170 DC, changing voltage with selector and dc offset when off.

[Jonas_Skaegg]

Hi guys!

I have a 170 DC I have been working on, on and off for 1 year or so. Used it some in between.

All caps changed, elcos and polypropylen at least. New speaker relay. new trimmer resistors for dc offset, new output transistors. The last thing I did now was changing the big filter cap to 2 15000 uF Nichicon Gold Tunes.
Quite many new resistors on both sides as well, making the bias match.

I also changed the 2 safety caps over the power switch (e version). I put in 0,01 uF 440VAC Film caps.

However, after a couple of days use working very well, I heard it made a frying sound when off. I discovered that it was the voltage selector on the back that had a bad connector and was arcing. It actually broke off inside when I started cleaning it. By changing from 220 to 240 volts I can bypass this bad connector and use of only good connectors for it. As I have around 238 Volts in my outlets It should be fine, But to my first question.

What happens with the voltage in my regulated supply, and how do I check and maybe adjust it? The secondary side of the transformer should see a voltage drop, right? I see no trimmer for it in this amplifier. The service manual doesnt mention it at all either, as it usually does for the marantz stuff I have been working on?

Also, when powered off I can measure dc offset over the speaker terminals, like a couple of hundred mV. Then when I power it on it goes to close to zero. Is this normal behavior? I measured now, after the amp has been shut off for 1 day, and I had 100 mV offset. Both sides do this.

Thanks!

 

[KutzlerTrans]

Check your speakers and see if its a crossover cap discarging. Just a guess considering the relay should have cut any amp discharges.

Your voltage should work as long as your volt meter is accurate. Check your 238 vac under a load first . Plug a high amp draw into your outlet.
I am not familiar with Swedens power supply.
Your transformer will get warm, but it should not get real hot too fast.

 

[Jonas_Skaegg]

No speakers connected.

I measured after the rectifier, It measured 50,1V. The SM says 50V so I guess the voltage change is no problem.

Im still puzzled that it can show dc-offset...with no power.

 

[blhagstrom]

What does your meter show when not connected to anything but set to read DC?
What does it read when you short the probes when set to read DC?

I wonder if there is some parasitic voltage on the speaker terminals when the amp is off.

Since the voltage selector fried, I wonder if there is something terribly wrong that caused that.
Those selectors are quite sturdy.

 

[Jonas_Skaegg]

Thanks for the input!

The meters show a couple of mV in the air. Probes touching = 0,0 mV. Both my DMM show the same.

Today I measured when the amp had been unplugged for 1 day, it showed 30mV at one of the terminals. Then I plugged in the power cord, it shows 200mV! how will the AC react in my snubber caps? 0,1uf film caps rated for 440 VAC. Even if something gets past the power switch, the speaker relay is disengaged. hm.

About the voltage selector, I dont think it is connected to the other problem. It is actually really flimsy, and has probably been arcing due to bad connection for a while. when cleaning it and moving the selector I was thinking I should bypass ut completely and hard wire it instead, due to its weak connections.

 

[blhagstrom]

Capacitors pass AC, so a snubber acts as a path, I believe.

I ran into a hum problem once with capacitors on power switches when I miss-wired for a relay installation.

Large capacitors can actually acquire their own charge. I am not sure if its a parasitic thing, there is a term for it. But, the relay should be isolating the direct coupling to the filter caps, so I am out of ideas.

Maybe you've found a new form of free energy.