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Old 13th Jan 2017, 1:08 pm   #41
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Default Re: Bush A.C.91

Manufacturer's manual available here if required: http://www.service-data.com/section.php/909/1/ac91
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Old 13th Jan 2017, 8:26 pm   #42
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Default Re: Bush A.C.91

I've now got all the valves safely removed and bubble wrapped. I'll get around to cleaning them and other valve things later on.

Meantime hope you will indulge me as I embark on some circuit tracing and that someone can offer some advice on the attached picture. Hopefully this shows the V5 (Mullard CV 31) valve base viewed from underneath the chassis - the pencil tip, I think, is pointing at Pin 8 which is the Cathode? If this is correct then the + ve end of the new electrolytic capacitor 47 μF, 450 V is connected to the Cathode while the - ve end is connected to the chassis (more on that connection later). Is this a replacement for C29 which should be 32 μF, 275 V??

I can also see that Pin 5, the Anode, has connections as do Pins 2 and 7 for the Heater. I do, however, note that there are connections to Pins 1 and 3 which according to my research for the CV 31 are not used?? This is what confuses us beginners!

Won't go any further until I'm sure I'm on the right track!
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Old 13th Jan 2017, 9:25 pm   #43
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Default Re: Bush A.C.91

"Meantime hope you will indulge me as I embark on some circuit tracing and that someone can offer some advice on the attached picture. Hopefully this shows the V5 (Mullard CV 31) valve base viewed from underneath the chassis - the pencil tip, I think, is pointing at Pin 8 which is the Cathode? If this is correct then the + ve end of the new electrolytic capacitor 47 μF, 450 V is connected to the Cathode while the - ve end is connected to the chassis (more on that connection later). Is this a replacement for C29 which should be 32 μF, 275 V??"

"I can also see that Pin 5, the Anode, has connections as do Pins 2 and 7 for the Heater. I do, however, note that there are connections to Pins 1 and 3 which according to my research for the CV 31 are not used?? This is what confuses us beginners!"

The 47uF, if connected to pin 8 of the rectifier will be a replacement for the original 32uF reservoir capacitor, the fact that it's rated voltage is higher is no problem, the fact that its value is higher would probably be no problem either, if you look at the CY31's data sheet it gives the maximum reservoir capacitance for an anode voltage of 250 volts fed from a feed resistance of 125 ohms, in your receiver the anodes feed resistance is given as 150 ohms, any series resistance provided by the transformer can be added to that 150 ohms, that coupled with the fact that the rectifiers anode voltage should be 225 volts when the mains voltage selector is set correctly would probably offset the effects of that extra capacitance.

CY31 data sheets:

http://www.r-type.org/pdfs/cy31.pdf

Regarding the extra connections to the CY31's valve socket, unused connections are often used as extra connection points for other parts of the circuit so that's not unusual, but be aware, then can sometimes be a rake in the long grass, typical example is the old WW2 R1155 aircraft receiver, a particular valve in that one did not have an externally painted metalized screen but one of the common substitutes did, unfortunately that metalized screen was connect to a pin on the valve base which corresponded to one of the valve holder tags that was being used as an anchor point for some of the HT related circuit, if the power was on then grabbing the valve with one hand and the chassis with the other resulted in a shock...the rake had sprung.

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Old 13th Jan 2017, 10:44 pm   #44
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Default Re: Bush A.C.91

Thanks Lawrence - hope you understand my caution when confronted with connections not shown on the Service Instructions.

The solder connections to Pins 1 and 3 appear undisturbed do I am assuming they are original - I find it difficult though to reconcile these with the circuit diagram but perhaps I should just accept they are correct and move on?

Apologies for my typo that you corrected without comment, thanks - V5 is indeed a Mullard CY31 Half Wave Rectifier.

