There are many devices that would undoubtedly be OK, its a case of what you can easily get.
I'd be tempted to use TIP41C for the outputs and MJE340 and 350 for the drivers.
I'd be tempted to use TIP41C for the outputs and MJE340 and 350 for the drivers.
Well spotted. Yes it does look wrong even given the unconventional Vbe multiplier. To many 'diode drops' to overcome (four including the Vbe generator). The top half of the output stage wouldn't be current limited.
SCOTT A426
Dear friends,
I was busy, I couldn't share. a nice experience to you; BD243C is good for output. I used BC140 & 141 in the driver transistor part. It works well, the transmitter resistance should be updated with 0.25 ohm resistance.
Good luck
Dear friends,
I was busy, I couldn't share. a nice experience to you; BD243C is good for output. I used BC140 & 141 in the driver transistor part. It works well, the transmitter resistance should be updated with 0.25 ohm resistance.
Good luck
Dear friends,
I was busy, I couldn't share. a nice experience to you; BD243C is good for output. I used BC140 & 141 in the driver transistor part. It works well, the transmitter resistance should be updated with 0.25 ohm resistance.
Good luck
I was busy, I couldn't share. a nice experience to you; BD243C is good for output. I used BC140 & 141 in the driver transistor part. It works well, the transmitter resistance should be updated with 0.25 ohm resistance.
Good luck
Hi everybody,
I'm resurrecting this old thread cause I have a similar issue with a 1976 Scott A426.
My profile is also similar to the one of the author of the thread, I used to be an audio electronics hobbyst in the mid 70s, then I quit at the beginning of the 80s. Now I’m trying to take up again, possibly with a better knowledge, also thanks to the Internet.
Here’s my problem. The amplifier has a dead channel (left) and the other one only outputs a 50 Hz noise, no audio signal.
When the right ch. speaker is connected, the bulb tester lights strongly. On the phones out, left ch. is ok while the right one has the 50Hz noise)
I tested all the transistor in the power amp section (off the pcb) with a China component tester (T7) and all the transistors are recognized as good, but some of them show a wrong pinout (the tester shows base in place of the emitter, or collector in place of the base etc.).
All the other components on both channels look OK.
I came to the conclusion that the “wrong pinout” ones are damaged (It also happened with one of the rectified diodes, anode and cathode resulted inverted, though the tester recognized it as a diode). It turned out that the diode was damaged.
Here are the part numbers, according to the schematics:
Q806; Q816 = NPN 1860 .C1166
Q809; Q819 = PNP 1861 .A661
My problem is: the suspected transistors are nowhere to be found, so I would ask the post’s author: What did you replace the damaged transistors with?
@trtuncay: I couldn’t find neither BC140 nor BC141. Btw, they are both NPN. Shouldn’t the drivers be NPN & PNP?
Any help will be greatly appreciated, thanks.
I'm resurrecting this old thread cause I have a similar issue with a 1976 Scott A426.
My profile is also similar to the one of the author of the thread, I used to be an audio electronics hobbyst in the mid 70s, then I quit at the beginning of the 80s. Now I’m trying to take up again, possibly with a better knowledge, also thanks to the Internet.
Here’s my problem. The amplifier has a dead channel (left) and the other one only outputs a 50 Hz noise, no audio signal.
When the right ch. speaker is connected, the bulb tester lights strongly. On the phones out, left ch. is ok while the right one has the 50Hz noise)
I tested all the transistor in the power amp section (off the pcb) with a China component tester (T7) and all the transistors are recognized as good, but some of them show a wrong pinout (the tester shows base in place of the emitter, or collector in place of the base etc.).
All the other components on both channels look OK.
I came to the conclusion that the “wrong pinout” ones are damaged (It also happened with one of the rectified diodes, anode and cathode resulted inverted, though the tester recognized it as a diode). It turned out that the diode was damaged.
Here are the part numbers, according to the schematics:
Q806; Q816 = NPN 1860 .C1166
Q809; Q819 = PNP 1861 .A661
My problem is: the suspected transistors are nowhere to be found, so I would ask the post’s author: What did you replace the damaged transistors with?
@trtuncay: I couldn’t find neither BC140 nor BC141. Btw, they are both NPN. Shouldn’t the drivers be NPN & PNP?
Any help will be greatly appreciated, thanks.
