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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 29th Sep 2014, 1:21 pm   #121
ukcol
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Default Re: Ambassador 545

The heater of a valve changes resistance with temperature; when the heater is cold the resistance is quite low and the resistance increases when the heater is hot.

When you switch on your radio the current drawn by the cold valve heaters is relatively high (making your lamp limiter glow orange), then as the heater resistance goes up less current is drawn and the limiter lamp dims.

Finally when all the valves have "warmed up" HT current starts to flow; this increase in current makes the lamp limiter glow orange again.

I don't happen to use a lamp limiter myself but your results sound good to me.
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Old 30th Sep 2014, 4:40 pm   #122
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Default Re: Ambassador 545

Thank you!
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Old 30th Sep 2014, 10:25 pm   #123
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May I describe the results from my lamp limiter test, please?
I apply power from the mains, and the bulb filament glows orange,
At this point, the valve heaters are cold and their resistance is low. So they will draw maximum current. Of course, as more current is drawn from the secondary winding of a transformer, so the primary winding draws more current; reduced by the inverse of the voltage ratio. So 1 A on the 6.3V side would correspond to 1 * 6.3 / 230 = 0.0274 A = 27.4 mA on the 230 V side. And the primary current is also flowing through the filament of the limiter lamp. The more current is being drawn by the radio, the brighter it will glow. The current flowing through it can never exceed the normal operating current of the lamp. The price we pay for this double benefit of visual indication and circuit protection, is that the lamp robs some voltage from the appliance under test.
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then fades,
As the valve heaters begin to warm up, the current they draw reduces, and with it the amount of heating power; eventually reaching a steady temperature where the effects of increasing temperature and decreasing heating exactly cancel one another out.
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then after about ten - twenty seconds, the bulb filament glows again.
It takes some time after the heater current has stabilised, for the cathodes of the valves to reach operating temperature. Once they all begin emitting electrons (especially the rectifier: all the HT current flows through this), the HT current begins to increase to its final operating value; limited mainly by the bias on the output pentode. The frequency changer, IF amplifier, detector and preamplifier draw much less current between them from the HT supply than the audio output. The increased current causes the limiter lamp to glow more brightly.
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The light behind my radio tuning dial also lights.

Can I ask if this is good?!
Yes, it's pretty much exactly what should be happening. What's going on inside the set? Are all valves glowing nicely? No fireworks anywhere?

What wattage bulb are you using as your limiter?
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More importantly, what is happening?
See comments inserted inline.

Does your radio make any sounds when it warms up, if you increase the volume and wiggle the tuning? Do you get static? Power hum? A station coming through, even faint and distorted? (N.B. If you get a loud "farting" noise accompanied by pulsing of the limiter lamp, try again with a larger wattage bulb.)
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Old 18th Oct 2014, 5:11 pm   #124
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Default Re: Ambassador 545

Hello Guys

Sorry to trouble you, but I really am totally stuck, I still have a silent set, after all this time.
I don't want to bother you, once more, but I am desperate, is there anything I could do to isolate the problem?
I suspect something is amiss with inductor L13/L14, and the wiring with C24 & C25.
The radio powers, dial lights on, the lamp limiter experiment a success, but I have silence.
Fine if anything by the Spice Girls is being broadcast, but overall, not ideal.

I am close to tears, and can't see the wood for the trees!

PS - I have also struggled to locate L16, which is supposed to be on the top of my chasis? However, I can't find it, anywhere, and this has something to do with the switching of internal/external speakers. So, I am now thinking, is this L16 fella actually inside my speaker cone?? Is this possible!!?
Yours,
Mr Desperate
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Old 18th Oct 2014, 8:23 pm   #125
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Default Re: Ambassador 545

No, don't give up just yet. Your lamp limiter tests are very good. There probably isn't much stopping this set from singing its heart out.

L16 could indeed be part of the speaker -- the ERT service data makes reference to the fact that an energised-field speaker might be fitted. All modern loudspeakers use a permanent magnet to provide the steady magnetic field, but some older ones used to use another coil fed from DC. You will know which sort yours is by there being four terminals: the usual two connected by flexible tails to the moving coil behind the cone, plus an additional two somewhere around the back. (This was because it was hard in those days to make permanent magnets that were both strong and long-lasting; and also, the fixed coil of the speaker could be employed usefully as a choke in the power supply.)

