Need help with a SA300

[curtsad17]

Back at it again. I have changed R70( incorrectly shown as 390 ohm) to 33K. I now have 2 for each circuit.. The bias controls really interact, should that be? I also added a second 10 UF cap as heyraz had done.
With this set up I can get the bias voltages to match with some tinkering ( two voltmeters would make this easier) but on the high side 8.3 to 8.4V and no lower.

I changed out R 69 the 390 ohm to 100 ohm. I can now get the correct bias voltage 7.8V for each pair. Voltage between rectifier and R69 -58.8V

Any thoughts on why I needed the 100 ohm in place of the 390 ohm

 

[larryderouin]

Even tho you split the bias sections, they are still fed from one source. So there will be some interaction between sides. As for the 100 ohm vs. 390ohm the only thing I can think of is a difference in the voltage from the supply due to the difference in the resistor.

 

[curtsad17]

Fed from the same supply is what I was thinking, but did not expect it to be so interactive. If that is the case, I was thinking what if I used 2 diodes to supply each bias circuit in place of the single original rectifier?

Secondly I just measured the voltages again. Upstream of the rectifier I have 57.5vac down stream I have -59vdc.and 2+-vac.
Is my rectifier failing or my meter?

 

[curtsad17]

Here is a photo of the second bias pot installed. It was close but I was able to squeeze a second in and use the existing location for the other.

 

[heyraz]

I'm having trouble with my computer and can't access my notes and pictures of what I did when I worked on my SA300.
I kept a notebook with my handwritten notes and schematics as well, but that's in storage for the moment.

Could you please post a schematic of your mods. It's difficult for me to envision what you are doing because there are two schematics out there (SAMS and Fisher's) and I'm not sure which schematic you are referring to.

I suggest you follow the Fisher schematic (only) and it's parts list. That's what I did because I didn't trust SAMS.

Without opening my amp to take measurements, I can verify that once the bias supply has been split the two channels are interactive. It also takes a good 5 minutes for the amp to stabilize before you will get consistent readings.

If you think you have AC riding on the DC after the diode, you can bypass the electrolytics after the diode to ground with an 0.1uf film cap to drain off the AC snd pass it to ground. I would recommend something military style like a Vit Q because they have a high temp rating.
But the small amount of AC you read might just be your meter being too sensitive.
Sorry I can't be more help at the moment.

 

[dcgillespie]

Curt -- Neither. The AC you're reading is simply the ripple produced at the output of the rectifier.

Dave

 

[heyraz]

Dave,
I thought ripple produced by any diode's output would present as either positive or negative, yet Curt indicated it was "2+- VAC" which I interpreted as +/-2vac acknowledging the diode's 0.6 v voltage drop and latency.

Regardless, I think something else is at play because his overall results do not match mine, i.e. I did not have to replace the 390 ohm resistor with R100.
I think a schematic of his mod would help us figure things out.
Rich

 

[dcgillespie]

Hi Rich -- If the DC were perfectly filtered at the output of the rectifier then an AC indicating meter would indicate 0.00 vac, since there would be no fluctuation in the voltage to respond to. While ripple is hardly a nice smooth sine wave, it none the less represents a repeating fluctuation in the DC voltage present at that rectifier's output that an AC voltmeter will in fact respond to -- ergo the AC voltage reading.

To your point, the overall DC level present at the output of the rectifier in this case is always a negative value, but what it really represents is a base negative DC voltage level, with an AC component of ripple riding on top of it.

Dave

 

[heyraz]

the overall DC level present at the output of the rectifier in this case is always a negative value, but what it really represents is a base negative DC voltage level, with an AC component of ripple riding on top of it.

Hi Dave and thanks.
Maybe I was imagining how it would appear on an oscilloscope.
I pictured a base negative DC voltage accompanied by another negative DC voltage occurring at regular intervals as the diode's output pulsated as it switched on and off. To be honest, I didn't think of that pulsating voltage as AC, I thought of it as pulsating DC riding on baseline DC. I suppose it might look more like a (slanted) square wave than a sinusoidal wave, but nonetheless count as an alternating current.
I know this has nothing to do with the OP's question, it was thought provoking to me as I wondered why he measured a + and - voltage.
Thanks again, Rich.