Bush AC31 renovation

[Oldelectronics]

Hi folks.I have recently acquired an AC31 but a couple of things I found when cleaning it made me think it would worth asking if anyone had done one before & what was normal. I think the set which allegedly was working has been repaired in the past the set uses rimlock valves and one of the sockets is clearly not rimlock and the locking rings on the others are missing!
My questions are about what I assume are R6 and which on later sets was uprated to 1 Watt and R15 6 watt these resistors appear to have been running hot is this normal or do I need to look for another fault or is it a case if it aint broke dont fix it?

Regards

Kev

[Wellington]

Kev - EBF80 is correct - have a look here.

[mark pirate]

The resistors should be fine, they will certainly run cooler once those leaky old caps are replaced.
The fact the rings are missing is a bonus, they serve no real purpose and can nip off the side pip on the valve. I usually remove them on my sets.

Mark

[Oldelectronics]

Thanks guys I appreciate that. I take itC3 and C15 (0.05) either side of R6) are actually connected to the tags they are opposite its not that obvious that they go down through the hole and back up the outside to the same tag and I was hoping to not have to take off the panel from the chassis to see.
Kev

[Nuvistor]

I would be surprised if they were not connected to the tag, the leads are not insulated so they would connect to the tag any way.
Shouldn't need to remove the panel, use a dentist mirror to look underneath if you wish to confirm the connections or a multimeter.

Frank

[Oldelectronics]

Ta Frank I do have a mirror somewhere!
regards

Kev

[gary_crutchley]

I think Bush used that technique of passing the wire through the hole on a number of sets.

[Oldelectronics]

Hi Gary they do it on the AC34 but you can see where it goes checking with the meter the tags either side dont register on the old AVO as Franks suggestion unless sharing chassis earth so it must be opposite tags.

Regards
Kev

[Oldelectronics]

I have found another anomaly with this set or rather haven't found it Thats C25 .001 1000v paper tubular Aerial isolating capacitor I cant see it on the schematic and on the set all the PTs taken off have been 500v max I was wondering if this was DAC31 only? But it does seem to indicate its for the AC31.It also does not seem to be on the Trader sheet?

[David G4EBT]

C25 - though on the maker's parts list is shown as being required for the AC31 & DAC31 - isn't needed for the AC31, and neither is C26, C27 or C28, which are absent from the AC31 circuit. In the parts list, the latter three are shown as DAC31 only, but C25 should also have been shown as DAC31 only.

C25 is the Aerial Isolator capacitor, and C26 is the 'True Earth' isolator capacitor. They're required as a safety feature due to the DAC31 being a live chassis model. Thus, if the mains is connected with live to the chassis, whilst the set will still work, without C25 the aerial would be live , and without C26, the mains would be connected to Earth. Hence, both C25 & C26 are essential safety components, which is why they're rated at 1,000V. Likewise, on the DAC31, C27 is the Pick-up tone corrector, C28 in the Pick-up Isolation capacitor, essential for safety reasons to isolate the pick-up from the mains if the chassis was wired to the line rather than neutral of the mains input. (As replacements, Class 'Y' caps would be a better bet for all of these nowadays).

None of this has any relevance whatsoever to the AC31 because the set uses an mains isolation transformer with no mains connection to the chassis, therefore both the external aerial & earth terminal are at ground potential. Hence, isolation capacitors aren't needed, which is why neither C25 nor C26, (nor C27 & 28) are shown on the AC31 circuit.

Hope that's helpful.

(For the avoidance of doubt, the above notations are for the Bush 10-page data-sheet - not the 2-page Trader Sheet).

[Oldelectronics]

Brilliant answer David I was beginning to doubt my observation skills thank you for explaining it.

Many Regards

Kev

[Oldelectronics]

Hi folks, the answer to this may seem a bit obvious but I thought I would check!the smoothing/reservoir capacitor when on low volts to slowly reform is getting vaguely warm just detectable with a finger(is this normal) if this gets worse when increasing power I may end up changing it (32/32) I have a replacement but the original only has two pins I take it can is earth and the replacement would use the plain tag as earth and just means a wire to a convenient earth point?

regards

Kev

[HamishBoxer]

The can very often is earthed.Just keep a finger on how warm and if in doubt change it.They go with a serious bang.Yes just find an earth point for that tag.

[Oldelectronics]

Good advice thank you G8JET I dont want any heart failure moments so any doubts and out it will come!No sign of life yet though just a bit of a crackle when useing the wave change switch and pulling the aerial cable in and out but only useing a 60watt bulb at the moment so I will have another look when I move up to 100w later this evening (I have to go out and wont leave it unattended!)
Kev

PS it did have a bit of a crackle session when turned off and the a few minutes later to do a photo of the underneath which I wanted to do before I forget!

[David G4EBT]

Yes, the original one used the can as the negative via the mounting clip screwed to the chassis, which is why the tag on the top of the old can was unused (as in the DAC90A for example). You could fit a solder tag on the nut and screw which holds the clip to the chassis and connect the negative tag of the new cap to that.

A point worth explaining is that we generally talk of ‘smoothing capacitors’, but more correctly, the first one - which does most of the work - is actually the ‘reservoir’ capacitor, and the second one is the ‘smoothing’ capacitor. In a full-wave rectifier circuit the output from the rectifier consists of a continuous series of positive-going half cycles, so the frequency of that waveform is 100Hz. On the rising part of each of those half-cycles the reservoir capacitor charges up, then on the falling part of each half cycle begins to discharge but will hold most of its charge by the time the next half cycle is rising, when it charges up again. The difference between the voltage when the capacitor is fully charged and when it is losing its charge is known as the ‘ripple voltage’.

The second (‘smoothing’) capacitor seeks to remove the residual ripple and doesn’t have to work quite so hard. Over time, when capacitors age, they lose their ability to hold the charge and that’s when hum can arise, but there are other causes of hum, such as an output valve which is starting to fail. (Oddly, though the AC31 uses a full wave rectifier, it’s in a half-wave configuration with both the anodes connected together, presumably to save a few shillings on the cost of the mains transformer secondary winding).

Often, the reservoir and smoothing capacitor will be of the same value – say 2 x16 uF or 2 x 32uF (as in your Bush AC31). The reservoir capacitor will often be identified by wording on the can with the term ‘Res’, but if not, the tags will usually be marked red, yellow and black, and the can may well state ‘Red Outer’, but if there is no wording, (as I don't think there is on your replacement one), red is the reservoir, and yellow is the smoothing capacitor (the one after the resistor between the two sections of the capacitor). The reason that it’s preferred to use the outer capacitor for the reservoir is that it works the hardest, and being the outer one, is more effectively cooled.

So, in your AC31, the red tag will be C24 on the Bush Data Sheet (connected to pin 7 of V4), and the yellow tag is C23, which is connected to the smoothed HT line - to the OPT, C21, R11 etc.

When the two caps are of different values, the quandary of which is the reservoir and which is the smoother doesn’t arise.

Hope that helps, rather than confuses.

(Nothing drastic will happen if the yellow and red tags are connected the other way around, and I don’t doubt that many restorers don’t bother to even think about it and assume that they're interchangeable).

[Oldelectronics]

Thats great information David thank you and makes it very clear do these capacitors get vaguely warm to the touch not hot by any means but just warm enough to tell with a finger?

Regards
Kev