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STA-300, default settings

Read more on the Optimus STA-300 in our home audio section

Paul Moore2010-03-13 14:09

When the radio comes on it goes back to the factory set default station. How do I change the default station.


Jazzjms2010-09-22 16:18

I am guessing that a small diode has failed on your Optimus unit due to age. This allows the unit to retain station memory presets when powered down and is also linked to the loudness function. It is very common that this will fail. It can be replaced, however it is not a simple fix as it is UNDERNEATH the circuit board and can not be accessed from the bottom. Everything has to come out to get at it. The part is only a couple of bucks but expect to spend $40-50 on labor. I have done this for BOTH Optimus units I own but consider it worthwhile as it is such a great little unit.

Keith2011-05-30 19:50

I have the same preset memory issue with my STA-300. I don't see any components on the bottom side of the PCB...altough you do need to remove the PCB to gain access to the solder joints. Can you help me? Which diode are you referencing? Is it in the power supply section? I see about 6 diodes there. Also, I get a little bit of AC hum at low volume. Is that a bad capacitor? Any idea which ones I should replace? Thanks for your time. Keith

Jim2011-05-31 16:32

To be honest I do not know exactly which one it is as I had given up looking due to frustration! I gave the unit to a (one guy who’s been fixin’ electronics for 35 yrs) local service shop who tracked down the culprit. I assume it is linked directly to the power supply as its job is to draw a small amount of juice when the unit is powered down to retain the preset memory function and loudness setting. I will see if I can get further information from the tech and report back. The hum could be related as power draw continues to the failed part.

Keith Miller2011-06-01 03:19

Jim...thanks for the prompt reply and effort. It would be nice if a schematic was available for this unit, but Radio Shack only offers an incomplete parts list and some basic instructions. The instructions state that if the unit is unplugged, the momory will only last about 3 days. The memory circuit must be drawing some stored power from a capacitor in order to do this. I'm guessing that either the capacitor can't store sufficient power or the stored power is getting used by some other component...maybe a diode gone bad. I look forward to your reply. Thank you.

Jim (jazzjms)2011-06-01 16:47

Keith--You are correct . . . a small amount of power is stored for the settings even if the unit is unplugged and will dissipate in a few days. I believe a semiconductor diode needs to be replaced which is located in the corner of the board with the solder points on the bottom. I will speak to the tech that fixed my units.

Keith2011-07-04 15:48

Jim...I figured it out...it's the short, 0.047uf 5.5V capacitor on the edge of the board. It almost looks like a stack of small watch batteries. Thanks.

Jim2011-07-04 20:29

Keith, I'm glad you got it--my repair guy only keeps (poor) paper records and it's hard to find anything. Yes that is what it looks like, kinda like a little fuse. I'm bummed--I had one of my two STA-300s at work--my department moved and everything was in boxes on the floor--the cleaning service THREW IT AWAY!!!

Keith2011-07-05 01:36

Sorry to hear that...at least you still have one STA-300 to enjoy. Ironically, I found this one on the recycle pile at work and it did not work initially. Something was rattling around inside. Three of the four nuts that hold the power transformer had come loose and one of them shorted a trace on the circuit board and blew the fuse. It wasn't until after I replaced the fuse that I found out that it didn't have any memory. I'm amazed at the sensitivity of the receiver. It's not as good as my Yamaha T85, but it is still very awesome and a fraction of the size.

Cp Brown2012-03-31 05:19

Have the Optimus sta-300 ... one of the output channels is dead (hope my terminology is correct) ... the left speaker doesn't work, right side works fine ... have been tapping wires from both speakers into the right side .. any easy fix to restoring the down channel ?


Ed2013-01-04 00:45

Memory problem solution:
Change C-167 (0,047F 5,5V) super cap.

Bob2013-02-14 03:02

Kith, thank you for your invaluable advice. I am by no means experienced in electronics repair, but I have basic soldering skills, so armed with your advice, I removed what seemed like 6 million screws removed the defective cap installed the 0.047uf 5.5v supercap I purchased on ebay, put it back together, with no extra parts I must add plugged it in and she works like new. Thanks again

Ken2013-06-09 15:24

Well folks, I acquired an STA-300 with similar loss of preset problems. I used this forum to help diagnose the problem and replaced C-167 with a supercap. Unfortunately it did not fix the problem.....but didn't make anything worse either. Where do I go from here? Well, I hope those of you with in-depth radio experience will chime in. This is a great little box and worth saving. I can live without preset savings, but it would be nice to see this one work as it should. Any help will be greatly appreciated. I sure do wish there was a service manual/schematics/diagrams available.

Ken2013-06-18 11:52

Here's an update...No one indicated that after replacing C-167 that it would take time to trickle charge the cap (back-up battery)! After almost 2 weeks of acknowleging failure, I guess it had enough time to charge and hold the memory presets and last station settings along with loudness either on or off. This box now works like it should and has rekindled my faith in my soldering techniques, etc. Hooray!

Randyman2013-08-01 15:27

....do any of you STA-300 owners have a unit that has been upgraded? Mine is probably as old as the rest of yours, but works fine. I am considering spending money to replace/upgrade parts instead of buying something new. I bought an Emotiva "mini" X-a100, 50wpc and frankly i can't tell any superior sound quality...will return it...used w/ fostex FE-103 full range; AR 18bxi, and JBL decade 26.Thanks.

