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
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
Attachments
Can you post more of the schematic...e.g. the amps, too...not just the power supply?
Update My Dynaco
Akitika GT-101
Update My Dynaco
Akitika GT-101
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
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.
Attachments
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..
BFC238320514 Vishay BC Components | BC1874-ND | DigiKey
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.
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.
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.
if it is a big thump...the cap in the switch will not solve this problem. That sounds like caps charging/discharging at turn off/on and in some amps this is common since they do not have output relays to avoid this. also with high SPL speakers you can hear it louder. I had an Adcom amp that had this thump noise at on/off it is not bad for speakers as long as you measure the voltage at the outputs when you turn it on/off and see it is safe for speakers. some amps designs are like this...
Don't really know...it was essentially dead when I got it.
I found 2 open 270ohm 2W power resistors on the output transistor stage which I replaced and an open e-cap in the audio board (the section before those output transformers).
I have some new .47ohm replacement resistors that I can replace at the output transistors as well as new .01uf film caps, but I haven't replaced those yet.
I did replace all the small e-caps on the audio board. And as it sits right now both channels work now.
I also replaced the PS e-caps to see if that would improve anything but nothing really changed there (that I can tell)....I tested the old PS filter caps and only 1 was bad (open). it was C431 the 50uf 50V between the +17/-17VDC rails.
After replacing that the rails now measure +19/-19VDC. Before they measured +24 and -13VDC.
Also the thump when you switch it on is not too loud, but the pop when you switch it off is really loud even with the volume turned all the way down!
I've been thinking about putting the old filter caps back in. I replaced them with same values and voltage. I used radial Nichicon VZ and and axial VX series. These were much much smaller then the original axial style metal cans that were in there.
All the older caps also measured much higher when I removed them than they're listed values...i.e...both 1000uf read 1650uf.
I do have a couple of 2200uf 25V SM series that I could try in place of the 1000uf....if you think that would help?
Is it possible my replacement filter caps are causing this problem?
Or is it something to do with the output transistors and maybe I should replace those .47ohms and caps?
try disconnecting the pilot lamps. they would have a large current draw on switch on, delaying power up times.
otherwise build a dual tracking dc servo for the return legs of the driver transformers
There is only one pilot lamp installed right now. It's the only good one left.
The others including the stereo indicator lamp were burned out.
I'm sure your correct whizgeek that this may be the way it is per design.
I'm not in a position to spend more money on this unit and was hoping for a cheap easy fix. Hard to believe it has such a nasty pop!
I suppose the cheapest workaround would be to put a switch on the back to the speakers and turn them on after the receiver is powered up and switch them off before it's switched off.
Would using a standard DPDT toggle switch for each channel be suitable for that.
I'm willing to try something simple if it is something I have on hand and won't cause the thing to melt down...
I have these here...would there be any harm or damage caused in trying one across the power switch contacts?
http://www.digikey.com/product-sear...6AD0417-D23F-40FB-A642-F8D8A33F49FAcs8pp2mfAi
You need a 100n 250v X-class in series with a 100 ohm resister across the live wires after the switch to take care of the switch off click. A speaker protector with turn on delay is the only solution for your turn on thump.
Mac
The Book Worm - DIY Audio Ebooks and Manuals - Speakers, Moscode Amplifiers, Electronics, Map Reading, Free Capacitor Intro.
Mac
The Book Worm - DIY Audio Ebooks and Manuals - Speakers, Moscode Amplifiers, Electronics, Map Reading, Free Capacitor Intro.
This is the basic problem with any mass produced solid state amp. in the silicon era before the the general use of relays around 1980. Even capacitor coupled output designs could deliver a strong thump.
Some high quality units seemed to have had care taken to reduce DC offsets and the imbalance in the charging currents to the preamp and ancillary circuits (this usually causes most of the noise) but all amplifiers had some degree of "thump/crack" and it was once part of everyone's audio experience. I notice that a lot of people who weren't around then, fret about damage to their speakers or just the surprise of it as they aren't accustomed to this in modern audio systems which have muting/protection circuits.
There is an important issue with re-capping that DIYs often ignore by fitting low ESR caps everywhere. Be careful buying caps on a popularity basis - you don't want low ESR cap grades for filtering DC (the big reservoir caps) and they will probably make the noise pulses shorter and louder. You'll have to replace caps anyway, but just use regular types, a little larger in capacitance (<= 100%) may actually be helpful.
As in previous posts, this muting is in the form of a speaker protection relay disconnecting the speakers until a few seconds have elapsed from turn-on and dropping out immediately the power is turned off. You can purchase pre-built universal modules of these (a PCB with ready built circuit) on E-pay for peanuts or you could DIY completely from schematics shown regularly on the forum, if you wish.
Unfortunately, short of just reducing the sharpness or "crack" of the sound , you can't make the effect disappear with a simple tweak or part replacement. I think though, that unnerving as the effect may be, any speaker that can't handle the typically brief burst of energy at moderate level is sure not going to handle music too well either. The rest is down to you doing the proper mod. or simply getting used to retro audio.
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