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Old 14th March 2013, 01:30 PM   #71
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Sanyo TR36643 - PartSimple.com
36643 NTE Equivalent NTE128 NPN audio transistor... - Wholesale Electronics
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Old 14th March 2013, 01:37 PM   #72
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I went ahead and ordered the Sanyos, the only ones in the world I could find
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Old 14th March 2013, 01:39 PM   #73
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p.s. Ian I think you missed also that I did actually pull the T03's which are also socketed, deoxed the sockets, checked their resistances and put new silicone thermal compound on both sides of their mica gaskets. Thinnest possible coating...
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Old 14th March 2013, 05:19 PM   #74
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From #66, I deduced the problem was most likely drivers, not output transistors and I was aware you had gone through cleaning and remounting the outputs earlier.

For the present, getting the right parts is all that can be done to get operational but likely the hum will return and as you confirm, this varies with the bias current setting.
That means that the more current drawn through the output stage, the more hum and that hum is not signal from the input stage, which is unchanged, but from the power supplies to the output stage, where current does change.

We'll have to look at that again when you have the parts or you could work on just this channel.
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Old 14th March 2013, 06:00 PM   #75
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Quote:
Originally Posted by Ian Finch View Post
From #66, I deduced the problem was most likely drivers, not output transistors and I was aware you had gone through cleaning and remounting the outputs earlier.

For the present, getting the right parts is all that can be done to get operational but likely the hum will return and as you confirm, this varies with the bias current setting.
That means that the more current drawn through the output stage, the more hum and that hum is not signal from the input stage, which is unchanged, but from the power supplies to the output stage, where current does change.

We'll have to look at that again when you have the parts or you could work on just this channel.
I did not run the amp for very long with the driver transistors swapped, but from what I could tell, the hum switched channels with the transistors. Hoping that's the case. Can a transistor cause a hum or pick up power supply hum in any way? Maybe wishful thinking Thanks Ian.

p.s. it may be a week or two before I get the Q's so if you don't hear from me in this thread for a bit that will be why...
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Old 14th March 2013, 07:55 PM   #76
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No problem - it's the way of our modern globalised supply world. Huge market and huge delays. Funny, it takes only 4 days for Mouser, Texas to deliver a carton with 40 items to me in a rural district 350 miles from a major city airport and 10,000 miles distant, but 2 -5 weeks to airmail a couple of tiny signal transistors from Hong Kong

We were looking at this hum issue before the incident with the driver and if you just consider this section of the amplifier, the causes can only be due to the supply and a simple list of parts and connections. If, when you replace the drivers, this small residual hum doesn't transfer with a driver swap, you can be fairly sure you have a resistor issue and one is open.

The lesser possibilies of a fault in the previous stage could be eliminated by pulling both input stage transistors (Q9,Q11 or Q10,Q12) and rechecking hum. Note,that R58 should also be lifted or the connection GG lifted too, to ensure no possible supply hum from the small remaining current through the transformer primary there.

The transistors don't generate or receive hum at this stage of the amplifier where voltage gain is only 1, so as I'm saying, the influence is from supply current flowing where it shouldn't, as when parts have shorted or gone open circuit. The rectifier switching noise that is always on the power supply rail, depends on caps and resistors to attenuate and keep it out of the input stage where voltage gain raises it to audible. When the current exceeds the design level, the caps are less effective at their filtering job and hum rises, as you hear. You've replaced the caps, but that may have just exposed another fault that has been there for years. The obvious place to start is at C16 by simply paralleling another large electro. of sufficient voltage rating across it. Then you know where the problem is - or not.

If it were my amp, I would pull all the old resistors in the output stage, bar the 0.5 ohm output emitter ones and replace with cheap and cheerful half or 1W carbon film types from your local store. They will be more reliable, at least. That'a long and somewhat expensive process though, if you have to buy packs of more than 2 parts from Radio Shack etc.

Re: matching - #69 already suggests that there will be little benefit and you can't match parts without a large stock to sort from. With these parts the only matching would be the process wafer quality and packaged lots, which are still quite good matching methods. From just a few loose parts stock? Nope.
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Old 14th March 2013, 08:43 PM   #77
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OK Thanks again Ian, I'll let you know when the new transistors are in place and I see what's what... Greatly appreciate your time thinking about this and sharing your wisdom.
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Old 18th March 2013, 04:35 PM   #78
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Well the 36643s fell through as I'd feared. So I've ordered the NTE128's, hope they work. Should they be biased the same as the originals? Mouser had the NTE's so I should have them by end of this week...
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Old 18th March 2013, 08:44 PM   #79
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Hi, It wouldn't be the first time hopes are raised by some false listing on the net.
Sorry about that but I think NTE 128 will work OK in most conditions though I ran a short thread in the Parts Forum to ask about these RCA 3xxxx codes which don't appear in contemporary RCA general catalogues but their later replacement SK 3024/128, which lasted until the 90's apparently, did. RCA transistor - vintage search

I prefer 2N3019 as a heavier duty model than NTE128 which would suit 4 ohm speakers and today's bass levels better with its higher current rating. 'Just my view, mind you.
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Old 18th March 2013, 09:15 PM   #80
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Quote:
Originally Posted by Ian Finch View Post
Hi, It wouldn't be the first time hopes are raised by some false listing on the net.
Sorry about that but I think NTE 128 will work OK in most conditions though I ran a short thread in the Parts Forum to ask about these RCA 3xxxx codes which don't appear in contemporary RCA general catalogues but their later replacement SK 3024/128, which lasted until the 90's apparently, did. RCA transistor - vintage search

I prefer 2N3019 as a heavier duty model than NTE128 which would suit 4 ohm speakers and today's bass levels better with its higher current rating. 'Just my view, mind you.
OK Thank you SO much for researching this for me! I'll probably go ahead and find some 2N3019's and order them next time I'm ordering parts. The NTE's will arrive sooner and I can at least use them to work on the hum. I don't really want to run the amp at all the way it is right now. Thanks again.
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