Error in protection circuit? (771)

[surfnsound]

Sansui 771, here's what's happening: From tim to time (not every time) when I turn the unit on, during the press of the power button, I get a loud heavy pop/distortion/noise delivered to my right speaker for a split second. Only afterwards the really clicks and all normal.

During the pop/distortion/noise, I see my woofer move in and out +/- 1/2 inch, I am afraid it will get shot, I assume thats what the protection circuit is for?

I cleaned the relais contacts and thought the problem is solved, but no. It can be normal on/off - and then suddenly when I turn it on, again the short loud distortion. It will soon kill my AR 4ax woofer...

Any ideas?

 

[Hyperion]

Error in protection circuit? - possibly, the scenario here is that the relay may be briefly closing at power on thus giving you a burst of sound while the amplifier is 'settling down' during power on. Cleaning relay contacts seldom provides a lasting cure for any intermittent relay behaviour, however this isn't the problem you have.

To address this you really need to replace the relay and refresh the components (electrolytic capacitors) in the protection circuit, this may solve the issue but isn't guaranteed to.

 

[surfnsound]

Thanks Hyperion... that's about what I was worried about

I read here: http://audiokarma.org/forums/index.php?threads/fyi-sansui-protection-circuits.52137/

"D.C. Current Detection: The purpose of the D.C. current detection circuit is to prevent pop-noise generated when the power supply is turned on, and to protect the speakers from being damaged when a D.C. voltage is produced across the speaker outputs."

Also info here:

• http://www.canuckaudiomart.com/forum/viewtopic.php?t=28209
• https://www.reddit.com/r/vintageaudio/comments/2jtlkp/speakers_popping_when_i_turn_my_receiver_on/

That suggests:

• Check DC
• Check caps

So i'll check DC offset voltages according to service manual... and if not better, I'll replace caps on power board.

 

[surfnsound]

Okay, no luck with DC adjustments :-( Here is what happens when turning the receiver on:

• Should I change the caps now?
• Which ones?

This receiver is one of the best I have, this pop started suddenly.... Here's a video with the 771 connected to an el-cheapo speaker in the shop:

 

[Hyperion]

Some questions
Were you able to set the DC offset to zero? and if so was it stable?
After the power on pop, the protection circuit never kicks in is that correct?

You are probably going to end up completely re-capping the protection circuit (or having it done for you), and this may spread to most if not all of the receiver.

How confident are you about ordering new capacitors, removing old capacitors, and soldering in the replacements? - how 'handy' are you with this sort of stuff?

 

[surfnsound]

John,

Were you able to set the DC offset to zero? and if so was it stable? YES, I did this some months ago with the service manual and its still very stable, has not changed. I rechecked all fours measurements.

After the power on pop, the protection circuit never kicks in is that correct? The pop goes to the speaker.... and some seconds later (as normal), you hear the relay click and the sound then is a great as ever.

You are probably going to end up completely re-capping the protection circuit (or having it done for you), and this may spread to most if not all of the receiver. How confident are you about ordering new capacitors, removing old capacitors, and soldering in the replacements? - how 'handy' are you with this sort of stuff? I did recap my Wharfedale Dovedale 3's... So I guess I take a shot on it. But there are a lot of caps, maybe I should start with the most obvious ones? I ordered caps in the UK, but we have Mouser in Belgium, so I can order here.

Question: Shouldn't I start replacing the relay?

David

 

[Hyperion]

This is the bit of circuitry you are interested in...


The important ones (for your problem) are C07,C08 and C09.

Make C07 a 100µF, 50V, C08 a 220µF, 16V and C09 same value/voltage (but can be a film capacitor).

If this doesn't fix it then I would change the transistors, and then the relay.

 

[surfnsound]

Okay, lets do it ! I order the caps. Will change them first. Have already a relais ordered on ebay from Germany. Seems that many people also with Sansui 800 or similar have the pop problem, lets try to solve it. On my side, DC seems stable and +/- 10 mA around zero. Ordering the caps and replacing will take a week. I report back.

Thank you already !

 

[Hyperion]

Excellent, I strongly recommend you take some nice clear pictures of those capacitors as they are now, so you will have a pictorial record to follow when you put the new ones in.

 

[Overundr1]

Hmmm, Have you checked dc offset at initial power up (not after the relay clicks). You may have a differential pair of transistors that are not tracking well when initial voltage is applied which would allow dc to shift from a 0 value towards o as they settle in.

 

[surfnsound]

It works again !! I changed the relay tonight. All as normal after the relay change. I thought I first gonna change the relay as I ordered it already and then the caps. But all works after the relay change !!

• The relay: http://www.befr.ebay.be/itm/Sansui-...566847?hash=item35f3cd06ff:g:b40AAOSwA4dWIjoG

How to: The relay change was really simple: Just remove bottom cover, there is full access to the 8 solder points of the existing relay. Desolder, change relay, resolder, basta. 30 minutes opening to close receiver.

Before the relay change

After the relay change

Old relay (bottom), new one (top)

New relay in place

Hope it stay that way, otherwise I'll report back. So far so good, no more popping !! Sound come exactly with relay click.

Thanks for all your help !

 

[Bassblaster]