GETTING A BUSH VHF41 WORKING

Front view [9K] Rear view [12K]

The BBC's first experimental V.H.F. broadcasts began in 1950, though at this time it was .A.M. However in 1951 the B.B.C. published plans for a national V.H.F. F.M. network although it was not until May 2nd 1955 that the first of the nine proposed V.H.F. F.M. transmitters went into regular service.

This set is probably Bush's first V.H.F. radio. Released in November 1954, 6 months before the start of regular broadcasting, it cost 20 15s 4d which is only a couple of quid more than an equivalent non-VHF radio of the time.


INITIAL INSPECTION

This isn't my radio; a mate asked (on behalf of the real owner) if I'd take a look at it and see if it was worth getting working, but with strict orders not to actually start repairing it just yet.

As ever, the first thing was a general look around, checking for signs of obvious faults and for previous work. The following were noted :-

  1. Obvious sign of wax having dripped out from the mains transformer (there were a couple of stalegtites hanging down).
  2. The total replacement of all connections between the main chassis and the off-chassis components (namely the output transformer, loudspeaker earth and front bulbs)
  3. Insulation breaking down on some of the wires around the mains transformer, rectifier valve and output valve
  4. The "white stuff" on a couple of trimmer caps looked like they may have been disturbed.
  5. One of the main HT electrolytic capacitors had been replaced.
  6. Obvious messing around with the wires on the mains switch. This switched both live and neutral and both switches were open circuit.
  7. Possibly missing screen can on the second VHF valve (the socket looked like it was meant to have a can fitted).
  8. Whoever it was that did this obviously couldn't afford a soldering iron so it was just as well that they had that blowtorch to hand, otherwise they would have been stuck ! Seriously, wherever wires had been soldered nearby components were half melted away and insulation was stripping off from nearby wire. Only half of the coupling capacitor to the volume control remained, literally !

At least with item (8) its easy to spot where works taken place !

Having the 'Trader' service sheet to hand (curtosy of CD#1), I checked out the modified wiring, which seemed to match the circuit diagram as best as I could tell. However the main concern really was the obvious heating of the mains transformer and its surroundings (melted wires and capacitors are simply nuiscences) so I decided to dig a little deeper. The service trader sheets are very useful as they invariably give you the DC resistances of the various transformer windings. The HT and mains windings seemed about the right resistance (the filament winding is too low resistance for me to measure) so I decided to risk connecting the mains. Having confirmed there was no capacitor across the mains input (some sets do have one, usually wax, but should ALWAYS be replaced), and a quick check of the mains wiring seemed OK (but see later!). The original mains lead was removed and thrown in the bin and a temporary mains lead was soldered to the mains switch (on the transformer side). A TEMPORARY BODGE ONLY!!!

In this set, all of the valve heaters are in parallel, so you can remove one valve and all the others will still work.. I therefore removed the HT rectifier (IMPORTANT) and gradually applied the mains via a variac. By about 100v there was a definate buzz from the transformer. But also from the speaker ??? Eh ??? S**t ! SMOKE ! Power-off pronto ! However, I couldn't help thinking the smoke did not come from the transformer. I decided to try again with careful monitoring ; if it was the transformer then it was dead anyway, otherwise there were only wires and an odd cap that could possibly have smoked. It turned out the smoke had come from one of the modified wires that went off to the front panel. I snipped the wire and tried again. Almost total silence, no smoke and as I got to around 160v I could see the valves begin to light (except the HT valve ... that was still on the bench, remember?). A check of the center tapped HT secondary revealed identical voltages on both halves, indicating this was probably healthy. So there was a very good chance the mains transformer had survived.

With the mains removed, I connected my variable DC supply up to the HT and gradually wound it up to around 120v over 20 minutes or so (120v happened to be about when I got bored!). This indicated 1.1mA of leakage, but in this set biasing resistors would acount for ~700uA of this. Not too bad. Various bias voltages were checked ; leaky decoupling caps would cause voltage drops when in series with a resistor, so this is a useful check on the condition of such capacitors. All seemed OK.

With the HT rectifier still on vacation, mains was applied to heat up the valves. After a couple of minutes I gradually wound up the variable power supply connected to the HT until the HT current matched that stated in the service data. This resulted in a 100V H.T (normally this would need to be somewhere around 250v) so clearly some biasing had gone astray. There was also a slight buzzing from the speaker despite the fact that the HT was coming from a regulated D.C. supply. There were two possible reasons for this buzzing I could think of :-

  1. A valve somewhere had a heater-cathode short circuit, or -
  2. SOME DIVVY HAD THE WAVEBAND SWITCH SET TO 'GRAM' !

Switching to medium wave killed the hum, and tuning around suddenly brought in a station ! In fact, very clearly ! And long wave ! And VHF ! Not only that but you could still pick up stations on MW with only 25v of HT.

