So I expect there will be some eye rolling at your end but I've got to ask someone - where exactly am I to measure the bias/idle current?
I've done this a total of once before which was several years ago and someone had to tell me where to take this measurement.
On the board of the SC335 are two posts per side each marked "Idle" + and -. I would like to confirm that this is where the measurement is obtained. On the SC335 schematic sent to me is the hand written step 5 addition to NOTES reading "Bias Adj. 10mV Idle + to Idle -" which I presume are the two posts indicated above.
As well, does that 10mV figure apply to any and all transistors or will those I plan to install need a different setting?
I've done this a total of once before which was several years ago and someone had to tell me where to take this measurement.
On the board of the SC335 are two posts per side each marked "Idle" + and -. I would like to confirm that this is where the measurement is obtained. On the SC335 schematic sent to me is the hand written step 5 addition to NOTES reading "Bias Adj. 10mV Idle + to Idle -" which I presume are the two posts indicated above.
As well, does that 10mV figure apply to any and all transistors or will those I plan to install need a different setting?
Put 1 multimeter lead on the idle pin, and the same for the other . Doesn't matter which is which, just do both.
Turn the trimmer until you see 10 mV (or -10) on your meter. Go slow, only move the VR microscopically little each time, and wait for the value to settle.
Also spray the VR with contact cleaner before even touching it.
This is a risky procedure. Amp may get killed.
Turn the trimmer until you see 10 mV (or -10) on your meter. Go slow, only move the VR microscopically little each time, and wait for the value to settle.
Also spray the VR with contact cleaner before even touching it.
This is a risky procedure. Amp may get killed.
Yes just measure first to get idea.
Trimmers can be stuck, further clockwise was lowering bias here I remember, that should be safe direction to turn trimmerpot.
You remounted transistors on heatsink? Fresh paste and new eg. mica's ?
Trimmers can be stuck, further clockwise was lowering bias here I remember, that should be safe direction to turn trimmerpot.
You remounted transistors on heatsink? Fresh paste and new eg. mica's ?
I will measure at those points first prior to doing anything. Since there looks to be locking compound on the VR's, maybe I won't need to mess with them. The only other time I had to adjust bias current, there was no mention of endangerment from rotating while testing. I recall turning them end to end to see how much variance was achieved. In any case, if need be I will remove the locking compound first then adjust the pots in small increments. And if I must go that route, I will use contact spray first.
Regarding the heatsink mounted transistors, I was not planning to disturb those. I checked that paste and it the same consistency as new. That being so, what benefit would there be in re & re-ing the paste and micas as you suggest?
Regarding the heatsink mounted transistors, I was not planning to disturb those. I checked that paste and it the same consistency as new. That being so, what benefit would there be in re & re-ing the paste and micas as you suggest?
Leave the output transistors alone. As long as you don't disturb the thermal compound, it will be all right. If you do move it, you need to clean it all off and apply new stuff. The mica sheets don't wear out so there is no need to change them.
Turning the bias VR from end to end while the thing is running is a great way to kill the amp.
Turning the bias VR from end to end while the thing is running is a great way to kill the amp.
Thanks! And as planned, I will not be disturbing those transistors.
I had no intention of exercising the bias pots in this amp extreme to extreme unless for cleaning purposes with power off and then only if needed.
What I referred to was a different amplifier in which the more I think about it, the correct bias current could not be achieved due to an elusive short and that was why I wound up swung that pot end to end.
I had no intention of exercising the bias pots in this amp extreme to extreme unless for cleaning purposes with power off and then only if needed.
What I referred to was a different amplifier in which the more I think about it, the correct bias current could not be achieved due to an elusive short and that was why I wound up swung that pot end to end.
If you measure bias now you may find it to be 25mV, wouldn't that be interesting?
If you feel more heat from heatsink than vumeter bulb lights, reread first sentence and know at 10mV the meterlights burn more than you'll feel at the heatsink.
If you feel more heat from heatsink than vumeter bulb lights, reread first sentence and know at 10mV the meterlights burn more than you'll feel at the heatsink.
Well, you were correct. After letting the amp warm up a bit, I did find bias current higher than 10mV and was in fact higher than than 25. 35mV on right channel and 33mV on left.
