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Old 23rd Jun 2017, 1:03 pm   #161
DonaldStott
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Default Re: Bush A.C.91

Two questions following further research: -

1. Whether or not I really need to bother with C30 (Mains R.F. by pass) - do I require an X-type capacitor fitted across the mains tags between Live and Neutral?

2. The location of the switch S3 means that the mains is switched on the neutral - unless I am wrong this means that the wiring highlighted in RED in the attached image will stay live when the switch is in the off position?

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Old 23rd Jun 2017, 1:09 pm   #162
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Default Re: Bush A.C.91

The set will wok perfectly without C30, but as you have it, it's probably best left in place.

As an alternative you could wire C30 across the incoming supply, but this means it will always have a voltage across it.

The original mains connector was unpolarised so either L or N could be switched. You can change over the wiring so the L is switched if you wish.
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Old 23rd Jun 2017, 1:25 pm   #163
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Default Re: Bush A.C.91

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Originally Posted by DonaldStott View Post
The location of the switch S3 means that the mains is switched on the neutral - unless I am wrong this means that the wiring highlighted in RED in the attached image will stay live when the switch is in the off position?

Attachment 144941
Not only what's highlighted in red, the chassis will be 240 volts AC with reference to earth.

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Old 23rd Jun 2017, 2:27 pm   #164
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Default Re: Bush A.C.91

If I get stuck like this, I print off the circuit, then as I check each connection one at a time, I go over the circuit with a highlighter pen so I know where I've checked. In a set which is working correctly, the lamp limiter should glow dimly, but with a 60 Watt bulb, enough to see the glow. As was pointed out by Tony in post #150, when the set is switched off, in no circumstances should the lamp glow - however dimly.

At some point it would be useful to measure the HT current being drawn by the set. You can conveniently do this by putting your multi-meter on say the 200V DV range, applying the test prods to each end of R18 - the 10K Ohm HT load resistor - and reading off the voltage dropped across R18. Then applying Ohms law, divide that Voltage by 10,000 (Ohms) and it will give you the current in Amps (or a fraction of an Amp in this instance).

Hopefully, with careful scrutiny you'll find a misplaced wire here or there and after putting the right, the lamp limiter will stop drawing excess current. At that point, even if the set still isn't working, you'll be able to check the voltages of the screens and anodes of the valves and compare them with the maker's data to give you some pointers, but of course, with the lamp limiter in circuit, the voltages will read maybe 10 - 20% low, but that wouldn't stop the set from working.

As far as you can, it's best to do as much testing with the set switched off and unplugged. You can measure the continuity of wire connections, coils, and resistors, and as you go along, can write the results down, and can highlight the circuit diagram to show what you've checked.

In a dead set, having checked voltages etc, I always use a signal injector/tracer to try to discover which stage(s) is faulty. For example, if a signal is injected at the volume control and you get a loud tone from the speaker, that shows that the audio stage is working, if only after a fashion. If you don't have a signal injector/signal generator, an alternative approach is to scratch the centre tag of the volume control with a screwdriver and you should here loud scratching noises from the speaker.

To check the RF/IF/Detector stages, if you apply a signal tracer to the centre tag of the volume control and can tune stations in, that means those stages are working even if the . If you get signals on one band but not on another, or if you get signals on AM but not on FM (in a set which has both modes), that helps to narrow down where to look for the fault.

As to the mains switch being in the neutral lead rather than line, it was quite commonplace on AC/DC sets. I recall someone suggesting that the technical reason for this was to prevent arcing of the switch contacts when the set was used on DC, but whether that was so, I've no idea and it's irrelevant to us with AC only, so I invariably change it over and put the switch in the line rather than neutral as I instinctly dislike the idea of part of the set being live when switched off.

