July 19, 2009 at 5:05 pm #1420
I have just finished re-building a small Rogers 5 tube chassis, Type 563, dating from the 1933-1934 model year. I acquired this set for a ‘nominal sum’ at a CVRS meeting in Burnaby earlier this year. The tube line-up is a 6A7S pentagrid converter, 88S (or 78S) IF amplifier pentode (at 465kHz), a 6B7S duo-diode-pentode second detector/AGC and first AF, an 89S AF output pentode and an 84S full-wave rectifier. This chassis type was used in Rogers Models 910, 911, 912, 915 and 916, however, I am not sure exactly which model of set I have – it is very similar to a 911 (for which I have a photo) – but it is not quite the same . If anyone can identify it from the photos I would be most grateful! The cabinet is in not too bad a condition, though is lacking two small escutcheons for the tuning and volume controls (maybe also for the side-mounted tone control?) and knobs. There are four small screw holes present where the front panel escutcheons would have been secured.
The chassis was probably the worst case of ‘rats nest’ construction I have ever come across for such a diminutive set. It has obviously had a hard life with many of the original passive components having been replaced, including the wiring, both IF transformers and its AF output transformer. Whoever had made the last attempt at getting it to work had spent some time tracing and labelling various wires, but must have given up – the form of construction used meant there were many wires running all over the underside of the chassis to two tagstrips that contained most of the resistors and capacitors. One of these ran transversely across the chassis obscuring the wiring layout beneath. Also, many of the replacement wires looked like they belonged in a washing machine, being 14 gauge, thickly-insulated types. These had suffered many soldering iron contacts, melting the insulation through to the copper wires beneath, as had several of the (replacement) capacitors. Most of the soldered joints were dry, or the stranded wires had frayed around them, making them messy and subject to shorting. On closer-inspection, I found several mistakes in the wiring and the two tagstrips (which were really only Paxolin insulation strips with holes to take the tag-leads of resistors) were in very poor condition. All-in-all a real mess!
So, what to do? – re-store the original form of construction, re-build or chuck into the dumpster? Luckily I found the schematic in my RCC data, so at least I knew what I was trying to achieve… After a few minutes of assessing this and getting very confused with what wire went where, I could see why someone had identified many of the wires (and components) with small labels made from masking tape. I detest throwing things (especially radios) into dumpsters, so I decided give this poor little set a new lease of life by stripping everything out from beneath the chassis and starting again with (mostly) all-new wiring and passive components. The first job was to take out the first IF transformer (the large oblong box under the chassis) to allow access to the converter and IF stages. This transformer had been riveted in place, with one rivet in a very awkward spot, resulting in some mangling of the case before it came free. Once out, this allowed good access to the 6A7S and 88S tube bases. The two tagstrips and under-chassis wiring was then chopped-out. I tested the two (replacement) 10uf electrolytics that had been installed at some time in the past and these were ok, so I decided to re-use them, along with one of the silver mica caps (C3): all the other capacitors and resistors were discarded, all either being in poor physical condition or testing out of tolerance.
I then tested the power transformer – it worked (thank goodness!), it has a nominal 300-0-300 volt and two 6.2 volt windings (the 84S rectifier is a 6.3 volt filament). However, I noted that the high voltage winding was not balanced around its centre-tap, being more like 285-0-305 volts – this could either be poor tolerance during manufacture or a shorted winding. I left it running (no load) for a half hour or so and it was still cool, so I decided to press on with the re-build. The wires emerging from the power transformer are wrapped in what looks like bandages, however, some of this looked suspiciously fibrous, possibly containing asbestos. Not being able to remove this without dismantling the transformer, I stabilized the material in-situ by applying several coats of clear nail varnish.
I started replacing components/re-wiring at the front-end of the set (6A7S) and worked back towards the AF stages and finally the power supply. The first thing I noticed was that the colours on the insulated wiring emerging from the two IF transformers were incorrect – I concluded that these were likely both replacement items, this being supported by the crude way they had been re-mounted to the chassis (pop-rivets). The secondary of the second IF transformer (the small round metal can beneath the chassis) was found to be open-circuit and this was tracked-down to the fine coil wire becoming detached from the thicker lead-out wire – fairly easy to repair, though a bit fiddly. I decided to mount as many of the new components as possible directly to the tube bases, minimizing the number of wires beneath the chassis – this also makes for a more easily-understood layout when servicing in the future. Two small tagstrips were needed however, one serving to mount the AF and AGC resistors and capacitors, the other the bias supply resistors and filter capacitors in the power supply. New one watt metal-film resistors (from Lee’s Electronics in Vancouver) were used throughout, along with polypropylene, polyester or ‘Mylar’ capacitors (all from JustRadios), a couple of ‘NOS’ silver micas, plus the two 10uf electrolytics recovered from the ‘rats nest’ (this chassis appears not to have had can-type electrolytics fitted – unusual for an early-1930’s design). The ‘washing machine’ wiring was replaced with new cloth-cover wiring (from Radio Daze). Heat-shrink tubing was used to tidy-up frayed-ends on the little remnant original wiring, eg. to the speaker field coil and AF output transformer, plus some of the power-transformer wiring. I installed a 1A fuse line fuse beneath the chassis as some added protection. In order not to miss replacing anything, I pre-selected all the components from stock and placed them in a parts ‘bin’ on the workbench, ticking off each one on the schematic as it was installed, together with any replaced wiring (otherwise it is all too easy to miss connecting something up!). I did not bother with drawing a layout diagram as the circuit is fairly simple, so I just decided where things should go as I went along, minimizing leads lengths and number of grounding-points. Only one of the grid leads (top-cap connections) was replaced – the others were re-furbished using heat-shrink tubing.
