Replacing 2sc644 and 2sc828 transistors with NTE 199

[jheu02]

Why replace the C644s when they measure that high? You're getting about half the gain using the C1845s as you are with the original Qs. You don't even have to gain match them as it's not a differential pair.

 

[dlucy]

Yes, the output is Considerably lower on PHONO. Roughly as an example, FM would play at a certain volume at 1/4 volume. To get that same volume with PHONO the knob would have to be at half turn instead of 1/4.

In general, that is not an apples-to-apples comparison.

Each and every phono cartridge generates a different amount of output voltage. When you review the specs for any given cartridge, you should find its output voltage. The PHONO stage you amplify that output with will always amplify the same amount. The point here is a phono cartridge run through a receiver and its PHONO stage will have an output that is likely to be different (volume) when compared to all other inputs running through that receiver. You get whatever gain the manufacturer put into the PHON stage times... whatever output level that specific cartridge makes.

That said, the overall gain of the PHONO stage is important. I encourage you to run a phono-cartridge-level 1 kHz signal (i.e. from a signal generator) into your PHONO stage, with the high gain original transistors still in there, and measure the output voltage (with the Volume knob at a set point). Then replace those high gain transistors with the standard KSC1845F you have and measure the output again.

This would be a excellent example for all of us here on AK. I'm super interested in the results.

 

[SweetVintage]

Why replace the C644s when they measure that high? You're getting about half the gain using the C1845s as you are with the original Qs. You don't even have to gain match them as it's not a differential pair

Noise mainly. After the re-cap there was noise throughout so I did a sweep and found quality was far better (my ears) but that could have all been coming thru the tone board for all I know and the only ones I changed out were any 644's. I may end up putting them back in (644 (T)'s) but I ordered 2N5088's and we'll see what gain those have on the tester that everything else has been measured on.
I'm not a fan of simply changing every transistor by far but known to go bad and noisy are usually targets. A tech went thru this unit sometime in the past and replaced only one of the outputs (2SC897) with a 2SC793 so I changed all of them out to the Onsemi's I had here.

 

[SweetVintage]

In general, that is not an apples-to-apples comparison.

Each and every phono cartridge generates a different amount of output voltage. When you review the specs for any given cartridge, you should find its output voltage. The PHONO stage you amplify that output with will always amplify the same amount. The point here is a phono cartridge run through a receiver and its PHONO stage will have an output that is likely to be different (volume) when compared to all other inputs running through that receiver. You get whatever gain the manufacturer put into the PHON stage times... whatever output level that specific cartridge makes.

That said, the overall gain of the PHONO stage is important. I encourage you to run a phono-cartridge-level 1 kHz signal (i.e. from a signal generator) into your PHONO stage, with the high gain original transistors still in there, and measure the output voltage (with the Volume knob at a set point). Then replace those high gain transistors with the standard KSC1845F you have and measure the output again.

This would be a excellent example for all of us here on AK. I'm super interested in the results.

ok, I can do this but I'll wait for new x-istors to come in before I open it up again.
Install new 1845fta's, measure in and out.
put in originals and measure in and out.
Should get mouser order before next weekend.
Thanks for the info too!