Chattering Relay in Krell KSA-250

[K-amps]

So I take the plunge and buy a Krell KSA-250 from ebay. Powered it up and find one channel's relays are chattering. Bzzz Bzzz Bzzz

(applying finger pressure to the cap of the relay mutes the sound but I can still "feel" the buzzing)

I have not checked DC offset or DC at the outputs and am "hoping" that this is only a symptom of a dried out smoothing cap on the relay's coils.

Any experiences are welcome:

Additional note: It is the speaker engaging relay; not the power up relay. This is the one that engages after 2 sec delay.

thanks!!

PS: Written to the seller.... no reply yet... why am I not surprised?

 

[johnnyx]

Buzzing relay

Does it buzz after a 2sec delay or does it buzz immediately?
If it's 50 or 100Hz buzz, then it could very well be a dried up cap.
If it's on a seperate "protection" pcb then it will make checking the components easier.
It could also check for a dc offset at the output though, so it's a good idea to measure that, but I don't think that would give a buzz. It's probably not a big problem to fix.
Good luck🙂

 

[K-amps]

As I said, the buzz is not immediete but after the protection circuit engages the speaker relays after 2 seconds or so.

Thanks for your comments. 🙂

 

[destroyer X]

Do a fast test, a dirty solution

Put a diode in relay coil, also invert it fast to see if it works without noise.... no result?.... put small condenser and will increasing it till 22 uF... no result.

DAMAGE!, real problem, DC off set...over current, over voltage, Lower voltage, transistor that controls the relay, or need a finger on it (ha ha ha, smile, please).... sometimes a rubber can be good to avoid vibrations.... because to much search, sometimes we make a short with the multimeter points and...bummmmm!

Carlos

 

[K-amps]

Carlos,

I am sure Krell would have put a diode on the relay coils... the condensor/ capacitor is what I hope is the issue.

No DC offset at all, the amp plays good.

K-

 

[destroyer X]

That's ok, you are doing a good job for him

So, go on doing this good job.

Thank"s to inform me that he already knows.

Carlos

 

[K-amps]

Carlos,

You look much more distinguished with a greying beard/Gotee ... the older photo does not have much character.

Off to the nearest Churrascaria!!!

 

[EchoWars]

If the relay circuit uses a transistor to engage (and it likely does), then I'd suspect it's either got too much C-E voltage on it (won't saturate because of age), or just plain leaky from C-E. Pretty common.

 

[K-amps]

Echo,

It does engage the relays, now as I keep it on for a while, the sound of buzzing is low... the amp plays ok...

Wouldnt you say that the chances of a aged cap is higher?

K-

 

[EchoWars]

From my own experiance...no. I mean, both are certainly possible, but for relays, the engaging transistor operates continually in a saturated state. C-E voltage rising or leakage from aging is pretty common...and on units I restore, the driving transistor gets replaced as a matter of course.

Chattering is not good for the thin plating on the relay contacts ..it will pit and wear them quickly.

 

[johnnyx]

This kind of circuit senses the ac voltage from the power transformer independantly of the main reservoir caps. It will probably use a large series resistor and cap to give the 2 sec delay. there will probably be a diode in reverse to discharge this cap quickly when switched off. I don't know the actual circuit, but they all provide the same function. The time constant is chosen so that it will give a steady output voltage, and hence relay switched on, during the 10mS pulses from the transformer.
time = 1/f = 1/100Hz = 10mS
If the time constant is say 12ms, then the output will be steady. When the power is switched off, the relay switches off after 12mS.
If the fault reduces the time constant to say 5mS, then the relay will see pulses instead of a steady voltage, causing relay chatter.
There are many ways to achieve this. there may be two seperate timing devices, one for turn on delay, the other for turn off delay.

Try to identify and change the components in this circuit.
Krell might use a tricky-dicky circuit, just because it's Krell.

Hope this helps.

 

[K-amps]

Thanks Echo and Johnny,

Looks like I will need to spend more time than I thought initially.... I just hope there are no custom IC's ... I suck with IC's

 

[ericwwk]

Thanks Echo and Johnny,

Looks like I will need to spend more time than I thought initially.... I just hope there are no custom IC's ... I suck with IC's