Now for C27 which should be a16 μF, 275 V but has been replaced by a 15 μF, 400 V - let's see what that's connected to?
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Old 14th Jan 2017, 2:32 pm   #45
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Default Re: Bush A.C.91

Quote:
Originally Posted by David G4EBT View Post
With the meter set on the Ohms range and the set disconnected from the mains, with one test lead to the chassis (doesn't meter which one) and the other at the 250V tapping on the transformer, it should measure 210 Ohms. You'll need to have the valves removed to carry out that test, otherwise the series valve heaters would be in parallel with the transformer winding across points B & C. You'll see what I mean from the attached sketch. If by any chance it reads open circuit, the individual resistances of each tap are on the circuit data so you'd need to check to see where the break is.
So with all valves removed and a quick clean up job on the auto-transformer tags I'm getting 199 Ohms at the 250V tap - as this is within 10% of 210 Ohms I'm assuming that's acceptable?

Next job is measuring the output transformer.
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Old 14th Jan 2017, 4:00 pm   #46
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Default Re: Bush A.C.91

If quoting resistance readings, we need to know which 2 points you are measuring between, not just one. But that sounds OK for across the transformer whole winding, ie. 250v tap to 0v (ground) end.
Using unused pins on valve holders as junctions for components is common practice, the CY31 valve has no connection on these #1 &#3 pins. If in doubt, consult the valve data and it will become clear. Many octal valves are missing pin 6 altogether!
Try to follow the circuit diagram, not a physical layout which often will confuse you. The layout of components fitted in a chassis bears no relation to the position on the circuit, but the point to point connections are as the circuit diagram.
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Old 14th Jan 2017, 4:02 pm   #47
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Default Re: Bush A.C.91

Your transformer resistance measurement looks ok to me.

Unless I'm wrong there's an error in the manual I'm looking at, Page 6 shows the total winding resistance as 200 ohms, but under the schematic it shows the total winding resistance as 210 ohms.

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Old 14th Jan 2017, 5:06 pm   #48
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Default Re: Bush A.C.91

Quote:
Originally Posted by Boater Sam View Post
If quoting resistance readings, we need to know which 2 points you are measuring between, not just one. But that sounds OK for across the transformer whole winding, ie. 250v tap to 0v (ground) end.
Sorry Sam, I should have been more specific - it was indeed from the 250v tap to 0v (ground) end.
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Old 14th Jan 2017, 8:42 pm   #49
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Default Re: Bush A.C.91

Spent the afternoon circuit tracing and feel that I have now got to grips with my two “mystery” electrolytic capacitors and one of the “mystery” tubular capacitors – these had been replaced before the set came into my hands and were causing confusion. There is still the issue of one “unknown” tubular capacitor but more on that later.

Forgive the slow rate of progress, only my second restoration, but this has really helped me understand what might be going on: -

C29 - Electrolytic capacitor 32 μF 275 V (H.T. line smoothing) – replaced with 47 μF, 450 V.

The +ve end is connected to Pin 5 (Cathode) on V5 (Mullard CY31).

The +ve end is also connected to the Output Transformer.

The –ve end is connected to an earth screw on the chassis.

The –ve end is also connected to the –ve end of C27 (see below).

The –ve end is also connected to the enclosure of the on/off switch/volume control – this connection is not shown on the circuit diagram – see first picture below??


C27 - Electrolytic capacitor 16 μF, 275 V (H.T. line smoothing) – replaced with 15 μF, 400 V.

The +ve end is connected to Pin 4 (g3 – Screen Grid) on V4 (Mullard CL33).

The +ve end is also connected to R18 (10,000 Ω 2 W – H.T. Line smoothing).

The –ve end is connected to the earth screw as detailed above.


C30 - Tubular capacitor 0.1 μF 500 V (Mains R.F. by pass) – replaced with an RS capacitor with the markings .1 / 20 1500V.

This is connected to the on/off switch/volume control (S3) and to the right hand tag (when viewed from underneath) on the mains connector – is this the LIVE connection?? See second picture below.


