Hello,
First time here on Diyaudio.
Here's a quick intro before asking what really is on my mind.
I'm living in Lyon, France so you'll have to excuse me for a few mistakes from time to time.
I have been around audio from the 70's and 80's for a long time, the music as well as the equipment and recently I discovered the pleasure of opening them up and try to understand them a little more, repairing and recapping when needed. So I am looking around on the internet for nice pieces.
I currently have a Technics su 8080 with a pair of ditton 66 legends, a technics sl-B2, a teac UD-H01 dac and a philips CD 624.
As secondary setup I have a Dual Pa 5060 saved from being thrown away with technics speakers also saved from a horrible death, a technics SL-PG580A and an ITT turntable (quite rare apparently).
I'm here to learn mostly but will do what I can with my little knowledge to help.
So now the real deal, I'm looking for help with a Scott A 426 I picked up recently.
When I got it nothing worked so I started by isolating the problem : the power amplifier section. I checked all the transistors and most of them were shorted, never seen that much in one single amp. Also a few resistors and diodes were bad so I replaced everything (some with originals some with equivalents but same on both channels).
(the red circled ones have been replaced)
But after all of this the problem doesn't seem solved, I powered it up with a lamp connected in series and it lit up so there has to be something else.
I couldn't find anything so if anyone here has any idea of what could be causing this I would be very grateful.
Thanks
First time here on Diyaudio.
Here's a quick intro before asking what really is on my mind.
I'm living in Lyon, France so you'll have to excuse me for a few mistakes from time to time.
I have been around audio from the 70's and 80's for a long time, the music as well as the equipment and recently I discovered the pleasure of opening them up and try to understand them a little more, repairing and recapping when needed. So I am looking around on the internet for nice pieces.
I currently have a Technics su 8080 with a pair of ditton 66 legends, a technics sl-B2, a teac UD-H01 dac and a philips CD 624.
As secondary setup I have a Dual Pa 5060 saved from being thrown away with technics speakers also saved from a horrible death, a technics SL-PG580A and an ITT turntable (quite rare apparently).
I'm here to learn mostly but will do what I can with my little knowledge to help.
So now the real deal, I'm looking for help with a Scott A 426 I picked up recently.
When I got it nothing worked so I started by isolating the problem : the power amplifier section. I checked all the transistors and most of them were shorted, never seen that much in one single amp. Also a few resistors and diodes were bad so I replaced everything (some with originals some with equivalents but same on both channels).
(the red circled ones have been replaced)
But after all of this the problem doesn't seem solved, I powered it up with a lamp connected in series and it lit up so there has to be something else.
I couldn't find anything so if anyone here has any idea of what could be causing this I would be very grateful.
Thanks
When faced with something like this, the only answer is methodical fault-finding. If you have a circuit that we can see then I'm sure we could advise better on how to go about it.
(and well done for using the bulb tester)
(and well done for using the bulb tester)
I actually have a second amp, same model but working and I have been comparing every single components resistance between the two and have found no difference except for a few ohms on one resistor.
What do you mean by circuit, the other side of the board or the schematics ?
What do you mean by circuit, the other side of the board or the schematics ?
Schematics. Its the only way to do it on here... different story when its in front of you.
First thing would be to disable one channel by removing the output devices and work on the other. You could short out the vbe multiplier or whatever arrangement is used for biasing (this is why a circuit diagram is needed, because I can't determine that without having the board in my hands to spin around and study) and begin fault finding from a known starting point.
First thing would be to disable one channel by removing the output devices and work on the other. You could short out the vbe multiplier or whatever arrangement is used for biasing (this is why a circuit diagram is needed, because I can't determine that without having the board in my hands to spin around and study) and begin fault finding from a known starting point.
Here they are (if the resolution isn't high enough I can send the hole pdf file.
I tried that but not the way you say; I just unplugged the +/- 30V coming from the filter caps for one channel and then the other but both times the problem remained the same.
I tried that but not the way you say; I just unplugged the +/- 30V coming from the filter caps for one channel and then the other but both times the problem remained the same.
OK, that looks pretty straightforward. Q804 on the top channel is the vbe multiplier, Q814 on the lower. If you link collector to emitter that would force the bias to zero whatever other problems there may be. If you do that on both channels and the bulb still lights brightly then there is a problem with the driver or output transistors. No speakers connected at any point while working on it.
I'll be back later......
I'll be back later......
I just tried that, the bulb still lights (not very bright but as bright as before).
I checked for the voltages of power supply and I only have 11.6v against the 30v I should have and do have whenever I disconnect the power amplifier board. So the problem doesn't seem to come from q804 nor q814.
I checked for the voltages of power supply and I only have 11.6v against the 30v I should have and do have whenever I disconnect the power amplifier board. So the problem doesn't seem to come from q804 nor q814.
I would have been surprised if they had been faulty, but what shorting them out does is force a zero bias condition.
Leave the short in place on both channels (so that is as if that 0.047uf cap across those transistors was shorted out).
You need to disable one channel to work on this. Whichever channel you choose, you need to lift one end of both of the 0.27 ohm emitter resistors in that channel. You also must lift R817 and R820 or R842 and R844 (both 330 ohms) depending on the channel you decide. You can then work on the other channel to repair it.
You need to bring it to that state before we can go much further. If the bulb still lights then there is a problem with the drivers and/or output transistors on the channel you are repairing. With the vbe multiplier shorted they can not conduct and draw current between the rails.
See where you get to with all that 🙂
Leave the short in place on both channels (so that is as if that 0.047uf cap across those transistors was shorted out).
