Sansui QRX7001 repair
- Thread starter SPXER
- Start date
Help Support QuadraphonicQuad:
My desoldering iron just uses a bulb at the handle end. Not real fancy but for anything with a lot of terminals it's handy.
Owen Smith
1K Club - QQ Shooting Star
I have a spring loaded suction device, but keeping the heat on the hole and sucking at the same time is hard with my poor manual dexterity (I've always been clumsy). And the amount of copper on this PCB is ridiculous, Arcam often used extra thick copper for their high end stuff which this is being £600 tuner in 2008. Add in that the PCB is double sided plated through hole and it's a soldering nightmare, getting things hot enough to melt all the way through the holes without using so much heat you damage things is a challenge.
I guess I'm the outlier here... I've always had good luck using Solder Wick braided copper. It acts like a heat sink too, drawing away heat from the PCB. Once upon a time I tried a suction bulb. You know the cheapest of cheap where you hold the soldering iron in one hand & the bulb in the other. It just didn't seem to suck enough, in a good way, to be effective. Probably the unit described above with a true vacuum pump would be best of all.
Mine is just an old Radio Shack piece.
That looks like a medical device. Ouch!
I have replaced the electrolytic caps on the F-2428 Filter and power supply board. It feeds 25V to the 2088 QS phase control board. Both QRX Restore and 4channelsound stress the importance of 25v. Before the rebuild I read 26v with the 2088 board in place. Checking the output on pin 5, I have 29v with the pot turned all the way down. This is with no boards in place. Is this normal? Will it settle down with the 2088 board in place? I am reluctant to put the 2088 on with 29v present. Did the new caps change the voltage?
You should have 25V. It might be that an output capacitor is holding the voltage high with no load. I would power it down and wait for the voltage to drop back down and then power it back up. It should be lower than 25V if the regulator is working and you turned the pot the right way down. You could clip a load resistor across the regulator output if you don't want to connect the board, and see if the voltage is adjustable. If it won't adjust check the transistors and zener diode.
The decoder chips are designed to work properly at 25V as specified by Sansui. All their Vario-Matrix decoders use the same spec. I don't think that powering up with 29V will damage anything but it would still be best to verify that the regulator is working properly before reconnecting the boards.
Last edited:
A vacuum powered desoldering station is a must if you are doing a lot of repair work. Second best is Solder Wick as Sonic said.
I have used the spring loaded type as well, works OK if you are only desoldering a few connections. The bulb type does not always give enough suction power and would be my last choice.
I have used the spring loaded type as well, works OK if you are only desoldering a few connections. The bulb type does not always give enough suction power and would be my last choice.
I had 26v before the cap replace with the 2088 in place. after the cap replace and without the 2088 in place I had 37v. I adjusted the pot down to 29v, as far as the pot goes. I will do what you suggest with the power back up and or resistor. A high value resistor I presume.
Checking the schematic it shows 38V output from the regulator. That voltage is fed to pin or terminal number 40 via a 180Ω resistor (25V) and to 4 via a 150Ω resistor (24V). Those other two voltages will read high (38V) without the board attached. I would reinstall it and adjust 40 for 25 volts.
Last edited:
Owen Smith
1K Club - QQ Shooting Star
I've never tried solder wick, it just looked like a bad option to me. Maybe I'll give it a try, but given I'd have to buy some I'd be likely to skip that and just buy a vacuum desoldering station. Having one less thing to hold in my uncoordinated two hands really appeals.
About that 25V setting. It's critical to ber as close as possible. Too low & decoding will be wonky. Too high & it will fry the VM chips. Sansui suggests running equipment for about 20mins before testing to "season". They said this even back in the days of the QS-1. And it is good to play actual music through the decoder when testing as real use of the chip circuitry will tend to cause a tiny bit of voltage drop. It doesn't have to be any kind of quad signal just stereo played through basic QS mode is fine. And it doesn't need to be played loud through speakers just something to get the chips going.
@par4ken can you dig up the VarioMatrix Survival Guide you posted around here? Might be some good background for @SPXER .
@par4ken can you dig up the VarioMatrix Survival Guide you posted around here? Might be some good background for @SPXER .
I changed TR01, so that may have changed the values. Will I have 38v without the output connector plugged in? I will check those voltage drop resistors. Can I provide a dummy load to adjust the 25v? I am scared for the 2088 board.
I am very impressed all you posters are so willing to help. I was a common industrial electrician when I worked, so I am over my head with this. But, hey, I am learning and I'm not too old to learn yet. The only other component I have rebuilt was a tube amp.
Last edited:
Checking the voltage drop resistors I get 147ohms for the 150 ohm.
I get 150 ohms for the parallel pair of 180 and 820 ohms. A bit high when it calculates to 127. They must be both at the top of their tolerance.
I get 150 ohms for the parallel pair of 180 and 820 ohms. A bit high when it calculates to 127. They must be both at the top of their tolerance.
Last edited:
The voltage will stay at 38 until the load (board) is connected. Your voltmeter is a high impedance so will not drop any voltage when connected to the power supply (the 180Ω and 150Ω resistors will have virtually no effect on your reading). You will get the same reading at all three test points with the board disconnected.
You should just connect the board. If you want to be extra safe set the voltage low 29V first, then turn it up slowly to get 25V at (40). Let things warm up as Sonic suggested and readjust as necessary. Don't worry if the 38V and 24V are a bit off, they should still be close.
If you want to apply a load resistor to test first you just use ohm's law to calculate the resistor required. It works out to 347Ω to get 25V at (40) and 257Ω to get 24V at (4). I don't see the point of doing that, it has to work!
I didn't notice that they were in parallel, the scanned manual from Hifiengine is not very clear, some lines are even missing! Likely still OK but if voltages are too far off you can replace the resistors with something with better tolerance, or else tweek the pair with another parallel resistor.
barfle
2K Club - QQ Super Nova
I’ve been using a copper braid to remove solder for about 50 years. I find it far more useful than the vacuum (solder sucker) devices. There’s always the danger of lifting a pad, no matter what tools you use, but I’ve had good luck with the wicking braids.
quadsearcher
1K Club - QQ Shooting Star
- Joined
- Nov 8, 2010
- Messages
- 1,198
This kind of regulator, I suspect needs original cap values. No increasing the capacitance of the 220uFs that hang off the outputs. I once toasted a dropping resistor by using something higher. But, yes the 24 and 25 depend on the load to be part of the circuit.
Similar threads