Unknown tubular capacitor – this is a 0.047 μF 1500 V which connects the earth tag on the mains connector to the chassis. This is not shown on the circuit diagram – see third picture below??

Once I get past this stage hopefully most everything else is a standard restoration (if there is such a thing?) in terms of waxy caps, out of tolerance resistors, chassis cleaning, rewiring etc. etc. Might need some help with the wiring associated with the two large aluminium coil enclosures?? Looking forward to tackling the metallised valve coatings further down the line and the cabinet is also in need of some attention.

So any help with the above items would be most welcome – thanks in advance.
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Old 14th Jan 2017, 8:56 pm   #50
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Default Re: Bush A.C.91

Pin 5 of the CY31 is the rectifiers anode not the cathode, I hope the reservoir capacitor (C29 47uF) isn't connected to that, if it was then switch on could be spectacular, hopefully it's just a typo error.

I would remove that tubular capacitor from the IEC sockets mains earth tag, nothing should be connected to that tag.

Not sure if that other tubular capacitor across the mains is an X type which it should be, someone else might be able recognize whether it is or isn't.

Your doing ok so far, keep at it.

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Old 14th Jan 2017, 9:59 pm   #51
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Default Re: Bush A.C.91

Ah, well spotted Lawrence - it was a typo and my picture in Post 42 above clearly shows the connection to Pin 8 of the CY31, the Cathode.

Any idea why someone would install a tubular capacitor from the IEC sockets mains earth tag?

There is also still the mystery of the –ve ends of C27 and C29 which are connected to the enclosure (is that the right word) of the on/off switch/volume control – this connection is not shown on the circuit diagram??
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Old 14th Jan 2017, 10:29 pm   #52
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Default Re: Bush A.C.91

The metal volume control enclosure/body on the pot was normally electrically connected to the chassis due to the way most pots are mechanically constructed and secured to the chassis, the metal body was often used as a connection point for other components or wires that needed a return to chassis connection, those connections are not normally depicted in schematics. Its possible that there might not have been anything connected to the pots metal body originally and that it has just been used as a convenient point to connect the -ve ends of C27 and C29 to, having said that, looking at the photo that short bare link wire connected to the pots body does seem to have an original look to it.

The tubular capacitor from the chassis to the mains earth tag would have been fitted so that from an RF signal point of view the chassis would be earthed if a 3 core mains lead was fitted to the IEC plug, an automatic isolated earth connection to chassis if you like, it would be to save running a separate "real earth" wire to the receivers "real earth" connection socket on the receiver, which is also isolated from the chassis by C3, if you decide to retain C3 then it should be replaced by a Y Class type only.

Lawrence.

Last edited by ms660; 14th Jan 2017 at 10:37 pm. Reason: addition
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Old 14th Jan 2017, 11:43 pm   #53
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Default Re: Bush A.C.91

47uF is too much for a CY31. Mullard quote max C of 32uF. You might get away with it, but it would be better and not particularly costly to replace it.
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Old 15th Jan 2017, 12:51 am   #54
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Default Re: Bush A.C.91

Thanks Lawrence -I understand your point about the connection to the pot and will probably just leave that in place if, as you say, it is original?

Your explanation about the tubular capacitor from the chassis to the mains earth tag has gone a bit over my head - in the Service Sheet C3 is listed as a tubular capacitor that should be 0.005 μF, 500 V but has now been replaced with a 0.01 μF, 1500 V??

Earlier advice also seemed to suggest that there should be nothing connected to the earth terminal on the IEC socket?

Thanks Chris - as you say it wouldn't be that difficult or expensive to replace the capacitor that's been installed with one having the proper values.
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Old 15th Jan 2017, 11:35 am   #55
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Default Re: Bush A.C.91

Donald, regarding C3 and the tubular capacitor connected to the earth connection on the IEC socket, there's nothing too complicated about it, on your receiver there's an earth socket on the back of the chassis, in the schematic it shows that it's for a connection to "real earth" by real earth they mean a connection to an earth spike or rod banged into the ground, the function of that is to enhance the receivers reception but because the receiver is a live chassis design and the fact that it's earth connection socket is exposed to you or anyone else then a capacitor is fitted between the chassis and the receivers earth socket, that capacitor is C3.

