30V-45V AC on Speaker Terminals (KA-2000A)

[Birdpirch]

Goooooooood evening my friends!

This is my first post, I promise I scoured for info before making it.

I have just pulled apart and put back together a Kenwood KA-2000A which I'm gifting to a mate.
I never really used it, and suspected it had an issue tripping the houses RCD when connected. This was never confirmed, so don't let that throw you off.

After putting it back together tonight, I decided to power it probe it and see if I could detect any AV voltage.

I did. It started with 45V AC across the ground post on the amp and one of the speaker terminals. As I left it plugged in and did some googling, it dropped to 30V. I am puzzled as to why this is the case. I checked my other vintage amps, and don't experience any AC voltage when they are powered on (without playing anything) and so I have come to the conclusion the amp is a hazard?

Any and all info is very helpful.

Thank you!

 

[Birdpirch]

capacitor coupled output these ?, put a resistor across the o\p and see if its still there

Hey mate, my electrical knowledge is a little rusty.
Am I checking for a coupling resistor? Or checking if its dead?

While googling this topic I found pretty much no info, only a vague mention of a coupling capacitor allowing AC flow while blocking DC current.

This was only a couple minutes of looking, but found it strange that no one else has experienced the same issue.

More info about what to look for would be ace!
You can look up the circuit diagram online for this amp if you need.

Thanks!

 

[dfoulk]

Hey mate, my electrical knowledge is a little rusty.
Am I checking for a coupling resistor? Or checking if its dead?

While googling this topic I found pretty much no info, only a vague mention of a coupling capacitor allowing AC flow while blocking DC current.

This was only a couple minutes of looking, but found it strange that no one else has experienced the same issue.

More info about what to look for would be ace!
You can look up the circuit diagram online for this amp if you need.

Thanks!

Put an 8 ohm resistor across the + and - of one of the speaker connections and see if that gets rid of the DC voltage. Are you sure you have 45 volts AC? There should be a coupling capacitor inside for each channel, something like a 2000uf maybe 25-35 volt rating that filters out the DC which would fry your speakers.

 

[Birdpirch]

Put an 8 ohm resistor across the + and - of one of the speaker connections and see if that gets rid of the DC voltage. Are you sure you have 45 volts AC? There should be a coupling capacitor inside for each channel, something like a 2000uf maybe 25-35 volt rating that filters out the DC which would fry your speakers.

Ill give that a try tomorrow! Thank you.

And yeah pretty certain. I mean, I have a $10 multimeter which has an AC setting, and it reports AC voltage as stated haha. I switched it back to DC to confirm at least if it was reading correctly and I tested the 20V DC mode on my spare car battery - so as a bare minimum it is reading DC voltage (whether it's accurate is another question) and I am pretty confident that it is actually reading an AC voltage.

It's odd. I will check what you've mentioned above.
Thanks for the help, if you think of anything else let me know.

 

[cronie]

Very strange, where would this amp even GET 45 vac? I would like to know the frequency of this voltage. O-scope.

 

[Birdpirch]

I actually may be able to snag a scope from work!
Im going to check that out tomorrow night.

Very strange, where would this amp even GET 45 vac? I would like to know the frequency of this voltage. O-scope.

 

[gadget73]

you might get a bunch of AC if its humming loudly for whatever reason. Without a scope or something this is all guesswork though.

 

[dfoulk]

Ill give that a try tomorrow! Thank you.

And yeah pretty certain. I mean, I have a $10 multimeter which has an AC setting, and it reports AC voltage as stated haha. I switched it back to DC to confirm at least if it was reading correctly and I tested the 20V DC mode on my spare car battery - so as a bare minimum it is reading DC voltage (whether it's accurate is another question) and I am pretty confident that it is actually reading an AC voltage.

It's odd. I will check what you've mentioned above.
Thanks for the help, if you think of anything else let me know.

I have a little Kenwood KA-2002 and without the load of the speakers it has something like 20 volts DC, but with the speaker load, the DC is filtered out by the coupling capacitor.
I was not familiar with this behavior of a single ended amp without the speaker load before and when I first fired it up after re-capping and up-dating, I wondered what the heck was wrong.
A posting of my problem provided the answer of needing the speaker load present!

 

[Ylli]

I mean, I have a $10 multimeter which has an AC setting, and it reports AC voltage as stated

A good meter will ignore DC on the AC ranges. Some of the cheap meters allow a DC input to 'bias' the AC voltage readings. It is unlikely that what you think you are seeing is actually there.

Borrow (or buy) a better meter. Put a resistive load on the speaker terminals - anything from 8 ohms to 1000 ohms will be sufficient to bleed the charge off the coupling caps. Then take a reading of AC voltage and DC voltage across that load resistor (speaker terminals).