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Old 29th Jun 2022, 12:50 pm   #1
martinhog
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Default Roberts R800, Quiet audio, only on FM

Hi,

I have been reviving a Roberts 800 transistor radio. It had suffered from being in a smoky atmosphere, needing 3 "doses" of foaming cleaner to remove all the nicotine.

The first fault found was the radio only stayed on if you held the on/off switch down; several blasts of servisol failed to cure it. I was going to change the whole switch assembly but then noticed the on/off switch is a two pole switch with only 1 set being used, so the "spare" set were joined with two wire links and all was well.

The radio is now working well on AM, however on FM, the audio seems much quieter than AM, sometimes barely audible with the volume control set to maximum. Strangely, if you retract the telescopic aerial, the audio is noticeably louder ! .

Does any one have any ideas on this problem ?

All advice and comments gratefully received, Thank you .
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Old 3rd Jul 2022, 11:31 am   #2
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Default Re: Roberts R800, Quiet audio, only on FM

Bump, thanks
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Old 8th Jul 2022, 3:37 pm   #3
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Default Re: Roberts R800, Quiet audio, only on FM

Update:

I had a good look around the front end. Checking both C301 and C302, these checked out OK. A did a few basic checks on the surrounding circuitry. I noticed that R304 on the service sheet is listed as 4K7 whilst on my radio, it had a 680R fitted. I looked at another R800 and R304 was 4K7 in that one too. I changed mine to a 4K7. It seems quite a bit better, if you touch the aerial it makes a difference now, however it still doesn't sound/feel right. The voltage on the emitter of T301 is only 0.4 volts, the service sheet says 0.72 V.

All comments and opinions gratefully received, thank you.
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Old 8th Jul 2022, 6:42 pm   #4
roffe
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Default Re: Roberts R800, Quiet audio, only on FM

If you look closely at the circuit diagram,you'll notice that all three transistors in the front end get their base voltages through three 10k resistors.Now these three resistors are all connected to D304,a 1,4V zener diode.What's the voltage across this zener?
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Old 9th Jul 2022, 9:01 am   #5
martinhog
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Default Re: Roberts R800, Quiet audio, only on FM

Thanks for your reply, I will check it on Monday and let you know.
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Old 11th Jul 2022, 11:16 am   #6
martinhog
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Default Re: Roberts R800, Quiet audio, only on FM

Good Morning roffe,

Thanks for your advice.

The voltage across D304 is 1.18V, the emitter voltage on T303 is 0.35 V and on
T304, it is 0.376 V. So all a bit low ?

The supply voltage from the built in mans power supply is 10 volts. When running on a battery, the radio is drawing around 20mA.

All comments appreciated, thank you.
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Old 11th Jul 2022, 12:15 pm   #7
ms660
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Default Re: Roberts R800, Quiet audio, only on FM

What does the B+ (D302 cathode) to the VHF tuner measure WRT ground ?

Lawrence.

Last edited by ms660; 11th Jul 2022 at 12:29 pm. Reason: shuffle
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Old 11th Jul 2022, 12:45 pm   #8
martinhog
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Default Re: Roberts R800, Quiet audio, only on FM

Hi Lawrence, it measures 5.92 V

Many thanks

Martinhog
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Old 11th Jul 2022, 12:46 pm   #9
ms660
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Default Re: Roberts R800, Quiet audio, only on FM

That should be ok.

Lawrence.
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Old 15th Jul 2022, 11:51 am   #10
martinhog
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Default Re: Roberts R800, Quiet audio, only on FM

Hi,
I wonder if I am missing something ?

I got out a known good Roberts R800 and did some similar voltage readings to those I took on my faulty model. These were also about half what is shown on the Service sheet (Voltages with respect to 0V).

T301 emitter is only 0.4 V (Service sheet says 0.72 V)
T302 emitter is 0.35 V (Service sheet says 0.74 V)
T303 emitter, it is 0.376 V.(Service sheet says 0.79 V)

The above voltages are comparable to the faulty model

Across D304 (1.4 V Zener) 0.61V , this was much lower though than the faulty one I am looking at.

The DVM is a recently calibrated ISOTech IDM 91E.

Maybe one or more of these transistors is low gain ?

All thoughts appreciated

Last edited by martinhog; 15th Jul 2022 at 12:07 pm. Reason: Missed a line of text
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Old 15th Jul 2022, 7:37 pm   #11
roffe
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Default Re: Roberts R800, Quiet audio, only on FM

A tuner can actually work with lower currents than intended.This gives often lower noise but at the prize of lower gain and lower ability to handle strong signals.
The only tip I have to give is to check the ceramic filter by bridging it with a 10nF capacitor.
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Old 21st Jul 2022, 1:22 pm   #12
martinhog
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Default Re: Roberts R800, Quiet audio, only on FM

Thanks for your reply Roffe. Bridging the filter did make a very slight difference. In the end I changed T301, originally a 2SC1417F, I used a 2N3904 as a substitute, this so far, has worked well, with a much better audio response. I did consider the same for T302 but this has a coil assembly wound round and sealed with wax, so I think I will leave it alone.

I am wondering of the Transistor voltages quoted on the service sheet are "base-emitter" rather than 0v- emitter, as following this idea, the voltages measure are as per the service sheet ? Reading other posts, I see other contributors have questioned these voltages.

Thanks for all of your suggestions.
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