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Old 18th Jun 2018, 2:10 pm   #1
johnny english
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Default Murphy B841A transistor radio

Hi
First time attempt at fixing this type of radio and it has not responded to my attempts to get it going. The b841a is basicly an b831 but with a chromed plastic bezel around the dial face. I looked through a post by Howard from 17th december 2007 which was very helpful as he had similar problems to myself with a b831. I downloaded the b831 service manual from the Radio museum website as well as the b841 from vrsd, I decided to use the b831 manual as it was more informative. Powered up and no signals being received on vhf/mw nor long wave just a noise through the loudspeaker so I started tapping the af series transistors to no effect no crackles or anything. I then noticed that the set was drawing 65ma and rising referring to the manual I tried to set the Q current via the 200 ohm pot (rp3) but this was proving to be faulty so I ordered a new one along with af 125/126/127/178 replacement transistors. During the wait for parts I checked out the resistors and found only one which was out of spec and a couple of e caps which I replaced. The volume control did`nt seem to be working so I checked the resistance and it worked fine although it made no difference to the level of sound.
The spare parts arrived so I fitted the 200ohm pot this worked fine but still could`nt set the Q current to manufacturers spec, so I set about the transistors I resistance tested old and new and found one old af116 which was totally u/s the rest showed some shorts and high leakage. After replacing a transistor and so on I then checked the sets performance which remained the same as before. I have though managed to set the Q current to the specs.
Q=18ma
vhf=21ma
mw=18ma voltage is set at 8.7v at the junction of r44/45 and c61.
So to sum up I am more or less back to where I started. Voltage checks using the schematic all seem to correspond to specs give or take a volt or two. I hope I am not the victim of the phantom twiddler as a realignment of this radio is quite a task. What do you think I should do next. Btw there is no voltage present on the volume control.

Last edited by johnny english; 18th Jun 2018 at 2:12 pm. Reason: to add a character to script
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Old 18th Jun 2018, 2:33 pm   #2
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Default Re: Murphy B841A transistor radio

Inject an audio signal at the volume control, confirm the AF stage is working.
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Old 18th Jun 2018, 3:40 pm   #3
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Default Re: Murphy B841A transistor radio

Doh!!
Will do Frank but it will be later on today. Thanks will report back.
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Old 19th Jun 2018, 12:20 am   #4
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Default Re: Murphy B841A transistor radio

I injected an audio signal through and got nothing except a crackling thru the speaker as the probe touched the contacts. Would this suggest the output driver trannies (ac127) and or the (ac128) output transistors need replacing. Heres the measurements of the ac127s of which there are two.


tr7
e=14.81v
b=8.72v
c=17.86v

tr8
e=18v
b=15v
c=14.81v The following measurements are for the AC128 X2

tr9
e=14.94v
b=13.63v
c= 0v

tr10
e=14.77v
b=14.85v
c=18v
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Old 19th Jun 2018, 8:49 am   #5
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Default Re: Murphy B841A transistor radio

The readings on Tr's 7 & 8 don't seem right to me. Between the base and emitter of any germanium transistor in normal operation, there should only be about 0.2v, so Tr 7 in particular is suspect, since, from your figures, there is just over 6 volts difference between b & e. In fact none of those readings are correct, so I'd be checking all 4 transistors and their associated components. Is this a D.C. coupled amplifier, b.t.w? If so failure of one transistor can have a 'knock-on' effect on all the others.
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Old 19th Jun 2018, 2:28 pm   #6
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Default Re: Murphy B841A transistor radio

I don`t know if this is a dual dc amp, but I will follow the circuit through each stage and check the voltages using the schematic. I have ordered replacement output transistors ac127-8 these should arrive in a couple of days. Thanks for your guidance so far and I will report back with good news I hope.
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Old 19th Jun 2018, 3:20 pm   #7
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Default Re: Murphy B841A transistor radio

Those transistors are know to short to the case. If they are mounted on a common heat sink, lift them off it and test again, for a short test they should not overheat.
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Old 19th Jun 2018, 4:28 pm   #8
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Default Re: Murphy B841A transistor radio

The ac128s are coupled to a heatsink bolted to the frame so I wil do as you suggest later this evening. Thanks Ed.
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Old 24th Jun 2018, 11:22 pm   #9
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Default Re: Murphy B841A transistor radio

Hi.
I tested for shorts on the transistors and there was non, replaced the ac127s no change so onto the ac128s Tr8 the first one removed tested faulty fitted the replacement and the radio sprang to life mw/lw/vhf all corresponded with the dial markings so I set about setting the QC that was fine but noticed when the volume was at minimum there is a noise through the speaker which sounds like when you put a seashell over your ear, its also behind the radio signal on all wavebands and raising the volume doesnt mask it. I checked my work making sure the transistor leads are in the correct positions on the circuit and took voltage readings which slightly more or less match up with the manufacturers specs. What do I do now.
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Old 24th Jun 2018, 11:41 pm   #10
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Default Re: Murphy B841A transistor radio

Check for out-of-tolerance resistors. You will have to unsolder one end to test, to avoid turning on semiconductor junctions (not an issue in valve sets, as a cold valve cannot pass any current). Start with the largest and smallest values, since they tend to be the "driftiest" (if there be such a word); values from 1kΩ to 100kΩ are the most likely to be within tolerance, but are not beyond reproach.

If no joy, work backwards through the amplifier towards the detector diode, shunting the base of each transistor to one side of the battery with a large-ish non-electrolytic capacitor -- 100 to 470nF should do. This will narrow the fault down to a particular stage. Try replacing the transistor and if still no cure, any associated passives.
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Old 26th Jun 2018, 1:12 pm   #11
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Default Re: Murphy B841A transistor radio

Will do. Besides the 2 ac127s and 1 ac128 replaced also faulty where 3 af116s 1 af115 and 1 af117, will probably be a couple of days before I report back. Thanks everyone for your help help so far.
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