RAP Transatlantic Restoration

[Oxford1940]

Originally Posted by Silicon

Can we assume that you are using an 'energised' or 'field coil' loudspeaker with this set?

Do you have a voltmeter?

After doing the screwdriver test, can you measure the HT voltage and anode voltage of the output valve.

This is the speaker with the set. Apologies i'm unfamiliar with the terminology!

[cathoderay57]

Yes, that's definitely a field coil speaker. All it means is that the magnetism is produced by an electromagnet which is a large heavy coil of wire inside the circular metal shield at the back of the speaker housing. It fulfils 2 purposes: one is to magnetise the speaker so that it works; second it acts as a smoothing choke passing high voltage HT to most (but not all) of the circuit. It is the coil shown in your circuit immediately below LS1. The loud buzz proves that the field coil is working otherwise the audio amp wouldn't work. Mounted on the back of the speaker is the audio output transformer. This is the device with the red, black, blue and yellow wires soldered to the 4 terminal posts. If you set the multimeter to the dc volts range covering up to 450v then put the black meter probe on the chassis and the red probe onto the 4 terminal posts you should get a range of high dc voltages ranging from maybe 350 to 250v. One of these wires will be the one that goes to the anode of the output valve. The range of voltages should give enough info to indicate if all looks good. The other thing to mention is that to receive your 1MHz oscillator signal you should have an aerial wire plugged into the aerial socket, and the set needs to be tuned to Medium Wave at 1MHz (or 300 metres) on the MW scale. Plugging in an aerial wire of say a couple of metres in length should also generate some background noise. Last question for now is do you get clicks or pops from the speaker when you rotate the waveband switch? Be careful plugging in the aerial wire because there are 2 more sockets for an extension speaker that are wired across the primary of the output transformer and will therefore be live at 300-odd volts. To make sure you have the aerial socket double check the voltage between it and chassis using the meter before plugging in the aerial wire. Cheers, Jerry

[Oxford1940]

Thank you for that explanation Jerry, it was very useful.

So the voltages read are as followed:

Brown(Black): 890V
Red:713V
Blue:673V (Crackle on the speaker when I touched this one with the red probe.)
Yellow:720V

Now, compared with your predictions I find the readings quite alarming! Also adding to the fact that my multimeter says it only reads up to 600V DC... Does this identify a fault?

Yes I had the 1Mhz antenna plugged directly into the aerial socket using a 4mm banana clip. Fortunately the ports are labelled still. The L.S. ports give a reading of around 22V AC.

I did manage to get music playing via the PU port by using a aux lead running from my laptop. I tested the PU port prior to ensure no current, AC or DC, was running through them before plugging the aux wire in.

I get a pop on the switch when I turn it from the far right position (which is the PU position) one place left. Unfortunately the dials have no markings so I'm unsure what band this is. Altogether there are five positions on the switch. I attempted to ID this switch on the schematic but with it showing six switches and the radio only having four I presumed some have multi-functions and are therefore depicted twice for ease of reading. I have however attached photos of the band change switch and what it leads to in the the chassis. I've also included a snippet taken from the diagram of SW2/1B to attempt to help demonstrate better the position that causes the popping sound.

I did watch a few Youtube videos on how to read schematics but I feel this one is a little complex for that!

[cathoderay57]

There would appear to be something wrong with your meter. Those voltages are not feasible. I recommend that you get the meter checked out, or invest in a new one. It's going to be difficult doing any more tests until that gets sorted. What I would be tempted to try next would be to see if you have any HT voltage on the oscillator valve anode. The mixer/oscillator valve is V1 which is at the front of the chassis alongside the tuning capacitor. The oscillator anode is fed via a separate HT feed from the rectifier cathode via R14 (37k) and R12 (40k). You should be able to identify these components fairly easily. R14 should be connected between the live terminals of the HT smoothing capacitors 8uF and 2uF. I don't know if you replaced these? R12 looks like the orange tubular component with a white band at one end, going through a hole in the second coil from the left on the top group of coils shown. If your multimeter has an Ohms range that works then with the radio unplugged from the mains you could try measuring the resistance of these 2 components. Unless anyone has a better idea it's probably worth waiting until you have the meter sorted. Do be careful doing voltage and resistance checks because often the HT capacitors will retain a charge even after the set has been switched off which could give you an unpleasant electric shock. Cheers, Jerry

[cathoderay57]

Just a thought - are you sure you are not misreading the decimal point, i.e. that the readings are not 89.0, 71.3, 67.3 and 72.0 ??

