Radiomobile 108S Intermittent Radio Reception - UK Vintage Radio Repair and Restoration Discussion Forum

Magical · 5th Mar 2021, 1:36 am

I have a "strange" problem with the radio on a Radiomobile 108S 8 track. It took me some time to get the radio going, but not by changing things, just by trying to align and by getting the 8 track working....somehow the radio started, but intermittently.

The radio will fade in and out and when it's playing it plays well but after a few minutes it quietens down to nothing. Leave it on a while and it may kick back in. The interesting thing is that when I activate the 8 track and go immediately back to the radio the reception is restored. Then the cycle starts again.

Sometimes the play from the radio is out of one speaker only but doing the 8 track starting trick restores sound from both speakers.

I suspect it's not a reception problem but maybe some problem with the transistors in the preamp....but why would they work ok on the 8 track?

Any help or advice would be much appreciated.

Cheers,

Cobaltblue · 5th Mar 2021, 11:09 am

Have you cleaned the switches?

Use a proper switch cleaner like Servisol and be careful not everything is resistant to cleaners.

This has the hallmarks of switching or dry joints although there are other possible causes.

Cheers

Mike T

G6ONEDave · 5th Mar 2021, 12:03 pm

I would also check the coupling capacitors in the audio pre-amp stage for being leaky. A voltage leak could possibly be biasing the transistors off or causing them to self oscillate at an inaudible frequency. An oscilloscope would prove the self oscillation theory.
Dave

Magical · 5th Mar 2021, 12:42 pm

Hi,

Thanks Cobaltblue, I’ll clean the switches. Thanks G6ONEDave, what you propose sounds feasible. When I measure the voltage across the base and the collector of each of these pre amp transistors sound returns to the speakers then disappears when I remove the probe. I’ll set to on the capacitors.

Thanks to both of you again,

Nick.

Magical · 5th Mar 2021, 10:49 pm

Hello again,

All connections re-cleaned but the problem still there. I've checked the capacitors in the area where I'm sure the problem is but I don't trust the results as this is an intermittent fault....everything came out as "good". The two electrolytic caps in that area were changed but no improvement. There are a number of tantalum capacitors, which also checked out on my multimeter and ESR gadget....don't believe that either. Are tantalum caps prone to intermittent faults? I've measured the voltages on the transistors, (LH and RH channel) and the read b= 0.055v, e=0.66v and c=6.8v when they're working but when they drift out the base and emitter go to 0.011v and the collector to 11.8v. Therefore, could it be the tantalum caps even though they appear to check out ok?

Thx again.

5th Mar 2021, 1:36 am	#1
Magical Triode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 33	Radiomobile 108S Intermittent Radio Reception I have a "strange" problem with the radio on a Radiomobile 108S 8 track. It took me some time to get the radio going, but not by changing things, just by trying to align and by getting the 8 track working....somehow the radio started, but intermittently. The radio will fade in and out and when it's playing it plays well but after a few minutes it quietens down to nothing. Leave it on a while and it may kick back in. The interesting thing is that when I activate the 8 track and go immediately back to the radio the reception is restored. Then the cycle starts again. Sometimes the play from the radio is out of one speaker only but doing the 8 track starting trick restores sound from both speakers. I suspect it's not a reception problem but maybe some problem with the transistors in the preamp....but why would they work ok on the 8 track? Any help or advice would be much appreciated. Cheers,

5th Mar 2021, 11:09 am	#2
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,865	Re: Radiomobile 108S Intermittent Radio Reception Have you cleaned the switches? Use a proper switch cleaner like Servisol and be careful not everything is resistant to cleaners. This has the hallmarks of switching or dry joints although there are other possible causes. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

5th Mar 2021, 12:03 pm	#3
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,701	Re: Radiomobile 108S Intermittent Radio Reception I would also check the coupling capacitors in the audio pre-amp stage for being leaky. A voltage leak could possibly be biasing the transistors off or causing them to self oscillate at an inaudible frequency. An oscilloscope would prove the self oscillation theory. Dave

5th Mar 2021, 12:42 pm	#4
Magical Triode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 33	Re: Radiomobile 108S Intermittent Radio Reception Hi, Thanks Cobaltblue, I’ll clean the switches. Thanks G6ONEDave, what you propose sounds feasible. When I measure the voltage across the base and the collector of each of these pre amp transistors sound returns to the speakers then disappears when I remove the probe. I’ll set to on the capacitors. Thanks to both of you again, Nick.

5th Mar 2021, 10:49 pm	#5
Magical Triode Join Date: Jun 2020 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 33	Re: Radiomobile 108S Intermittent Radio Reception Hello again, All connections re-cleaned but the problem still there. I've checked the capacitors in the area where I'm sure the problem is but I don't trust the results as this is an intermittent fault....everything came out as "good". The two electrolytic caps in that area were changed but no improvement. There are a number of tantalum capacitors, which also checked out on my multimeter and ESR gadget....don't believe that either. Are tantalum caps prone to intermittent faults? I've measured the voltages on the transistors, (LH and RH channel) and the read b= 0.055v, e=0.66v and c=6.8v when they're working but when they drift out the base and emitter go to 0.011v and the collector to 11.8v. Therefore, could it be the tantalum caps even though they appear to check out ok? Thx again.