Roberts RP26B No MW and LW
 

Hello, we have 3 of these radios and each one has some kind of problem on AM. All of them have extremely noisy volume pots. I dismantled one of them yesterday, fixed the pot noise with no problems, then examined the AM section. The tuning system on both FM and AM is by means of potentiometers (I think this is called varicap, is that correct?). On MW there is only static, which changes in pitch very slightly as the tuning is altered. On LW there is the merest hint of RTE 252 which doesn't alter with tuning. On both bands, touching the aerial coils increases the level of noise. I've noticed that lack of AM seems to be a common fault on these radios, looking at eBay adverts and past postings on this site. My experience so far has been of amplifiers, record players, tape players, etc, and I've never really tackled an RF fault so I thought this would be a good starting project. In the schematic I've marked in red the circuit path when LW is selected. My thoughts are that the likely cause is a fault with the oscillator section - I'm assuming L6 is the oscillator coil? Would I be on the right lines or am I overlooking something obvious? As usual, any advice greatly appreciated

Re: Roberts RP26B No MW and LW
 

Download the datasheet of the AM-FM chip of the radio:
https://pdf1.alldatasheet.com/datash...M/BA4236L.html

The datasheet explains in detail the working of the chip both in AM and FM mode.
Your instinct seems correct about following the AM oscillator circuits.
Depending on the test equipment available you can verify their operation.
You could also check out the working of the AM-IF stage by injecting AM modulated signal e.g. at Pin 4 of the chip.

Regards, Peter

Re: Roberts RP26B No MW and LW
 

I’m not familiar with this set but agree L6 is the AM oscillator inductor.
Do give the M1, M2 switches a good clean. Can you measure a changing tuning voltage at Test Point 5. Also check the voltages on IC2.

Regarding the circuit, I’m wondering what is the purpose of R24? It appears it mutes AF (via TR4) when the set is off and current will continue to flow via R24,R23 & R22 in this state!

Edit; I should add, if the set is powered by the battery.
Edit;Edit; Also true if left permanently plugged into the mains.


Vic

Re: Roberts RP26B No MW and LW
 

Thanks for the advice, will check out all of that. Now the next question may be a bit like asking how long's a piece of string but here goes: I don't have a signal generator but I've been thinking about investing in one for a while. I built myself a simple injector / tracer for fault finding in amplifiers, but it's just a straightforward single audio tone, so I need something much better for radios. What kind of spec. should I thinking of, bearing in mind it's unlikely I'll ever use it for anything other than this level of work (i.e. no TV stuff) - and it is just a hobby?

Re: Roberts RP26B No MW and LW
 

Quick update - the problem on the Roberts was quickly sorted, turned out to be the skeleton preset pot - (VR8 at the very bottom left of the schematic), was virtually open-circuit, thanks again for the advice