The pots received a shot of cleaner and I eased them down to 10.
Before I made the adjustment, the heatsink was warm to the touch but not overly. At 10mV, it's slightly warm but cooler than it was.
I can't make that comparison with the lamp bulbs. Both meters were defective and I've removed them from the amplifier. Some previous owner attempted replacing the meter lamps and did not do it well which may have damaged the mechanism.
I expect some new illuminated meters to arrive in a week or so.
I will check when the time comes, but in your opinion, will I be re-adjusting the bias again once the new transistors are intalled?
The pots received a shot of cleaner and I eased them down to 10.
Before I made the adjustment, the heatsink was warm to the touch but not overly. At 10mV, it's slightly warm but cooler than it was.
I can't make that comparison with the lamp bulbs. Both meters were defective and I've removed them from the amplifier. Some previous owner attempted replacing the meter lamps and did not do it well which may have damaged the mechanism.
I expect some new illuminated meters to arrive in a week or so.
I will check when the time comes, but in your opinion, will I be re-adjusting the bias again once the new transistors are intalled?
Here ~60% of outputtransistor metal and mica had thermal paste contact but maybe Welcome sold you your Toshiba and already checked.
Meters will also change bias left channel.
How much dc you measure on speaker outputs ?
Meters will also change bias left channel.
How much dc you measure on speaker outputs ?
Presently there are -31mV at the left speaker terminals and -30mV at the right.
No, I did not buy the amplifier from Welcome. I found it at a second hand store about 80km from where I live.
That is interesting what you say about re-adjusting bias on the Left channel when meters are installed. Before I moved them, the pots were unevenly set when looking at them but now, after setting for 10mV, they are more or less the same.
No, I did not buy the amplifier from Welcome. I found it at a second hand store about 80km from where I live.
That is interesting what you say about re-adjusting bias on the Left channel when meters are installed. Before I moved them, the pots were unevenly set when looking at them but now, after setting for 10mV, they are more or less the same.
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Both channels do seem healthy at the moment.
(if you put like 3k9 resistor parallel to r9/r10 if I remember correctly, you'll be single digit close to zero mV, preferably positive, because I remember it dropping a little with temperature??)
((Lamp temperature-rise affects bias a little))
(if you put like 3k9 resistor parallel to r9/r10 if I remember correctly, you'll be single digit close to zero mV, preferably positive, because I remember it dropping a little with temperature??)
((Lamp temperature-rise affects bias a little))
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I'm willing to try paralleling those resistors, but I need to buy some 3.9k ohm. The closest I have on hand are 3.3k.
So just try the 3k3, nobody knows for sure how your amp will react, I think it will be above 10mV positive LOL, tell me 🙂
There you go, surprise for me, still negative 🙂 Close to no time or money spent it is nice result you can see. Most will say 30mV is low enough, now everybody say it is low enough.
I picked up the transistors on Thursday and installed them this morning - checked the idle current and re-adjusted it for 10mv. So far, so good - it's been on and as yet, no nasty noises or smoke.
As it's been on my dining table (aka workbench), I'm only using Minimus 7's for speakers but I shall hook it into my system with Infinity Kappa 7.1's. I tried a NAD 3020 with those and it worked surprisingly well so I hope the Toshiba SC335 will not run into difficulty. it will not be replacing my main amp but
Thank you for help, patience and advice.
It crossed my mind to ask how or why you became so familiar with this particular amplifier?
To me, these seem to be quite humble and not something that many would take notice of. I've known of their existence probably since new but they're not often found at least not those with analog meters. It was the scarcity that attracted me to it.
As it's been on my dining table (aka workbench), I'm only using Minimus 7's for speakers but I shall hook it into my system with Infinity Kappa 7.1's. I tried a NAD 3020 with those and it worked surprisingly well so I hope the Toshiba SC335 will not run into difficulty. it will not be replacing my main amp but
Thank you for help, patience and advice.
It crossed my mind to ask how or why you became so familiar with this particular amplifier?
To me, these seem to be quite humble and not something that many would take notice of. I've known of their existence probably since new but they're not often found at least not those with analog meters. It was the scarcity that attracted me to it.
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