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Old 15th Oct 2020, 9:27 pm   #21
cathoderay57
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Default Re: Marconi 346 Radio

What rectifier are you using?The specified U12 is directly heated but if you have replaced it with an indirectly heated valve that might explain why the HT is significantly higher than expected, on account of its lower series resistance (even allowing for the use of a high impedance DVM). Might not be helping things.
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Old 15th Oct 2020, 10:03 pm   #22
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Default Re: Marconi 346 Radio

Hi Jerry, I replaced the 0.05uF but as there are bass capacitors bridged across (which are original TCC Mica) if one of those is leaky then it would be the same effect. I'll have a look at this tomorrow and see if they are faulty. From memory the rectifier is a MU14
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Old 16th Oct 2020, 10:52 am   #23
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Default Re: Marconi 346 Radio

OK, the MU14 is indirectly heated and is probably the reason for the high HT. It's a long-shot but before doing anything else check C44 the speaker field shunt capacitor since if it has gone very leaky it will be reducing the voltage drop across L35. To bring the HT voltages closer to spec I would recommend inserting a couple of dropper resistors as follows. In between the top of T1 primary and the HT rail insert a 1k 5W which should drop the N41 anode voltage by about 40v. Make sure the new resistor is only in the part of the circuit that drops the voltage to T1 primary because the HT feed to the rest of the set could be taken from wires also soldered to T1 primary tags. Re-measure the cathode voltage and if it isn't less than 5v then increase R37 from 100R in small steps until it is, to increase grid bias and reduce the anode current. Also insert a 1.8k 2W resistor between C45 (west of the connection to T1 primary) and the HT feed to the rest of the set. The total HT current budget for V1, V2a, V3, V4 and V5 screen is about 23 mA so a 1.8k should drop about 40v. There is a separate HT feed to V2b oscillator to improve frequency stability but the excess voltage on the oscillator anode is probably OK to leave as-is. Cheers, Jerry
P.S. Also worth checking the dc resistance of L35 the speaker field coil. It should be 1.1k but if the speaker has been replaced at some point it's possible somebody fitted an energised speaker with a lower field coil resistance.

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Old 16th Oct 2020, 3:22 pm   #24
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Default Re: Marconi 346 Radio

Thanks Jerry I can give this a try. I'm inclined to find an original U12 and see what it is like with that first, but they seem to have fallen victim to audiophile prices in recent years. I do have a directly heated rectifier in my cossor, it will be a 442BU equivalent but I don't know how that will fare in the HMV should I try it, and I certainly wouldn't want to cause it any damage. If I can't find one then I'll go ahead with the mods and get back to you.
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Old 16th Oct 2020, 4:28 pm   #25
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Default Re: Marconi 346 Radio

I ventured into the loft to find the 442 equivalent, turns out it is actually a U12 which is fairly lucky. I've fitted this to my set and the HT has dropped a bit, I've noticed the voltages vary depending on whether the radio is switched between radio or gram so with the U12 fitted I've measured both as follows

N41 Radio Cathode v: 5.93v Anode v:255v
Gram: Cathode v: 6.39v Anode v:270v

Also checked coupling caps etc and there is no leakage, C44 is also okay.
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Old 16th Oct 2020, 8:47 pm   #26
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Default Re: Marconi 346 Radio

Ok, how is the distortion now on audio output? If you can put a scope on it with a clean sinewave input to the amp it might give a clue. I'd still be tempted to drop the HT a bit but if the set otherwise works OK then maybe it's time to quit! Cheers, Jerry
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Old 17th Oct 2020, 3:50 pm   #27
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Default Re: Marconi 346 Radio

I've scoped the output with a 1k sine wave and it looks fine, I wonder now if the speaker has a problem with voice coil rubbing. I've attached a photo of the waveform and also noted that with the treble control on the lower two settings there is more hum which is making it sound more distorted, but the waveform still looks nice and free from distortion. These two treble capacitors I have replaced so it isn't a problem with leaky caps but I wonder where the exess hum is coming from with these two.

Strange though that the radio side of things seems to sound less distorted than the gram input. Next job is to connect a speaker to the external LS socket and see if that works okay, if it does then I suppose I'll need to source a replacement speaker unless this has an adjuster for coil seating.
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Old 17th Oct 2020, 4:03 pm   #28
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Default Re: Marconi 346 Radio

Re: hum. Since you've replaced the volume control, with modern replacements the pot's metal shield doesn't always make a good earth via the mounting nut and so it might be worth soldering a bonding lead from the pot case to chassis.
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Old 17th Oct 2020, 4:09 pm   #29
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Default Re: Marconi 346 Radio

Good point I'll do that when the chassis is out again. Connected up a speaker via the external LS socket and that is fine so there's a problem with the original internal speaker, either a shorting coil or rubbing. I'll get it out and investigate. From memory there may be an adjuster screw but I can't be certain, looks like it'll be a replacement speaker otherwise.
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Old 17th Oct 2020, 4:59 pm   #30
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Default Re: Marconi 346 Radio

Happy to report back that the speaker responded well to a re-alignment. The underside of the cone has two card springs held by washers and nuts, I loosened these off and lined the gap between magnet and coil with some thick paper before retightening the nuts and removing said paper. It now sounds great.

Now I'm happy with where the sound is I just need to sort out SW1 with a new 10pF capacitor (that is on the way) and then re-alignment of the other wavebands. Some new screws and washers and a reproduction back and it'll be finished.

Thanks Jerry for all of your advice and wisdom, it's a shame I didn't try the speaker earlier, must remember this on future projects. I'll try and find a U12 to keep in this or a 442BU for my Cossor, which may be easier to source without a ridiculous price tag.
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Old 17th Oct 2020, 9:26 pm   #31
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Default Re: Marconi 346 Radio

Nice work, well done
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Old 17th Oct 2020, 10:22 pm   #32
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Default Re: Marconi 346 Radio

I had a look in the spares box and I have a U12. On the VCM163 when tested at 60mA it registers 60% on both anodes. However, it glows blue as you can see from the pic when I tried it in my Murphy A50. The radio worked fine though, and there were no flashovers inside the valve. I think it means the rectifier has gone gassy. I leave it up to you if you want to risk it - others might wish to comment on how sensible that would be. You can have it for £4 postage if you would like.
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Old 17th Oct 2020, 11:42 pm   #33
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Default Re: Marconi 346 Radio

That's a great offer and very kind of you to test and sort it out for me. I'm not sure if it will be a problem but I can rig up a psu and give it a long test for reliability first before putting it in the set, that way I can just put this U12 back in the cossor. I'll send you a PM over in the morning
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