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Old 26th Jun 2018, 11:37 am   #21
ms660
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Default Re: Marconiphone 4351 radiogram popping when turned off

Just in case you're not aware, the polarity of the output voltage from the meter terminals on ohms test is usually opposite to what the meter terminals would indicate, eg: positive meter terminal is -ve voltage out and negative meter terminal is +ve voltage out, also the transistors in the output stage of your unit are a mixture of PNP and NPN types, you might also get misleading indications when try to measure the resistances of components when they are still connected in circuit.

I would go with Post#13 first.

Lawrence.
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Old 26th Jun 2018, 12:02 pm   #22
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Default Re: Marconiphone 4351 radiogram popping when turned off

Hi again
Here is the readings taken this morning , I have taken the reading using the
meter probe positive as shown, the reading seem consistant across both pairs of transistors, so does this mean they may be OK and should I look elsewhere on the board?
I hope I am not being a pain to you all
many thanks
Frank
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Old 26th Jun 2018, 12:06 pm   #23
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Default Re: Marconiphone 4351 radiogram popping when turned off

When dealing with suspect germaniums in this kind of kit, it usually pays to unclip them one at a time from the heat sink then briefly power on and test. This should reveal which if any have an internal tin whisker short to case. If this is inconclusive you will need to measure the voltages at the legs.
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Old 27th Jun 2018, 1:46 am   #24
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Default Re: Marconiphone 4351 radiogram popping when turned off

Quote:
Originally Posted by frankw678 View Post
Here is the readings taken this morning , I have taken the reading using the meter probe positive as shown, the reading seem consistant across both pairs of transistors, so does this mean they may be OK and should I look elsewhere on the board?
On the top transistor you have put a 12. Then on it's sister you have put 13.5.
So that does NOT match up.
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Old 27th Jun 2018, 5:48 pm   #25
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Default Re: Marconiphone 4351 radiogram popping when turned off

thank you , Grubhead but you did say in post no 16
"if the all get roughly the same readings then they are OK."
could this 13.5Ohm reading point to a faulty transistor
perhaps i should buy a better multimeter and try the diode test setting to check voltages
as I saw on youtube yesterday, what do you think?
am i getting out of my depth
frank
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Old 27th Jun 2018, 5:57 pm   #26
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Default Re: Marconiphone 4351 radiogram popping when turned off

I would suggest taking VOLTAGE readings with power applied. You should be able to do this from the component side of the board, but sleeve all bar about a millimetre of your probe to prevent shorting of the transistor leads.
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Old 27th Jun 2018, 8:17 pm   #27
frankw678
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Default Re: Marconiphone 4351 radiogram popping when turned off

Thank you Graham
will do ,I assume I am looking for difference in voltages or no voltage at all
I will sleeve as suggested
once again many thanks, I hope I am not keeping this posting running too long

Frank
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Old 27th Jun 2018, 10:26 pm   #28
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Default Re: Marconiphone 4351 radiogram popping when turned off

Post your readings on here and you find plenty of members who'll analyze them for you and make a diagnosis.

We really like helping those who help themselves.
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Old 29th Jun 2018, 8:06 pm   #29
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Default Re: Marconiphone 4351 radiogram popping when turned off

Those black cased electrolytic capacitors to the right of the P/C board are ripe suspects in all Thorn products They are often marked 'Callins Eire'. They can go O/C or leaky. Note all other styles of Callins capacitors are very good quality. It is just these black case types that are a problem. J.
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Old 13th Jul 2018, 5:47 pm   #30
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Default Re: Marconiphone 4351 radiogram popping when turned off

Hello again, sorry for the delay, I have finally tested the voltages from the two output transistors AC141 and AC142 (possibly)
I don't know if this is correct way to test them but I have tried the recommended Diode multimeter testing setting and this does not seem to make any sense with the transistors in place.
I have obtained some new /old transistors and intend taking the old out and testing out of circuit.

voltages tested with the set on.

transistor 1
ground to B = 4.68v
ground to e = 4.81v
ground to c = 13.1v

transistor 2

G to b = 4.69v
G to e = 4.50v
G to c = 5.6 v

many thanks Frank
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Old 15th Jul 2018, 7:25 pm   #31
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Default Re: Marconiphone 4351 radiogram popping when turned off

Frank on your post 18 you posted a circuit of the device. This had the correct voltages on them. It's a bit small to see, but if you have the original you should be able to see them and compare them with your own.
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Old 16th Jul 2018, 8:02 pm   #32
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Default Re: Marconiphone 4351 radiogram popping when turned off

Hello again Grubhead,
thanks for the reply post
when I look closly at the circuit diagramm I can see 10.5v between Gnd and the two resistors betweeen T12 and T14 but cant see any voltages for gnd C & E.

sorry to be a pain again but I am not giving up on my gramm I have been doing more ressearch on the web regarding testing transistors and tomorrow I intend pulling the base connection out and testing again with Diode setting on analogue MM, I also have a new DMM with singnal trace function which I ma going to try out, I have enough capacitors to renew all of those , albeit slightly higher values and all electrolytic. and I have some new AC141k and AC 142K
transistors which check out as working. so any more info or help would be much much appreciated.
last week I called to see Mr Birkett the famous seller on the straight where I have been going since I was a boy. He has a supply of germaniums 141 but can't remember where he put them, but I think he is now in his 90's bless him and still opens the shop with assistance as his sight is nearly gone...
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Old 23rd Jul 2018, 5:34 pm   #33
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Default Re: Marconiphone 4351 radiogram popping when turned off

Well i bit the bullet and removed the base leg of the two germanium transistors, tested them with the diode setting on a DMM and they tested working , same values as new/old stock ones. So put back the legs.
Next i removed the black 'callins Eire' capacitor, as suggested in post 29.
This tested ok with an analogue MM on ohms it charges and leads swappped it showed a voltage discharge. So i tried a replacement electrolytic but it made no difference, so i put back the original one. Next i removed the small yellow capacitor and put in a new Electrolytic but still not working so I tested the old one and it showed charge and discharge so i put that back. Next i took out the large electrolytic near the power transistors and replaced that with a new on. No change, and the old one showed no fault so i put that back in.
So i am stumped , does anyone have any more suggestions ?
Anyone know of an engineer in the midlands area who repairs amps.
Best wishes ... But not very hopeful.....Frank
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Old 23rd Jul 2018, 7:54 pm   #34
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Default Re: Marconiphone 4351 radiogram popping when turned off

Did you carry out the test I suggested in post#23 , i.e.e unclip the transistors from heatsinks and try it?
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Old 24th Jul 2018, 7:04 pm   #35
frankw678
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Default Re: Marconiphone 4351 radiogram popping when turned off

Hi Ben Yes I tried that just after it was posted, No effect at all, also I suppose if 'tin whisker' was present it would have shown a "short to case" in my later testing of the transistors. But thank you for the advice.

I begin to think I may have to look for a small separates amp that will fit in the spare compartment of the gram and wire it to the deck and speakers.
AT present I have the stereo from the deck shorted and passed through the one remaining channel, i.e. MONO, but it is still a great sound. I did try connecting both speakers in series to the one channel but lost a lot of quality.

once again cheers for the reply, do you know anyone in the midlands area who repairs transistor equipment?

Frank
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