The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Please Help!
Uniden PC244
Symptoms: Radio does not transmit on Am or SSB. The rig does key and still has Rx. Just no output. I don't have a Schematic and not real sure where things are located, like the final Transistor.

It is a nice radio and I would really like to fix it. Any direction would be greatly appreciated!!

hello,,if your radio is keying up dead carrier,,you can rule out the final and driver..if it is recieving with clear receive audio,,you can rule out the audio chip...id be looking in the area of the audio chip..probably a shunt capacitor...check the large capacitor next to the small transformer,,audio transformer,,it could be burned open...

Thank you for the reply ramblingman!

I checked the audio transformer(the only transformer I see in the radio) and have 13.8v in and 8.2v out, it appears to be working. It also looks like the capacitors are ok as far as I can tell. The 2 large ones next to the transformer are 50v 1000 uf. are these the ones I should be checking?

Just to clarify the radio does switch to transmit(the TX light on the front comes on and the Rx switches off) the output must be very small. It does the same thing on SSB the radio will key but no output is present. There must be some small amount of output, because a near by radio will RX a weak signal(with no audio). But it is to small for my watt meter to detect. The RX seams to be working.

Please hang in there with me, I'm just starting to learn something.

what is your voltage readings at the audio chip

I assume that you mean on the uPC1242H.

Here the measurements are:

Pin 1: .1v (will cause the speaker to hum when checked)
Pin 2: 2.04v
Pin 3: 1.39v
Pin 4: .078v
Pin 5: .093v
Pin 6: 7.03v
Pin 7: 12.77v
Pin 8: 13.78v

This radio has no audio transformer, it uses a pass regulator for the audio.

A common fault with this radio is TR544 (2SA1012) bolted to the side of the chassis you may also find TR37 (2SB525) o/c.

Checked the TR44. It tested ok! I'm not seeing the other transistor? If I can find it I'll check it, or is it not nessessary knowing that TR44 in ok? I'm thinking of getting the UNIDEN EXPORT SERVICE MANUAL to help me out. What do you think?

Thank you for all time you guys have spent on this problem.

does not this radio have the pc-833 p.c.b.

The Uniden Export Manual will not help.
Sorry about the transistor number I was working on a radio and TR37 got stuck in my memory, the transistor should be TR43.

What is the voltage on TR44 Collector in TX?
What is the voltage on the Driver and Final, Base and Collector in TX?

The collector voltage on TR44 is 6.04 RX
The collector voltage on TR44 is 5.95 TX.

I assume the driver and final are the transistors mounted to the back of the chassis along with TR44?

I'm not sure which is which or the transistor #'s or pinouts. Could I do damage to the transistors by checking voltage on all three pins? I don't think so I just want to be sure?

Well I couldn't wait for the answer. The two transistors on the back of the chassis measured the same in TX. I think the pinout is this:

Pin 1 B:.71v
Pin 2 C:5.99v
Pin 3 E:.002v

RX voltage on these transistors is:
Pin 1 B: 0v
Pin 2 C: 6.09v
Pin 3 E: 0v

Hope this helps you trouble shoot for me!

It looks like the Final has gone south, it would be best to test it before replacing it, to test the final, do a Google search on test transistor. The type you are testing is known as NPN, this site may help.
http://www.anatekcorp.com/qdmmvom.htm
It may be worth testing the driver as well.
Let us know how you get on.

Rick,
I tested the transistors and found that the one closest to the radios left side looking from the back at the bottom of the PC board(speaker down)(do you know if this is the final or driver?) is not testing open from the emitter to the collector in one of the polarity directions. It appears that this may be the problem.

I don't have much experience with transistor failure. So if you have time please enlighten me. Is this common for them to fail in this way. Meaning: the E-C not open in one polarity? Seams funny to me that if it is not open one way it should be not open the other. Likewise if open in one polarity it should be open in the other polarity also.

Let me know what you think?
Thanks again for all the help.

Thought I would post a pic to help out.... I also needed a site to store JPGs!

http://www.geocities.com/poikaa/location.JPG

I have had similar problems with radios and it sounds like the driver or final to me. By pulling the jumper board you can take the DC voltage off the driver and final. In the pic power is from the right. Then you can use a jumper lead to apply power to one or the other transistor to see if one is shorted or open.
You can also operated QRP (low power) about 100mW using this, er mod! ....

poikaa

if i remember correct,,you said that you can hear a faint carrier in a nearby receiver,when keying radio...that pretty well tells me that the driver is working..that means it is most likely the final...the final will be the transistor furthest from the side rail..is this a double final radio or a single ..if this is a double final radio,id suggest changing the center final,this is generally the one to go..

Well here it is. C103(100uf 10v)! I lifted the cap and I now have my carrier back. I Don't know much about the cap, it was a suggestion from a local radio tech. It was shorted. Although it looks like this will fix it I'm not quite certain.

If anyone has any details on this cap I would like to know more! As soon as I get a new one installed I'll let you guys know how the radio works. I didn't want to operate it much without it, it's obviously in there for a reason.

Boy what a learning experience! I thank you all again for the help. If it wasn't for people like you, this site wouldn't exist and I wouldn't learn a thing.

Well, back to the drawing board!
I changed the cap and now have my full power carrier back but no modulation on AM or SSB. Is there another bad cap somewhere?

Any suggestions?

Thnx