Etronic RA640 re-visited - UK Vintage Radio Repair and Restoration Discussion Forum

Beardyman · 7th Feb 2020, 9:38 am

It was a while back when this radio was finished & lately I've been going through the collection doing a bit of cleaning/dusting etc. I never really finished the wooden case of the RA640 so I thought, "That's a good thing to begin with, should keep me quiet for a few weekends". Whilst I was seeing to the case I powered the chassis up on the bench & was listening to some 40's music via the pantry TX. After about maybe 10-15 minutes my inner supervisor said, "That sounds different!", it was very thin & reedy, virtually no bass notes at all then it would behave itself then thin & reedy again. To cut a long story short, I found the KT61 output valve had an intermittent heater/cathode short. Looked on line & could not believe the price these valves are going for! Ridiculous! Off to the valve museum & with a change of cathode resistor a Mazda 6P25 will be a good substitute at a far more reasonable price. Should be here early next week but all that aside apparently the KT61 had a bit of a history of this fault. With nothing to lose I applied a PSU set to about 3V but high current (>5A), it drew around 2-3A for a few seconds then cleared to zero. Thinking I'd either removed the short or blown the filament I checked again, no short & filament intact! Result I thought. No, within a few seconds it returned. I think that's the thing I like about valve radios, they just keep giving!

Leon Crampin · 7th Feb 2020, 6:59 pm

The poor quality of MOV valves never ceases to amaze me. Metallised types are now nearly always bare, and even when new, many MOV types barely make their specified gm on the tester.

Mazda, I think made the best valves apart from a few flaky rectifiers (U801 etc.) and most Philips and Brimar types were OK.

I would fit the 6P25 unless originality is important - a wayward output valve can damage transformers.

Leon.

Beardyman · 10th Feb 2020, 11:21 am

Originality is sometimes important but in this case not. I refuse to pay £60+ for a valve that has an unknown history & doubtful construction. From what I've read the 6P25 has a better reputation & is significantly cheaper (approx. £18). If this set was a high end model I may have been tempted to "splash the cash". Yes, output transformers are easily damaged & I'll be keeping a weather eye on that when the new valve arrives. All the changes will be documented so if in the future the next owner wishes to put it back to original then they can. BTW, the case is looking much much better! Have a good day.

mark2collection · 10th Feb 2020, 11:35 am

Sound-wise, we had a similar thing recently with our Philco 444, reaching the point when warm, as through someone had 'flicked' an internal tone switch to make it sound quiet, dull, flat & no low frequency response at all. Imagine a pillow being placed over the loudspeaker.

Looking at the set yesterday, the output valve wasn't fully home & the centre of the 'speaker magnet was touching the voice coil former(?) The loudspeakers magnet assembly runs warm in use, I'd imagine the expansion was enough to upset the clearance, coupled with the lumpy/rusty bits too.

Having reseated the output valve, de-rusting the inner metal parts of the 'speaker & re-centering the voice coil, it's never sounded so good

My initial fear was, duff output valve.

They keep you on your toes, that's for sure.

Mark

Beardyman · 14th Feb 2020, 6:00 pm

Well, the 6P25 arrived today. Plugged it into the chassis and, after a few double checks, applied power. Everything looking good, no odd smells or smoke. Gradually a hiss from the speaker, tuned into the pantry tx, we have bass! Actually we have a very nice sounding radio again. Its been running on the bench for a couple of hours & nothing is distressed, mains & output transformers the same temperature as surrounding metal work. All in all I'm one happy bunny. Now to get to grips with the random pops, clicks & f*rty noise the Defiant is making! As I said before, I love valve radios, they just keep on giving! Have a good weekend one & all, keep safe.

