Analogdino
Active Member
I have two Sansui 5050's both with burned out pea (rice grain ?) filiament lamps in the tuner pointers. I replaced them with LED's.
Here's how:
First and obvious (but it has to be said), don't just connect in a miniature LED - it will burn out!
I used a HLMP-1340 LED (made by HP, 3-5 Systems Inc, Quality Techn and Chicago Miniature, perhaps others.) Most, if not all, similar substitutes will work. It's an ultrabright red (wavelength 635 nm), non diffused. It has a maximum forward rating is 30 mA with a drop of 2.2 (min), to 3.0 (max) VDC, but a PIV of only 5 VDC (all LED's have very low PIV ratings) We will run it well below this current.
The supply to these lamps is 6.3 VAC so the PIV could be 8.9 volts - too high.
To solve this we will connect a "back to front" diode across the LED, i.e. anode to cathode (see below.) Use any small, general purpose Si diode, such as an IN5392, or similar. Do NOT use a point contact detector diode (quite a different animal.)
Installation
1. Extract the dead lamp from the pointer carefully (don't sever or damage the tuning drive string that is over the top of the wire.)
2. Take a pair of very thin insulated flexible wires abut 9 inches long (I use 30 AWG flex, not solid, that we have in the lab) and solder one to each LED wire. Don't let the LED overheat, do it quickly and cool the joint with a wet sponge. Put about 2 inches of sleeving on, right up to the LED glass. Use the smallest sleeving that goes over the flexible wire and the soldered joint. Do not use thicker flex as it has to bend as the pointer moves and thicker wire can displace the pointer carriage on its track. The maximum size is what Sansui used - you may be able to salvage what was on the original lamp and just extend it for connection the other end.
3. Bend the LED wires to a right angle near (not at) the LED glass. You want the LED to go to the bottom of the hole in the plastic pointer and have the wires lie flat and level with the top of the pointer carriage. Easist way is to put the LED in, then bend the wires flat, then pull out the LED and set the right angle firmly with long-nosed pliers so that the bend in the wires and sleeving does not pull the pointer away from the vertical. Twist the wires together and cut them to the right length for free movement of the pointer to both ends of the scale, plus about 1 1/2 inches. Strip the wire ends carefully (only one chance!) Put the LED back in the pointer and dress the sleeving beween the tiny brass tabs; bend them in to hold. Lay the sleeved wire over the pointer carriage and tie it down to the center tiny dial string tab with a twist of thin copper wire. Keep the total height low, the box top is very close here. Test that there is no connection to ground from either thin wire with the DVM ohmmeter (the risk spot is the right angle bend of the LED connecting wires - they are very near the pointer carriage.
4. Connecting the PIV safety diode:
Connect the other diode across the stripped wire ends. When you have done this check with the diode check function of a DVM. When you test the pair together you should get a 0.6 VDC or so forward drop of the logic diode one way round and a 1.6 VDC or so forward drop of the LED the other way around. If you get 0.6 VDC and infinity, you have the diode the wrong way round - 50/50 chance! Reverse it. DO NOT omit this very important test!
5. The series resistor
Target around 12 mA for the LED "on" current (a 1/2 wave rectified AC.) Voltage to be dropped is 6.3v minus, say, 2v for the LED, or 4.3 volts, thus R = 4.3/12 Kohms = 358 ohms. But, the 5050 and 6060 models have an 18 ohm resistor in the pointer lamp cicrcuit, so we need a net 340 ohms. 330 ohms is a standard so use that, close enough. (The 7070's, 8080's and 9090's may be the same, but I've not seen the schematics.
6. Connections
Use the tags on the PCB under the white plastic lamp housing (any way around - it's AC but one side is grounded.) Connect one end of the resistor to either. Solder the diode (and flex) across the other end of the resistor and the other tag. Sleeve the resistor-to-diode bare wire. Dress the thin twisted flex to clear all obstacles as the pointer moves over the scale.
7. Test.
Switch on receiver. Pointer should glow red. If not, switch off immediately and trouble shoot.
