Bush BS35 (Ireland) - new restoration

[paulsherwin]

The cabinet may have been made locally in the Republic to satisfy local content rules. Several of the big boys had Irish factories but I don't think Bush did. A local furniture maker could have made the cabinets and fitted the chassis inside though.

[turretslug]

Are you sure about the mains neutral connection? A bit of cross-checking reveals this set as having a mains isolating transformer, with a single tapped secondary feeding series heaters and HT- hence half-wave rectifier- which makes sense in a bean-counter sort of way, if not from the engineering elegance view-point. Peering closely at the under-chassis view appears to confirm this, with red and black from on-off switch going to centre two tags on the transformer underside and not one of them connecting to chassis, as would be the case with "universal" or "AC/DC" practice.

As for use of two-core or three-core replacement mains lead, this subject gets aired here every now and then and the last time it did, the conclusion was that most folk here would go for three-core mains lead and earthing the chassis, but with a minority strongly holding the opposite view. Personally, I take the view that all the old radio junk we play around with isn't worth a candle compared to the safety of human beings and mains-isolated sets get earthed- if a transformer goes leaky to frame, better a residual current device/circuit breaker/fuse gets tripped rather than a chassis floats up to high mains voltage for yonks until an unsuspecting fixer down the line takes the back off/removes a knob and earns a nasty surprise. If a mains transformer has weak insulation, better that it reveals itself to be discarded or rebuilt.

[paulsherwin]

If the chassis is isolated, earth it. If it isn't, don't.

[Mulcher]

I checked the wiring and found as follows: the mains live and neutral are connected to the on/off switch and from there to the transformer primary. One of the transformer secondaries (the one that feeds the display bulbs) has one side connected to the chassis. As I understand it, that means that the chassis is isolated form the mains, so the correct action would then be to earth it. I had overlooked the fact that the transformer isolates the radio circuitry from the mains. Thanks again for advice and comments.

Regarding the work, the speaker and UM4 wiring have been replaced. Also, the replacement capacitors are due in a day or two, so I will soon know if the radio is a dud or not!

[paulsherwin]

It sounds as if there is an isolating mains transformer but the HT is being half wave rectified as is standard practice in transformerless AC/DC sets. There will be a secondary winding of about 130V feeding the valve heaters in series. This is an unusual arrangement but a few other British manufacturers used it at times, notably Ekco.

[Mulcher]

Regarding the operation of the radio, I posted this a few days ago:

For testing the mains transformer I used a variac and with the primary AC voltage at about 64V I get 2.2V on one secondary and 76V and 47V on two other leads - there’s a blob of wax over the transformer so I cannot see where the different leads are connected on the transformer. One 2.2V line goes to the display lights and another 2.2V line goes to the heater of UBC41 and from there to UCH42 and then UM4. The 76V line goes to pin 2 of UY41. The 47V line goes to the heater of UY41 and from there to UL41, UF41 and another UF41.

The above is what I found a few days ago with about 64V on the transformer input. The valve heaters appear to be divided into two groups rather than in a single series. I'll double check them again.

[paulsherwin]

You were measuring the voltages with only 64V on the primary. Scaling up, you will find your 76V winding goes to the UY41 anode to provide HT, and your 47V winding goes to the heater series string.

Some AC/DC export sets did have odd series/parallel heater arrangements to support different mains voltages, but a transformer removes the need to do that. However, it's possible that the heater winding was intended to support a standard superhet without the RF amp or magic eye, and some jiggery pokery is taking place to support the extra valve heaters. The UM4 and UF41 need 12.6V each.

[Mulcher]

Those numbers look like they fit generally. About the heater connections, I think I may have been looking at "the two ends of a single piece of string" - I could have missed a link between two of the valves (some are well buried below the components and wiring). Another check to be made.

[Mulcher]

Checked again and found that the heaters are in a single series, so normal.

I've been encouraged by the comment that the UM4 base can be a bit loose even though the tube is OK. Earlier, I had found that one wire to the UM4 had been left disconnected after the previous 'repair'. I could see from the solder joints which pin it had detached from but nevertheless I'd like to check that the following looks correct. There are some resistors across some of the pins - details of resistor values below.

Pins 1 and 8 - heater
Pin 2 - connected to chassis and through resistor to pin 7
Pin 3 - connected through resistor to pin 4
Pin 4 - connected to red contact of double capacitor 'can' and to pin 3 and pin 6 (through resistors)
Pin 5 - connects to some components in circuitry
Pin 6 - to pin 4 - see above
Pin 7 - to pin 2 - see above

The measured resistor values are not the same as the colour code:

Between pins 2 and 7: code 100 ohms, measured 375 ohms
Between pins 3 and 4: code 1 Megohm, measured ~1.6 Megohm
Between pins 4 and 6; code 1 Megohm, measured ~1.6 Megohm

I would be grateful for comments on whether these differences in resistor values are important and also whether I should go the whole hog while I have it all opened up and change all the resistors.

Thanks again for all the help

[turretslug]

1 M going up to 1.6 M is absolutely typical of this type and age of resistor- the effect will be to make the deflection greater for a given signal strength (AVC voltage), which isn't really a problem- though you might want to change them and get it right long-term. The presence of a resistor between pin 7 and chassis I would characterise as "unusual", rather than "anomalous" or worse- the intention may have been to alter the effective signal strength vs. deflection angle characteristic and it's probably not critical.