Here's the reason, and it's a very important safety reason as to why a capacitor such as C3 is fitted to a live chassis design. As you should know by now the chassis of a receiver which is of the live chassis design can become live, that's to say if, for whatever reason, the mains live was connected to the chassis and power was applied and the receiver switched on then there will be full mains voltage between the chassis and earth, a potentially lethal voltage. The value of C3 is selected so that at mains frequency any current flowing via it will be low enough to be regarded as a safe level while at the same time providing an effective path for an earth connection for radio frequency currents.

A rake in the long grass waiting to be stepped on with your receiver is that even if the mains lead is wired up correctly the chassis can still become live if the receiver is still connected to the mains, that's because its on/off switch only switches the neutral and therefore with the receivers on/off switch switched to off the chassis will become live via the path provided by its mains transformer, you need to be aware of that.

The probable reason that the tubular capacitor (0.01uF) was fitted is as I explained in an earlier post, with a 3 core mains lead connected, the receiver would be earthed from a radio frequency point of view via the houses protective earth, which at some point will be connected to the "real earth" somewhere along your electricity supply line, the upshot of that is there would be no need to have a separate earth spike and its connecting wire which would normally be the case.

As said earlier I would remove that 0.01uF completely and if C3 is the original one then I would replace it with a Y class type of the same or almost the same value such as a 0.0047uF, or if you don't intend to use the earth socket you can omit C3 altogether.

Hope this helps.

This post might cross.

Lawrence.
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Old 15th Jan 2017, 11:51 am   #56
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post

C29 - Electrolytic capacitor 32 μF 275 V (H.T. line smoothing) – replaced with 47 μF, 450 V.

C30 - Tubular capacitor 0.1 μF 500 V (Mains R.F. by pass) – replaced with an RS capacitor with the markings .1 / 20 1500V.

This is connected to the on/off switch/volume control (S3) and to the right hand tag (when viewed from underneath) on the mains connector – is this the LIVE connection?? See second picture below.

Unknown tubular capacitor – this is a 0.047 μF 1500 V which connects the earth tag on the mains connector to the chassis. This is not shown on the circuit diagram – see third picture below??

So any help with the above items would be most welcome – thanks in advance.
As others have explained, C29 ought not to exceed 32uF, as per the valve data.

As C30 is across the mains, it should b an 'X' Class capacitor. Worth checking if it says 'X' on it. They're commonplace, widely available and are AC mains rated. Typically used on such appliances as electric drills, vacuum cleaners, hair driers and so forth.

As to the unknown cap connected from the earth pin of the IEC connector to chassis, in trying to second-guess what was in the mind of the previous owner, I can only assume that he thought it was a handy way of providing a substitute for a 'true earth' to improve reception, rather than plugging in an earth wire to the earth socket adjacent to the external aerial socket. He'd already ensured that by fitting a non reversible connector only the neutral side of the mains input was connected to the chassis, so I doubt he considered it in any way a safety feature. I'd be inclined to remove it.

Hope that helps a bit.

As regards connections to unused valve socket pins, that was a favourite of Bush for anchoring components not associated with that valve and can be confusing as the circuit will show no connections to that pin of the valve (which in fact there isn't as the valve has no internal connections to that pin). The output valve of DAC90As has one disused pin on the valve socket for that purpose - can't remember which one off-hand, but some restorers claim that it's best to detach the solder tag from the valve socket.

Good luck with the restoration, which you're now well into!
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Old 16th Jan 2017, 5:33 pm   #57
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Default Re: Bush A.C.91

Where is the best source of information on the modern equivalents for capacitors in vintage radios?

