Customer sent me this amp stating that it would whine and squeal. Most likely due to RCA shield disconnects. Now I'm almost done with this one. I replaced PS caps, R52 and R53 as they were missing. Previously the PS fets were replaced by someone else but that repair seems to be OK. Lots of other misc stuff like RCA shield connections were repaired, a power terminal replaced, Rem tab, etc. Normal RF commonalities. The channels have never been rebuilt.
On the test bench, the amp has a TON of hissing noise through speakers. If I un-plug RCAs the noise goes mostly away but is still present. I tried different RCA Y cables and flipping the 2/4 input switch and there is no difference. If driven harder the noise goes away. Also if I reduce the input voltage to <13v the noise also goes away. Seems the noise is only present when the PS is regulating. I replaced the 1n4003 (D3) diode as I thought it was shorted but I was fooled by having RCAs connected. Engaging the LP crossover eliminates the noise. FP/HP and the noise is present. The board has no damage and all the caps even the tiny ones look and smell OK.
I'm coming up blank as to why this is happening. Any suggestions?
On the test bench, the amp has a TON of hissing noise through speakers. If I un-plug RCAs the noise goes mostly away but is still present. I tried different RCA Y cables and flipping the 2/4 input switch and there is no difference. If driven harder the noise goes away. Also if I reduce the input voltage to <13v the noise also goes away. Seems the noise is only present when the PS is regulating. I replaced the 1n4003 (D3) diode as I thought it was shorted but I was fooled by having RCAs connected. Engaging the LP crossover eliminates the noise. FP/HP and the noise is present. The board has no damage and all the caps even the tiny ones look and smell OK.
I'm coming up blank as to why this is happening. Any suggestions?
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Yes. Noise only when it modulates. When input voltage drops to about 13v the amp no longer modulates and the noise goes away.
R23 is 100k and measuring well.
C1 off the PCB is measuring 0.15uF. With C1 removed, amp still produces noise at >13v.
Question: I had found R53 missing, and also R52 partially detached. Should I remove these maybe? R12 and R55 are 220ohm not 22R like the 800a4 schematic is suggesting. Is there something perhaps going on in this area of the PS transformer snubbers?
C1 off the PCB is measuring 0.15uF. With C1 removed, amp still produces noise at >13v.
Question: I had found R53 missing, and also R52 partially detached. Should I remove these maybe? R12 and R55 are 220ohm not 22R like the 800a4 schematic is suggesting. Is there something perhaps going on in this area of the PS transformer snubbers?
Do you see a lot of ripple on the rail voltage when it makes noise?
Those should be 220 ohms (that's what most Rockfords of that era use). Is this a PC-3085?
Those should be 220 ohms (that's what most Rockfords of that era use). Is this a PC-3085?
I removed R52 and R53 and the ripple improved, and noise to speakers is now completely GONE. Amp seems to be perfect like this actually. Thoughts?
That may be a good solution. I don't know what that network does and I don't see it on many other amps.
Confirm that there is essentially no high-frequency ripple on the rails when running the amp hard, into a load to confirm that the rail caps aren't bad.
Confirm that there is essentially no high-frequency ripple on the rails when running the amp hard, into a load to confirm that the rail caps aren't bad.
4-ohm bridged. 1khz sign wave. The bottom line is negative rail. Both probes calibrated. 10v/Div on DC coupling. First image is sign wave hitting clipping. Second is just before clipping. This is at 14.5v in put DC. Amp draws 45A at clipping across 2 channels. Left 2 other channels open no load.
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I wouldn't expect that much ripple on the rails.
Is the positive rail that bad?
Could you have a rectifier with an open internal diode?
Does this change at lower input voltage where the amp is at full duty cycle?
Are C1 and R23 back in the circuit?
Is the positive rail that bad?
Could you have a rectifier with an open internal diode?
Does this change at lower input voltage where the amp is at full duty cycle?
Are C1 and R23 back in the circuit?
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The negative rail is about the same as the positive rail.
C1 and R23 were in circuit.
This amp only has one positive and one negative rectifier. They appear to be OK in circuit.
No change at solid 14.4v vs 12.2v.
C1 and R23 were in circuit.
This amp only has one positive and one negative rectifier. They appear to be OK in circuit.
No change at solid 14.4v vs 12.2v.
Do you see a similar waveform/ripple at the 12v input to the amp?
The ripple on the rails should be fairly smooth (not showing the notches in the waveforms you posted).
The ripple on the rails should be fairly smooth (not showing the notches in the waveforms you posted).
Yes same scope images for 12v and 14v. The notches appear when the amp is brought to clipping. It’s drawing 50A only loading 2 channels normal.
What about just before clipping.
Previously, I was referring to the ripple on the 12v supply input feeding the amp.
Previously, I was referring to the ripple on the 12v supply input feeding the amp.