Hot plate for PA device replacement in pallet amps
Posted: Sat Apr 03, 2021 7:28 pm
A need is said to be the mother of invention - when I popped one of the PA devices in my Nokia UMTS unit I was lucky enough to find a replacement (Thanks Jim 
) Not so keen on cooking the whole PA over the gas stove I remembered a surplus 'burger grill' that I picked up from our local Freecycle group. It had removable plates and at the time I needed some means of heating candle wax. It didn't suit the task (I used 3-D printer heat beds instead) so it was relegated to the 'move it on pile'.
I decided to dismantle it and see what heating elements it used. A pair of 115V 800W units, with a microcontroller providing the temperature control. Seemed ideal, so I dug out a piece of 1/4" alloy plate and did some thinking. Although the control board was packing a microcontroller, the element switching was by a simple relay and further dismantling showed the control knobs had fatigued from heat and burger fat etc. I decided to junk the control board and replace it with a cheap PID control unit with a SSR for switching. Mechanical work was mainly cutting, drilling and tapping the metal plate to secure the element to the bottom of it, re-using the original clips. A bit of rewiring and it was ready to test. As its only a 115V element I ran it off my variac and after emitting some 'cooking odours' it proceeded to warm up nicely. I put a small piece of solder on the hotplate and as it reached 190C it obligingly melted. I turned the input voltage down to about 30 and it seemed to reach a steady state at about 200C. If I can work out how to programme the controller (under £15 on eBay with a large SSR and 0-400C thermocouple included) in PID mode so much the better. Its just in off/on mode at the moment but after the first overshoot its reduces the cycle to a few degrees.
Anyone any advice of using a hotplate to repair boards? (Heatsink paste under device for removal).
Gareth
) Not so keen on cooking the whole PA over the gas stove I remembered a surplus 'burger grill' that I picked up from our local Freecycle group. It had removable plates and at the time I needed some means of heating candle wax. It didn't suit the task (I used 3-D printer heat beds instead) so it was relegated to the 'move it on pile'.I decided to dismantle it and see what heating elements it used. A pair of 115V 800W units, with a microcontroller providing the temperature control. Seemed ideal, so I dug out a piece of 1/4" alloy plate and did some thinking. Although the control board was packing a microcontroller, the element switching was by a simple relay and further dismantling showed the control knobs had fatigued from heat and burger fat etc. I decided to junk the control board and replace it with a cheap PID control unit with a SSR for switching. Mechanical work was mainly cutting, drilling and tapping the metal plate to secure the element to the bottom of it, re-using the original clips. A bit of rewiring and it was ready to test. As its only a 115V element I ran it off my variac and after emitting some 'cooking odours' it proceeded to warm up nicely. I put a small piece of solder on the hotplate and as it reached 190C it obligingly melted. I turned the input voltage down to about 30 and it seemed to reach a steady state at about 200C. If I can work out how to programme the controller (under £15 on eBay with a large SSR and 0-400C thermocouple included) in PID mode so much the better. Its just in off/on mode at the moment but after the first overshoot its reduces the cycle to a few degrees.
Anyone any advice of using a hotplate to repair boards? (Heatsink paste under device for removal).
Gareth
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