551. KSC2690AYS is also the same package style, not that it necessarily matters.