Bush SAC35.

[ekjdm14]

As well as the Sobell mentioned in my previous thread, I have also today bought a Bush SAC35. It was purchased because I liked the look of it & it seemed generally "OK" condition, but otherwise I knew nothing about it.

Having got it home, checked out behind the back cover & done a little looking around though I have to wonder if it's maybe quite a rare set. RMorg doesn't have a photo (I'll upload some photos here shortly) and not a huge amount of info on it.

Certainly on further inspection it's in better condition than I thought, I initially thought the dial material had gone cloudy over time but on looking inside it appears to actually be built that way & has a novel way of displaying the frequency tuned. It has a pair of lightbulbs covered by 2 rotary drums attached to the dial cord, each with a slit in it to let the light shine on the back of the frosted dial material in the correct position. This feature has endeared the set even more to me than it's seemingly clean and original condition.

It also has an electromagnetic speaker, a Rola Model 'F', which is something I've not owned/worked with before. Apart from the obvious things like checking transformers aren't open, check/replace caps as needed & replacing degraded rubber wiring is there any advice/tips I should be aware of as a relative novice, for the restoration of this set?

Aside from the usual HT safety related stuff, of which I'm aware, really I just would appreciate some sanity checks/gotchas with equipment of this age to ensure against (at least lessen the chances of) ruining a set which I'd very much like to have working, not to mention there may not be too many of them...

Cheers

[Nuvistor]

Manual.
https://www.service-data.com/product...28/1012/m10628

[ekjdm14]

Thanks & Purchased! Never even thought to look there...

One thing I'd already gleaned is that I have an Ex-RAF rectifier (pre CV VU39) and the output has been replaced with a PM1LF which appears pin-compatible but not sure if any other mods would have been required. I would suggest (well, guess at the moment) that this was subbed for a dead AC044 on grounds of availability & hopefully the cause of it's failure repaired also before further use.

Alternatively, I again guess, the set was largely original and had it's output valve pilfered and then thrown into auction with a replacement that would fit. Anyhow, enough guessing. Now I have the paperwork on it I'll get checking through it and see where we are.

Very, very tempted to save powering this one up until my variac arrives. Dim bulb just feels a bit "rude" for a nice set like this.

[Nuvistor]

Couple of more unusual features compared with later sets.
Band pass tuned circuit before the mixer, no RF stage so don’t let the 3 gang fool you.
Variable IF selectivity and 123Kcs IF.

I know little about pre late 1940 sets so it will be interesting following the thread.

[Cobaltblue]

The PM1 isn't a sub in any way for An AC044 otherwise known as a PX4

About the only thing they have on common is they are both directly heated triodes.

The PM1LF is a small signal valve for a battery set designed to drive headphones or horn speaker the AC044 is a mains powered power output valve.

Cheers

Mike T

[Herald1360]

Oh dear.... it should have an AC044 aka PX4 as its output valve- this has clearly been removed and the set sold on.....

A PM1LF is a battery 2V filament triode and is not at all compatible- it just happens to have a B4 base.

Replacing the original will either cost LOTS or you may be able to acquire a part functioning (half the filament gone) PX4 or PX25 one for a reasonable sum which will work, albeit at reduced power. Otherwise you'll need to modify the set to take some other 4V heater output valve, probably needing a change of valveholder too- something like an AC/PEN maybe- if triode connected you could leave the original holder and make up a B7 to B4 adaptor with the additional links etc to do the mods. There's been quite a bit of discussion at time about what best to do in these circumstances- quite possibly simply fitting (reversibly) a small modern audio amplifier just to get the set working would be the cheapest option.

[ekjdm14]

Looking at some data on r-type (what a useful site) made me suspect as much, I wonder what has gone on there then & if there's bodges to be undone. :/ will have to pull the chassis tomorrow and compare what I see to the schematic.

Is it likely (even possible?) that the radio was modified to use the current valve, or not possible and more likely a case of "jam one in that fits the socket" to make it look complete? Hope it's the former, otherwise will be looking for an expensive valve...