Under the circumstance, I would leave the original 1M resistors in place, at least until I had the set going well in other respects, but tack something like a 150 ohm (uncritical) across the rather drifted original 100 ohm for the time being just to see that the eye was basically working.

Whilst magic eyes must be one of the most appealing spin-offs of the thermionic era (much more "soul" than a meter!), it's an unfortunate fact that the lovely intense green glow is rather short lived and you may well find that this one is tired to near- or actual invisibility- darkened room is often needed here! The UM4 is one of the best of the genre, with dual sensitivity shadow areas (hence the two 1M load resistors for the different gain driver triodes looking across the AVC- unfortunately, this translates to "sought after and expensive" as a replacement.

Don't change all the resistors- for several reasons! Again, it's all too easy to introduce a fault that wasn't there to start with. Valve circuitry like this is largely uncritical (with a few exceptions) as to precise resistor values and will continue on happily with resistors that are 50% or more off much of the time. Removing too many components wholesale is a recipe for broken tags, valve socket eyes, coil/IFT pins etc.- that gets really boring and demoralising and it's likely to be unnecessary to boot.

[paulsherwin]

I agree that you should not start changing resistors in the absence of a fault unless the values are massively out.

Actually, the magic eye should be the last thing to bother about as it is largely cosmetic. You just need to ensure that all of the valve heaters are OK, as any open heater will stop all the other heaters working.

[turretslug]

The vintage TV guys (and gals) have a useful rule of thumb summed up in the phrase "first light"- i.e. do the basic safety checks, change/check that handful of critical components (in the case of the majority of radios, that's change "that cap" and check/reform/change the smoothers), connect up via a lamp limiter and apply power with trepidation/glee and just see what happens. I'll confess that I'm a bit of a cautious sort of chap and basically go over things with an ohm-meter- but that needn't take more than 10-15 minutes for a typical radio.

That is, do the basics and if there's at least a background hiss from the speaker, maybe a bit of soft hum accompanying it, you're probably winning, better still if there's even occasional faint reception here and there. The point is, you know that the fundamental, what some might term the go/no go components, such as mains transformer and output transformer are in good order. Once that's established and you feel that you like the set/feel you could get the bug for vintage restoration, you can then go to town and do a thorough overhaul/cabinet refurbishment and so on when you know you're not sending good money after bad.

Perhaps there should be a "first hiss" rule-of-thumb by analogy?

[Mulcher]

I've taken your advice and left the resistors as they are (except I will add a second to get the 375 ohm down towards 100). I'm part of the way through replacing the capacitors and it sure is a tedious job - I now know the wisdom of leaving as much as possible of the radio as it is. Another lesson learned.

Thanks again for your advice and help.

[turretslug]

It's very likely that all of the waxies would have been leaky to a greater or lesser extent, anyway- but there is something of a pecking order as far as urgency of replacement is concerned. Most here would treat "that" cap as a change anyway before first powering- compared to pretty much all output valves and certainly all output transformers, a modern plastic film capacitor, even a quality type with ample voltage margin, is of trivial expense. Once the set is confirmed as a goer, the rest can be progressively worked through- more therapeutic than making a formidable initial marathon of something that's an unknown quantity.

At least next time....

[paulsherwin]

I change all the suspect ones in one go, but it's better for a beginner to change one at a time to check a mistake hasn't been made.

[Mulcher]

Nearly finished the capacitor replacements, and should be able to power it up (carefully) at some stage soon. Time for this work will be a bit scarce over the weekend and next week so it could be over a week before I get back to it.

Thanks to forum members for advice - much appreciated.

[Mulcher]

There's one question I forgot to ask about the EU35. Beside the double capacitor there are two other smaller metal 'cans'. These can be seen in the picture of the EU24 that Synchrodyne posted. There are four or five connections to each of them underneath. I would like to know what type of components these are, or if they are some combination of several components.

Thanks

[Mulcher]

I made a mistake in previous post - there isn't a pic of the EU24 from Synchrodyne - sorry for misleading you. The second pic in my post of Feb 6th shows the top view of the chassis with the two rectangular metal boxes visible (as well as the usual double capacitor 'can') - those are the bits I would like to have identified. Thanks again.

[paulsherwin]

What you describe sound like IF transformers.

[turretslug]

I reckon so too. Without delving too deeply into the theory, they serve to pass the signal between the UCH42 frequency changer and the UF42 IF (intermediate frequency) amp and thence between the UF42 and the UBC41 with its detector/AVC diodes whilst isolating the differing DC voltages between the stages and filtering the wanted station from adjacent unwanted ones.

They contain a couple of coils and a couple of small-value capacitors and are generally pretty reliable, unless the set has been in very damp conditions for some time (I realise that I'm setting myself up as a hostage to fortune here!). Indeed, they're best left as they are, as excess dismantling is likely to de-tune them. The internal capacitors are generally a trustworthy type, rather than horrible waxies. Sometimes the lead-out pins can be a tad brittle, so gently does it when replacing any attached components.