Good luck in your endeavours Alistair.
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Old 23rd Jun 2017, 2:40 pm   #165
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Default Re: Bush A.C.91

I too deplore the neutral mains switch and usually rewire them to be in the live and also rewire the voltage selector to be in the neutral if it is unshielded and easy to touch inadvertently.
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Old 23rd Jun 2017, 2:46 pm   #166
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Default Re: Bush A.C.91

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Originally Posted by David G4EBT View Post
At some point it would be useful to measure the HT current being drawn by the set. You can conveniently do this by putting your multi-meter on say the 200V DV range, applying the test prods to each end of R18 - the 10K Ohm HT load resistor - and reading off the voltage dropped across R18. Then applying Ohms law, divide that Voltage by 10,000 (Ohms) and it will give you the current in Amps (or a fraction of an Amp in this instance).
Don't forget V4's anode current.....

Lawrence.
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Old 23rd Jun 2017, 2:51 pm   #167
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Default Re: Bush A.C.91

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I too deplore the neutral mains switch and usually rewire them to be in the live and also rewire the voltage selector to be in the neutral if it is unshielded and easy to touch inadvertently.
It's an auto transformer....

Lawrence.
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Old 23rd Jun 2017, 4:07 pm   #168
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Default Re: Bush A.C.91

I know that........& its Friday.....
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Old 23rd Jun 2017, 4:14 pm   #169
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Default Re: Bush A.C.91

"It's an auto transformer...."

"I know that........& its Friday....."

Can someone translate for us neophytes - thanks.
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Old 23rd Jun 2017, 6:01 pm   #170
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Default Re: Bush A.C.91

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"It's an auto transformer...."

"I know that........& its Friday....."

Can someone translate for us neophytes - thanks.
Yes, it was regarding connecting the Neutral to the voltage tap, ok on a transformer that has primary/secondary isolation but not really practical with the auto transformer that's fitted in your receiver.

Lawrence.
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Old 24th Jun 2017, 4:32 pm   #171
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Default Re: Bush A.C.91

Quote:
Originally Posted by David G4EBT View Post
At some point it would be useful to measure the HT current being drawn by the set. You can conveniently do this by putting your multi-meter on say the 200V DV range, applying the test prods to each end of R18 - the 10K Ohm HT load resistor - and reading off the voltage dropped across R18. Then applying Ohms law, divide that Voltage by 10,000 (Ohms) and it will give you the current in Amps (or a fraction of an Amp in this instance).
Firstly, for resistance R18 is measuring 9.91kΩ so well within tolerance.

Secondly, for the HT current R18 is measuring 135.5 with the multimeter on the 200V DC range - using Ohms law = 0.01367 Amps I think?
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Old 24th Jun 2017, 5:40 pm   #172
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Default Re: Bush A.C.91

Is that using the lamp limiter?

The HT current you have measured is only part of the picture as it's not the total HT current that flows through all the amplifying valves, the largest contributor is normally the sound output valves anode current because it's operating in class A with a low value resistive part of the load (The output transformers primary resistance)

You need to measure that current and add it to the 13mA that you've already measured/computed.

Lawrence.

Last edited by ms660; 24th Jun 2017 at 5:46 pm.
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Old 24th Jun 2017, 6:20 pm   #173
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Default Re: Bush A.C.91

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You need to measure that current and add it to the 13mA that you've already measured/computed.
No lamp limiter...!

I'm assuming that's the V4 anode current you mentioned previously??
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Old 24th Jun 2017, 6:35 pm   #174
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Default Re: Bush A.C.91

Yes, V4's anode current as mentioned previously.

Normal routine is to measure the cathode resistor and if that's ok measure the voltage across it, from that you can work out what the combined anode and screen current is, if it's not excessive then all's normally well.

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Old 24th Jun 2017, 6:38 pm   #175
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Default Re: Bush A.C.91

Yes, near enough 14mA, which is not excessive, so should not cause the lamp limiter to glow brightly, so if it is, that's puzzling. (The heater chain current is 200mA, total voltage dropped in the chain is 72.6V AC plus 3.5V for the dial lamp in series with the heaters - 76V in all). A defective C29 could cause excess current to be drawn. 135V dropped across R18 accords with the maker's data, which states 245V on the anode of V4, and 115 on the screen, so 135V drop across R18 in spot on.

I'd be inclined to check the voltages on the anodes, screens and cathodes of the other valves and compare them with the data on page 6 to see if anything stands out. Also, the cathode currents are listed, but if the voltages on the cathodes are fairly close to the data, then so should be the current. (Dividing the cathode resistor into the voltage gives the current. So for example, the cathode voltage of V4 is listed as 4V, across R17, 150R, so 4/150 = .027A, (27mA) - the data shows this as 30mA, which is near enough.