Jazzjms2013-08-02 16:24

I don't what you mean by upgraded. I have had to replace a cap on my two STA-300s to regain memory function and they both work like new. No other additions or modifications.

Randyman2013-08-03 14:23

....I mean, is it foolish to have a tech. install premium parts to replace caps., resistors or possibly installing a Plitrontoroidaltransformer.Thanks.

Trog2013-09-02 19:27

When I put my STA-300 in storage 10 years ago everything was working fine. Now the FM is working fine but there is just noise on AM. Any ideas? Thanks.

Mike2013-12-16 21:30

My STA-300 lost it's ability to remember station settings years ago. After finding this thread, I got a new .047F super capacitor for less than $2.00. There were indeed a lot of screws to remove to allow access to the bottom of the circuit board - I'm going to guess 20+. I used a solder sucker to remove the old C-167 and put the new one in being sure to observe the proper polarity. There are some small signal diodes around it but I didn't change any of those. After putting it all back together, powered it up for a few minutes, shut it down for a minute, turned it back on and it remembers the last station just fine. Thanks for the good thread!

Randy Waters2013-12-16 23:00

My unit (STA-300) was unplugged when I moved. I reconnected, and after a few days all my presets returned. Thanks.

Mike2013-12-16 23:47

Trog - regarding your AM not workging ... I think the FM and AM antennas are separate - maybe you are missing something for a.m. ...

Jazzjms2013-12-17 14:31

Am & Fm antennas are separate. Typically AM uses a small Ferrite "bar" antenna that is internal to the radio/receiver.It has been some time since I replaced the caps in my STA-300's (the memory function still works) and I do not remember if this unit has a built in AM antenna. As the STA-300 has external connections I use an AM "loop" antenna with mine. This is the small plastic antenna (with a 2-wire lead) that used to come packed with receivers back in the day. Most people just threw them away or they became lost in the junk drawer. I had one and the other I picked up at my local electronics repair shop. They had a couple and just gave me one as few people use them any more. The link below will take you to Amazon and show you what one looks like. The plastic âbaseâ folds up so the antenna will stand. I would ask at your local repair shop(s) if they have any in their junk box. Mine work great. AM does not travel in a straight line like FM. An AM âloopâ can be rotated for best reception.

Jazzjms2013-12-17 14:35

Mmmmm--looks like quote marks don't always work here! Funky characters...

Lauren2016-05-10 22:23

Hi all,

I have the same issue with my amplifier. One question:

Keith & Bob recommend a capacitor of 0.047uf
Ed and Mike recommend a capacitor of 0.047F

Which one is it?

Eduard Dresden2016-05-11 00:48


Hello, I still hold the original piece in hand and rechecked the value as per your question.

I read: = 0.047F 5.5V

(known as "super cap" type)

Good luck!


Steve2016-12-06 18:00

Several years ago I bought my STA-300 new. 6 or 8 months after I bought it I discovered it wouldn't hold the preset stations. At the time it was still under warranty so Radio Shack repaired it. Fast forward to spring of this year and I turned it on one day and it appeared to power up but no sound from either channel. Without a schematic I decided to look for a unit on E-bay but the one I bought had no output on one channel and no preset memory. I found this forum and remembered when Radio Shack fixed my unit 20+ years ago they told me it was a defective capacitor without telling me which one. Now I know it is C-167. I am planning to remove C-167 from my totally unresponsive original unit and put it in my unit that has the one channel working. At least most of the functions will work, I hope.
It is hard to believe that there isn't a schematic out there somewhere for this terrific little receiver.


Steve2016-12-07 19:57

Well, as the doctors say...the operation was a success. I removed C-167 from my original unit and put it into the unit I purchased on E-bay and it works fine.

I also got my original unit working but now it has no preset memory because it has no C-167!
I just ordered 10 capacitors from E-bay and will get my original working again.

If any of you guys need a .047F 5.5V Supercap I will have them in a week or so. I will sell them cheap because I won't live long enough to use them all.

By the way, there are 28 screws to remove in order to access the bottom of the circuit board so that C-167 can be changed. It's not too difficult to take apart but it takes a little time and knowing where the screws are located.


John Friese2017-03-18 18:11

I replace that .047mfd with a 1 farad @ 5.5v supercap. With supercaps the bigger the better.

John Friese2017-03-18 18:13

That dead channel can be fixed by replacing the dead output IC. There's one for each channel. I think Mouser sells them quite cheaply.


Garry2017-04-28 01:15

Thanks guys for the info. I have 2 of these great receivers with this problem. Time to geek out on capacitor soldering.

Ed2017-09-10 22:26

Thanks so so much for identifying the C167 fault.

Ed2017-09-10 22:50

P.S. I have scanned my copy of the STA-300 service manual. If someone could recommend a website that I could upload it to, I will share my scan of the manual.

Steve2017-09-12 18:05

Ed, responding to your 9-10-17 contribution. I would love to see the service manual for the STA-300. However, I wouldn't know anything about uploading the manual to a web site. I certainly could download it if anyone else could provide a site.
Hope to see it soon.

Ed2017-09-12 19:28

if you need a manual I can email it to you. One file is the complete manual scanned at 200x200 then there is another file that is a 600x600 scan of just the schematic. email me your request at eekizer@yahoo.com

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