Marconi TRG39A (TARG39A) restoring? - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

matthewhouse · 16th Sep 2014, 11:24 pm

OK. Lets start right at the beginning.

First look at the circuit diagram. Ignore everything to the left of the PU socket for the time being. It's important to get everything to the right working correctly first. If everything on the right works it will amplify a signal connected to the PU socket. (unless there is a catastrophic fault on the left hand side)

V3 contains a triode which in this set is used as a preamplifier. It also contains 2 diodes which in this set are both used for AM detection.

V4 is the output valve, in this case a tetrode. It amplifies the signal fed into its grid from V3.

V5 is a full wave rectifier, it contains 2 diodes, and it's heater is fed from a separate winding on the mains transformer.

The first task is to get all the valve heaters lit and the 2 dial bulbs as well, as they are also connected to the same wiring. You mention a valve with a broken pin, replace it, the valves used in this set are common and easily available. The set in my avatar uses the same valves and a very similar circuit.

Check your replacement bulbs are good and screwed in firmly, they should be 6.5Volt 0.3Amp.

As you are having burning and sparking, unplug the U149 rectifier valve V5, this will disconnect the HT supply from the set. Make sure the fuse in the mains plug is no more than 3 Amp, then connect the mains supply. Valves 1 to 4 should start glowing after a few seconds and the 2 dial lamps should be lit. The room may need to be darkened to see some of the heaters due to the silvering, but they all should be visible.

If not, switch off and unplug the mains. Set the chassis upside down securely so you can get to the underside. Connect the mains supply again, and you should be able to carefully measure approximately 6.3V AC with your meter connected to pins 1 and 8 of each valve, and from the centre contact and thread of the bulb holders. I would recommend partly insulating the probes of your multimeter so that only the tips are bare, this will help reduce the risk of shorting something out again.

If you have 6.3V but no heater, check the valve socket contacts and valve pins are clean. You can check if the heater has continuity by unplugging the valve and with your meter set to ohms should give you a low reading when connected to pins 1 and 8, if not the valve needs replacing.

If the 6.3V is missing at one or more positions the wiring will need to be traced to locate the problem.

Now the heaters in valves 1 to 4 light, as well as the dial bulbs, the next stage is to get the HT supply working.

Unplug from the mains supply and disconnect the wires to the cathode (pin7) of the rectifier valve V5 and make sure they can't touch anything. Plug the rectifier and connect the mains supply. All the valves should light and there should be no sparking in the rectifier valve, if there is, replace the valve.

Taking great care as there are high voltages here, connect your meter, set to read DC volts, with the black lead to the chassis and the red lead to the cathode (pin7) if the valve is working you should have 250 or so volts here, it may be more as no HT current is being drawn.

Return with your results and we can go from there.

G8UWM-MildMartin · 17th Sep 2014, 12:33 am

Excellent advice from Matthewhouse.
I'll just add that internal sparking in U149 rectifiers is a known failure mode, even if there isn't another fault.
You've probably noticed when looking it up that 7Y4 is the USA equivalent, and may be easier to find, and similarly 7C5 for the N148.
P.S. your paper labels on these two will smoke horribly and char when they warm up!

matthewhouse · 17th Sep 2014, 8:44 pm

Yes good points! The U149 in my avatar radio fireworked every time the radio was switched on I did eventually replace it.

mushibu10 · 19th Sep 2014, 2:15 pm

Oh fantastic advise.

easy to understand.

Okay I will follow the advice.
i am on a limited budget the double cap i had to wait till i had no bills to get it. so it's a slow process. now i have a 5amp in. I will change that too a 3amp.

so the internal sparks are 'normal' to an extent. (nice to know it's not me.
I will look for a new valve for the suspected broken pin now.

am not at home at the moment. i'll do it tonight, now to insulate the probes more? would insulation tape be okay? (was thinking of attaching crocodile clips to the ends of probes?)

the dial lamps, can you confirm its a E10 thread type. before i go ordering. (how long are the bulbs suppose to last?)

i never thought of the labels being the reason for the smoke. (trouble is the fire alarms in my building are very sensitive blowing a candle out will set them off.) so i'll remove the labels, how do i out the names back on? (sharpie pen?)

