I also fail to understand how this thing is still working, really. I will definitely use a light bulb tester for troubleshooting. The BD139 is on hand so I'll try that first - the SD414 needed a full washer set - mounting hole as well as from the sink, and it is identical to the BD139 package, so I guess it will have to do as a temp replacement. In any case this amp is from 1986, so the original's not a hot young thing either 🙂
Do you not suspect Q455/456/459? As there is no bias at all to begin with? I will double check capacitor C456, that could also be shorted out (does that actually happen? It looks absolutely in the pink of health...). Anyway I will check a couple of times to be sure.
Do you not suspect Q455/456/459? As there is no bias at all to begin with? I will double check capacitor C456, that could also be shorted out (does that actually happen? It looks absolutely in the pink of health...). Anyway I will check a couple of times to be sure.
Do I suspect them... not yet 🙂
Reasons why...
The volt drop across R472 of about 0.7 is proof that 7 ma or so is flowing through R472 and that's exactly what is expected because the base of Q459 is held constant by those two diodes and the vbe drop and the drop across R472. So Q459 is OK.
Q455... is the VAS stage and the amp wouldn't play at all if that were faulty, plus there would be a large DC offset at the output... so that's OK.
Q456... well, that's possible to be faulty but unlikely. If it were short circuit C-E and the base open circuit (I'm just trying to imagine all the scenarios) well I think the amp would work as "normal", but it's a bit unlikely.
C456... highly unlikely to be faulty. If it were short or leaky though it would remove the bias.
For the bulb (bearing in mind safety implications of bare wire etc) I just have a 100 watt bulb with wire soldered to the base and just remove the main fuse in the amp etc and solder the bulb across... very naughty but very quick and effective 🙂
All right then, I'll focus on the Vbe multiplier area. I will probably use a 60 watt bulb, and some cable-mounted sockets and plugs to ensure I reach my next birthday with all my bits intact. I'll be trudging to the local market tomorrow and trying for the replacement transistors, plus the bulbs and electrical wiring stuff, and try and fix her up on Sunday. Hope to have the next reports in a day or two 🙂
Lol 🙂 yea be careful
60 watt bulb should be fine. I would turn down the bias on the good channel to zero while the bulb is in use too.
I'll look in again tomorrow 🙂
60 watt bulb should be fine. I would turn down the bias on the good channel to zero while the bulb is in use too.
I'll look in again tomorrow 🙂
Well it looks like everything is on hold while I figure out a source for the drivers. My local market does not have anything equivalent - sad state. So I'll be poking around in the rest of the country, or look at a foreign source. Connexelectronic has the Toshiba parts in stock (I hope) and I'll update once this lady is back on her feet.
One question - for the purposes of troubleshooting, would just removing the collectors of the output transistors be enough for me to poke around the circuit without blowing up anything in case the bias overshot its mark significantly? Or would I also have to disconnect the driver transistors? Say the resistors connecting the emitters of the pre-drivers - 470 and 100 ohms in the output stage...
It would seem so.
Actually, the voltage across the emitter resistors changes rapidly when the volume is cranked, at normal listening levels it is well over 100mV so I would assume the output stage is OK. If I had not let my soldering iron slip, even the fried output pair and the driver would be OK, I guess.
Actually, the voltage across the emitter resistors changes rapidly when the volume is cranked, at normal listening levels it is well over 100mV so I would assume the output stage is OK. If I had not let my soldering iron slip, even the fried output pair and the driver would be OK, I guess.
If you are really determined to play around, here's what you can do.
Remove Q462/463/464 and 465 leaving just the output in circuit. They are OK ?
Lift the ends of R480 and R481 that go to the base of the outputs.
Then connect the Base of output Q466 to the Collector of Q461 and connect the base of Q467 to the emmiter of Q461.
Set the bias pot to minimum (which is the pot on max resistance) and useing bulb switch on. Now see if the bias pot works.
Remove Q462/463/464 and 465 leaving just the output in circuit. They are OK ?
Lift the ends of R480 and R481 that go to the base of the outputs.
Then connect the Base of output Q466 to the Collector of Q461 and connect the base of Q467 to the emmiter of Q461.
Set the bias pot to minimum (which is the pot on max resistance) and useing bulb switch on. Now see if the bias pot works.
🙂 if you are sure Q462/3 are OK then you can just remove Q464/5 AND R479 and then place links across Base and Emmiter of where Q464/5 were and test.
OK, you mean just provide the bias directly to the output pair - I don't need to go that fancy. They are mounted off-board, and the pins are soldered to the component side of the board. I just need to lift the legs with the tweezer and solder another 10 ohm resistor to the base of the outputs (yes, they are fine, brand new Toshiba 5200/1943 pair 🙂 changed on both channels).
I ordered the drivers already, hopefully they should be here in a week or so...
Would the bias pot be at maximum resistance for minimum bias? I thought it was the other way around - minimum resistance would shut down the transistor, and increasing the resistance would turn it on. Or have I got that backwards?
I ordered the drivers already, hopefully they should be here in a week or so...
Would the bias pot be at maximum resistance for minimum bias? I thought it was the other way around - minimum resistance would shut down the transistor, and increasing the resistance would turn it on. Or have I got that backwards?
Yes maximum resistance for minimum bias.
Max resistance of the pot means Q461 turns hard on via the two diodes and R474. As you decrease the pot resistance you are turning OFF Q461 so more of a volt drop (the bias voltage) is developed across it.
Yo can try providing the bias directly to the outputs as in post 30 or add one driver stage as in post 31
But use the bulb for all testing like this.
Max resistance of the pot means Q461 turns hard on via the two diodes and R474. As you decrease the pot resistance you are turning OFF Q461 so more of a volt drop (the bias voltage) is developed across it.
Yo can try providing the bias directly to the outputs as in post 30 or add one driver stage as in post 31
But use the bulb for all testing like this.
or else - 
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