7th Feb 2020, 9:38 am	#1
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 428	Etronic RA640 re-visited It was a while back when this radio was finished & lately I've been going through the collection doing a bit of cleaning/dusting etc. I never really finished the wooden case of the RA640 so I thought, "That's a good thing to begin with, should keep me quiet for a few weekends". Whilst I was seeing to the case I powered the chassis up on the bench & was listening to some 40's music via the pantry TX. After about maybe 10-15 minutes my inner supervisor said, "That sounds different!", it was very thin & reedy, virtually no bass notes at all then it would behave itself then thin & reedy again. To cut a long story short, I found the KT61 output valve had an intermittent heater/cathode short. Looked on line & could not believe the price these valves are going for! Ridiculous! Off to the valve museum & with a change of cathode resistor a Mazda 6P25 will be a good substitute at a far more reasonable price. Should be here early next week but all that aside apparently the KT61 had a bit of a history of this fault. With nothing to lose I applied a PSU set to about 3V but high current (>5A), it drew around 2-3A for a few seconds then cleared to zero. Thinking I'd either removed the short or blown the filament I checked again, no short & filament intact! Result I thought. No, within a few seconds it returned. I think that's the thing I like about valve radios, they just keep giving! __________________ The difference between stupidity and genius is that genius has its limits. (Einstein)

7th Feb 2020, 6:59 pm	#2
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Etronic RA640 re-visited The poor quality of MOV valves never ceases to amaze me. Metallised types are now nearly always bare, and even when new, many MOV types barely make their specified gm on the tester. Mazda, I think made the best valves apart from a few flaky rectifiers (U801 etc.) and most Philips and Brimar types were OK. I would fit the 6P25 unless originality is important - a wayward output valve can damage transformers. Leon.

10th Feb 2020, 11:21 am	#3
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 428	Re: Etronic RA640 re-visited Originality is sometimes important but in this case not. I refuse to pay £60+ for a valve that has an unknown history & doubtful construction. From what I've read the 6P25 has a better reputation & is significantly cheaper (approx. £18). If this set was a high end model I may have been tempted to "splash the cash". Yes, output transformers are easily damaged & I'll be keeping a weather eye on that when the new valve arrives. All the changes will be documented so if in the future the next owner wishes to put it back to original then they can. BTW, the case is looking much much better! Have a good day. __________________ The difference between stupidity and genius is that genius has its limits. (Einstein)

10th Feb 2020, 11:35 am	#4
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 470	Re: Etronic RA640 re-visited Sound-wise, we had a similar thing recently with our Philco 444, reaching the point when warm, as through someone had 'flicked' an internal tone switch to make it sound quiet, dull, flat & no low frequency response at all. Imagine a pillow being placed over the loudspeaker. Looking at the set yesterday, the output valve wasn't fully home & the centre of the 'speaker magnet was touching the voice coil former(?) The loudspeakers magnet assembly runs warm in use, I'd imagine the expansion was enough to upset the clearance, coupled with the lumpy/rusty bits too. Having reseated the output valve, de-rusting the inner metal parts of the 'speaker & re-centering the voice coil, it's never sounded so good My initial fear was, duff output valve. They keep you on your toes, that's for sure. Mark __________________ Slowly turning the 'to-do', into 'ta-dah'

14th Feb 2020, 6:00 pm	#5
Beardyman Hexode Join Date: Jan 2018 Location: Portsmouth, Hampshire, UK. Posts: 428	Re: Etronic RA640 re-visited Well, the 6P25 arrived today. Plugged it into the chassis and, after a few double checks, applied power. Everything looking good, no odd smells or smoke. Gradually a hiss from the speaker, tuned into the pantry tx, we have bass! Actually we have a very nice sounding radio again. Its been running on the bench for a couple of hours & nothing is distressed, mains & output transformers the same temperature as surrounding metal work. All in all I'm one happy bunny. Now to get to grips with the random pops, clicks & f*rty noise the Defiant is making! As I said before, I love valve radios, they just keep on giving! Have a good weekend one & all, keep safe. __________________ The difference between stupidity and genius is that genius has its limits. (Einstein)