I had great success making the three speed turntable working again, once i finally got the platter off haha! It's still rusty but its working and i oiled(not using WD40)

thank you once again. there maybe a delay in results as mentioned above, but it's greatly appreciated.

Leo

PS is it possible to make a amp for the turn table? just connect the pick up line to the input of amp?(i have audio mixer i use for all my TV and so on,)

am i correct your avatar radio is the T29A? (one in Bristol for sale (working))

matthewhouse · 20th Sep 2014, 8:57 pm

The sparks are due to either a faulty valve or a fault causing excess HT current to be drawn, they should not be considered normal. If replacements are needed search for the equivalent types if you can't get hold of ones with the Marconi numbers.

A few layers of tape will be OK, just make sure the tape is replaced if it gets damaged or worn. Most high quality leads come with clip on shrouds which leave only a tiny point exposed, or can be removed if a large contact is needed. Fully insulated crocodile are good as they allow you to connect them with the power off and take measurements with your hands safely out of the way. I have a crocodile clip on the black lead allowing it to be connected to the chassis while I probe with one hand, keeping the other well out of the way. This reduces the risk of shock across the chest.

A pen or pencil will often be fine to mark these valves, write on the metal bit rather than the glass. Once you have worked with valves for some time it's often possible to fit a set of unmarked valves in their correct sockets just by looking at their electrodes.

Yes it's possible to make an amp and so on, but I'm not the person to ask about that. I repair things rather than make them.

Yes it's a T29A. It was my first vintage radio, it's not the one for sale.

16th Sep 2014, 11:24 pm	#41
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Marconi TRG39A (TARG39A) restoring? OK. Lets start right at the beginning. First look at the circuit diagram. Ignore everything to the left of the PU socket for the time being. It's important to get everything to the right working correctly first. If everything on the right works it will amplify a signal connected to the PU socket. (unless there is a catastrophic fault on the left hand side) V3 contains a triode which in this set is used as a preamplifier. It also contains 2 diodes which in this set are both used for AM detection. V4 is the output valve, in this case a tetrode. It amplifies the signal fed into its grid from V3. V5 is a full wave rectifier, it contains 2 diodes, and it's heater is fed from a separate winding on the mains transformer. The first task is to get all the valve heaters lit and the 2 dial bulbs as well, as they are also connected to the same wiring. You mention a valve with a broken pin, replace it, the valves used in this set are common and easily available. The set in my avatar uses the same valves and a very similar circuit. Check your replacement bulbs are good and screwed in firmly, they should be 6.5Volt 0.3Amp. As you are having burning and sparking, unplug the U149 rectifier valve V5, this will disconnect the HT supply from the set. Make sure the fuse in the mains plug is no more than 3 Amp, then connect the mains supply. Valves 1 to 4 should start glowing after a few seconds and the 2 dial lamps should be lit. The room may need to be darkened to see some of the heaters due to the silvering, but they all should be visible. If not, switch off and unplug the mains. Set the chassis upside down securely so you can get to the underside. Connect the mains supply again, and you should be able to carefully measure approximately 6.3V AC with your meter connected to pins 1 and 8 of each valve, and from the centre contact and thread of the bulb holders. I would recommend partly insulating the probes of your multimeter so that only the tips are bare, this will help reduce the risk of shorting something out again. If you have 6.3V but no heater, check the valve socket contacts and valve pins are clean. You can check if the heater has continuity by unplugging the valve and with your meter set to ohms should give you a low reading when connected to pins 1 and 8, if not the valve needs replacing. If the 6.3V is missing at one or more positions the wiring will need to be traced to locate the problem. Now the heaters in valves 1 to 4 light, as well as the dial bulbs, the next stage is to get the HT supply working. Unplug from the mains supply and disconnect the wires to the cathode (pin7) of the rectifier valve V5 and make sure they can't touch anything. Plug the rectifier and connect the mains supply. All the valves should light and there should be no sparking in the rectifier valve, if there is, replace the valve. Taking great care as there are high voltages here, connect your meter, set to read DC volts, with the black lead to the chassis and the red lead to the cathode (pin7) if the valve is working you should have 250 or so volts here, it may be more as no HT current is being drawn. Return with your results and we can go from there.

