My trusty and warm (literally!) Krell KMA-160 monoblocks have finally reached the point where I turned one on a few days ago and was soon greeted with a descending whistle sound which means that it's time to recap the amps before things get serious.
I can hardly complain. I bought the pair back in, I think, 1996 and they have 1988 date codes on the PC boards.
You MAY recall that I had a story to tell about these two. I bought the pair for 500 bucks plus shipping costs.
But they'd been in a house fire. They hadn't been burned but they had been hosed down and put out with the fire, flooded and full of water and debris from the fire, and then left to rot and corrode in a basement for two years before I found and bought them. I still think it's lucky they weren't just scrapped!
40 hours of my own hard labor later, spent almost entirely doing a complete teardown and cleaning of each amp, I had them both back together and singing in my system. Electrically speaking one amp never had any problems and the other had one open power transistor out of 24 and its emitter resistor was open, too, so it would have worked like that, just down by maybe 6 watts in output power.
So they've definitely been a bargain!
I never have gotten around to rebiasing them to run in Class A for only the first 10 to 20 watts, which would certainly make them run cooler. Perhaps I should do that this time around. But my speakers are inefficient at 87 dB rated sensitivity (Aerial Acoustics 10Ts) so if I want to play fairly loud I'm sure the amps would drive past a 20 watt class A limit pretty easily. It might be best just to leave them in the factory bias setup.
But later with all that.
I'm here to ask for advice. The red bakelite Roederstein caps on the control board are no longer available and since they're now 30 years old I plan to replace them as well as all the electrolytics aside from the main reservoir caps, which should last practically forever, being computer grade.
The Roedersteins' value is 470 uF 40 volts.
Please recommend an alternate high quality brand and type for replacement. I'd very much like to retain the radial capacitor form factor if possible because I don't like it to look like redneck engineering has taken place.
Heat really isn't a problem with these amps. Oh, they may generate a lot, but my thermocouple probes and calibrated meter have not found any spots on the amps that run higher than about 125 degrees F at maximum. That's not even 52 degrees C so even 85 degree C rated caps should last a very long time. And since they're all original and the amp is 30 year old, I'd say there's evidence to support that assumption. Nevertheless I'm going to go with 105C rated and long life ratings, too, wherever possible, for all electrolytics.
While I'm at it, there's a shortage of documentation on the KMA-160 and KSA-80. I've found two adjustable trim pots on the protection board and two on the driver/preamp board as well. But no clear information on their parameters and adjustment/biasing procedures.
I would expect that the current limiters are set via those variable resistors on the protection board. I'll leave them as they are. But information on setting the bias, drive, and DC offset would be appreciated if you have it.
I can hardly complain. I bought the pair back in, I think, 1996 and they have 1988 date codes on the PC boards.
You MAY recall that I had a story to tell about these two. I bought the pair for 500 bucks plus shipping costs.
But they'd been in a house fire. They hadn't been burned but they had been hosed down and put out with the fire, flooded and full of water and debris from the fire, and then left to rot and corrode in a basement for two years before I found and bought them. I still think it's lucky they weren't just scrapped!
40 hours of my own hard labor later, spent almost entirely doing a complete teardown and cleaning of each amp, I had them both back together and singing in my system. Electrically speaking one amp never had any problems and the other had one open power transistor out of 24 and its emitter resistor was open, too, so it would have worked like that, just down by maybe 6 watts in output power.
So they've definitely been a bargain!
I never have gotten around to rebiasing them to run in Class A for only the first 10 to 20 watts, which would certainly make them run cooler. Perhaps I should do that this time around. But my speakers are inefficient at 87 dB rated sensitivity (Aerial Acoustics 10Ts) so if I want to play fairly loud I'm sure the amps would drive past a 20 watt class A limit pretty easily. It might be best just to leave them in the factory bias setup.
But later with all that.
I'm here to ask for advice. The red bakelite Roederstein caps on the control board are no longer available and since they're now 30 years old I plan to replace them as well as all the electrolytics aside from the main reservoir caps, which should last practically forever, being computer grade.
The Roedersteins' value is 470 uF 40 volts.
