|N9EWO Review :
The MW122A Regulated Power Supply : Modified
for use with Grundig Satellit 800, Eton e1, Drake SW-8
The low cost Chinese MW122A "117 VAC transformer type" regulated power supply can be modified for use with the Drake SW-8 , Grundig Satellit 800 , Tecsun HAM2000 , eton e1 receivers that require a pretty good amount of current at 9 VDC (for a portable). As stock it operates way too hot at 600 ma current and I would expect failure in time if modifications are not done . We removed the unstable selector switch (now 9 volt output only) , changed the secondary transformer tap for cooler operation. Now no more headphone hum and much more stable output . Once modified we found it to be a very useful and clean supply for these receivers . NOTE : Availability became very scarce starting in early 2017. (N9EWO Photo)
I will NOT be held responsible
for any info that is listed here.
Regulated Power Supply : Modified for use with the Grundig Sat
800, Tecsun HAM2000, Eton E1, Drake SW-8 receivers.
WARNING : The following information and modifications are for the person handy with basic electronics and soldering skills. Of course pass this up totally if you are not, or find a friend who is !! DOING THE FOLLOWING IS TOTALLY AT YOUR OWN RISK . I WILL NOT BE HELD RESPONSIBLE FOR ANY NASTY PERSONAL INJURY AND OR FRIED EQUIPMENT FROM USING THIS INFORMATION !!
A low cost
regulated "higher current" power supply, but not cool
out of the box even at 600 ma current.
The Drake SW-8 , Grundig Satellit 800 , Tecsun HAM2000 and eton e1 portable receivers all operate on nominal 9 VDC and require a good healthy 600~700 milliamps (ma) of total current for proper operation. The included UNREGULATED floor/wall warts are all rated at 1 amp of current and even at full load give over voltage to the set. So the receiver internal voltage regulators must work harder with more "voltage burn off" difference and this is in the form of excessive heat from these regulators.
If one operates the eton e1 for a few hours using it's included AC adapter one will feel this excessive heat on the rear side around the DC jack. With the Grundig Satellit 800 and Tecsun HAM2000 its harder to detect, but feel on the top part of the cabinet just above the S-Meter after being on for awhile. It's on the warm side and any PC board is quite a ways from this point. But you get the idea and excessive heat is never a good thing in any electronic device.
Of course the solution for less internal regulator heat is to just use a nice 9 VOLT REGULATED linear-transformer type supply that is good for at least 700ma of current . Of course using any low cost switching type supply (or a linear one that uses switching regulators) is not a good idea with any Long-Medium-Short wave receiver !! That is unless you love self-inflected interference.
Good Old Headphone Hum With Included Power Supplies
Another issue that makes it very desirable to use a regulated supply is the fact the internal audio amplifiers normally do not see any of the filtered current from the lower voltage internal regulators in the receiver. It's just the raw voltage off right off the external DC jack. So one can hear a background hum especially with headphones in use. Have verified this in the eton e1 and Grundig Satellit 800 and it drives me crazy. Using a regulated power supply will eliminate this.
Tough finding anything "9 Volts , Pre-Built" at the required current....
Now the real problem is finding such a power supply as "pre-built" off the shelf (but not too big either). One could be build from scratch for those handy enough, but at a high cost if you have to purchase most of the parts. In the past we have used and recommended some of Jameco's linear regulated power supplies wall warts, however after my own experience they have started to use very noisy switching regulators in these adapters (without any word that it was changed...nice guys). So that takes those off the list fast (sorry Jameco).
OK, so we see this MW122A multi voltage "high current" regulated transformer linear supply around. In fact this is the ONLY such regulated linear power supply to have enough current that we have been able to find. It's sold under various brands such as LKG-Philmore etc. This power supply uses a LM317K type regulator (TO220 case) , and thank goodness is NOT a switching type .
UPDATE : Sadly this power supply has become more unavailable in early 2017. Hunt around for MW122A's which again is / was also sold under other names.
IMPORTANT NOTE : There is a 220/240 volt version of this supply marked as the MW2122A model. Sorry , I cannot say if this is 100% identical and will these issues and/or mods still be valid ?? You are totally on you own with this one (and do your own web page searches for more information).
Runs Way Too HOT as Stock at only 600 ma current !!
