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Marconiphone T38A

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HuntsSmoothingBomb
(@huntssmoothingbomb)
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Topic starter
 

Hi all,

I've been a long member of Paul Stennings Forum and have dipped in and out of here from time to time but never really posted anything, now however I'm after a bit of advise regarding the titled set that I'm taking a look at as a favour for a work colleague (the set is actually at work so no access to scope, sig gen etc.)

The set did function to a degree in the fact that the wave bands produced plenty of static and digital noise when scanning over the bands but not a single station could be found. (not even with an external long wire aerial.)

The HT was a little down due to the normal leaky coupling cap connected to audio output valve, all six of the wax caps in this set were leaky to varying degrees from 5M right down to 600k, these have been replaced with good quality equivalents. All the resistances were checked and found to be within tolerance. Voltages are now up to where they should be.

The problem now is that when the set is switched on and warmed up it "motorboats" at about 2-3Hz and receives nothing, I have found that this pulse is on the AGC line and you can see the voltage pulsing on a meter, if I disconnect C15 in the attached  diagram it stops oscillating and it receives once more but only static, still no stations.

Any ideas to what is possibly happening here before I cart the thing home to my workshop and connect it up to the bench?

T38A

Cheers Lee

 

 
Posted : 30/04/2019 11:11 am
Nuvistor
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Welcome to the forum Lee.

First thought is to check the work around the common screen grid decoupling circuit on the FC and IF amp.

The HT line is decoupled by a 20 mfd electrolytic, if this is not fully functional it is a feedback path for audio from the screen of the output valve back to earlier circuits.

Appreciate you are experienced in fixing radios but worthwhile checking for mistakes.

Fix the motor boating, then fix the lack of stations, maybe different faults. It’s possible the motor boating is now due to the higher correct HT voltage, the low HT masking other faults.

 

 

Frank

 
Posted : 30/04/2019 12:11 pm
crustytv
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For those that wish to follow or assist Lee, the full T38A circuit ? 

mcont38A

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Crusty's TV/VCR Collection: View my collection

 
Posted : 30/04/2019 12:49 pm
PYE625
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As Frank mentions above, C20 could be the cause due to lack of decoupling if it is defective.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 30/04/2019 4:08 pm
HuntsSmoothingBomb
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Hi chaps,

Thanks for the replies, and thanks Chris for the full circuit, I wasn't sure whether I should post or not.

I will check this out today over my lunch break and report back.

 

 
Posted : 01/05/2019 7:08 am
HuntsSmoothingBomb
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Thanks very much for your replies and to Nuvistor for hitting the nail on the head! Sound advice regarding C20, this dual electrolytic (also housing C23) has been replaced at some point maybe in the 60's judging by the design of the Radio Spares logo.

I naturally dismissed it being at fault but on close scrutiny I found a hairline crack in the solder joint to  C20's positive tag, a quick wobble of the component proved that the tag was not electrically connected to the circuit. A quick resolder has restored operation and good strong output from the loudspeaker but still only static at the moment with one very faint station mumbling in the background (too faint to be identifiable)

So that's that part sorted, I just need to turn my attention to the lack of stations now, I have been thinking about the L.O. Can I connect a frequency counter to try and measure the frequency or will it load the L.O. to heavily and give an incorrect reading?

I may take it home at the weekend when I have the car (I cycle to work so can't transport it in the week) and get the scope and sig gen on it to see if it will tell me anything.

 

Cheers Lee

C20 & 23
 
Posted : 01/05/2019 12:05 pm
Nuvistor
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Good news, I will have another look at the circuit this evening, out and about with family at the moment.

Frank

 
Posted : 01/05/2019 12:45 pm
Nuvistor
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You can get a rough idea of the LO Frequency with another radio, I would concentrate on MW and LW for now. Set your faulty set around a 1Mhz  MW and tune around 1.470Mhz on the monitor radio bringing it close to the faulty one, a transistor radio with ferrite rod is ideal. Hopefully hear the LO on the monitor.

You may well find it easier in the long run to take it home, you can then check the IF and detector stages before delving into the LO, a sensitive frequency counter should be able to pickup the LO by loose coupling.

 

Frank

 
Posted : 01/05/2019 1:42 pm
HuntsSmoothingBomb
(@huntssmoothingbomb)
Posts: 5
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Hi Frank,

Great advice, cheers! Yes I think you're right and that I will need to take the set home, I have all the equipment there that I will need, I'll try and sort this out this weekend.

 

Cheers

Lee

 
Posted : 02/05/2019 11:12 am
HuntsSmoothingBomb
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Hi chaps, 

Well, good news, the set is working fine, I have to admit it and that is I have been led up the garden path!

I powered up the set in my workshop at home and loads of stations! It would appear that at work I can only pick up the most misserable of signals which equates to a mumbling in the background due to all the hash and other airborne rf noise. Anyrate as I had the set in my workshop I thought I may as well give it the once over and checked the local oscillator frequency, Smooth operates on 828khz here and with my probe connected to my frequency counter and waved around the base of the frequency changer I obtained a reading of 1297khz so IF was operating at 469khz (supposed to be 470khz so not far out) a quick adjustment made to the oscillator core of the medium wave coil brought this back on track, I peaked up the IF's and it has improved sharpness of tuning and volume and that's it really. I have to say I'ts a little begrudging to give this radio back to the chap at work now as he was just going to boot sale it. I might see if I can buy it from him! 

 

Cheers

 
Posted : 05/05/2019 3:49 pm
Nuvistor
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Hash and interference, a real problem for radio reception. Good it’s fixed, a boot sale wouldn’t bring much money, you may be lucky and you get it as a nice present.

 

Frank

 
Posted : 05/05/2019 4:55 pm
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