Author Topic: Old Amplifier and Relay Issue  (Read 582 times)

Offline B_Unit

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Old Amplifier and Relay Issue
« on: May 19, 2017, 08:12:29 PM »
Hey Guys.

I have an old Kenwood ka-6100 Amplifier. I bought it a while ago off ebay in very good condition.

After a year I started hearing distortion on the left channel after inspection I had found the idling current on the left channel was unstable and fluctuated.

After this I decided to recap the entire Amp and also found a transistor gone bad.

I replaced it and then all was well until recently the distortion came back and now when I push the amplifier the protection relay cuts the connection and then reconnects after a few seconds.

The amp also runs fairly hot.

Could anyone suggest what could be happening or what I should begin to test.




« Last Edit: May 19, 2017, 08:18:42 PM by B_Unit »

Offline Steerpike

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Re: Old Amplifier and Relay Issue
« Reply #1 on: May 19, 2017, 11:46:00 PM »
Cm81 and Cm82 might be old, dry and leaky. The DC sensing time constant will then be way too short, making audio transients cut off the output, instead of genuine DC.

That has nothing to do with the distortion though, or the high temperature; is the idling current now correct and stable? Other leaky capacitors in the output bias circuit could be the cause of that.

Offline KOD

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Re: Old Amplifier and Relay Issue
« Reply #2 on: May 20, 2017, 11:12:20 AM »
Both bias pots VRm5 and 6 should be replaced with sealed multi-turn types for good measure.
Grundig/Eddystone/B&O/Montgomery Ward/NAD

Offline B_Unit

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Re: Old Amplifier and Relay Issue
« Reply #3 on: May 20, 2017, 01:53:54 PM »
Ill do some testing on those caps (CM81, CM82 and CM 83)

At investigating the actual board Diode, DM18 has been put into any RC configuration Were there is just supposed to be a Diode.

It seems like someone added a extra resistor in series with RM133.

Would this effect the cut off voltage/current for the relays control circuit since it looks like DM18 has control over DM19.

Ill replace the BIAS pots.

Thank you for the advice guys,

Byron

Offline B_Unit

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Re: Old Amplifier and Relay Issue
« Reply #4 on: June 18, 2017, 05:20:13 PM »
Update on this the Biopolar Capacitor (CM82) went dicky and lost a lot of its original capacitance.

Offline Steerpike

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Re: Old Amplifier and Relay Issue
« Reply #5 on: June 19, 2017, 12:14:43 AM »
DM18 is there to monitor the presence of mains, and cut the speakers off as soon as AC mains is too low, or vanishes. If the amp did not do this sensing, you get a situation where the amp continues to operate on the charge stored in the power supply capacitors, for a minute or so after the mains is switched off.
They may have needed to make it more sensitive to low AC supply, and so modded the diode.