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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 22nd Jun 2018, 1:09 pm   #1
brightsparkey
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Join Date: Sep 2017
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Default Roberts R66 sevicing and trader sheet differences

So here are some pics of my R66, serial number R8351. Dark green leather covering - strap has failed and has been removed in these photos. I un-soldered the washers on the inside and removed the brass pins - now looking for a leathercraft person to make another one as I have a complete set of broken parts to use as a pattern.

The case work is all intact though there are numerous white dots of paint - this must be so common - which radio do you use whilst painting the ceiling - the oldest one..

The HT battery connector is broken, missing a leg. No idea where to get one of those ! The LT battery connector had come un-soldered at the ends - easily fixed. I powered the set from bench supplies and it basically worked. The mains supply was another story..

I obtained the trader sheet 1273 for the set. Over the course of tuning up the set I discovered the set is slightly different to the one used for the sheet..

One thing to be wary of is the HT of 120V is right at the edge of the chassis on C28 and easy to touch when moving the set. The Mains switch disconnects the HT as well as the mains leaving the capacitor charged and with no discharge path. I fitted a temporary 1M bleed resistor across the cap while working on the set for safety.

Both selenium rectifiers were more or less open circuit - Having isolated them I tested them with a bench supply with current limit set to the set's expected current draw. With 50V on the HT rectifier, no forward current measurable. Same for the LT - even 10V and only 1mA still. On disassembly both were very corroded.

I dismantled the LT rectifier and re-used the plastic shaft, two terminal tags and the paxolin washer to provide support for the two 1N4007's. I measured R15 and R16 LT droppers and both were high. These were replaced with 5R6 resistors which brought the LT to 1.4V (from ~2.8V at the diodes). The trader sheet mentions their set having 1.5R of extra resistance wire, so there may have been a design issue with the LT voltage initially. That wasn't present on my set. The HT rectifier was placed inside the original's case using the original black and red end plates with 100R series ballast resistor. A bit of sleeving and some heat-shrink for insulation inside. Looks authentic enough.

With the rectifiers out I tested the electrolytics. In all 4 cases they were fine - less that 100uA leakage at rated voltage, so they were left alone. The output pentode cathode capacitor C24 was leaky and was replaced as was the only wax cap, c23, more or less on principal. The 1kV LCR is a bit overkill

I wont go over the fun and games I had with V1 as this is described in the earlier thread... now that the tube is replaced the set functions reliably. Whilst trying to work out what was going on, I adjusted the oscillator and discovered the trader sheet differs materially from the set in front of me.

I have attached a pic of the difference - the layout around the oscillator coil is subtly different. The main difference is that the transformer is reversed/inverted so that L5 core appears on the wiring side and L6 is accessible from the valve side on the top of the can. This means that tuning is a bit confusing if you follow the sheet's instructions. The pins of the IF can are likewise reversed so that R4 goes to the south side and C5 to the north. R1 is also in a different place. I can only assume it was slightly less cramped in that area with it this way around..

The loudspeaker was covered in white insulting tape - very gooey and untidy. When removed much of the label came with it I recovered what I could. The speaker was loose in the baffle so a pair of nuts and bolts were provided to hold it still as originally intended.

Alignment was easy - the IF was spot on - no need to touch it. The RF was out and the pointer was scraping on the dial. I opened the scale up with the four screws, shook out the spiders and wiped away the dust. Cleaned the point then set the pointer as per the instructions having straightened it out - very nice and solid brass pointer construction - really quite luxurious.

Then once having realised the coils were reversed, the alignment was pretty easy - not much to do as the set is very simple. I did discover that C12 had been screwed right down hard, causing the ceramic disc to crack. Unwinding the screw and removing the bits - was able to super-glue them back together and the trimmer works OK again. With the pointer in the right place there is no need for violence with the trimmers - the screws were chewed so someone must have had a go..

And so it a happy radio now, in full working order, just waiting for a nice new handle. I rather like the cream octagonal knobs and the bold curve of the scale lines. Shame there's so little to listen to on MW and LW these days.

Kevin.
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Old 22nd Jun 2018, 1:12 pm   #2
brightsparkey
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Default Re: Roberts R66 sevicing and trader sheet differences

Some more pics of the rectifiers, tuning cap.

It has a license paid label too for A7S - glued in not pinned.

K.
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Old 22nd Jun 2018, 1:15 pm   #3
brightsparkey
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Default Re: Roberts R66 sevicing and trader sheet differences

Oh and the new rectifiers too..
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