Roberts R303

[David G4EBT]

From what you say it sounds as though the audio stage is most likely working, but that the coupling to it via R2 is suspect (given that it's fallen apart!). If you use your K7000 to inject a signal into the base of TR1 and get a loud tone from the speaker, as you have tunable signals at slider of the volume control, that more or less narrows it down to R2 as the most likely culprit. (Less likely, C5).

Well done on the progress so far.

[DonaldStott]

Let's cut to the chase - replaced R2 with a new 2W 10k resistor and the set literally roared back into life!!!

SUCCESS!!!

Here is the culprit:-



I was extremely careful when removing the broken resistor and soldering the replacement to ensure that I didn't disturb the PCB tracks - not easy!

Rewind to where I was at Post #24 above -

"The set is performing well although probably sub-optimally and I am going to walk away and leave well alone until such times as an actual fault occurs!

So I've just got to finish off cleaning up the case etc. then reassemble this neat little transistor radio - I'll Post some pictures when I'm finished."

A huge thanks to all those who responded and steered/nudged me in the right direction. A special mention to David G4EBT for keeping me on track with his methodical diagnostic approach and for introducing me sometime ago to the wonderful world of the Velleman K7000 Signal Tracer/Injector which has been indispensable here! The AF117s have lived to fight (fail?) another day but at least I'll know what the problem is when the time comes!

[ms660]

That's a common failure mode for those resistors, amongst others, BRC/Thorn used them by the million, often one of the end bands would fracture.

Lawrence.

[poppydog]

Glad you have finally sorted it. The resistor that gave me a real headache was the 2K2 emitter resistor on the BC148 that caused a really nasty crackling when the volume was turned up. It too, was falling to pieces.
I also have a EKCO A455 that has had a problem with those types of resistor in the past, the previous owner has somehow managed to “poop” some solder over the affected end of the resistor, ive not had any trouble with it yet though.

Regards Poppydog

[DonaldStott]

Originally Posted by ms660

That's a common failure mode for those resistors, amongst others, BRC/Thorn used them by the million, often one of the end bands would fracture.

Thanks Lawrence - I'll watch out for those in future!

Originally Posted by poppydog

Glad you have finally sorted it. The resistor that gave me a real headache was the 2K2 emitter resistor on the BC148 that caused a really nasty crackling when the volume was turned up. It too, was falling to pieces. I also have a EKCO A455 that has had a problem with those types of resistor in the past, the previous owner has somehow managed to “poop” some solder over the affected end of the resistor, ive not had any trouble with it yet though.

I suppose I could have tried to reassemble the resistor from the broken parts but replacing it with a new 2W 10k resistor at a total cost of 1p-2p seemed the optimal route to go?

[DonaldStott]

And finally some pictures of this refurbished and fully working R303:-

[DonaldStott]

Roberts 303 Revisited !

This restored set is now making a very loud noise on start-up and when switching waveband before it settles down and plays normally!

Warning - loud noise!

https://youtu.be/6AbxkcjjevI

Suggestions please?

[Andrewausfa]

New volume pot required?! Seems as if the section just after the on/off indent is very worn.

Andrew

[AD360 Rob]

That's not the volume control. Sounds like it could be an electrolytic on the way or a bad joint, possibly even one of the AF117s in the module . Time to dismantle it and do some fault finding.
These are really great sets when working but they are almost completely ignored in comparison to the R200/R300. I have on the same colour but in poorer condition due to wear and tear so at least it's had a life.

[Davewantsone]

Like I said in an earlier post. Replace those AF11X transistors with BC557.

[DonaldStott]

It's definitely not the volume pot as the noise is heard when switching on and changing waveband.

Also when I leave the volume at normal listening level and disconnect then reconnect the battery the noise is heard (thanks to David G4EBT for that tip).

I'm no expert (!) but it sounds to me like something is discharging before the set settles down to normal operation? It's back on the shelf until such times as I can dismantle it again for yet another look! Thanks to above Posts I know to look for dodgy electrolytics, bad joints or (dare I mention it) replacing the AF117s in the module!