The reason the bulb lights when a speaker is connected will most likely be because of a large DC offset at the output. Failed output transistors/drivers would do that.
The output devices are both NPN (its a standard Quasi Complementary output stage) and the upper driver is NPN and the lower one PNP.
The picture in post #1 shows the outputs look to be TO220 outline parts. Do they look like this:
If so then you could try TIP41C devices but only buy from authorised suppliers. Do not buy off eBay.
For the drivers you could try the BC639 and BC640
https://www.digikey.co.uk/en/htmldatasheets/production/63749/0/0/1/bc639
The output devices are both NPN (its a standard Quasi Complementary output stage) and the upper driver is NPN and the lower one PNP.
The picture in post #1 shows the outputs look to be TO220 outline parts. Do they look like this:
If so then you could try TIP41C devices but only buy from authorised suppliers. Do not buy off eBay.
For the drivers you could try the BC639 and BC640
https://www.digikey.co.uk/en/htmldatasheets/production/63749/0/0/1/bc639
Hi Mooly,
Thanks for your quick reply.
Yes it’s exactly what I thought, DC offset on the right ch. and possibly open or shorted drivers/output on the left ch.
While the output transistors look OK (at least according to the China tester), the drivers reported a confused result (the actual pinout doesn’t match with what shown on the tester’s display.
Yes, the output transistors look like the image shown, and are secured with a screw to the big heatsink. The pinout is the same (BCE, front view). They look OK on the tester, should I replace them anyway?
Also, should I modify some resistor values if I replace both drivers and output devices?
Big thanks,
M
Thanks for your quick reply.
Yes it’s exactly what I thought, DC offset on the right ch. and possibly open or shorted drivers/output on the left ch.
While the output transistors look OK (at least according to the China tester), the drivers reported a confused result (the actual pinout doesn’t match with what shown on the tester’s display.
Yes, the output transistors look like the image shown, and are secured with a screw to the big heatsink. The pinout is the same (BCE, front view). They look OK on the tester, should I replace them anyway?
Also, should I modify some resistor values if I replace both drivers and output devices?
Big thanks,
M
I would test the transistors with them removed and test them on a DVM on diode range for basic checks. I would be surprised if the outputs were OK but its good to test them. These usually fail short circuit C to E.
The outputs will be insulated from the heatsink with insulating kits so you might need those as well. These days we usually use synthetic flexible pads rather than mica and heatsink compound. If the transistors come off cleanly and the mica is undamaged then they can be reused but will need fresh compound applying.
Again if it were me and I would probably also replace the VAS transistor (Q803/813). These can be damaged as output stages fail. Certainly worth checking anyway. A BC640 should be workable for those if needed.
The outputs will be insulated from the heatsink with insulating kits so you might need those as well. These days we usually use synthetic flexible pads rather than mica and heatsink compound. If the transistors come off cleanly and the mica is undamaged then they can be reused but will need fresh compound applying.
Good question. If it were me I would link out one of the two series connected diodes on the vbe multiplier to give a reduced bias current. The design is unusual in that it runs fixed bias and that may need tweaking when its all working. Either diode of the two can be linked out, it doesn't matter which.
Again if it were me and I would probably also replace the VAS transistor (Q803/813). These can be damaged as output stages fail. Certainly worth checking anyway. A BC640 should be workable for those if needed.
Another useful method of trimming the bias down a bit is to replace a regular junction diode with a small signal schottky. A lot of bigger power transistors just don’t need as high a turn on voltage at mA levels (half a volt vs. the usual 0.6 or 0.7). You can get the same effect by using a 6 or 10 amp diode, but those never fit the PCB.
Hi Mooly,
Thanks for your advice. I’ll buy the suggested replacement devices and let’s see what happen. I’m also going to check Q803 and Q813 off the board.
I see the component tester is not totally reliable, so I will check semiconductors with a multimeter too.
I’ll keep you updated, thanks.
Thanks for your advice. I’ll buy the suggested replacement devices and let’s see what happen. I’m also going to check Q803 and Q813 off the board.
I see the component tester is not totally reliable, so I will check semiconductors with a multimeter too.
I’ll keep you updated, thanks.
Q813 and Q803 might possibly be intermittent. Those two will run quite hot so also look for any dry joints on them before you remove them. Anything like that can cause failure of the output stage.
