Kenwood KA-801: powers on intermittently

[Stack]

I've been having a very strange issue with my Kenwood KA-801 recently and was wondering if somebody could point me in the right direction for starting my troubleshooting:

Recently, the amp has been having intermittent issues powering on, and it's been getting worse. 1 time in 10 the amp will power on with no issues, but 9 times out of 10 it won't power on at all.

Power runs through the unswitched outlet just fine, but not the switched when it's in the failure state. VU lights don't come on, relay doesn't click, it seems like it's flat out not getting power. I may have seen the VU needles twitch but I'm not sure of this.

When it powers on fine, it's 100% good. Works flawlessly. I've moved and toggled every switch, jostled the power switch, and shimmied every connection; I can't get it back into the failure state except by switching it completely off.

My assumption is that it has something to do with the power switch itself and that's probably where I'll start, but wanted to run it by the community to see if anyone else has run into something similar and maybe the power switch is a red herring. Also maybe worth mentioning that it's a dual voltage model, and the power switch does look like it was replaced at some point in the amp's history, because it seems to be one off of a 9100 with the end filed down.

Thanks!

 

[DatunWalnut]

Could be a red herring. When you switch it on, do you hear a buzz from the mains switch? That's sometimes an indicator of a switch that's carbonised or not operating properly. Incoming fuses on the main board (if it has any) can be a problem. Sometimes the fuse holders lose springiness and intermittently don't touch the fuse. Also, the voltage regulation section may be a problem. I've not worked on your amp but sometimes the power section runs so hot that over the years, a transistor will desolder it's self. And will now and again be disconnected. Above all, don't take the lid off without disconnecting from the mains.

 

[kwayde]

Unswitched outlets always have power regardless of the power switch position. The switched position is what it says. The power switch cuts off/turns on power to that one.

The switch contacts are either bad or dirty from arcing. It will need to be opened up to see what the problem actually is.

 

[Stack]

Unswitched outlets always have power regardless of the power switch position. The switched position is what it says. The power switch cuts off/turns on power to that one.

The switch contacts are either bad or dirty from arcing. It will need to be opened up to see what the problem actually is.

The more I think about it, the more I think this is very likely the case. There's no other reason that the problem would only occur when switching off and back on that I can think of.

My first line of testing was just going to be spraying some deoxit in the switch to see if it temporarily helps the problem, but my last can sprouted legs and disappeared so I have to wait for another one to arrive.

 

[Stack]

Could be a red herring. When you switch it on, do you hear a buzz from the mains switch? That's sometimes an indicator of a switch that's carbonised or not operating properly. Incoming fuses on the main board (if it has any) can be a problem. Sometimes the fuse holders lose springiness and intermittently don't touch the fuse. Also, the voltage regulation section may be a problem. I've not worked on your amp but sometimes the power section runs so hot that over the years, a transistor will desolder it's self. And will now and again be disconnected. Above all, don't take the lid off without disconnecting from the mains.

I did try to reseat the fuses, but they seemed to be fine. I feel like if it were a loose transistor the failure mode would be different -- I would probably see it acting up while the amp was on and running, but it never does. It's only between switching off and on.

Given this, and how it's failing, I'm pretty set on it being the switch now. I might take some voltages to verify this, but it just seems most likely

My first line is going to be trying to clean out the switch with contact cleaner. If that doesn't work I might try bypassing the switch to see if that changes anything, which would rule out the switch as the problem once and for all. I'm avoiding this because getting the switch out is an annoying process, but if it must be done it must be done.

My next action would depend on whether or not the problem is the switch. I do have a spare, but the paddle is broken off on it, and it doesn't look like you can easily disassemble these to take parts from one to put in another.

 

[p4spooky]

Bypass the power switch and put it on a power strip with a switch. See if you are able to power it on consistently. If it works, you need to replace the switch or move to unused contacts if the switch has one.

 

[loudnoises]

If there is a power relay not clicking in an older piece like this, it's next in line to check after that switch. A signal relay shouldn't affect anything but the output signal ideally.

It's not the most common, but the closer to the coasts you are, the more likely. If the rca plugs on the back have a layer of corrosion the relay contacts could as well. Especially if a unit sits in storage long term. Frequently used relays seem to burn off the corrosion or something.

 

[Stack]

Update: it was the switch.

I pulled it apart, and the contact leafs were completely burned through on one side. the points were very dirty, so I assume that it's probably been making very poor contact for a very long time.

I fixed it by scrubbing the contact poi ts clean amd flipping the leafs over so that the unburned side is making contact now.

Thanks for the support on this!

 

[redk9258]

You should consider doing the triac mod.

 

[Stack]

You should consider doing the triac mod.

That's definitely on my radar after seeing the state of that switch. It's very lucky that the mechanism reused parts of a 3 position switch and I could reverse the leaves.

It's not even something I had thought to think about until suddenly it was a problem.

 

[DatunWalnut]

That's definitely on my radar after seeing the state of that switch. It's very lucky that the mechanism reused parts of a 3 position switch and I could reverse the leaves.

It's not even something I had thought to think about until suddenly it was a problem.

I've modded a Yamaha that way. The mains switch was completely useless. I managed to find a much lower amperage Yamaha switch from a cassette player that fitted the fascia. That mod did the trick