Thanks guys.

[PJL]

It sounds like instability. It could be caused by a poor contact? Is the battery still good? I would try replacing C6 first if you have not already done this, and C13 simply because it may improve the tone.

[DonaldStott]

While waiting on parts for other Projects I decided to disassemble the Roberts R303 in an attempt to locate the very loud noise problem highlighted in Post #47 above.

I examined all components on the circuit board and tapped the LP1159/1 IF module but this made no difference. As previously I worked my way round each component, tapping it with the flat blade of a screwdriver and found that R7 crackled in much the same way as R2 did. So this 6.8kΩ resistor will be replaced.

Some of the components were touching each other and everything was straightened up - this appeared to cure the very loud noise problem so I'm assuming something was shorting out somewhere?

While the board was out the case I measured some of the electrolytic capacitors: -

C2 : Spec 125μF
Measured : 231μF
ESR : 0.53Ω
VLoss : 19%

C6 : Spec 200μF
Measured : 256μF
ESR : 0.36Ω
VLoss : 28%

C14 : Spec 200μF
Measured : 292μF
ESR : 0.39Ω
VLoss : 17%

So everything seems a bit high but I don't want to start blanket replacement as I have a working set, again!

Same goes for the 'infamous' AF117 transistors as the LP1159/1 IF module does not yet exhibit any of the well known symptoms associated with these transistors. I've looked at the board and module and can't figure out how to disassemble these?

[DonaldStott]

A quick follow up in the hope that I can elicit some responses: -

1. I've assumed that the measurements I've taken for the electrolytics tends to imply that they need replacing, especially the high value ones?

2. What would be appropriate values for these caps and what voltage?

3. I've tried to find reasonably priced 125μF and 200μF electrolytic caps online but I'm really struggling - does anyone know of a suitable source? I've assumed that there is no reason why radial caps can't be fitted instead of vertically stacked axial ones??

4. How does one 'carefully' disassemble the LP1159/1 IF module from the main board?

5. If I were to replace the AF117 transistors would BC557 be an appropriate choice or something else? Transistor swapping not one of my strengths!

Any information would be very helpful as well as being informative so thanks in advance!

[AD360 Rob]

Your electrolytics are fine according to your readings, it is very common for them to read higher in value than the stated figure on the can sometimes by over 50%. However if you wanted to replace them, use 100uF and 220uF, these being the closest values readily available today to those fitted. As for replacement AF117 transistors, I use the russian GT322B transistor, it's a 4 pin germanium device available for a reasonable price on eBay. Pin out isn't the same as an AF so beware if you do get any but they do work well

[DonaldStott]

Originally Posted by AD360 Rob

Your electrolytics are fine according to your readings, it is very common for them to read higher in value than the stated figure on the can sometimes by over 50%. However if you wanted to replace them, use 100uF and 220uF, these being the closest values readily available today to those fitted. As for replacement AF117 transistors, I use the russian GT322B transistor, it's a 4 pin germanium device available for a reasonable price on eBay. Pin out isn't the same as an AF so beware if you do get any but they do work well

Thanks Rob - I found some 100uF (25V) and 220uF (16V) electrolytics in my capacitor stash so these would do while I've got the set disassembled?

I'll have another look at how easy it would be to tackle the AF117s as I think I may also have some BC547s if they are suitable?

[DonaldStott]

As I mentioned in Post #53 above I worked my way round each component and found that R7 crackled when tapped!

So this 6.8kΩ resistor was replaced, just in time :-



Some of the electrolytics were playing up as well so these were also replaced.

Good news is that all the cracking has gone along with the very loud noise problem and we appear to have a happy Roberts R303 on both LW and MW. So time to move on quickly before anyone asks about the AF117s !

[G6Tanuki]

Those resistors with tubular stamped-brass ends are well known for being problematic: the brass tubular-bit often splits along its length and so stops gripping the ends of the carbon 'stick'.