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Post by Marcel Bogert on Dec 13, 2021 17:19:48 GMT
Working on a 546 amp out of a 2900.
recapped, converted the output to silicon all output transistors were bad.
AGC repaired seems to be working D1 shorted D1, VR1 and VR2 replaced with small signal diodes in series.
AMP is running has good volume and runs stable and sound is clear at mid and high volume.
That's all nice but as soon as i turn down the volume control below midrange distortion starts.
Increase volume distortion goes away even at full bass.
Anyone has any suggestions before i power up my o'scope.
(same issue before AGC repair.)
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Post by jukenorman on Dec 13, 2021 20:05:49 GMT
Hi Marcel,
When you changed the output transistors to silicon, did you change the 1.8 ohm 2W resistors (R28) to 9.1 ohms?
Norman.
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Post by Marcel Bogert on Dec 14, 2021 13:56:57 GMT
Hi Norman, yes i did to compensate for the GE / Si difference
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Post by jukenorman on Dec 15, 2021 9:52:09 GMT
If it's the same on both channels, you might just have to get out your oscilloscope!
Norman.
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Post by Marcel Bogert on Dec 18, 2021 17:42:26 GMT
Issue related to SI change, have harmonics on the output stage.
Pre-amp has a clean signal.
Need to tweak the resistors Jay confirmed this change for 547 548 and 549 if i remember correctly.
Never needed to change to SI as I had plenty of Ge output transistors, can't make any matched sets anymore with low leakage.
546 has a different resistor setup for the output stage. Need to work on this a little further to get it to work or source some 2N2526, ASZ15, NTE121 (these work as replacement) that are still ok and have a realistic pricing.
2n2526 is actually the original part Wurlitzer used for 125703, but this is easily found online...
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Post by jukenorman on Dec 19, 2021 10:49:02 GMT
Hi Marcel,
I thought that the 546 and 547 looked identical at the output stage but I don't have access to good quality drawings (only what is available on Arcade Archive) so cannot read component values. I suppose worst case, you could configure the output stage as a 547? Of course, maybe you don't have a 547 schematic?
Norman.
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Post by Marcel Bogert on Dec 22, 2021 21:44:53 GMT
Hi Norman, I have very many schematics collected over the years.
THe 546 546b and 547 look the same and are not my favorite amp to work on.
Later amps use an SI for the driver which is a great improvement.
Ge tends to get unstable especially with age. (as we all do :-))
I had planned to change to 547 design as an escape, this seems to have been used on some later models too.
But as i'm measuring anyway i will dig a but further, should have more time next week.
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