17th Sep 2014, 12:33 am	#42
G8UWM-MildMartin Heptode Join Date: Jan 2004 Location: Stockport, Greater Manchester, UK. Posts: 827	Re: Marconi TRG39A (TARG39A) restoring? Excellent advice from Matthewhouse. I'll just add that internal sparking in U149 rectifiers is a known failure mode, even if there isn't another fault. You've probably noticed when looking it up that 7Y4 is the USA equivalent, and may be easier to find, and similarly 7C5 for the N148. P.S. your paper labels on these two will smoke horribly and char when they warm up!

17th Sep 2014, 8:44 pm	#43
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Marconi TRG39A (TARG39A) restoring? Yes good points! The U149 in my avatar radio fireworked every time the radio was switched on I did eventually replace it.

19th Sep 2014, 2:15 pm	#44
mushibu10 Retired Dormant Member Join Date: Jun 2014 Location: Devizes, Wiltshire, UK Posts: 42	Re: Marconi TRG39A (TARG39A) restoring? Oh fantastic advise. easy to understand. Okay I will follow the advice. i am on a limited budget the double cap i had to wait till i had no bills to get it. so it's a slow process. now i have a 5amp in. I will change that too a 3amp. so the internal sparks are 'normal' to an extent. (nice to know it's not me. I will look for a new valve for the suspected broken pin now. am not at home at the moment. i'll do it tonight, now to insulate the probes more? would insulation tape be okay? (was thinking of attaching crocodile clips to the ends of probes?) the dial lamps, can you confirm its a E10 thread type. before i go ordering. (how long are the bulbs suppose to last?) i never thought of the labels being the reason for the smoke. (trouble is the fire alarms in my building are very sensitive blowing a candle out will set them off.) so i'll remove the labels, how do i out the names back on? (sharpie pen?) I had great success making the three speed turntable working again, once i finally got the platter off haha! It's still rusty but its working and i oiled(not using WD40) thank you once again. there maybe a delay in results as mentioned above, but it's greatly appreciated. Leo PS is it possible to make a amp for the turn table? just connect the pick up line to the input of amp?(i have audio mixer i use for all my TV and so on,) am i correct your avatar radio is the T29A? (one in Bristol for sale (working))

20th Sep 2014, 8:57 pm	#45
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Marconi TRG39A (TARG39A) restoring? The sparks are due to either a faulty valve or a fault causing excess HT current to be drawn, they should not be considered normal. If replacements are needed search for the equivalent types if you can't get hold of ones with the Marconi numbers. A few layers of tape will be OK, just make sure the tape is replaced if it gets damaged or worn. Most high quality leads come with clip on shrouds which leave only a tiny point exposed, or can be removed if a large contact is needed. Fully insulated crocodile are good as they allow you to connect them with the power off and take measurements with your hands safely out of the way. I have a crocodile clip on the black lead allowing it to be connected to the chassis while I probe with one hand, keeping the other well out of the way. This reduces the risk of shock across the chest. A pen or pencil will often be fine to mark these valves, write on the metal bit rather than the glass. Once you have worked with valves for some time it's often possible to fit a set of unmarked valves in their correct sockets just by looking at their electrodes. Yes it's possible to make an amp and so on, but I'm not the person to ask about that. I repair things rather than make them. Yes it's a T29A. It was my first vintage radio, it's not the one for sale. Last edited by matthewhouse; 20th Sep 2014 at 9:03 pm.