[Cobaltblue]

Sorry Dan it's likely to be the second option jam anything in the socket

Cheers

Mike T

[Silicon]

Is the valve holder for the output valve a B4 or a B5 (complete with 5th contact)?

A 5th contact would give you more options.
The PM1LF has a 2V filament.

If you do not intend to buy an expensive triode, you could fit a B7 valve holder and fit a 4V pentode.

If you test the radio without a suitable output valve the HT will probably rise too high and damage the other valves.

A pair of 12k 10W resistors in parallel could simulate the current taken by the output valve.

[ekjdm14]

Thanks for the advice, and even the bad news! I have an eye on an S30C currently that's not yet stratospheric in price but will check the socket (pretty sure it's B4 although will check).

Will get the chassis pulled tomorrow and check my transformers, filaments of correct valves etc & if things look reasonable I may consider bidding on the valve. Would like to keep it as original as possible at this point, pending what I find under the chassis but good to know there's options to have it functional without spending hundreds on a set that admittedly cost me £32.

Edit- Valve socket is in fact B5.

[ekjdm14]

OK I've got the chassis out & had a closer look, so far things of note are-: The set looks largely original except for C24/25 being replaced with a pair of Hunt's electrolytic cans on a plywood base (mounted quite nicely in all honesty & if they reform they will stay as an example of a period repair). Overall condition is decent and doesn't seem to have seen a lot of damp thankfully.

Also C19 and C20 (I think) have been replaced for some far more modern looking, maybe late '80s items. I also noted that the covers over 2 of the coils under the chassis were loose.

The rest of the caps look to be original, a combination of Dubilier "Domino" style and a couple of multiple oil-filled blocks that are spewing everywhere and will have to be replaced. I won't consider restuffing anything as yet, but will keep old parts with the set in case this is something for the future.

Main transformer tests OK initially, lots of gummy/crusty/leaky wires everywhere but impedances all roughly where they ought to be. Speaker field tests good but output transformer I think is open (surprise surprise).

I don't see evidence of other mods made to the chassis so Mike T is correct & looks like the poor set has been "harvested" of it's valuable output valve. Mind you, given the possible open output Xfmr, I wonder how good the thing was anyway!

Other observations, the 3 original valves (FC4/VP4B/TDD4) have pencilled percentages & dates on the painted sides of their envelopes, 2 say 100% and the FC4 says 90%, I can't make out the date but it could be either 16/10/38 or 16/10/88.

One other possible date pencilled on the underside of the chassis, looks like xx/xx/13 so I wonder if that was when it was last diagnosed & thence robbed of it's PX4. And I also have to wonder now, was it harvested by it's owner or by an unscrupulous "repair tech" when the owner didn't want the output rewound?

Final thoughts on the set are, I'm impressed with the construction method and how neatly it's all been done. There's some innovative stuff in there what with the scale/cursor lamps and the way most of the passive components are mounted on one panel/board. Very ahead of their time with that one. I'm becoming more and more fond of the set as I get into it & really hope it comes through and performs as well as it's construction would suggest.

Anyhow, lots more research/testing to do now so will end this post here. Got fingers tightly crossed for a reasonable priced valve today, and If the output primary is indeed o/c it looks like I might be after a winding setup as I have a Fidelity item to do as well. Cheers,

[ekjdm14]

Originally Posted by Herald1360

Replacing the original will either cost LOTS or you may be able to acquire a part functioning (half the filament gone) PX4 or PX25 one for a reasonable sum which will work, albeit at reduced power. Otherwise you'll need to modify the set to take some other 4V heater output valve, probably needing a change of valveholder too- something like an AC/PEN maybe- if triode connected you could leave the original holder and make up a B7 to B4 adaptor with the additional links etc to do the mods. There's been quite a bit of discussion at time about what best to do in these circumstances- quite possibly simply fitting (reversibly) a small modern audio amplifier just to get the set working would be the cheapest option.

Apologies, I completely missed this post yesterday. Must have posted while I was typing.