The result is a chassis that works well and which can now be serviced relatively easily, albeit not that original underneath. The set worked first time on applying power (rather cautiously with a variac) and appears very sensitive, even without any re-alignment. I plan to re-align the set once I have decided how to replace the missing tuning scale/escutcheons.
Click on the schematic and photos for larger versions.
EdJuly 21, 2009 at 6:14 am #1423
Not good news I fear regarding the transformer in the Rogers set – I think it may have a partially-shorted secondary as suspected. I left the set running for a couple of hours and the transformer reached a temperature around 64C – a bit on the hot side – and was very slowly still creeping-up. It ran cooler when I fed it 105v from a variac. The set was still working ok, but I decided to switch off and only assign the set to ‘light duties’ – max an hour on or so – or fork out $50 or so for a new transformer, use a ‘bucking’ transformer, high-wattage series resistor or use the ‘dual zener trick’ in the primary.
I did some slight re-furbishing of the cabinet – limited to toning-in some scuffs and scratches with a touch-up pen and then a quick polish. Also installed a scrap of cloth over inside of the speaker aperture grill.
Next job wil be making the missing escutcheons for behind the tuning and volume control knobs (if you look carefully on the photos below you can see four small holes that these used to be screwed into). In the meantime, I have been working on drafting some tuning dial artwork – I used a signal genny/DFM and marked out the dial at 100kHz or 50kHz intervals onto a piece of paper placed behind the tuning knob. I did not align the set at all before I did this and it looks pretty good for broadcast band coverage as it is, tuning from 500kHz to 1700kHz.
In use, I have noticed that the set has some heterodynes present when tuning-in stations – maybe due to the RF, IF and detector tubes being un-metalized types. (these should be 6A7S, 88S (or78S) and 6B7S types). I may try wrapping some aluminum foil around the tubes that are in there to see if it makes a difference. If it is not this causing the problem I will try aligning it and then adding some additional by-passing.
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Download Rogers Dial Draft 1.jpg. (Caution: This file may not be virus scanned.)July 25, 2009 at 7:48 am #1429
Well, how lucky am I? – I located the exact same model radio in the SPARC radio musum in Coquitlam. I scanned the dials and tidied the scans with Photoshop before printing.
Attached are some photos of the repro PhotoShop dials fitted to the radio – unless you look very closely you cannot tell the difference (except the repro ones look to be in much better condition!). I printed them using an inkjet printer on HP Premium photo paper. I ran a black marker pen around the edges once I had cut the dials to shape to prevent the white paper showing through. I simply stuck the repro dials onto the cabinet front with Pritt-Stick and model train rail tacks! Also attached is a photo of the originals – look a bit scruffy by comparison… I borrowed and cleaned-up the knobs from the SPARC-owned set for these photos (and I will keep looking out for some the same), but it does not look too bad with the black ‘chicken-heads’ fitted as shown on the earlier photos.
By the way, the dials have a purplish tint on the photos taken under artificial light but appear solid black in reality.
Also, I fitted a ‘Goat’ shield to the converter tube and borrowed metallized tubes to try in place of the remaining plain glass tubes – cured the heterodyning dead.
Download DSC00158 [1024×768].JPG. (Caution: This file may not be virus scanned.)
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Download DSC00167 [1024×768].JPG. (Caution: This file may not be virus scanned.)August 2, 2009 at 5:40 pm #1439
Wow – lucked-out again! Called at Phil’s house last Sunday and he had found a couple of the correct octagonal knobs and a selection of 6A7S tubes to try out! Set is now looking and working as it should! – many thanks for the help Phil.
EdAugust 11, 2009 at 9:14 pm #1452Robin in KansasForum Participant
It looks great! My mantle has the same dials. They were nearly off entirely so to keep them from damage I took them of and found very thin foam underneath. I suppose it was to keep the relief from getting smashed.
Anyway, I saw a picture of a model someone listed as a 4511. If you overlook the dial cutouts, feet and inlay it’s the basically the same body.
Oh, well. At least close counts in horseshoes, hand grenades and nuclear attacks, right?
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