UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

David G4EBT · 3rd Jul 2018, 4:52 pm

There's a tendency where Roberts are concerned, to focus on the Mullard units (where fitted) or the AFxxx transistors as know culprits. However, as far as the R505 and R700 are concerned, electrolytic capacitors pose a considerable risk of causing damage. I was given a non-working R505 a few months ago by a neighbour who was about to bin it. On switch on, it came on briefly for about 2 seconds, then nothing. On a metered power supply it was evident that there was a short, and TR11 & TR12 (AC187/Ac188) were getting very hot. However, on testing them, they'd survived.

The fault was due to C40 - 680uF - connected to the emitters of TR11 & TR12, it having gone short circuit, registering as a 0.53 Ohm resistor. I replaced that, which restored the set to working order, but given that C41 & C42 (150uF & 470uF) could also put a dead short across the battery, I replaced those too as a precaution.

Let's not forget that these sets are 50 years old, so to not replace those caps as a matter of course is just inviting trouble - if not now, then later. As to the R707, if C42 (250uF - modern equivalent 220uF) goes short circuit, it will place a dead short across the two 6V batteries in series, which could have interesting consequences. Likewise, if C41 goes short circuit, it will short the emitters of TR6 & TR7 to ground, which will make them feel very unhappy.

For the small amount of work and expense involved, I'd change the electrolytic caps on sight, but at the very least, check their capacitance, leakage, & ESR.

I've attached relevant snippets of the circuits of the R707 and R505 to illustrate what I mean.

Also the readout of C40 from the R505, which shows that it is no longer a capacitor - it had morphed into a 0.53 Ohm resistor.

The R505 service data states that a scope should be connected across the speaker, a 1KHz sine wave injected between S1e and chassis, then R36 adjusted to show a symmetrical waveform at the onset of clipping. Not a bit of use to anyone who has neither a signal generator or scope. I have both, but rarely use them, though out of curiosity I did check the R505 and found it to be in need of adjustment as illustrated in pics 5 & 6 - before and after adjustment.

I guess the moral of that is that if we have test gear, for the small amount of effort involved, why not use it?

Hope that's of interest.

3rd Jul 2018, 4:52 pm	#6
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,766	Re: Roberts R707 There's a tendency where Roberts are concerned, to focus on the Mullard units (where fitted) or the AFxxx transistors as know culprits. However, as far as the R505 and R700 are concerned, electrolytic capacitors pose a considerable risk of causing damage. I was given a non-working R505 a few months ago by a neighbour who was about to bin it. On switch on, it came on briefly for about 2 seconds, then nothing. On a metered power supply it was evident that there was a short, and TR11 & TR12 (AC187/Ac188) were getting very hot. However, on testing them, they'd survived. The fault was due to C40 - 680uF - connected to the emitters of TR11 & TR12, it having gone short circuit, registering as a 0.53 Ohm resistor. I replaced that, which restored the set to working order, but given that C41 & C42 (150uF & 470uF) could also put a dead short across the battery, I replaced those too as a precaution. Let's not forget that these sets are 50 years old, so to not replace those caps as a matter of course is just inviting trouble - if not now, then later. As to the R707, if C42 (250uF - modern equivalent 220uF) goes short circuit, it will place a dead short across the two 6V batteries in series, which could have interesting consequences. Likewise, if C41 goes short circuit, it will short the emitters of TR6 & TR7 to ground, which will make them feel very unhappy. For the small amount of work and expense involved, I'd change the electrolytic caps on sight, but at the very least, check their capacitance, leakage, & ESR. I've attached relevant snippets of the circuits of the R707 and R505 to illustrate what I mean. Also the readout of C40 from the R505, which shows that it is no longer a capacitor - it had morphed into a 0.53 Ohm resistor. The R505 service data states that a scope should be connected across the speaker, a 1KHz sine wave injected between S1e and chassis, then R36 adjusted to show a symmetrical waveform at the onset of clipping. Not a bit of use to anyone who has neither a signal generator or scope. I have both, but rarely use them, though out of curiosity I did check the R505 and found it to be in need of adjustment as illustrated in pics 5 & 6 - before and after adjustment. I guess the moral of that is that if we have test gear, for the small amount of effort involved, why not use it? Hope that's of interest. Attached Thumbnails __________________ David. BVWS Member. G-QRP Club member 1339.