Yes DRC,that makes sense.No,I did not observe the polarity of the Q615 Vbe.I have had too much beer now and can't be bothered,maybe tomorrow.
When i measured Q613 Vbe,as I pressed meter probes against component, distortion"came and went".I thought cracked PCB tracks/bad solder and scraped away solder resist and made real good solder connection to PCB.Measured again .Same.Part must have some sort of internal bad connection.
Will new transistors of different gain group badly or slightly effect the performance of my amp?
When i measured Q613 Vbe,as I pressed meter probes against component, distortion"came and went".I thought cracked PCB tracks/bad solder and scraped away solder resist and made real good solder connection to PCB.Measured again .Same.Part must have some sort of internal bad connection.
Will new transistors of different gain group badly or slightly effect the performance of my amp?
Yes,Ostripper,very helpful advice.Thanks.
But the datasheet for 2SC1826 quotes min hFe 40,max 400.No typical.I could take out all four existing TR's and check the average hFe on them I suppose.
Schematic from Rotel says gain group O.Y.
I have been unable to find full datasheet for 2SC1826,only rough specs and don't know what is gain for group O.Y( or does this mean O or Y?)
If I put in transistor of higher or lower gain than recomended by Rotel,would only effect be a higher or lower bias voltage?
But the datasheet for 2SC1826 quotes min hFe 40,max 400.No typical.I could take out all four existing TR's and check the average hFe on them I suppose.
Schematic from Rotel says gain group O.Y.
I have been unable to find full datasheet for 2SC1826,only rough specs and don't know what is gain for group O.Y( or does this mean O or Y?)
If I put in transistor of higher or lower gain than recomended by Rotel,would only effect be a higher or lower bias voltage?
IF you replace all 4... ,set bias to lowest- then for .22ohm
resistor get 20mv across it ,(90+ma bias idle) or if schematic has
recommendation go with it. TIP29A is more powerful than your present
outputs so it probably won't matter.I had a little Kenwood receiver
with same circuit in 1978 except it used NPN/PNP the tip29/30
worked perfect to replace the japanese transistors..
Min/Max - most outputs go to lower range of Hfe when they drive
low imp. load (speaker)tip29A= 40/80 combined (darlington) with
bc637=125/400 they should make the grade.
(What I matched your devices with are the closest I could find,
they are slightly overrated but that is good!)
OS.
resistor get 20mv across it ,(90+ma bias idle) or if schematic has
recommendation go with it. TIP29A is more powerful than your present
outputs so it probably won't matter.I had a little Kenwood receiver
with same circuit in 1978 except it used NPN/PNP the tip29/30
worked perfect to replace the japanese transistors..
Min/Max - most outputs go to lower range of Hfe when they drive
low imp. load (speaker)tip29A= 40/80 combined (darlington) with
bc637=125/400 they should make the grade.
(What I matched your devices with are the closest I could find,
they are slightly overrated but that is good!)
OS.
You beat me to it 
Aidan -- Stop worrying, just get e'm fitted. Set the bias as per the manual making sure it doesn't go into thermal runaway.
If one output transistor is higher or lower in gain it just means the driver has to supply more or less current but it's all relative anyway. The gain or hfe varies greatly with temperature as does vbe etc. It's a very simple circuit and very non critical on parts.
Have fun
DRC said:
I would think Q615 gets 'reversed' biased (did you note to polarity when you measured ?) as a result of the failure of Q613. 2.5 volts shouldn't make it avalancheso its it may be OK
Aidan -- Stop worrying, just get e'm fitted. Set the bias as per the manual making sure it doesn't go into thermal runaway.
If one output transistor is higher or lower in gain it just means the driver has to supply more or less current but it's all relative anyway. The gain or hfe varies greatly with temperature as does vbe etc. It's a very simple circuit and very non critical on parts.
Have fun
Fixed!But not without problems first.
Transistors arrived yesterday.They were fitted.Straight after switch on I went to set bias in case it was grossly out after transistor change.It was about 10mV instead of 4.4mV.I began to adjust down bias.
There was a strange noise from left speaker then silence.All this within about 20 seconds of power on.
Q613(location of the original suspect) was short C-E!! Thinking this was caused by too high bias I put in another new transistor(good job I bought 8 of them).
Power on,bang,funny smell,2 fuses blown.Before throwing it out the window I decided to have one more go.Q613 and his mate Q615 short C-E.The cause was bias adjust pot wiper open circuit.Maybe it was just dirty and intermittant but new one cured fault and my Rotel lives on with a mighty and clean sound.
Thanks everybody for your advice.
Transistors arrived yesterday.They were fitted.Straight after switch on I went to set bias in case it was grossly out after transistor change.It was about 10mV instead of 4.4mV.I began to adjust down bias.
There was a strange noise from left speaker then silence.All this within about 20 seconds of power on.
Q613(location of the original suspect) was short C-E!! Thinking this was caused by too high bias I put in another new transistor(good job I bought 8 of them).
Power on,bang,funny smell,2 fuses blown.Before throwing it out the window I decided to have one more go.Q613 and his mate Q615 short C-E.The cause was bias adjust pot wiper open circuit.Maybe it was just dirty and intermittant but new one cured fault and my Rotel lives on with a mighty and clean sound.
Thanks everybody for your advice.
That's good Just looked at the circuit again. A dicky wiper etc would definitely cause that. Nowadays we would never wire the pot up like that because of that very eventuallity. Wire it like this and if the pot goes open circuit the transistor is turned hard on removing the bias voltage.
Just looked at the circuit again. A dicky wiper etc would definitely cause that. Nowadays we would never wire the pot up like that because of that very eventuallity. Wire it like this and if the pot goes open circuit the transistor is turned hard on removing the bias voltage.Attachments
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.