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Crate GX-1200 - Problems persist after new parts
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Originally posted by g1 View PostDo not connect the speaker or any load and solve the issue of DC at the output. Once that is solved you will not have high current when speaker connected.
You will not be able to get proper DC conditions til the supplies are balanced.
If you have a variac, you do not need the 15W limiter bulb, are you still using it?
Currently it's at +3.01 and -3.5x
The 500 millivolts is enough to throw it off? Agreed?
Al
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Do either of you have a 1200 to take some reading on, especially around the Zener diodes and R89 and R90 for the +-16 rail supply voltages?
What voltage drops do you measure across R89 and R90 with 120VAC input and 33VAC input.
Having some idea of what the normal current is through R89 and R90 will help me decide what to do.
I've already more soldering on this PCB than I originally planned. I gotta figure out which current node to chase on the +16 rail.
That would a lot, I hope.
Thanks AlLast edited by skidmark; 03-14-2025, 01:02 AM.
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Originally posted by skidmark View Post
So we're in agreement, the unbalanced +- 16 is the immediate culprit?
Currently it's at +3.01 and -3.5x
The 500 millivolts is enough to throw it off? Agreed?
Al"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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Well I'm inclined to agree with you, however I'm facing a quandary. At 30% variac I got over 1 amp line current. The amp is fused at 2 amps, and if I leave it there very long
the heat sink get scalding hot. So I can't hit it with full line voltage.
All 4 finals are getting turned on bucking each other.
Can the design of this amp handle cranking line voltage up via a variac?
One thought I had was to lift one end of all 4 emitter resistors on the finals, then crank it up and try to troubleshoot.
This is why I was hoping one of you had a 1200 to get some readings for me.
AlLast edited by skidmark; 03-14-2025, 03:07 AM.
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1 amp at 30% variac is too much. Anything making the heatsink hot with no load connected it too much.
Yes, you can try disconnecting the 4 emitter resistors at the output devices. Bring it up slow and see what you get. IC3 outputs are what is probably turning on outputs too hard.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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No I don't have an amp here, I doubt anyone will. Where are you at with variac and power supply voltages? 1.3V at IC is already too much, assuming one is plus and one minus.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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What kind of variation can you get from those IC3 voltages by adjusting the bias trimmer all the way to each end?Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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If you can remove the output transistors, unsolder the emitter resistors or some other method of disabling the outputs and get the amp up to full line voltage, recheck the voltages coming from IC3. See if it still puts out enough voltage to turn the outputs full on. Also, have you tried turning the bias pot all the way off to see if that allows you to bring the amp up without overcurrent?"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22
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I like the idea already suggested about making sure bias pot is at its coolest setting. Also make sure d13 d14 and possibly d29 are good. Removing IC’s is hard on the board too. Is the IC getting warm?
Edit: what about D11 & D12? We’re they tested?Last edited by DrGonz78; Yesterday, 02:04 AM.When the going gets weird... The weird turn pro!
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Originally posted by DrGonz78 View PostI like the idea already suggested about making sure bias pot is at its coolest setting. Also make sure d13 d14 and possibly d29 are good. Removing IC’s is hard on the board too. Is the IC getting warm?
Edit: what about D11 & D12? We’re they tested?
IC3 pin 2 = 229 - .215 mv
IC3 pin 1 = +1.39. No change from end stop to end stop
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