You need to disable one channel to work on this. Whichever channel you choose, you need to lift one end of both of the 0.27 ohm emitter resistors in that channel. You also must lift R817 and R820 or R842 and R844 (both 330 ohms) depending on the channel you decide. You can then work on the other channel to repair it.
You need to bring it to that state before we can go much further. If the bulb still lights then there is a problem with the drivers and/or output transistors on the channel you are repairing. With the vbe multiplier shorted they can not conduct and draw current between the rails.
See where you get to with all that 🙂
Okay, so I disconnected the resistors on one channel, bulb still lit.
Then I disconnected both channels and no problem, but when I reconnected the first channel the bulb lit again so I assume both channels have faulty output and/or driver transistors. The drivers are q816 and q813 for one and q806/q803 for the other channel right ?
So far you have been a lot of help so thanks, I hope we get somewhere with this 🙂.
I might see your next answer tomorrow as it is evening here and I work early.
Then I disconnected both channels and no problem, but when I reconnected the first channel the bulb lit again so I assume both channels have faulty output and/or driver transistors. The drivers are q816 and q813 for one and q806/q803 for the other channel right ?
So far you have been a lot of help so thanks, I hope we get somewhere with this 🙂.
I might see your next answer tomorrow as it is evening here and I work early.
Yes, it sounds like there is a problem with the output/driver combination. The drivers are Q806 and Q809 on the top channel. If you reconnect the 0.27 ohms on the channel you are working on and remove the drivers (Q806 and Q809 on top channel) then the bulb should remain not lit.
If it lights, then both outputs are faulty. (and probably the drivers are damaged as well)
If it doesn't light then it means that at least one of the two outputs is OK, and possibly both.
The output transistors should read open circuit between collector and emitter with the red lead of your meter on the collector. That applies for using the diode range or higher resistance ranges on your meter. The transistors must be isolated to test them. You can not read them in circuit... to much interaction.
If it lights, then both outputs are faulty. (and probably the drivers are damaged as well)
If it doesn't light then it means that at least one of the two outputs is OK, and possibly both.
The output transistors should read open circuit between collector and emitter with the red lead of your meter on the collector. That applies for using the diode range or higher resistance ranges on your meter. The transistors must be isolated to test them. You can not read them in circuit... to much interaction.
Most of these amplifiers will 'work' into a high impedance load without output transistors. Disconnecting them will minimize the fire danger and allow the rest of it to be effectively troubleshot. The DC feedback shoud 'center' the output and it should pass a signal. If it doesn't, the VAS or LTP need attention and this needs fixing first. If R819 or R815 smoke or get hot, one or both of the drivers are bad - or the bias stack is open. Once the rest of it is working, install known good output transistors.
Thanks for your reply wg_ski but I just found the problem and fixed it. I will definitely keep this in mind though for the next time I encounter something similar.
I followed your directives Mooly and it turned out that the two driver transistors q806 and q816 were shorted, I found the replacement parts in a near electronics shop, replaced them and everything is working fine now, I'll check some voltages across the board before removing the lamp though.
Next step will be recapping the whole thing, I will post some pictures once I'm done.
Thank you for guiding me.
I followed your directives Mooly and it turned out that the two driver transistors q806 and q816 were shorted, I found the replacement parts in a near electronics shop, replaced them and everything is working fine now, I'll check some voltages across the board before removing the lamp though.
Next step will be recapping the whole thing, I will post some pictures once I'm done.
Thank you for guiding me.
Thats excellent news, well done.
(You will need to recheck and possibly adjust the bias... the manual should give the recommended value and procedure. If not then we can probably come up with typical value... the theoretically correct bias and what should be used due to heatsink limitations may be somewhat different from each other. Always recheck bias again when on full mains and without the bulb)
(You will need to recheck and possibly adjust the bias... the manual should give the recommended value and procedure. If not then we can probably come up with typical value... the theoretically correct bias and what should be used due to heatsink limitations may be somewhat different from each other. Always recheck bias again when on full mains and without the bulb)
The manual does say how to measure and which value the bias should be (12mA between the emitter of Q807 and the collector of Q810) but the problem is that I don't have any adjusting point on the board. Also isn't the bias usually in volts and not amps ?
3mV across R821. You would have to trim R808. I hate it when there is no pot, and I hate it even more if the pot is of low quality or takes disassembly to get to.
I'll probably have to check anyway after replacing all the caps so I might as well wait a little.
It's the first one that doesn't have one for me, but it seems to be quite often with these Scott models even if the schematics say it should be there.
It's the first one that doesn't have one for me, but it seems to be quite often with these Scott models even if the schematics say it should be there.
Scott sets of that era were made in the same Japanese factory as Lenco amplifiers, receivers and tuners. Lenco sets were quite popular in Europe for their nice looks and moderate prices. My R25 receiver has quite the same AF section as your Scott, and is identical to the Scott R316 inside. The Lenco lacks the bias pot as well, and its idle current is way too low, but it's not difficult to fit one: the layout is very tidy and spacious. I should find some time for this improvement.
Last edited:
I've also seen some of them actually being made in Belgium as well as a lot of scott speakers but that might not be around the time the A426 came out. The board has a slot for it but there just isn't any so maybe I will try to install one.
12 ma is pretty low and so as long as it is in the range of say 9 to 20ma then I would suggest leaving it be. If you do alter R808 then fit a preset, say 10k in value, and start with it on max resistance (10k) which would give the lowest current. Make sure the speakers are not connected for this as any DC offset will skew the reading due to the small current flowing in the speaker coil. To get an accurate reading I would solder wires to each end of the pair of 0.27 ohm resistors (so that's Q807 emitter and Q810 collector) and read the voltage across both. That would be around 6.5mv DC for a current of 12 ma.