The radio did not work. Replaced all electrolytic and wax capacitors. Still no go. Checked choke found to be open circuit and also found audio transformer open circuit on input. Replaced these 2 items still no go. Replaced all valves. Very low output from speaker but still controlled by volume control. Checked volume control OK.

Question? The original speaker was dead. Replaced it with a new speaker with the same resistance - 4ohms but the new speaker is a 60 watt. Will the radio give me good volume with this speaker. If not that's my problem- if yes it should give a reasonable volume than I still have a problem.

Seeing that the choke and audio transformer were open circuit cold my problem be the transformer.

Will the radio give me good volume with this speaker.

Generally, the danger in using a speaker whose power rating is way over that of the amplifier is that the amp will reach full output way ahead of the speaker's limit and an amplifier at full tilt can produce what are effectively square waves due to clipping. Square waves are essentially pulsed DC and high DC current will quickly burn out the speaker coil. This is especially the case with solid state amplifiers.

So, if the correct unit cannot be sourced it's generally safer to use a lower powered speaker than a higher powered one.

Very low output from speaker but still controlled by volume control.

You say that you replaced the output transformer. Did you replace it with one that matched the output impedance requirement of the audio amplifier valve?

As for diagnosis, the usual thing to do is to quickly determine if the low volume is due to low signal from the RF stage or a low amplification from the AF stage. You can do that by injecting a test tone or "finger hum" into the AF section usually at the wiper of the volume pot. If the injected signal is loud at the speaker then that would indicate the AF section is probably okay, so the RF signal is probably too weak.

You should also check that the B+ voltage is correct.

Hi GTC,
Thanks for your response. I replaced the output transformer with one that had the same specs that you mentioned earlier 5500 ohms and the output has four leads and I used the two for 4 ohms.

Can you point me in the right direction with regards to testing RF and AF as well as B+ voltage

I'm not sure exactly which Classic chassis you have. What's the model number on your schematic (which I assume matches the actual set)?

Edit: I note that back in August I sent you 4 schematics for Classic models of the era.

I trust that one of them exactly matched your chassis?

Yes 3 of the diagrams matched as far as I can tell.The fourth diagram has the 6X5GT valve. Mine has the 5Y3GT valve.

There are some slight differences among them, but for the time being I'll assume that you are using the schematic that exactly matches your chassis.

Re: B+ ...

Without there being voltages on the diagram, I'll go by the general specifications for anode voltages of the valves themselves, as follows:

6AQ5, Va = 250 volts (anode is pin 5)
6BD7, Va = 300 volts (anode is pin 1)
6AD8, Va = 250 volts (anode is pin 6)
6AN7, Va = 200 volts (anodes are pins 7 and 8)

These are guide figures only, but you should find voltages around these if your power supply is working properly.

Re the RF/AF split test ...

The 6BD7 is the detector and AF amplifier. The signal is fed to the grid (pin 2) via the volume pot. If you touch that pin with a screwdriver *** you should inject some mains hum into the 6BD7 and that should be amplified if the audio section is working properly.

*** Be very careful to correctly identify the pins, which are viewed from underneath the chassis and numbered in a clockwise fashion from the divot/notch in the socket. Don't put your finger on something if you're not 100% sure of what it is. Measure with a voltmeter first.

Also, I would make sure that the radio/gram rotary switch contacts are clean. I use CRC contact cleaner and work the switch back and forth a number of times.

And, I should have asked earlier: do you get any signal from the gramophone pickup when switched to gram mode?

Thanks a lot GTC. I will have to put it on the back burner for 3 weeks as I am flying out to Penang/Taiwan tomorrow. When I get back I will keep you posted.

Regards Andy

"Be very careful to correctly identify the pins, which are viewed from underneath the chassis and numbered in a clockwise fashion from the divot/notch in the socket. Don't put your finger on something if you're not 100% sure of what it is. Measure with a voltmeter first.."

Another safety rule to always follow is to have one hand in your pocket, when poking around high voltage circuits. The objective here is to avoid having a shock current going through your heart. Footwear, long sleeves and long pants also help. If you then do touch with a finger some high voltage, you'd become like a bird on a power wire, but if there's no path through your body for the high voltage to get to its return (like a ground or the chassis) you won't get a shock. But don't get careless here.

Thanks for all the help guys. I am used to working on Thermo King transport refrigeration units and some of these have mixed circuits 12 volt dc 415volt ac.

