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Perry Babin 29th August 2020 03:52 AM

What's the rail voltage?

Is it regulating when you increase the 12v supply voltage?

objext 29th August 2020 04:37 AM

Looks like I am getting 29.04V for rail voltage. My power supply is not great - it's not variable and looks like it's only putting out 11.46V with the current 1.39A idle load.

objext 30th August 2020 11:57 PM

6 Attachment(s)
Pulled the board and reworked the transformer down to a reasonable position. Also noted the heat discoloration on the board from the U3 LM317.

Filter caps all read ~400ohm and then gain up beyond 2000+ on either my meter's 2000ohm setting or diode mode.

Perry Babin 31st August 2020 12:34 AM

Level the cap with the heatshrink that's pulled up and put a drop of adhesive under the points that would touch the board if the cap was pushed to the side.

objext 31st August 2020 12:50 AM

Good idea. Hot glue perhaps? Any other thoughts on the LM317 heat?

Perry Babin 31st August 2020 01:17 AM

Never use hot glue. Use something like goop, e6000 or even silicone. All you need is a tiny dot on opposite sides.

I'd expect the regulators to run hot but I don't know what's normal.

What's the DC voltage on terminals 1 and 2 of the 494?

objext 31st August 2020 05:07 AM

TL494C voltages to B- are:

Pin1: 2.22
Pin2: 2.22
Pin3: 1.54
Pin4: 0.10
Pin5: 1.47
Pin6: 3.26
Pin7: 0.00
Pin8: 13.52
Pin9: 4.52
Pin10: 4.50
Pin11: 13.51
Pin12: 13.52
Pin13: 4.84
Pin14: 4.84
Pin15: 4.84
Pin16: 0.00

objext 2nd September 2020 05:55 AM

Is it strange that pin1 and pin2 are the same voltage? Would failed/failing electrolytic caps cause this issue with heat? I get mixed results/opinions on whether to recap amps that don't show leakage or swelling...

Perry Babin 2nd September 2020 06:09 AM

The same voltage on pins 1 and 2 indicates that the regulation of the rail voltage is working properly.

The majority of the large capacitors on the B+ or the rails are used to filter high frequency (in the neighborhood of 30kHz) ripple. With a scope, you can get a very good indication of whether the caps are good enough by looking at the ripple on the rail and B+ voltage. It's normal to see ripple that's related to the frequency of audio. The problem is when you see high frequency ripple along with the audio related ripple. This can be very difficult to see on digital scopes. On analog, CRT scopes, it's immediately obvious.

objext 11th September 2020 06:54 AM

I picked up a power supply with some voltage adjustment. Regulation holds 29.04V from 12V to 14.4V. The 317 & 337 seem to get equally hot at either end of that range.

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