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Another Power Switch for the Adcom 5800

MRC01

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#1
I bought this amp new in 1992 and it's been running like new ever since. I've used nearly every day for over 25 yeras. But every 10 years or so, it fries the power switch and get stuck in the "ON" position. Just happened again tonight. So I'll be getting the 3rd replacement. The part costs about $25 - $30 (for single unit) and is easy to replace. Carling PA955.

Specs rate it at 100k cycles mechanical / 50k cycles electrical but IME this is WAY optimistic. At 10 years with daily use it's more like 355 * 10 = 3.5k cycles. When you turn on the amp, the initial surge briefly dims the lights in the room and occasionally blows a 20 amp breaker. I suppose that takes its toll on the power switch.
 

RayDunzl

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#2
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restorer-john

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#3
Maybe you need a snubber.
He needs a slow turn on/soft start. Big amplifiers with tons of capacitance and a large toroidal usually do that with a series resistor and a relay bypass.

This was done in the 270wpc SX-1980 giant receiver in 1979. Very effective and elegantly simple. It prevents the main switch ever getting hit with obscene currents. All you need is a single pole relay with an appropriately derived series resistor for the coil and voltage threshold, a wirewound 3-4R 20W resistor for the soft start in series with the primary and for good measure, a thermal fuse sitting on the wirewound resistor in case the relay contacts ever fail.
1602398192489.png




Why not leave the amp switch on and use a relay switched outlet? That is the best way to save hard to get vintage power switches in big gear.
 
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MRC01

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Thread Starter #4
He needs a slow turn on/soft start. Big amplifiers with tons of capacitance and a large toroidal usually do that with a series resistor and a relay bypass. .. .Why not leave the amp switch on and use a relay switched outlet? That is the best way to save hard to get vintage power switches in big gear.
Yes, this amp does need that. It never really bothered me since I only have to replace it every 10 years or so, the switch isn't too expensive, and it is easy to swap. I wonder if a beefier switch would last longer in this application.
 

restorer-john

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#5
It never really bothered me since I only have to replace it every 10 years or so, the switch isn't too expensive, and it is easy to swap.
That's the thing eh. Every 10 years or so and a cheap switch is not too much to bear. It's a very good amplifier so I guess you can put up with it.

The turn on current would be in the region of >100A depending on where in the cycle the switch contacts close. A beefier switch will still eventually fail. A slow start modification would put the problem to bed for good.
 
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MRC01

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Thread Starter #6
Here's the 5800. I've read that the 32 output transistors must be hand-matched, so if one dies keep that in mind when servicing. None of mine have blown, so I haven't had to deal with that. I think one of the reasons this amp has lasted so long (28 years so far, knock on wood!) is because of its excellent cooling. Two large (and quiet) ducted fans with temperature-controlled speed, with big heat sinks. It can dissipate a lot of heat/power, 250 W when idle and up to 1800 VA max.
20201016_134825.jpg


Looks like the power switch already has a capacitor across it. Maybe I should replace it with a bigger one?
20201016_134835.jpg


You can see how easy it is to replace. Just unscrew the terminals and connect the new switch.
20201016_135405.jpg


The only tedious part is adjusting the 2 lockwashers on the switch's threaded barrel to set just the right amount of set-back so the button activates the switch properly. But it's not really hard.

Ready for another 9 years now!
 
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restorer-john

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#7
Well done. :)

It looks like it has a PTC thermistor in series for over-current during switch on. (the black thing next to the suppression cap).

The two central electrolytics on the PSU board look like the sleeve has shrunk (heat), maybe check them out.

I've read that the 32 output transistors must be hand-matched, so if one dies keep that in mind when servicing.
It's not as critical as people like to think. As long as you are in the Hfe ballpark and match with a curve tracer to a reasonable tolerance, it's all good. The ones in the amp will be all over the place by now anyway.
 
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watchnerd

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#8
I bought this amp new in 1992 and it's been running like new ever since. I've used nearly every day for over 25 yeras. But every 10 years or so, it fries the power switch and get stuck in the "ON" position. Just happened again tonight. So I'll be getting the 3rd replacement. The part costs about $25 - $30 (for single unit) and is easy to replace. Carling PA955.

Specs rate it at 100k cycles mechanical / 50k cycles electrical but IME this is WAY optimistic. At 10 years with daily use it's more like 355 * 10 = 3.5k cycles. When you turn on the amp, the initial surge briefly dims the lights in the room and occasionally blows a 20 amp breaker. I suppose that takes its toll on the power switch.
I used to have an Adcom GFA-535 when I was in college.

I never turned it off.

Problem solved.
 
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MRC01

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Thread Starter #9
The two central electrolytics on the PSU board look like the sleeve has shrunk (heat), maybe check them out. ...
What would be the symptom for a failure there, how would I measure it?

When turned on, fully warmed up, no signal and speakers not connected, the DC offset measures 2 mV on one channel and 3 mV on the other. What other checks can I do?

I used to have an Adcom GFA-535 when I was in college. I never turned it off. ...
When the switch fails, it always fails "on". This makes it tempting not to replace, but it draws a lot of idle power. At $0.1 / kw-hr, it's 60 cents per day or $18 / month.
 

restorer-john

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When turned on, fully warmed up, no signal and speakers not connected, the DC offset measures 2 mV on one channel and 3 mV on the other. What other checks can I do?
The offset is perfect.

The caps are probably fine, their sleeves do shrink with age and heat. To be honest, just blow the dust out gently and put her back on the shelf. It's a rock solid amplifier and one that will be going long after many others are in landfill. :)
 

restorer-john

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#11
The gold electros look like Nichicon Fine Golds (UFG) and I'm pretty sure they are post 1992 style. Were they replaced by you? The Rubycons on the power supply board look 1992 era.
 

restorer-john

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#13
Also, you can buy soft start modules "off the shelf"
It has a PTC thermistor already for inrush current limiting.

Love the "english" translation or whatever it is :) If your amplifier is igniting, you've got bigger problems. I want a "fugitive pushbutton"! Every time you go to use it, it's escaped!

1602892192492.png
 

Count Arthur

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#14
Yes, no idea what a "fugitive" pushbutton is, maybe they mean a momentary on type?

Does a soft start or inrush limiter have other benefits, besides protecting the switch? I would have thought that it would be gentler on pretty much all the components in the amp.
 
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MRC01

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Thread Starter #15
The gold electros look like Nichicon Fine Golds (UFG) and I'm pretty sure they are post 1992 style. Were they replaced by you? The Rubycons on the power supply board look 1992 era.
It's all OEM, bought new in 1992. The only part I've replaced is the power switch. It's now on its 4th switch (3rd replacement).

I've had the bias checked a few times over the years and the shops always said it was in spec, nothing to adjust. I'd like to learn to do that myself some time.
 
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