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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 28th Dec 2018, 4:48 pm   #1
mark2collection
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Default Sobell 511W reworking the rework

Earlier this year, I finally acquired a Sobell 511W.

I say 'finally', as I'd been on the lookout for one for quite some time, they were either too expensive, too far away, or both. This being my first 'woodie' and first Sobell, it was sold as a 'worker' for a pretty sensible price, but it's always fun to haggle. The priced reduced and a deal done, I walked away a happy bunny. The mains lead was not original, incredibly short, but was told the set had been properly earthed.

Although sold as 'working' (having no reason not to believe the seller), I had to satisfy my curiosity to the overall safety of the set and make sure nothing had worked loose in transit. Removing the non-original back cover, the non-original base cover and carrying out some prudent checks with my multimeter, revealed all was seemingly ok to fire-up. Curious though, this is the first time I'd seen a selenium rectifier, which was still in circuit, bridged by a much newer 1N4007. Hmmmm(?) Cue the 'puzzled' expression.

Having attached a length of wire for the aerial, power was applied. The set crackled into life, the dial lighting up at one end of the scale (a duff lamp at the other end), tuning across the wavebands showed the set to be a lively performer, especially on Shortwave (tests carried out in May 2018). Only a few moments in, the 50Hz mains hum became a buzz, quite an annoying buzz.

There was however, another problem which became abundantly apparent on MW/LW, not so obvious on SW. When you found a station, within a few seconds of listening, the programme was interrupted by a steady 'ramped' noise through the 'speaker, the tone started at around 300Hz gradually ramping to the limit of the sets output stage/my hearing. Just as the tone seemingly went quiet, the was a gentle 'plop' through the 'speaker, where the 'ramp' would begin again. Set temperature would make no difference.

Over the course of a fortnight I acquired a datasheet for the set & checked voltages (all too high), plus the value of the capacitors which had clearly been replaced. These were new/red Wima capacitors, rectangular, short legs, designed for PCB 'through-hole'. Whilst the values/voltages were correct, the act of moving a capacitor to reveal its identity, legs would ping off from their location, in another moment, one cap' fell out all together. These parts were temporarily 'tacked' back in position, the set from here on powered via a Variac, reduced to bring the HT lines to within spec.

Shall we say the previous method of recapping was creative, with large bridges of solder to 'extend' the legs of the new parts. The only remaining wax capacitor appeared original to the set, connected from the Smoother positive to chassis ground, with both the HT/Smoother and waxie, being duffers! These were duly removed, and re-stuffed to their correct values, then reinstalled. These original parts read as resistors, the HT/Smoother reading, lowered as the part warmed in the palm of my hand.

Re-testing after this work revealed the 50Hz hum was still there, but greatly reduced. Still not convinced the hum should be this audible (during quiet passages on classical pieces), the HT/Smoother cap' had been grounded at its most convenient point, its own mounting bolt. Making the capacitors negative lead longer, adding suitable sleeving and soldering to the transformers 0V/Chassis ground point, the hum was much reduced. Carrying out further voltage checks, they were high again, variac reduced further still. The 'ramp' noise remained at this point. Some reading on-line and this very forum revealed a previous post some time ago from Mr Sherwin himself, with a suggestion of using a resistor in-line with the diode, in place of the selenium rectifier. After a bit of 'playing'/taking the selenium rectifier out of circuit, Paul, your suggestion of 220 ohms in said post brought all HT readings to spec at full mains! I've retained the original selenium rectifier in the set, but sleeved-up the new diode legs with clear PTFE sleeving, fed the diode legs though the wiring holes of the selenium rectifier, so it still looks right, but the selenium rectifier is not actually used, electrically.

After some careful cleaning of the chassis, switches, valve pins & sockets, a new set of dial lamps, yet another Wima replacement cap' came away in my hand. Only one thing for it, a set of yellow axial ones from RS (just a happy customer) arrived a few days later, then fitted sleeving where legs passed over each other/other components and installed in such a way as to keep AC pick-up to a minimum.

She has to be tickety-boo now, surely? Mmmmmm no, the 'ramp' was still there. After careful diagram/chassis layout scrutiny vs my set, it may well be I'd just made the same mistake as the previous repairer.

Remember the original waxie I'd so lovingly re-stuffed, a Hunts component, connected from Smoother + to transformer 0V? Turns out this should've been connected to the AGC line, and the 'original' HT/Smoother, was nothing of the sort, being a later (duff) replacement. Looking back at my 'before' photos, the waxie was indeed connected as above, I'd even connected it back exactly, where it came from.

The photo attached shows the Hunts waxie incorrectly connected, this has since been corrected as per the schematic. Needless to say, the 'ramp' has now gone, and my 511W with its 6V6 output valve, nice-sized 'speaker and wooden cabinet produces a wonderful sound. I've also fitted a P-Clip to secure the mains lead to the chassis.

It appears I'm the only one in the house who likes the style of the set, though we all agree it now sounds rather fabulous.

All in all, a great 'training' exercise.

Mark
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Old 24th Jan 2019, 10:10 pm   #2
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Default Re: Sobell 511W reworking the rework

Hello Mark,

Thanks for this story and write-up. I always enjoy it when a bit of sleuthing successfully solves a problem. Nice to see the set restored to full working condition! I hope you enjoy using it for many years to come.
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Old 31st Jan 2019, 11:56 am   #3
mark2collection
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Default Re: Sobell 511W reworking the rework

Hiya Al,

Glad you enjoyed the post, I had a few head-scratching moments, thankfully I took 'before' photos which have proven useful on many occasions/repairs.

I didn't post too much about the cabinet, which was given a gentle clean. Some of the lacquer (shellac?) has 'crazed' but given my lack of experience (the square root of zero) in wooden cabinet restoration, a going over with a slightly moistened cloth, given a dry and then a going over with wax has brought it up a treat, I think the cabinet has aged well and I fear any trials of restoration by me would otherwise ruin a nice looking set, plus the original logo is near-mint, and I would hate to damage that too.

The set does have a wonderful tone, and of all my sets with Shortwave, the Sobell is by far the most responsive, which has impressed a few family members.

I don’t know much about the brand Sobell, I have read-up about them on Graces Guide et al, I believe they started out selling sets via GUS Catalogues(?) You don’t see too many Sobells around, not sure they were produced in high volumes(?), they were certainly priced about right, back in the day, not budget yet not premium.

Quite a large set for me, being used to DAC-series Bush sets, and the occasional VHF-series Bush.

All in all, it’s great addition to the collection.

Mark
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