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SX-1980 USA Made Power Supply Causes Tuner Issues

thr33p4c

thr33p4c

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I had a couple of SX-1980s come across the bench recently with USA made power supply boards. This is not the typical Canadian made board that is prevalent nowadays. This USA board has four aux heat sink mounted transistors that are detachable using the same headers found on modern McIntosh equipment. The quality is excellent and removable screw down headers make removal a breeze.

The problem is, there is an error in the 13.5V regulator circuit which causes terrible buzz in the background when using the tuner. The resistor circled in green in the image below (R202) is supposed to be 3.3 ohm if we are staying true to the original schematic. I received the original documentation for this board and was able to identify the error. The provided schematic shows R202 as 3.3K, the parts list shows it as 22 ohm. In my case, both units had 22 ohm loaded in this position.

The resistor found in the unit was a CPF322R000FKE14 - 22 ohm 3W Metal film. I replaced it with the same resistor type in 3.3 ohm.

Correct Part: CPF33R3000FKRE6

After replacement, the regulator circuit is much happier, clean 13.5VDC with no ripple. If you come across a unit with this board installed and have a buzzing or poorly functioning tuner, this is likely your issue.

SX-1980 USA Power Supply.JPG
 
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EastPoint said:
Who's making these? I didn't know anyone was beside the Canadian ones.
Click to expand...

Probably this one:

txturbo

Post in thread 'SX-1980 power supply problems'

Getting close !!!

We received the boards in today..... Mitchel did an awesome job building the board up :beerchug:

We are missing one part that somehow did not get ordered, it's the 12 position header. It looks like the connector is plugged input there is no header. We also need to finish the 80V transistor install. I took a good look at the amp section and cleaned up a bunch on them, they should be ok for a test. Once the power supply is deemed solid I am going to recap the beast.

I can already see a couple things to change on the next board. The connectors on the left side...
 
Looking into this a little more closely. EW did recommend changing R202 to 22 ohm to help relieve the regulator pass transistor but apparently this does not work as intended with the new board. Going back to the stock value of 3.3 ohms seems to resolve the ripple but may mean that pass transistor will now run hotter than intended. I this case it is relocated to the output module heat sink so should be all good.
 
thr33p4c said:
The provided schematic shows R202 as 3.3K, the parts list shows it as 22 ohm. In my case, both units had 22 ohm loaded in this position.
Click to expand...
Ok.
thr33p4c said:
The resistor found in the unit was a CPF322R000FKE14 - 22 ohm 3W Metal film. I replaced it with the same resistor type in 3.3 ohm.
Click to expand...
Your post is a bit confusing mentioning '3.3K' and also 3.3Ω
 
Last edited:
The original design uses a 47-ohm/3W resistor for the input to the 8.5V regulator. The unregulated supply is around ~22VDc. I am not sure how much the @txturbo design follows the original design apart from changing the jfets to use the LED ring of two CCS, remote mounting of the series pass bjts.
 
Hyperion said:
Your post is a bit confusing mentioning '3.3K' and also 3.3Ω
Click to expand...

That is actually not an error in the post. It really is confusing. There are (3) resistor values in question. Original Value 3.3 ohm, txturbo schematic 3.3K, txturbo parts list 22 ohm.
 
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