Excessive current on start up/ shut down?

Billy Dio

Billy Dio

Well-Known Member
A stereo I just got needed deoxit badly and required some effort to get at the pots and toggles. Also had to resolder 2 wires that were weak.

Anyways, got it back together and noticed on start up it's sending a pretty big spike of current to the speakers with volume at zero. Definitely not normal, and I didn't mess with anything other than the 2 wires that were frail. Shut down gets a simple surge. Sounds OK otherwise. Thoughts?
 
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How are you seeing this? Output meters?

What receiver is this? Does it have a delay relay to switch on the speakers? i.e. do you get a click a few seconds after power-up.

Do you hear sound out of the speakers when this happens? What does it sound like?

Any chance there are boards or phenolic wafer switches that got a lot of solvent on them during the cleaning? Boards and switch wafers can soak up Deoxit and become somewhat conductive...
 
It's an MCS 3233. I can see and hear it happening. The woofers are getting thrust "out" and the sound is an electrical "vvvuuump". Lasted about 2 sec and I was surprised it didn't pop them.

I pulled all the switches out (still wired in) and stuck paper towels under them before using the deoxit. The FM, AM, AUX, etc. switch was disassembled and cleaned by hand as it was the worst. Pots seem to have the same material as a non coated circuit board, like a pressed board or something.

I gave the unit a good 8 hours to dry with a fan blowing over it. Used Deoxit D5. Can gives no warning of lingering? Will the stuff evaporate or what?


EDIT: Unit doesn't have relays that click.
 
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FWIW, my MCS 3212 & 3233 both "thumped" on start up. I thought it was just normal, as there isn't a relay for a soft start.
 
Nah, this isn't a typical "thump". My other 3233 sometimes had that thump IIRC and it was subdued and quiet. When this occurred with the current 3233, it was like an electrical transformer noise through the speakers, and both woofers were pushed so hard it distorted the rubber surround. I'm thinking it's something to do with the deoxit treatment, although I didn't spray the power switch, but I did fix one of it's wires that was brittle. It didn't do this before I opened the unit, but I was fairly careful with everything so trying to rule out the deoxit or part failure.

Figure it'll need all new caps anyways, but if something else was going, was hoping for insight.
 
D-5 will evaporate. F-5 leaves a slight lubricant on the affected area. I think the thump is exacerbated by the wire you resoldered. Possible the "WEAK" connections were acting as resistors and now are allowing full flow of current. REPLACE the MAIN FILTER CAPS and check for output coupling caps. If it has them replace them also.

You can turn the spkr switch to "OFF", turn on the unit, count to 5, and turn on the spkr's. Make sure the volume is down 1st.
 
larryderouin said:
D-5 will evaporate. F-5 leaves a slight lubricant on the affected area. I think the thump is exacerbated by the wire you resoldered. Possible the "WEAK" connections were acting as resistors and now are allowing full flow of current. REPLACE the MAIN FILTER CAPS and check for output coupling caps. If it has them replace them also.

You can turn the spkr switch to "OFF", turn on the unit, count to 5, and turn on the spkr's. Make sure the volume is down 1st.
Click to expand...

That makes sense. The wire was weak and brittle right at the solder point. I loosened up the twist ties and pulled the wire out more and severed back about 2" of wire that had a slight hardening of the rubber sleeve and soldered it back. Other power switch wire appeared OK.

I'll do the caps then and see if it subdues the turn on spike.
 
Is it possible a smoothing cap(s) failed or are faulty? Are they ever used to level out the signal to prevent extreme fluctuations? Just curious as I was reading about them.
 
The large "THUMP" on direct coupled or cap coupled units at startup and shut down is generally acknowledged to be leaky MAIN FILTER CAPS or output coupling caps. They are large enough to allow sufficient DC LEAKAGE during initial charging and again at shutdown.
 
Thank you for sharing that as I'd never heard of this before. I know with car amps, "thump" on is either a ground issue, or older head units that power up the remote power too early. I haven't tried turning this on since posting with the speaker switch "on" as the surge is WAY beyond a typical thump and I don't want the speakers coils getting cooked.

DC leakage makes sense too as that's what the speakers look like they were getting, that is a large spike of DC current that was holding the speakers in a "push" out for 2 seconds as opposed to just a quick "thump".
 
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