25ltd classic no recieve rx meter works fine

SoCal392 · Post by **SoCal392** » Wednesday 28th Feb 2007, 9:04

this radio seems to have had moisture contaminants. all pots check ok

Rick · Post by **Rick** » Wednesday 28th Feb 2007, 9:54

Try replacing the audio chip and maybe the cap around it.

Rick

SoCal392 · Post by **SoCal392** » Wednesday 28th Feb 2007, 19:40

this 25 tx is fine with good modulation, do you still think it could be the audio chip? thx

Rick · Post by **Rick** » Wednesday 28th Feb 2007, 22:12

Ok I should have ask if the TX audio was ok. Is it a case of the Squelch is on? check Q6 it is just used has a switch the collector should be 0 volts with it fully closed and about 2 to 3.5 volts open, also check Q10 it is used as the AF buffer and TX Mute switch, see if the Base voltage changes with you open and close the Squelch, also check diodes D4, 5, 6 and D7, if you have a small head phone try connect it across the Volume pot center pin and ground you should ear the incoming audio. Hope this helps, let us know how you get on.

Rick.

SoCal392 · Post by **SoCal392** » Thursday 1st Mar 2007, 6:30

ok, the base voltage of tr6 is zero when closed and 1.16volts when open. tr10 base voltage varies from 0-8.8volts. d4 thru d7 all ok. I noticed that i cant hear the pa thru an external speaker. also, I can hear incoming audio barely with earphone. your time is greatly appreciated rick. russ

Rick · Post by **Rick** » Thursday 1st Mar 2007, 8:24

From what you have told me about Q6, it appears to be turned on when it should be off try removing Q6 to if it now works.

I noticed that i cant hear the pa thru an external speaker. What do you mean by this?

If you put an external speaker in the PA jack and talk in to the mic can you hear you?re self from the pa.

I can hear incoming audio barely with earphone. Where do you try the head phone?

If you try the head phone across the Volume control you hear any incoming audio? Also can you hear any audio at Pin 4 of the AF amp (IC1 TA7222AP) try open and closing the Squelch control?

Hope this helps, let us know how you get on.

Rick.

SoCal392 · Post by **SoCal392** » Thursday 1st Mar 2007, 22:56

sorry about my vague descriptions. it was not intentional. "I cant hear pa thru ext. spkr" means: when I plug in an ext. speaker into pa jack and switch radio to pa, I dont hear my voice coming out of the ext. spkr. keep in mind, the radios tx and modulation work great. ref. your second reply, you said i should tap an earbud to the center post of the volume potentiometer, which is what i did to "hear barely incoming audio". tr6 was off by .5volts to your specs in your second reply, I may have typed it backward on accident. again, my apologies. I have since replaced tr6, tr10 to no avail. Tomorrow I will try pin 4 of audio chip for audio. thx again Rick, Russ

Rick · Post by **Rick** » Friday 2nd Mar 2007, 2:53

Ok for what you have told me it now looks like the fault is a bad audio transformer (T1) you can verify by checking for audio with your head phone at the input to the transformer connect you head phone to the negative side of C67 (407u 10v) and ground, you will need to use a coupling cap between you head phone and ground something like 100nf or .1u this is just isolate the DC from you head phone, or just try a audio transformer from a scrap radio as most transformer are a like. Let us know how you get on.

Rick.

SoCal392 · Post by **SoCal392** » Saturday 3rd Mar 2007, 22:26

well Rick, I replaced audio transformer and audio chip and nearby caps... still no recieve. any more options? again thx

Rick · Post by **Rick** » Sunday 4th Mar 2007, 1:42

Did you try verifying the audio transformer as I suggested? This will help if you can do this.

Can you let me know voltages at all the pins of the audio chip in TX and RX mode.

The strange think is that the PA is not working this is usually the audio transformer which has gone open circuit.

Rick.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Sunday 4th Mar 2007, 10:58

I am thinking that the issue may be volume control related, as the radio has good mod on AM, which SHOULD rule out the audio amp and transformer (although not completely).

Pots can go bad, or crack, and if that happens it could cause symptoms like these, as the volume is often used to control the PA volume, so I would start looking at the volume control pot, and its circuitry.

I will have a look to see if I have a good diagram for the radio, I have one for the old version (not a very good one though), and there may be some differences between the old version and the newer "classic", which is a remake.

Rick · Post by **Rick** » Sunday 4th Mar 2007, 23:08

I do not think it the Volume control has this is not in the PA chain, It could be that in audio chip is in Mute mode. The easy way to find out what is going on is to inject a 1KHz tone at the Volume control and Pin 4 audio chip if get no output then is time to check all the components around the audio chip. Is Q13 clamping the input to the audio chip?

Rick.

SoCal392 · Post by **SoCal392** » Sunday 4th Mar 2007, 23:38

Thanks for the input scott. this is a 1989 model. well rick, i will get the voltage readings from audio chip first thing tomorrow. I could just as well toss this pos but its personal now. cant let it win! thx peeps

SoCal392 · Post by **SoCal392** » Thursday 8th Mar 2007, 9:01

ive decided to put this on the back burner 4 a bit but this saga will continue. thx rick

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