I am working on a similar replacement for the stereo indicator. Will advise later.
Cheers,
Roger
Here's how:
First and obvious (but it has to be said), don't just connect in a miniature LED - it will burn out!
I used a HLMP-1340 LED (made by HP, 3-5 Systems Inc, Quality Techn and Chicago Miniature, perhaps others.) Most, if not all, similar substitutes will work. It's an ultrabright red (wavelength 635 nm), non diffused. It has a maximum forward rating is 30 mA with a drop of 2.2 (min), to 3.0 (max) VDC, but a PIV of only 5 VDC (all LED's have very low PIV ratings) We will run it well below this current.
The supply to these lamps is 6.3 VAC so the PIV could be 8.9 volts - too high.
To solve this we will connect a "back to front" diode across the LED, i.e. anode to cathode (see below.) Use any small, general purpose Si diode, such as an IN5392, or similar. Do NOT use a point contact detector diode (quite a different animal.)
Installation
1. Extract the dead lamp from the pointer carefully (don't sever or damage the tuning drive string that is over the top of the wire.)
2. Take a pair of very thin insulated flexible wires abut 9 inches long (I use 30 AWG flex, not solid, that we have in the lab) and solder one to each LED wire. Don't let the LED overheat, do it quickly and cool the joint with a wet sponge. Put about 2 inches of sleeving on, right up to the LED glass. Use the smallest sleeving that goes over the flexible wire and the soldered joint. Do not use thicker flex as it has to bend as the pointer moves and thicker wire can displace the pointer carriage on its track. The maximum size is what Sansui used - you may be able to salvage what was on the original lamp and just extend it for connection the other end.
3. Bend the LED wires to a right angle near (not at) the LED glass. You want the LED to go to the bottom of the hole in the plastic pointer and have the wires lie flat and level with the top of the pointer carriage. Easist way is to put the LED in, then bend the wires flat, then pull out the LED and set the right angle firmly with long-nosed pliers so that the bend in the wires and sleeving does not pull the pointer away from the vertical. Twist the wires together and cut them to the right length for free movement of the pointer to both ends of the scale, plus about 1 1/2 inches. Strip the wire ends carefully (only one chance!) Put the LED back in the pointer and dress the sleeving beween the tiny brass tabs; bend them in to hold. Lay the sleeved wire over the pointer carriage and tie it down to the center tiny dial string tab with a twist of thin copper wire. Keep the total height low, the box top is very close here. Test that there is no connection to ground from either thin wire with the DVM ohmmeter (the risk spot is the right angle bend of the LED connecting wires - they are very near the pointer carriage.
4. Connecting the PIV safety diode:
Connect the other diode across the stripped wire ends. When you have done this check with the diode check function of a DVM. When you test the pair together you should get a 0.6 VDC or so forward drop of the logic diode one way round and a 1.6 VDC or so forward drop of the LED the other way around. If you get 0.6 VDC and infinity, you have the diode the wrong way round - 50/50 chance! Reverse it. DO NOT omit this very important test!
5. The series resistor
Target around 12 mA for the LED "on" current (a 1/2 wave rectified AC.) Voltage to be dropped is 6.3v minus, say, 2v for the LED, or 4.3 volts, thus R = 4.3/12 Kohms = 358 ohms. But, the 5050 and 6060 models have an 18 ohm resistor in the pointer lamp cicrcuit, so we need a net 340 ohms. 330 ohms is a standard so use that, close enough. (The 7070's, 8080's and 9090's may be the same, but I've not seen the schematics.
6. Connections
Use the tags on the PCB under the white plastic lamp housing (any way around - it's AC but one side is grounded.) Connect one end of the resistor to either. Solder the diode (and flex) across the other end of the resistor and the other tag. Sleeve the resistor-to-diode bare wire. Dress the thin twisted flex to clear all obstacles as the pointer moves over the scale.
7. Test.
Switch on receiver. Pointer should glow red. If not, switch off immediately and trouble shoot.
I am working on a similar replacement for the stereo indicator. Will advise later.
Cheers,
Roger
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