700B Power Switch

[wattsabundant]

After about 6 years of fooling around I finalized a circuit board with a triac to save the Arrow Hart power switch on the 700B's. I listed it on EBAY tonight. Then I searched for it to verify the listing. The first search result surprised me. Here's the link. 700B Power Switch The $90 price tag certainly caught my eye. It has a type D shaft and the original is round. However I can see no way it will fit. I wrote to the seller to ask if he verified it.

I attached a screenshot of the inrush current (yellow) on a 700B with 15,000uf caps. The first half cyce, at 50A/divison is almost 150 amps. Look at how the voltage (blue waveform) crashed. My house is less than 100 feet form the distribution transformer.

 

[mlucitt]

I have often thought about an in-rush current eliminating circuit especially for the PL700, either triac, relay-controlled resistor bank, or several NTC Varistors. The consensus seems to be that no PL amp has suffered from the inrush current, so why do I want to make a solution for a non-existent problem? Placing a .01uF AC-rated ceramic safety capacitor across the switch contacts helps to reduce the arc between the contacts, I guess. A "QuenchArc" would be even better (Mouser P/N 539-104MACQRL150). Anybody want to take pictures of a skeletonized power switch at turn on?

I have a 60-amp 12V power supply I use with a HAM radio. When I turn it on the entire outer chassis flexes with the magnetic field expanding around the transformer - very intimidating. But it works, and works, and works.

 

[VSAT88]

Do I see a PL 700 in that pic ?

 

[marcok]

Use a 16 A relay with a cap connected to the power switch .
In other words you insert the relay circuit of PL 4000.
Ciao
Marco

 

[VSAT88]

Like a PS Audio 200c. Could use a solid state relay ? EDIT: Well, I guess the triac is the solid state solution. Very neat, very nice Mr. Imlay.

 

[VSAT88]

I was also reading and saw where the triac (as OP said) was used with great success over at AK. They were using it on a Kenwood KR 9600 and a few others. I know the in rush current cannot be the same on the 9600 as with the 700 though. I would imagine that the 9600 only has a wee inrush as compared to the 700. Same issue though. Burns out the switch.

 

[wattsabundant]

The thread about the unobtanium power switch at A. K. did get me thinking about the Arrow Hart switch in the 700B. The switch is rated 6 amps at 125 VAC. I think that switch was used because it was small enough to fit in the space. Anything larger conflicts with the light board. I was fortunate to find a small stash of switches at a surplus shop.

I'm very familiar with Quencharcs and contact bounce. At my day job we use them across coils for AC relays, gate/cathode leads on SCR's for noise suppression, across contacts that have to break up to 280 VDC, the list goes on. See the attached cut sheet, especially the scope photos. Quencharcs work. However, they can't protect an undersized switch.

For soft starts I don't like the current limiting resistor. It still requires a relay/timer. On P/L amps it would be a disaster. Assuming there is no output relay, as the amp comes up, the output goes DC until the differential amp is stable. However that DC goes into the speaker, the woofer, which doesn't usually have a coupling capacitor, loads down the power supply and the amp never stabilizes. That would likely lead to a resistor failure If the amp has an output relay then it would work.

 

[mlucitt]

The current limiting resistor circuit I have seen uses a resistor-capacitor charging network to operate a relay. The AC power initially goes through the resistor bank and after a few AC cycles the capacitor charges, the relay energizes, and full voltage is routed to the power supply. The downside here is that if the relay fails and de-energizes, the resistors are returned to the circuit and will quickly overheat and fail in a dramatic fashion. In your experience Don, are the relays we use fairly reliable?

The other option, which uses the NTC thermistors (Mouser P/N 995-SL32-5R020), is that they rely on heat to increase the resistance and reduce the inrush current flow. If the power to the amp is applied, then turned off and turned back on, there is no thermistor protection. This is a rare occurrence I imagine.

 

[Billboard]

I have a 700B WOPL, I use a smart outlet strip to turn my amp on, amp switch still has in rush current, but switch does not arc.
I have my amp on the switched side of the strip, and the SX 950 as a preamp on the master side.

To turn the system on first I switch the 700B on (nothing happens) then I switch the SX 950 on. At that point the amp turns on and all is well.
To turn system off, amp off first, then SX950. Because SX950 has a pop to the speakers when powering off, with amp off first all is quiet.

 

[wattsabundant]

My comments are in Yellow.

The current limiting resistor circuit I have seen uses a resistor-capacitor charging network to operate a relay. A simple RC network would likely cause chattering of the contacts. An RC driving a transistor would be more effective. The AC power initially goes through the resistor bank and after a few AC cycles the capacitor charges, the relay energizes, and full voltage is routed to the power supply. The downside here is that if the relay fails and de-energizes, the resistors are returned to the circuit and will quickly overheat and fail in a dramatic fashion. In your experience Don, are the relays we use fairly reliable? Generally speaking, relays by reputable manufacturers are reliable when properly applied. As for the relay boards I sell, I seem to recall somebody had one fail when an amp failed. The relay did it's job. It protected the speaker. That's the design goal. If anybody else has had a relay fail, I haven't heard about it.

The other option, which uses the NTC thermistors (Mouser P/N 995-SL32-5R020), is that they rely on heat to increase the resistance and reduce the inrush current flow. If the power to the amp is applied, then turned off and turned back on, there is no thermistor protection. This is a rare occurrence I imagine. If power is immediatly turned back on, the caps are still charged up and there likely is minimal inrush .

There are no soft start circuits I like. I think turning on a switch is the simplest. Everything else gets more complicated. There is a lot of vintage gear for which there is no replacement switch. For those applications, using the existing switch as a controller of a relay or SCR/triac will keep the gear original and extend the life of the switch. A triac is a great solution as it is compact and can be easily implemented.