SIT Startup Problems - What did I miss?

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dantwomey

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Joined 2002
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#1
I built the original circuit exactly as pictured here:
completeschemptone-1.jpg





I need help here figuring our what I missed when wiring my SIT.

Observations:

All four lamps light up what appears to be correctly and they increase or decrease in brightness when I adjust P1 on either channel. I also have an ammeter in line with the main power circuit so I can see current increase and decrease when P1 is adjusted.

Problem:

When drawing 2 amps bias I have 25vdc on the speakers terminals. If I increase the bias to 2.6 amps I get 8VDC on the speaker terminals.

Thoughts anyone?

Regards,
Dan 🙁
 
#5
Slipped in a spare 47000uF/25v electrolytic and got the same results. My current vote goes to some sort of miswiring. Bias circuits and main PS supply circuits seem to be behaving 'normally' so I'm inclined to look elsewhere. Since both channels are consistent I think it's safe to assume I made the identical mistake in both channels. Hmmmmm........😕

Regards,
Dan :hohoho:
 
#15
Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
 
#17
Blues said:
Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
Click to expand...

Will measuring from point "A" to ground be the right idea?

Test1-1.jpg


Regards,
Dan :hohoho:
 
#18
Yes that will work...just disconnect at the bulb to PS (removing bulbs from sockets work too) or at Source/GND connection to be sure there is no influence whatsoever. And you can measure main PS unloaded too.

Go Seahawks!!!
 
#19
Dan
1 Possibly have speaker connected to wrong side of c6.
2 speaker open or not grounded to the correct place.
are you measuring with a dvm? The 47,000uf 25V substitution
was too low of voltage may now be bad. With a dvm and the speaker
not loading the amp c6 would probably leak enough to show dc
voltage on both sides of capacitor.
good luck
Jim
 
#20
Blues said:
Dan,

You should test your power supply section before connecting the power amp section. You can detach at the bulb, at the wiper lead out, and keep the GNDs connected together. You can then measure the 7912 Vgs biasing regulator if it does go from max value to min of -12V by varying P1.

Once you are sure the PS section is okay then you can focus troubleshooting the PA section.
Click to expand...

Both bias circuits range from -2.0vdc to -11.9vdc when measured from point "A" to ground. An unloaded main supply measures 57.6VDC at point "B" on both channels. I'm starting to consider a different 'style' of ground connection between my bias circuit and main circuit.

Here's a more accurate schematic.

Test1-2.jpg



Regards,
Dan :hohoho:
 
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