Tram D300 HELP

[Unit 75]

I got this radio in on a trade without the side car speaker. The previous owner said it worked fine. It had output in all modes on the wattmeter so I took it in on trade. It wasn't until I got back home and hooked a speaker in the back that I noticed this problem because the wattmeter did show the needle swinging up from the carrier power. It has a 5w carrier on AM (with what sounds like a bad filter cap in the supply that produces that 60 cycle hum when you talk into the mic) and I almost have to scream into the mic to get any SSB output (about 8W of distorted audio). The same sound is there in PA mode. I then tried using an external 12v supply instead of the internal AC supply to eliminate any power supply issues but it is still there. When you turn the radio on it has a oscillating loud noise in receive mode thru the speaker no matter what speaker jack you have it plugged into (speaker one or speaker 2). So, I re capped the entire radio and the noise is still there. GRRRR! IC4 (TA7222AP) IC5 (MB3756) TR35 (2sc945) TR34 (2sc1419) have been replaced as well as ALL non electrolytic caps around the input and output of the TA7222A audio IC. I did find TR36 (2sc458) defective but after replacing it the sound is still there. I have never experienced a problem like this one. Main board is PC-385AE. The radio is in excellent physical condition and I wouldn't mind using it on the base if I can get it working. All voltages on all pins of the regulator and audio IC's are correct. Any help trying to find the problem would be appreciated. Thanks.

 

[nomadradio]

Uh, the mike socket hasn't been changed from the original 5 pins to a 4-pin socket has it?

73

 

[Unit 75]

No, sir. Still has the 5 pin.

 

[nomadradio]

Do you have a radio handy that can receive FM on CB channels? Key the D300 and switch the receiver from AM to FM. The hum will be louder one way than it is on the other.

If the hum is louder on FM than it is on AM, the problem is probably in the radio's PLL frequency synthesizer, not in the audio circuits.

73

 

[Unit 75]

Thanks for the replies, Chris. I don't have a FM mode radio handy at the moment but I can get use of one in the morning and try out what you suggest. I will post back in the morning with my findings Have a great evening!

 

[Unit 75]

For some reason things have changed. When I turned the Tram on this morning, the receive through the speaker is fine. No hum coming through at all. I thought that was strange but continued with the test. As soon as I keyed the Tram, it sounded like a loud machine gun noise coming thru the Tram receive speaker. Not something I expected to hear that early in the morning....crap. I had to unplug the Tram external speaker to continue due to the loudness of that sound. I also had noticed that the transmit had dropped from 5W to 3W on all channels but continued the test. So while transmitting with the Tram I switched the other radio between AM and FM modes and did not hear a difference. It seems there is no hum at all during transmit anymore. If I just key the mic with a carrier it is dead quiet on the other radio while switching back and forth from AM to FM modes. When I tried to modulate with the Tram, the audio seemed broken and slightly distorted like it was being attenuated some how but still breaking through, almost a crackling noise sound. So, as it stands, This weird noise is only coming through the Tram speaker during transmit and it is only putting out 3 watts of carrier.

 

[OldTech03]

I know you changed electrolytics but that machine gun sound is reminiscent of a bad electrolytic in the audio section. We actually referred to that as motorboating.

 

[Unit 75]

Yes, sir. This sound is different than the usual "motor boating" we are accustomed to. The hum sound is no longer in the RX or TX audio. The machine gun sound (which is different from the initial hum sound is happening when I TX a carrier with no modulation and only through the units own speaker but not on the receiving end of another radio. I'm still at it. Perhaps I will find something new and If I do I will update the thread. Thanks for your input.

 

[Unit 75]

It seems I found a ground problem on the chassis of the radio. Not where the main board is attached but the actual chassis. I removed all the screws holding the front face and rear panel and cleaned everything and put the screws back in. It is now putting out 5w again....but when the speaker is plugged in the back of the radio the wattage drops down to 4.5w. Unplug the speaker and back up to 5w. Looks like a RF issue. Still working on it.