One of the PX4's I missed (strangely the one that went for over £100!) had only part filament activity.

Anyway, it is possible I may have been kindly fixed up with a possibly-good PX4/AC044 for a sensible price but it is nice to know there are easily reversible options to get the set running if the valves don't perform. (for the moment thougn I'd absolutely refuse to put a modern amp in there, I don't need to listen to it as such, I have my Ekco A320 as a bedside set currently & a few others waiting in the wings for when I feel like capacitor-shooting. Also waiting on a Variac delivery from Spain of all places).

[Herald1360]

I did have a bit of a play to see if anything could be easily cooked up to sub a mosfet and a few bits on a B4 base to plug straight in but didn't get very far though I wonder if the kludge to get my ancient version of Simetrix working in Windows 10 64bit has its limitations. I couldn't get a transformer to make any sense!

[Mike. Watterson]

None of my old CAD/CAE programs work on 64 bit Win 7 or 64bit Win10. They do mostly work under WINE on 64bit Linux Mint with Mate desktop.

Certainly it's possible to use a Mosfet, though it needs a + drive for gate. I've tried 3 x 1p24b in parallel and it gives similar power to the earlier PX4, but with 2.4V 330mA filament (or 1.2V 660mA). So three resistors in the f- leads would allow 4V drive. It's a Russian tube, a rod Pentode, but the data sheet gives triodised data. 800W peak power for short life proximity radar (shells etc). Under £2 each. I've used a pair of the lower power 1j29b cousin to replace a 2V dual o/p pentode.

[Herald1360]

Got the simulation working.

No guarantees..... but on paper at any rate, it looks like the circuit attached would work as a PX4 substitute in this radio's circuit. Trickiest physical problem would be to heatsink the Mosfet well enough to cope with around 13W dissipation.

A couple of 10K/W TO220 twisted vane type heatsinks might do.
https://uk.farnell.com/aavid-thermal...t%2Fheat-sinks

Would be an interesting experiment, at any rate!

[Silicon]

I would add a zener across the gate - source junction to protect it.

The classic single transistor current limiter circuit could be added to this to protect the output transformer.

I think I would add a 10k gate stopper resistor.

[Alistair D]

Have a look at this thread for a solid state version that actually works. It was designed for a pentode but could be modified to suit a triode.

https://golbornevintageradio.co.uk/f...d.php?tid=6380

Al

[Station X]

Thread reopened at OP's request.

[ekjdm14]

Thank You.

During this enforced "play time" I've been slowly working through some of my round tuit projects & this set came to mind last night as one that deserves a shot at life again.

Firstly, thank you to those who suggested FET circuits & took the time to work things out back when I originally got the set. I'll be looking into this as a cheap way of getting the set functional once the rest of the work required is done, but first there's rotten rubber insulation and that OPT to look into. (the OPT is what made me shelve the set last year, but what better time than now to sit counting turns and bringing that back into order!)

Possibly more posts to come once I've dug the set out & re-inspected it with one years extra experience behind me.

[ekjdm14]

OK the set has been recovered from the round tuit pile, and I've gone over it one more time confirming my initial diagnosis that the output valve is missing (that was a real shame, wish it'd sprouted one in the time between first inspection and now!) , and the OPT primary was indeed open.

This evening I had a rummage through the shed & knocked up a rough jig for unwinding the transformer & counting the turns whilst locating the problem. So far, have just finished for bedtime but have unwound approximately 2000 turns (I estimate there are around 2500 total) and found the cause of death to be green spot.

Currently I've located about 5 affected portions of the winding, not really that bad of a failure rate considering. (assuming 2 more breaks left to find, that's about a 0.28% rate of failure for each turn over nearly 90 years).

Next up is determining the wire gauge for the replacement & building something less thrown together to enable rewinding in (hopefully) a neat enough package to fit back into the core again afterwards! Will post couple of photos of the makeshift "unwinding jig" tomorrow, it's pretty hilariously random in it's component parts!