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Old 6th October 2012, 08:04 PM   #1
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Default Thump on power up, pop on power down

Just finished changing caps on a vintage receiver Roberts Model 50.
I'm looking for a solution to the nasty thump on power up and a nasty pop when switching it off. I have attached a schematic of the power supply to see if someone can see something there I could do to stop this from happening.
I have 8ohm speakers connected and have tried the headphone jack as well, so the amp is loaded.

Perhaps there is some inexpensive design improvement(s) to this existing power supply that could be done. I don't really want to put any relays or speaker switches in unless that's the only fix for this.

I read an application note that said putting a MOV on the secondary side of a transformer would suppress transients, but I'm not sure if that's a proper solution or would do more damage than good. I've managed to repair a bad channel in this receiver and it is working good now except for this thump/pop so I don't want to ruin all my work I've put into it so far with some trial and error fix.

I do have a MOV available....P/N CNR 14D221K. But not sure if that would be suitable for this secondary suppression or maybe a MOV is better placed across the primary after the switch. Also it has been suggested to put a X1/Y2 type capacitor in line across the switch contacts.
Being an old basic design there is nothing on the primary side but the AC line coming in and the switch.
(See attached schematic photo)..
I'm worried I'm going to blow my speakers..
Thanks for any help
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Old 6th October 2012, 08:13 PM   #2
djoffe is offline djoffe  United States
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Can you post more of the schematic...e.g. the amps, too...not just the power supply?


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Old 6th October 2012, 08:21 PM   #3
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well, you can use one of these. it will cut in a shift and it will give a few seconds to power it up to speakers.The LSP-1 Loudspeaker Protection Board
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Old 6th October 2012, 08:28 PM   #4
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Quote:
Originally Posted by djoffe View Post
Can you post more of the schematic...e.g. the amps, too...not just the power supply?


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Sure...hope the photo is good enough to read..
Still a partial but it does show the rest of the power supply and the audio board.
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Old 6th October 2012, 08:48 PM   #5
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Quote:
Originally Posted by lanchile View Post
well, you can use one of these. it will cut in a shift and it will give a few seconds to power it up to speakers.The LSP-1 Loudspeaker Protection Board
I've thought about something like that lanchile....but it's more work and expense than I'm willing to mod.

I do have some 510nF (.51uF) 220VAC/630VDC film caps here. (8) to be exact.
One suggestion I got was to try a .01uF 600V cap across the switch contacts, which I don't have on hand.
Would one of these I have across the switch contacts be a safe and possible fix?
Like I said I'm a little skittish trying anything without some confidence that it won't blow things up. I'm so close to getting this fixed and running right, I'm afraid now of doing something that will be catastrophic!!

Thanks for all your suggestions..

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Old 6th October 2012, 09:08 PM   #6
djoffe is offline djoffe  United States
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Did it have this behavior before you re-capped?
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Old 6th October 2012, 09:12 PM   #7
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It could be the mains switch.
100nf across it or something similar would help.

Class AB amps are usually not too bad on thumps at power on/off.

I have seen serious problems with class D amps, i had to resort to a microcontroller holding the class d chip in reset on power up and down !
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Old 6th October 2012, 09:15 PM   #8
jaycee is offline jaycee  United Kingdom
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i'd say its likely to be sparking across the switch contacts, so yeah an X rated capacitor across it's contacts should help.
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Old 6th October 2012, 09:24 PM   #9
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Unless you are into some serious stuff, the turn on/off thumps are a fact of life for this amp. Its the in design of the signal path.
Every capacitor in the signal path needs time to charge up at power on, and during this time, the dc voltage on the previous stage is carried through to the next stage in the chain.
The 24v supply to the driver transistors will take much longer to charge up than the 17v supplies, so power to the outputs will be up before the driver stage has stabilised.
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Old 6th October 2012, 09:29 PM   #10
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Originally Posted by whizgeek View Post
Unless you are into some serious stuff, the turn on/off thumps are a fact of life for this amp. Its the in design of the signal path. .
I would still try the cap across the switch.
I had a disco with dual record decks and every time I turned on a deck i would get a huge crack through the speakers. I put a cap across the switches and that brought the crack down to an acceptable level.
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