If any of the measured voltages differ greatly from the data, that may help to pinpoint a problem. So much work has been done on the set, both before and after it came into your hands, that I'd be inclined to check any disturbed wiring/components and compare with the circuit - as I said earlier, by using a highlighter pen to show what you've checked.

If you hear no sound from the set when scratching the slider tag of the volume control as I mentioned earlier, then there's a problem in the audio stage. If you had a signal tracer - just a small amplifier - eg an amplified computer speaker or whatever would do as a makeshift tracer, and could apply that to the slider of the volume control and could tune in stations, that would mean that the frequency changer, detector and IF stages are all working, so that would be a big step forward.

Good luck in your endeavours Alistair - you've persevered with the set over so many months that you deserve a lucky break! Don't get despondent - think of it as a learning exercise - when you've sorted this set, anything else will be a doddle!
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Old 24th Jun 2017, 7:46 pm   #176
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Default Re: Bush A.C.91

Thanks Lawrence and David for your ongoing support and encouragement - there have been a number of occasions over the last few days when I've almost thrown in the towel!

I threw caution to the wind and unplugged the lamp limiter as I now think that may be faulty!!!!

Right at the start I should simply have removed all the previous "alterations" and trusted my own judgement in fitting replacements. I've only just ripped out all the cabling and components between the mains socket, auto-transformer and on/off switch - all that has now been replaced.

Similarly I (stupidly) retained C27 and C29 - both large electrolytic capacitors - but in hindsight those should have been ripped out as well. I'll do that later and do some measuring. What would be suitable replacements if I need them:-

C27 16 μF 275V

C29 32 μF 275V

Finally for now, what's the safest way to check the voltages on the anodes, screens and cathodes of valves - put simply, where do I put my probes?

Or maybe I should start looking for a "Painting by Numbers" Forum...?

Thanks guys.
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Old 24th Jun 2017, 8:09 pm   #177
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Default Re: Bush A.C.91

All the valve voltages are best taken with respect to chassis, so -ve probe on the chassis and measure the different voltages with the +ve probe.

Note that the voltages shown in the manual were measured with an AVO Model 7 meter, that meter has a low OPV (Ohms Per Volt) compared to more modern meters, can't remember offhand what the OPV was but probably somewhere around the 500 or 1000 ohms mark, the later AVO's had an OPV of 20,000 ohms, a typical digital meter would have a fixed input impedance of 10,000,000 ohms.

Typically 33uF and 15uF for the electrolytics at 400 or 450 volt working, plenty to choose from, radial, axial etc.

Lawrence.

Last edited by ms660; 24th Jun 2017 at 8:19 pm.
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Old 24th Jun 2017, 8:40 pm   #178
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Default Re: Bush A.C.91

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All the valve voltages are best taken with respect to chassis, so -ve probe on the chassis and measure the different voltages with the +ve probe.
Thanks Lawrence - that makes things easier as I can firmly crocodile clip the -ve probe to the chassis and use the +ve probe to measure - all this with the set live and chassis exposed!

I take your point about the AVO but I can only measure with what I have i.e. a modern digital multimeter.
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Old 24th Jun 2017, 8:51 pm   #179
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Default Re: Bush A.C.91

One hand in the pocket...

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Old 24th Jun 2017, 10:02 pm   #180
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Default Re: Bush A.C.91

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One hand in the pocket...
Thanks for the safety reminder.

Some measurements from V4 (CL33)

Anode (Pin 3) 255V (and some scratchy noise from speaker)

G2 Screen Grid (Pin 4) 138V

G1 Control Grid (Pin 5) 0V (hum and scratchy noise from speaker)

Cathode/G3 (Pin 8) 4.8V

R16 (47 kΩ) is connected from Pin 5 (G1) to the unused Pin 6 and measures 51.3 kΩ in circuit.

R15 (470 kΩ) is also connected to the unused Pin 6 while its opposite side connects to the chassis - this measures 558 kΩ in circuit.
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