So I reported back that this set was worthwhile renovating


IZZY WIZZY LETS GET BUSY

The owner did not want to spend much money on the set, so the plan was to confine work to faulty components plus anything to do with safety.

Modified wiring [14K] The first thing to sort out was the mystery wiring and why it had smoked, despit the fact that I had checked the connections. It didn't take long to discover why. As can be seen opposite, the wires (B) pass through the chassis inbetween the closely spaced HT recifier (C) and audio output valve(D). This had burnt the cable and created an internal short circuit, in this case shorting out the front bulb connections. However, this was clearly the wiring route intened by the manufacturer and so I guess this modified wiring was a replacement for the burnt out original ! This is definately a design fault, there being no obvious way in which to re-route or secure the wires safely.

Having noted the connections to the output transformer (A) in a log book for future reference, I disconnected the wires. This also enabled the chassis to be completely removed from the case.

When I eventually replaced these wires I used tie-wraps to strap the wires to the side of the transformer. Not particularly ideal but there was nothing else to attatch them to.

Whilst fiddling with the wiring it seemed a good opertunity to sort out the poor insulation on the wiring under the HT section. At this point I paniced ! You can just make out at point (D) in the picture on the left where the mains connections to the transformer ran. The wires had fused with a rather tired looking wax capacitor hidden underneath the tone control and really the only way to remove the wires was to remove the tone control and capacitor. And to access some of the other wires in the area it was clear that some of the tag strips would have to be temporarily detatched from the chassis and moved. What was I saying about the odd insultaion problem being a mere nuiscance ? Me and my big mouth, most of this area now seemed to have been stripped ! I'd missed this on initial inspection and wouldn't have let mains anywhere near the set if I'd noticed it earlier. Transformer [10K]

I followed the wires back to the transformer only to realise their connections were in fact buried in the wax drips. This leads me to what I would consider a second design fault. (B) is a non-insulated wire that connects several tags to chassis (0v). The mains connection connected to (A) runs under this wire and touches the tag (C) which is the second mains connection ! Given the state of the insulation in this area this was clearly dangerous and would need replacing.

The mains connection to (A) was rewired (taking the path up, left, down and round in previous diagram so as to ensure it did not rest against any exposed terminals) using a suitably rated wire.

This set utilises mainly those (I'm lead to believe) unreliable little black Hunts capacitors (of which the audio output coupling capacitor was one). However, some of the capacitors were so well buried that attempting to replace them all would require the desoldering of other wires and components, risking damage to both components and tag strips. Therefore I decided to replace only those caps whose failure would cause other damage. Specifically these were HT decoupling and any inter-stage coupling capacitors. This includes "that" capacitor that couples to the output valve ; just out of curiosity I put 100v on the HT (without lighting the valves) and checked the grid of the output valve, which was 5 volts instead of 0. With 220-250v HT this would become some 12v, 1.5 times the normal 8v cathode-grid voltage and would seriouly over-drive the output valve. If the set had been run in this state the catode bias resistor (220 ohms in this set) could be damaged and so I checked it ; 228 ohms and no sign of overheating so it was assumed OK. The wire-wound 1K resistor in the HT supply to the output valve was also checked and seemed OK.

With the capacitors sorted attention turned to the volume / (duff) on-off switch. It would have been nice to have replaced this but one wasn't available. In fact the one that was fitted wasn't original and had been bodged in place (one of the wires was too short so the pot had simply been rotated so that it reached). The rear mains switch was held by three lugs that could be bent out thus releasing the mains switch. As I removed the switch I had visions of springs and assorted bits of metal flying out all over the room, but in fact it was a simple arangement of a sliding mechanism and two spring contacts. A liberal dose of switch cleaner plus bending the spring contacts restored operation, though not sure for how long. On refitting the switch I shortened one of the leads so as to remove the melted portion of that cable and replaced the other lead that had been too short in the first place.

Time to power it all up. Again, the HT rectifier was removed and the mains gradually applied from a variac. Once the valves had been lit for a few minutes HT was gradually applied from a variable DC supply whilst monitoring supply current. The service sheet gave the various valve cathode voltages from which the expected current consumption could be estimated. The current seemed right and stations could be picked up on all wavebands. The set was ran like this for around half an hour, frequently checking the current.

On VHF it was found that the volume was intermittent, dropping by 1/2 every now and again. This could be prompted by rocking the waveband switch, which was subsequently cleaned, fixing the problem.

Finally, the radio was taken into a nice warm room and run continuously for around two hours by way of a soak test.


TODAYS LESSONS

  1. Thou shalt check every centimeter of mains wiring before applying the mains, regardless of how hidden the wires may be.
  2. If there is a 'Gram' switch make sure 'Gram' isn't selected !
  3. Bush used rhobust mains transformers. What a pity the same designer wasn't employed in their TV line output transformer department !

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J.Evans 2000,2001
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Last updated
17th September 2001