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 Post subject: Not much AM even after new AM RF transistor GE Superradio1
PostPosted: Jan Mon 15, 2018 1:11 am 
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So I recently got my hands on one of these fairly cheap. I like the reception of my Degen DE1103 and Tecsun PL380, but not the sound, so While the Super radio 1 that I now have sounds great, it only had FM when I got it. I read the write up on earmark about the common problems these radios have, and re-flowed the band switch, which had no effect, and replaced the AM RF transistor with an NTE 229. Now I can barely pick up a couple of stations towards the top of the AM band, but nothing else...and after about an hour lost them as well for the most part. I tried taking it outside to see if it helped, which it didn't.
I have a lot of high voltage electrical experience but as simple as this radio seems to be, I haven't had much experience troubleshooting things at this level. and as much as I enjoy watching Shango's repair videos I'm not sure it's helping me understand this situation I'm experiencing! I have some good multimeters, but no signal generator. I don't know of anywhere that still does electronic repair in the area either. Is this thing old enough to warrant a recap? Or do I need to think about doing an alignment since I replaced this transistor? If so...can you point me in a direction as to what I need and how do go about doing this? It's possible someone has worked on this before, but don't see anything that seems obvious inside that would indicate it. The radio is pretty clean. This will be a learning experience!


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Mon 15, 2018 1:51 am 
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FWIK the SR-1 is 30 Y.O., the electrolytic coupling caps are either dried out or leaking. Doubtful if any are good...

I have experienced internal failures of the I.C. after replacing caps no joy, take voltage readings on all the pins on the I.C. using a VTVM or a high resistance DVM. I f any are off more than 10% consider replacing the I.C. Since the board has to be cleaned up a bit, installing a gold flashed DIP socket may be helpful keeping the heat out of the new I.C.

YMMV

Chas


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Mon 15, 2018 8:56 am 
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Post back after you recap.

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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Mon 15, 2018 3:36 pm 
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Have you tried cleaning the bandswitch? If the switch was left in the FM position for all of its life, the AM contacts may be so corroded that any amount of exercising the switch may not be enough to make a good connection without solvent. I've had this problem with a few radios.

Dave


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Tue 16, 2018 2:23 am 
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Thanks for these suggestions... I will see what I can come up with!


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Tue 16, 2018 11:51 pm 
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Ditto on trying to clean the bandswitch. Older switches are famous for oxidation if they aren't used much....

I have two Superadios with those chips and haven't had issues with either them or the electrolytics.... That isn't saying it can't happen to a 30 year old radio, though, but I'd clean the bandswitch first, and then go from there.


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Wed 17, 2018 9:09 pm 
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Quote:
and replaced the AM RF transistor with an NTE 229.


Why?

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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Thu 18, 2018 11:24 pm 
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majoco: Not having much experience in this area and wanting to learn, I found a website called earmark that has the GE Superradio Tech page. On it, they say that one of the possible causes of the loss of the AM band is that the band switch may have caused the pc board to flex and break the connection loose, and it's found that the AM rf transistor can be damaged as a result. I didn't have any AM originally, now there's a little, but it's very quiet with the new transistor.

I have removed and tested the band switch to verify that it's good. I suppose the next step is a recap. I tried measuring voltage on the IC and did get some, but not exactly what is shown on the schematic. I may not have the right test equipment, I'm using a fluke 70III multimeter. I suppose if the caps are a little off that in turn could affect what I'm seeing at the IC. Since FM is loud and clear for the most part- there's kind of a distortion no matter what frequency I'm on...not sure if it's just normal or something else, but selectivity on FM is better than my other consumer grade radios if that's any indication.

I'm sure someone with more experience would have figured this out right away, but rather than trashing it, I'd like to see if I can get it working right again, and maybe learn something in the process.

Thanks for all the suggestions!


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Thu 18, 2018 11:41 pm 
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Super Radios have this kind of Class B audio, it sounds a bit funky at low volume level:

viewtopic.php?f=4&t=56282&start=80

It has been my experience that the large coupling capacitors in the output, if leaking, can reinforce the issue.

There is much to be found on the web about SR design. Some spins of the radio chips have leakage and can cause low audio or quit altogether. Such was the case of the SR II I bought for a dollar. Worked for a week, then within minutes the AM audio got distorted and faded down severely, almost like flat batteries. Ya, I went all over it hoping it was something simple, nope, its the chip...

Keep in mind the chip is obsolete, there is a replacement that may still be available from NTE. If you LIKE the radio, consider getting a chip. Once these are out of the mainstream market place the the chip bandits buy out the residuals, one will pay through the nose to get one.

I bought the chip some 8 YA and it was less than 10$ then and a bit hard to find.

YMMV,

Chas


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Sat 20, 2018 1:56 pm 
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Hi Jeff,

The NTE transistor has a different pin-out than the original transistor in the radio.
Do double check if it was installed correctly!

The other point is that there is certainly no need for such a "high performance" replacement transistor. The NTE is a VHF transistor, and the application, RF pre-amp on the BC band (Q5), does not see anything higher than 1.7MHz. Any jelly-bean Si transistor, like a 2N3904 (BTW, its fT = 300MHz!) would do there just fine.
Actually, I would prefer anything over a NTE transistor.

There are equivalents to the TDA1083 IC, the ULN2204, TA7613 are quivalents, and there is some "800-series" clone chip in my SR2 that I did not bother recording the actual number.

Good luck, Peter


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Tue 23, 2018 2:53 pm 
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Okay...It works! :D

I removed the band switch and verified that it works with 0.1 ohm resistance in both selections. I then ordered new electrolytics and changed them out. I used a digital capacitor checker that I had bought for work about a year ago to see how the new ones compared to the old ones, and in every case the new ones were closer to the rated capacitance. A few of the old ones were way out, but none appeared to be dead. After doing that the FM clarity had improved noticeably, but AM was still as it was after putting in the NTE229 as an AM RF transistor. I had a couple of stations more than in the very beginning, but they were pretty quiet just as before the recap and band switch check.

I then saw the comment about how the original and the new transistors have a different pinout which I had overlooked. :oops:

Now the new transistor is installed correctly, and after using a meter I can see that the original is open, so that transistor was the most likely culprit in the first place.

I know that I don't know what I'm doing, so your suggestions have been greatly appreciated as I fumble around! I only used the NTE229 because it was suggested on the earmark website. I don't know enough about transistors to know what brands, let alone what types to use as replacements. Maybe in a few years, and after some more projects like this then I will be slightly more knowledgeable!

Thanks for your help!!!


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Tue 23, 2018 7:30 pm 
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71Jeff wrote:
I don't know what I'm doing

Join the club! :wink:

Glad to see that you got it working, and learned a bit.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


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 Post subject: Re: Not much AM even after new AM RF transistor GE Superradi
PostPosted: Jan Wed 24, 2018 9:17 am 
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Joined: Mar Tue 27, 2012 1:24 pm
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Glad to hear you got the Superadio going.

Now get DXin' on it. :-)


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