If you're unsure, post some pictures of the speaker with notes as to what is connected where, and we'll take it from there. (Actually, please post pictures anyway; knowing whether the speaker is a permanent magnet or energised field type will give important clues for devising a suitable test sequence.)

With your multimeter, measure the DC voltages around the valves (black lead to chassis, red lead to pin of valve under test) and compare them to the figures on the wiring diagram. Also measure the AC voltages between the heater pins (1 and 8) of each valve.

If the voltages are all fine, we can test V4 as follows. (I'm assuming you don't have a signal generator, so this is a "poor person's equivalent" method you can use.) Get a cheap MP3 player or something with a headphone socket, and a pair of cheap headphones. Cut off one of the 'phones (if your source is a mono device and you only get sound from one headphone, this is the one you need to cut) and prepare the cable as follows. Connect the outer shielding to a crocodile clip, which should be attached to the radio's chassis; and the inner core to one lead of a 100 nF / 400 V capacitor (values not too critical but should ideally be rated as mch as the HT of the set). Connect the other lead of the capacitor to an insulated probe (this can be something improvised, such as an old ballpoint pen tube with a sewing needle with the wire soldered onto it glued into the end). With music playing on the MP3 player, its volume turned up and the radio powered, touch your probe onto the junction of R18 and R20. Does anything come through the radio's speaker? Does adjusting the tone control have any effect? (This is the only control in circuit at the moment -- we are injecting this signal downstream of the volume control and wave-change switches, so they will do nothing.) The speaker might burst into life at one end of the potentiometer's travel.

The next test will depend on the results so far, and also on whether the speaker is a permanent-magnet or energised-field type.
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Old 19th Oct 2014, 6:07 am   #126
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Default Re: Ambassador 545

Just reading your response fascinates me, AJS. (I am referring to the instructions regarding the MP3 player). Thank you!.

I am out all of today, but will definitely upload some photos as soon as I can.

Thank you once more.

PS There are only two wires coming off the back of the speaker cone, and they go straight into 'INT SPEAKER' sockets on the back of the chassis.
So, now I am puzzled, what have I done with L16!?

Cheers!
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Old 19th Oct 2014, 7:31 am   #127
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Hi again, just very quickly took pictures before I leave for the day, images of the speaker, the top of the chassis and the sockets for the speakers.
Apologies they are blurry, and forgive the red tape (embarrassed)

Thanks for any advice!
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Old 19th Oct 2014, 10:54 am   #128
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Default Re: Ambassador 545

That looks like a normal permanent magnet speaker, so it won't have an energising coil as a substitute for L16.

L16 should be connected to the positive ends of C24 and C25, possibly via long leads. It's also possible it's been replaced by a resistor. Check the resistance between the positive ends.

Get rid of that tape and replace it with silicone sleeving. The joints at the far left of the picture look like they may be shorting. Apart from that, plastic adhesive tape fails with age and heat, whereupon it drops off.
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Old 19th Oct 2014, 4:09 pm   #129
sooperrio
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Default Re: Ambassador 545

Hi Graham

Many thanks for your advice, I will definitely get the silicone sleeving, it would make it look neater too.
Which 'joints' are you referring to?!
Is there anyway you can identify them to me, please?!!!!

Also, yes, L16 should be at the end of C24/C25, but I will be darned if I can see it!!!
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Old 19th Oct 2014, 4:57 pm   #130
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L16 should be connected to the positive ends of C24 and C25, possibly via long leads. It's also possible it's been replaced by a resistor. Check the resistance between the positive ends.
Just follow the connections and leads from C24/C25 and account for every component they connect to. If there was no L16 or a substitute resistor the set wouldn't have worked for you, as there'd have been no HT except to the output valve.

Apologies if I've missed something, but I don't have the enthusiasm to read through all 129 posts in this thread.
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Old 19th Oct 2014, 5:44 pm   #131
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Default Re: Ambassador 545

O.K. That's a permanent magnet speaker. You can test that by disconnecting it from anything else, and applying a 1.5 V battery to the speaker terminals. You should hear a click, and another when you disconnect the battery. If not, the speaker is faulty.

If the speaker is OK, then while it is disconnected, measure the resistance of the secondary winding of the output transformer. You should get a low reading of just a few ohms.
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Old 19th Oct 2014, 6:22 pm   #132
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Default Re: Ambassador 545

Thank you, AJS, I will try that.

Graham, and everyone, I am really, really sorry for troubling you all.
Will step away.
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