But however it's not going to be that simple I'm afraid. It looks good enough on paper and while it can be used as is out of package.......well at least for awhile. Because after some time in operation the rear heat sink (LM317K IC Regulator) becomes extremely HOT at 9 Volt operation with any of these receivers. In fact it's gets so hot that if you touch the rear heat sink, I would expect one to receive a major burn !!
Another issue we have experienced with one sample was with the voltage selector switch that was very flaky. Jiggling it slightly did not always give the desired voltage (it went all over the place), but it would also erratically jump to it's unregulated voltage in the 20 + VOLT area too. Later tested samples were better in this regard, but would YOU trust it in time ?? Not good here in my view , and I simply will not trust it this way !
This low cost switch arrangement is comprised of finger leaf contacts (that are on the plastic voltage adjustment knob) that rub up against the pc board for contact. No wonder it can be flaky.
This rotary voltage selector adjustment internally actually switches 2 circuits at the same time when rotated. First side of it selects the proper resistor for selecting the LM317K's output voltage (resistor RX5) . Second side selects the proper secondary transformer tap which is supposed to select a lower voltage to keep the input voltage to the regulator more proper (less heat) .
In practice it falls way short and still too much transformer secondary voltage and HOT operation. So it's time to pull the plug from the wall and remove it's 4 screws to access the internal parts.
NOTE : Joel T. informs that his late 2016 sample used "Security
Type Screws" instead of the standard Phillips type screws that
were on both of our test samples. He had the proper "fork" type
screwdriver to get it opened, but this could be an issue without the
proper tools. This power supply is sold in a number of slight variants
, so this finding may vary (may or may not be a problem).
The PC Board before and after the modifications. We remove the not so stable selector switch. You can see my 2 added RED wires (after the removal of J1 and J2 jumper wires). One on the Transformer secondary wire to the 6 volt tap. The other added red wire to the RX5 resistor as shown. The 3 PC board screws (that need to be removed) are slighty hard to get at . See my hand scribbled notes here for more information . (N9EWO Photo's)
Time For The
So what we did is to remove the switch from it's front PC board (3 screws on the PC board holding that are a little tuff to get to and put back, but is not too bad). One screw holds the entire switch assembly. Next I removed the 2 bare jumper wires from the mid part of the board. After that we added 2 insulated jumper wires as to hard wire it for more stable and cooler 9 volt ONLY operation . To help reduce the power supplies heat sink heat, we found in extensive testing that using the 6 volt secondary transformer tap (yes for the 9 volt output) gave more than enough over voltage for proper LM317K regulator operation.
An alternative for the "transformer jumper wire" would be to skip this one and instead just move the RED pc board 6V tap wire over to the hole connection near the voltage bridge rectifier (see photo above, where the one side of the jumper wire is just below the rectifier) .
See my hand scribbled document here for additional information .
A few important notes :
- "Quality Control" (as it is on many low cost Chinese products) is fair to poor on these. We found the internal binding post nuts to be very loose on our latest test sample. So poor connections for sure here. So better have a look see at these as well as all other solder connections etc...etc.... before you close it back up.
- Power transformer in this beast runs extremely hot at even "no load" status. So watch out for that if you open it up after using it for awhile.
- When putting it all back together the hardest part will be replacing the 3 PC board screws to the front panel and getting LED back in place while struggling with all of the transformer wires all at the same time. It's a tight fit. Make sure the PC board is straight after it's screwed down and in place (don't bend it, it's not a glass board of course).
Still Not Real A Cool Supply But Now Tolerable , No More Hum
These modifications will NOT make the MW122A's rear heat sink run real cool by any means , but it is still much improved over stock and less of a chance for the internal LM317K regulator getting burned out and dieing (and maybe even kill the connected receiver too). You will now be able to touch the heatsink with these modifications (at least for a few seconds) now without getting a nasty burn.
We can now listen to headphones without hum and with less cabinet heat (so less stress on internal parts).
A couple of final notes. I would NEVER try and obtain anywhere near rated 2 Amps of current out the MW122A (let alone even one amp) , at least not without a fire extinguisher handy. But for the application we cover here at the 700 ma max current , it's just fine and very useful after the modifications . Remember just NOT to place it on any carpeted surface (give it lots of air under and around it) . Of course make note of the proper plug and tip polarity (+) as you connect it up .
: Sadly this power supply has become unavailable from MCM (and others)
in early 2017. Search around for MW122A's sold under other names.
c N9EWO, all rights reserved
I will NOT be held responsible
for any info that is listed here.
To Home Page