Hi Mooly,
I finally got the amp working. After replacing the drivers and the output transistor, the problem was still there: light bulb on and humming right ch.
It turned out that the problem were the VAS transistors (Q803 and Q813). After replacing them, no more hum and no more light bulb on.
But something weird happened: no sound on the left ch., although it was ok on the phones. I couldn’t believe the problem was in the speakers' switch.
By rocking it a bit, some sound seemed to appear, so I understood a contact cleaner spray wouldn’t have been enough. I had to un-mount and open the switch pair, they were extremely dirty and oxidized. I had to use isopropyl alcohol and a 2000 grit sandpaper to fix them.
Now the amp is working great. I ordered some heatsink for the small plastic transistor, cause the original ones are too large to fit.
Thanks very much for your advice, Mooly. I not only fixed the amp but, most important, I also learned something more about (class AB?) amplifiers.
PS Next episode will be a no-brand (Transylvania) 1970s cassette deck. No sound and no recording on left channel. No one in the world seem to have one and - of course - no schematics. I have been struggling with this piece of gear for months, but no luck. Stay tuned ;-)
I finally got the amp working. After replacing the drivers and the output transistor, the problem was still there: light bulb on and humming right ch.
It turned out that the problem were the VAS transistors (Q803 and Q813). After replacing them, no more hum and no more light bulb on.
But something weird happened: no sound on the left ch., although it was ok on the phones. I couldn’t believe the problem was in the speakers' switch.
By rocking it a bit, some sound seemed to appear, so I understood a contact cleaner spray wouldn’t have been enough. I had to un-mount and open the switch pair, they were extremely dirty and oxidized. I had to use isopropyl alcohol and a 2000 grit sandpaper to fix them.
Now the amp is working great. I ordered some heatsink for the small plastic transistor, cause the original ones are too large to fit.
Thanks very much for your advice, Mooly. I not only fixed the amp but, most important, I also learned something more about (class AB?) amplifiers.
PS Next episode will be a no-brand (Transylvania) 1970s cassette deck. No sound and no recording on left channel. No one in the world seem to have one and - of course - no schematics. I have been struggling with this piece of gear for months, but no luck. Stay tuned ;-)
Well done 👍 VAS transistors often run very hot and some types are prone to failure.
Record/play switches cause much trouble across all makes and models. Give them a good clean and move them back and forth 30 or 40 times to clean them up. Do not move them with a tape palying in the machine as it will cause the bias oscillator to start and will erase a moving tape even in play mode.
Record/play switches cause much trouble across all makes and models. Give them a good clean and move them back and forth 30 or 40 times to clean them up. Do not move them with a tape palying in the machine as it will cause the bias oscillator to start and will erase a moving tape even in play mode.
It will be what Chris says. Super common issue across all makes and models that use these multipole 'record/play switches'. Typically two of them side by side and of this generally outlIne. Some are bigger, some smaller.
Hi Mooly,
big thanks for your help & suggestion.
The rec/play multiple switch is the first thing I checked. I had to carefully open it and clean it with isopropyl alcohol, and now I think it works ok. Another common failure, according to what I found on the net, is the Dolby IC. I replaced it but unfortunately, none of them worked.
The unit has a weird wiring, part on pcb, part point-to-point, with a lot of intricate cables going anywhere. I will post some picture here.
Should I open a dedicated thread for this topic, so that people with similar issues can find it?
big thanks for your help & suggestion.
The rec/play multiple switch is the first thing I checked. I had to carefully open it and clean it with isopropyl alcohol, and now I think it works ok. Another common failure, according to what I found on the net, is the Dolby IC. I replaced it but unfortunately, none of them worked.
The unit has a weird wiring, part on pcb, part point-to-point, with a lot of intricate cables going anywhere. I will post some picture here.
Should I open a dedicated thread for this topic, so that people with similar issues can find it?
A new thread probably would be best going forward with it 🙂 The only other common-ish fault might be a wire to that channel where it solders to the R/P head. They sometimes fracture and break if there is movement on the wire as the head move when you press play. Easy to see visually.
Analogue Source is the one:
https://www.diyaudio.com/community/forums/analogue-source.11/
Analogue Source is the one:
https://www.diyaudio.com/community/forums/analogue-source.11/