The main rule I learnt is to treat ALL circuits as being live until proven different.

Regards
Andy

Hi Andy,

Your radio sounds like one I repaired recently, the amp side was fine but very faint radio.

After doing everthing you did I followed the above instructions given by members and found a broken wire internally on the IF coil .

Fortunatly it was visable and after repairing it the radio came to some sort of life.But it still was not perfect. I had to remove the internal capacitors and fitted external ones.

This fixed the radio and then I realigned it. Your radio to have blown chokes and transformers must have had really bad filter capacitors and used to death till it burnt up and stopped. This is what I found in the one I repaired.

Every clue helps , I have never come across a bad IF or RF coil but will be the first thing I check amoung my other checks I do.

Cheers
Vic

Hi GTC. Just got back into the country yesterday. I have taken those voltage readings. I assume these voltages should be static. Every voltage reading I took fluctuates from no reading up to 200 volts or more. Should they fluctuate or be static. I checked the gram and nothing. At the transformer there are 3 feeds. One for the lights which is ok and then another two. One of these has 2 wires going to the 5Y3GT valve. Going by the diagram their is a ground circuit in the centre and the voltage reading I got was 297 volts on each wire and 600 volts across the two wires. On the diagram it has 275 volts marked. For the third feed it also has two wires going to the 5Y3GT valve. The reading I got from this was between 4 and 5 volts. HELP!

The B+ voltages at each point should definitely be static, like a battery. This is meant to be DC, as rectified by the 5Y3 and smoothed by the filter comprising the 8μF cap, the choke and the 16μF cap.

That transformer has 2 filament windings: 5 volts AC for the 5Y3 and 6.3 volts AC for the other valves, and a HT winding. The HT is indicated as 2x275, which is otherwise notated as 275-0-275, and means that the voltage between the centre tap (0 volts) and each of the HT wires should be around 275 volts and the the voltage across the HT wires should be around 275+275 = 550 volts.

Let's take this one step at a time: What AC voltage do you get between pin 4 (anode-1) of the 5Y3 and chassis and also between pin 6 (anode-2) and chassis?

Then, what DC voltage do you see between chassis and whichever pin (2 or 8) of the 5Y3 that has the 8μF cap connected to it? (Note: black/negative meter lead to chassis, positive lead to that pin).

Then, what DC voltage do you get between chassis and the junction of the choke and the 16μF cap?

Hi GTC I must apologise first. When I did the first lot of readings I was lazy and used my old meter. It is now in the bin.Firstly I could only find one filament winding on my unit. The voltage reading between these 2 wires going to 5Y3GT is 5.2 volts AC. The readings I got for Pin 4 and Pin 6 to chassis on 5Y3 was 299 volts AC for each reading. This is where you lost me. The reading I got between the two terminals on 5Y3 for the filament wiring was 5.2 volts yet when I take a reading from terminal 6 which has one of the filament wires attached to chassis I get 299 volts and as mentioned there is no second feed to supply the other valves with 6.3 volts.

On terminal 8 on 5Y3 the reading was 338 volts DC and the reading for the junction of the choke and the 16micro farad cap was 306 volts.

AS mentioned for my first readings with my old meter I took the readings with my new meter and they are as follows. 6AQ5 pin 5 294 volts DC 6BD7 Pin 1 60 volts
6AD8 Pin 6 305 volts 6AN7 Pin 7 308 volts and for Pin 8 100 Volts.
As you can see the voltages are all over the place.

Okay, before we go any further, let me know which particular schematic you worked from when you re-capped the set.

That is, which PDF file name: TG (2), TG (3) or TG (4) ?

GTC you sent me 4 diagrams. Diagram PDF (1) is not the correct diagram as it has different valves. Diagrams PDF (4) and (3) are exactly the same diagrams but out of different publications. So that left me with 2 to pick from. I compared the 2 diagrams and as far as I could ascertain they were the same although some of the diagram is drawn different but still the same . I ended up running with PDF (4) down the bottom it has page 111 shown.
When I recapped I was fortunate to be able to read the cap specs on each one and I replaced them one at a time. But I still might have made a mistake.

Regards Andy

PS I found the 6.3 volt AC feed. All valves have 6.3 volt feed. All valves I checked between pin 4 and pin 5 except valve 6AQ5 I checked from pin 4 and chassis as pin 5 has another feed.