Please recommend an alternate high quality brand and type for replacement. I'd very much like to retain the radial capacitor form factor if possible because I don't like it to look like redneck engineering has taken place.
Heat really isn't a problem with these amps. Oh, they may generate a lot, but my thermocouple probes and calibrated meter have not found any spots on the amps that run higher than about 125 degrees F at maximum. That's not even 52 degrees C so even 85 degree C rated caps should last a very long time. And since they're all original and the amp is 30 year old, I'd say there's evidence to support that assumption. Nevertheless I'm going to go with 105C rated and long life ratings, too, wherever possible, for all electrolytics.
While I'm at it, there's a shortage of documentation on the KMA-160 and KSA-80. I've found two adjustable trim pots on the protection board and two on the driver/preamp board as well. But no clear information on their parameters and adjustment/biasing procedures.
I would expect that the current limiters are set via those variable resistors on the protection board. I'll leave them as they are. But information on setting the bias, drive, and DC offset would be appreciated if you have it.
I've actually started the work on these amps and it's evident that while I'd say I did a GOOD job of arresting any further rust or corrosion issues the first time I got into these amps in 1997 (I think it was that year) after rescuing them from the fire, there has been some additional corrosion since then, in hardware, so this time around I'm just going to be putting in all new hardware all around. Including some nylon screws where appropriate.
There's also still a fair amount of residual original dirt left over from the fire over 20 years ago which I'm going to be trying to chase out with a vengeance.
Some modest quality upgrades may be in order. While I'm generally satisfied with the circuit boards, I can think of plenty of better connector options than the Molex connectors used in these amps. If the effort and cost involved isn't great, I may be doing some upgrades there. And the internal wiring is a candidate for improvement as well.
The top rectifier PC boards are definitely going to be looked at with an eye toward DIY replacement. They did take the brunt of the chemical damage done in the fire and as a result I'm thinking that they should be replaced. I can draw up new boards and have them made fairly cheaply. And I'd go with larger, heavier traces. I can't question that the existing boards have been adequate but if I'm going to replace those rather well worn boards, I might as well improve them while I'm at it. These original boards are pretty heavily worn. Solder mask missing to a large degree, some traces still are fairly deeply corroded even after a flux based cleaning and brass brush scrubbing followed by retinning, and the copper bus plate between the four rectifier caps needs retinning, too.
Each rectifier board carries no less than four very large full wave bridge rectifier modules, for a very stout power supply capability.
It's really impressive that these amps have performed flawlessly for me for the last 20 years plus, despite the residual damage still in them that I'm just now deciding needs to be properly fixed.
I'm giving some consideration to the idea of even making new rear panels for the amps, to include Neutrik Speakon connectors in the place of half of the binding post pairs. It would not be a complex or expensive project.
There's also still a fair amount of residual original dirt left over from the fire over 20 years ago which I'm going to be trying to chase out with a vengeance.
Some modest quality upgrades may be in order. While I'm generally satisfied with the circuit boards, I can think of plenty of better connector options than the Molex connectors used in these amps. If the effort and cost involved isn't great, I may be doing some upgrades there. And the internal wiring is a candidate for improvement as well.
The top rectifier PC boards are definitely going to be looked at with an eye toward DIY replacement. They did take the brunt of the chemical damage done in the fire and as a result I'm thinking that they should be replaced. I can draw up new boards and have them made fairly cheaply. And I'd go with larger, heavier traces. I can't question that the existing boards have been adequate but if I'm going to replace those rather well worn boards, I might as well improve them while I'm at it. These original boards are pretty heavily worn. Solder mask missing to a large degree, some traces still are fairly deeply corroded even after a flux based cleaning and brass brush scrubbing followed by retinning, and the copper bus plate between the four rectifier caps needs retinning, too.
Each rectifier board carries no less than four very large full wave bridge rectifier modules, for a very stout power supply capability.
It's really impressive that these amps have performed flawlessly for me for the last 20 years plus, despite the residual damage still in them that I'm just now deciding needs to be properly fixed.
I'm giving some consideration to the idea of even making new rear panels for the amps, to include Neutrik Speakon connectors in the place of half of the binding post pairs. It would not be a complex or expensive project.
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