[Oxford1940]

There wasn't any decimal point on the screen. I had it set to 600V so i wouldn't think it would give such a detailed reading at that setting?

Alas, despite my better judgement I did buy a cheap multimeter. Coincidently the lead on the black probe began to fray today at the join (I've owned it less that a week...) so it is being returned. I will bite the bullet and invest in a decent Fluke model. I don't think testing equipment is the area in which to cheap out in!

Jerry thank you so much for your continued help! I'll procure a new multimeter and try for the readings again.

[Silicon]

Is there any chance you took the voltage readings with the selector switch set to AC Volts?

That sometimes gives a high reading.

Can you verify that the battery in the multimeter is OK?

[paulsherwin]

Cheap multimeters work well enough within their limitations. I agree that your readings are distinctly odd and unlikely to be produced by a fault in the radio. I don't know this set, but you would normally be expecting 250-300V on the output transformer primary after the rectifier has warmed up. Check that you understand what the meter is set to, and that you understand what you're trying to measure.

The fact that you're hearing a crackle when you turn the wavechange switch to the gram setting suggests that the audio stages are basically working.

[Herald1360]

Isn't the "600V" range on a budget DMM really a 2000V FSD range? The 600V is just the maximum permissible voltage that the meter is rated for. I just "Meggered" my cheapy which happily displayed 794V, confirmed by the 1000V rated DMM in parallel with it.

Interestingly the 10M input better meter showed 1050V when on its own, the 1M of the cheapy was obviously overloading the 1000V range of the little Robin tester!

So, readings of 800V or so are possible though still rather unlikely to be real.

[Station X]

Using the wrong meter range, an inappropriate meter range, misinterpreting the reading or plugging the probes into the wrong sockets seems to be a very common problem with new members. That along with measuring resistance with power applied, measuring voltage with no power applied and measuring current by connecting the meter in parallel with a circuit rather than in series with it. The only way to be certain as to what's happening is to get the member to post a picture of his meter showing the range setting and reading.

My pet hate is readings quoted without units!

[Oxford1940]

Hello all,

Attached is a photo of the meter I used. The setting I selected, 600V DC, is highlighted. The black lead was connected to the COM/Common socket and the red lead V/Ohms/mA socket. I read the manual and watched a few Youtube videos before using it so I hoped I'd cracked the basics. I'm not one to ask for help until I've tried to figure it out myself first.

I'm confident there were no decimal places in the readings. I distinctly recall mistakenly using the 600V AC first, as this did not produce a reading. So i'm confident I had it set on DC when I recorded the readings shown above.

Does that all sound and look correct or had I made a mistake?

Unfortunately I can't show you the actual meter used as it was sent back to Amazon due to the black lead fraying after only one week of use.

Thanks,

Oxford

[cathoderay57]

Sounds like you did everything right. Let's hope for more reasonable readings with the new meter. Stick with it - you will get there in the end. Cheers, Jerry

[paulsherwin]

Yes, the procedures you describe are correct, but it's difficult to imagine how a domestic radio could produce those voltages even under a fault condition.

[Herald1360]

The meter pictured looks like a slight variation on the usual 1M input cheapy in the case department, otherwise the ranges look very familiar. These meters are usually pretty good within their limitations of physical flimsiness and low category overload capabilities. (My one has been my general everyday toolbox meter for most jobs for at least 5 years now.)


First thing I did with mine was chuck the supplied test leads. Even home made ones with banana plugs and (insulated) croc clips survive better and they can be any length you want!

[cathoderay57]

I think the 80 rectifier and the 16+16 HT cap would both have popped if 890v were true!

[Station X]

Originally Posted by Oxford1940

Hello all,

Attached is a photo of the meter I used. The setting I selected, 600V DC, is highlighted. The black lead was connected to the COM/Common socket and the red lead V/Ohms/mA socket. I read the manual and watched a few Youtube videos before using it so I hoped I'd cracked the basics. I'm not one to ask for help until I've tried to figure it out myself first.

I'm confident there were no decimal places in the readings. I distinctly recall mistakenly using the 600V AC first, as this did not produce a reading. So i'm confident I had it set on DC when I recorded the readings shown above.

Does that all sound and look correct or had I made a mistake?

Unfortunately I can't show you the actual meter used as it was sent back to Amazon due to the black lead fraying after only one week of use.

Thanks,

Oxford

It certainly sounds like the meter was faulty. We'll find out when you get a new one. It might be worth doing a few basic tests with it like measuring 1.5V, 9V and 12V batteries, and the mains supply voltage at the input to the radio.