Noisy resistors - Nordmende Globetrotter 4/601 - UK Vintage Radio Repair and Restoration Discussion Forum

sant_ka · 3rd Jan 2017, 6:55 am

I wanted to know if anyone else has come across noisy resistors ?

I was restoring an early model Globetrotter (4/601). All the electrolytics on the audio board were completely dead due to which the set was motorboating badly.

So I had removed the audio board for the repairs, but when I turned it on after replacing the caps, I noticed a prominent hiss and crackle. This was not 50Hz hum, it was more like the crackling static that one hears on the MW band. The noise would go away completely whenever I grounded the audio section's input. Apart from this problem, the audio section was working correctly and the voltages everywhere looked correct.

Ok, so it must be the 1st Preamp transistor (AC163) that's about to give up the ghost. But replacing this did not solve the noise problem. After racking my brains for a day and trying out different substitutions I was left with just one suspect -> The bias resistors on the base of the 1st preamp transistor.

And Voila, replacing them solved the noise problem completely !! The original resistors were Carbon Composition types and although they had not drifted off value badly they were generating a lot of noise

Nuvistor · 3rd Jan 2017, 8:43 am

Yes it's a known problem, well done on repairing your radio.
Frank

sant_ka · 3rd Jan 2017, 9:43 am

Thanks Frank

BTW, there was another interesting problem on this radio. Every time I powered up the radio the RF section would work for a second and then die. It was happening on all bands. Turned out that biasing of the RF transistors is regulated by the V7 zener diode and it had gone badly off spec generating 4V instead of 1.54V. Replacing it with a Red LED solved the problem.

3rd Jan 2017, 6:55 am	#1
sant_ka Diode Join Date: May 2016 Location: Bangalore, Karnataka, India Posts: 7	Noisy resistors - Nordmende Globetrotter 4/601 I wanted to know if anyone else has come across noisy resistors ? I was restoring an early model Globetrotter (4/601). All the electrolytics on the audio board were completely dead due to which the set was motorboating badly. So I had removed the audio board for the repairs, but when I turned it on after replacing the caps, I noticed a prominent hiss and crackle. This was not 50Hz hum, it was more like the crackling static that one hears on the MW band. The noise would go away completely whenever I grounded the audio section's input. Apart from this problem, the audio section was working correctly and the voltages everywhere looked correct. Ok, so it must be the 1st Preamp transistor (AC163) that's about to give up the ghost. But replacing this did not solve the noise problem. After racking my brains for a day and trying out different substitutions I was left with just one suspect -> The bias resistors on the base of the 1st preamp transistor. And Voila, replacing them solved the noise problem completely !! The original resistors were Carbon Composition types and although they had not drifted off value badly they were generating a lot of noise __________________ Cheers, Sant

3rd Jan 2017, 8:43 am	#2
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: Noisy resistors - Nordmende Globetrotter 4/601 Yes it's a known problem, well done on repairing your radio. Frank

3rd Jan 2017, 9:43 am	#3
sant_ka Diode Join Date: May 2016 Location: Bangalore, Karnataka, India Posts: 7	Re: Noisy resistors - Nordmende Globetrotter 4/601 Thanks Frank BTW, there was another interesting problem on this radio. Every time I powered up the radio the RF section would work for a second and then die. It was happening on all bands. Turned out that biasing of the RF transistors is regulated by the V7 zener diode and it had gone badly off spec generating 4V instead of 1.54V. Replacing it with a Red LED solved the problem. __________________ Cheers, Sant