For example, I need to replace the 32μF 275V H.T. Line Smoothing electrolytic capacitor but can't find one with the equivalent voltage. I know the rule is to go high but how high on such occasions.

Similarly with the tubular capacitors I know that the existing 0.05μF 350V will be replaced with a 0.047μF, but what voltage is appropriate.

Finally, what should the voltages be for these capacitors: -

0.1μF, 500V Mains R.F. By Pass capacitor - X-Type

0.0005μF, 500V True Earth Isolating capacitor - Y-Type

More generally, apart from experience, how do you determine when a capacitor should be X-class or Y-class??
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Old 16th Jan 2017, 5:52 pm   #58
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Default Re: Bush A.C.91

The following link was very useful to me when starting out and may answer some of your questions: http://www.justradios.com/captips.html

For voltage rating, going higher does not hurt but will generally increase the physical size of the capacitor. This may be a concern if you are 're-stuffing' old capacitor cans with modern replacements, but generally modern caps will usually be smaller than the originals, even if the voltage rating is higher. This is due to improvements in materials over the years.

For electrolytics in an HT smoothing role, subject to high voltages, I tend to use 350-450V replacements to give some margin (compared with an HT of maybe 270V). These are still usually small enough to stuff inside the old cans if required.

For general use wax capacitors (apart from mains r.f. bypass) most people use 630V polypropylene types. This is largely because 630V caps are available as axial types (i.e. with leads sticking out from either end), thus matching the shape of the originals. They also tend to be roughly the same size or smaller. With the 630V rating you can’t generally go wrong for replacing waxies on radios like the AC91. Personally, I use these: http://www.ebay.co.uk/itm/10x-Capaci...-/190915805343


For r.f. bypass and other capacitors connected across the mains, you need to use X or Y rated caps. Generally these are rated for 250V AC. The 500V rating of the originals is a DC rating which I don’t think is terribly important here. If it’s going across the mains, the AC rating is what matters and X/Y types will be 250V AC for work on the UK mains. The differences between X and Y are detailed in the link at the start of my post.

I hope this helps. The capacitor minefield can be a confusing place!
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Old 16th Jan 2017, 5:56 pm   #59
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Default Re: Bush A.C.91

X class capacitors are capacitors that are connected across the mains between live and neutral normally after the on/off switch, X = "a cross" there function is for interference/transient suppression.

Y class capacitors are capacitors that might be connected between mains live and chassis and between mains neutral and chassis, there job is the same as above.

Y class capacitors connected between the chassis and the outside world such as a receiver's pickup sockets, earth socket etc are there to provide isolation, mostly in live chassis design receivers.

32uF at 275 volts would normally be replaced by a modern 33uF at 400 or 450 volts.

0.1uF X types should be readily available.

0.0005 sounds a bit low, 0.005 maybe, if so the modern equivalent Y class type would be 0.0047uF.

Hope this helps.

EDIT; Whoops...Post crossed.

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Old 16th Jan 2017, 6:47 pm   #60
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Default Re: Bush A.C.91

Regarding electrolytic smoothing/reservoir capacitors, some restorers simply disconnect the old one, leave it in place above the chassis and fit replacements beneath the chassis. Others, myself included, prefer to remove the innards of the old one and 're-stuff' the old can with new capacitors if there is space in the can. In recent years, high quality 'Rubycon' small capacitors with a high ripple rating have become available, in part, I think for use in flat screen TV repairs. For the last two or three years I've used them quite a lot and they've proved excellent.

I don't want to see this thread taken off at a tangent as to the arguments for/against re-stuffing old smoothing/reservoir capacitors, but there are links at post No 8 in the forum thread below as to where to obtain them. For anyone who prefers to simply fit them beneath the chassis, as they're so compact it's an asset if space is limited, as it often is. Not expensive either:

https://www.vintage-radio.net/forum/...d.php?t=121958

(They're available in a wide range of values and voltage ratings - not just the ones in that thread).

Hope that's of interest.
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