Marconiphone 4351 radiogram popping when turned off

[frankw678]

Hi I have a 10970's Marconiphone Radiogram No 4351
with a type 50 Chassis and standard BSR deck
Everything works fine except this small thing, and I wonder if anyone can explain or describe why it happens or if anything can be done.
The fault (if it is one) happens whenever I switch off the gram using the one/off/tone switch there is a loud-ish pop from the speakers. Also when the BSR deck turns itself off after playing a record there is an even louder pop or thump from the speakers. It worries me that some damage may be done by this surge like effect.
I have researched the net, I have purchased a circuit diagram and have bought a replacement smoothing capacitor to replace the large can capacitor on the DC side of the mains (4700mF 25V) but have not touched it yet as I do not know if that is the trouble? Can anyone help or advise please. I am quite good with the soldering iron and have repaired a few Roberts radios with badly soldered joints etc. but iI don't want to ruin what is a very good Radiogram by meddling with something that is normal behaviour for a gram of this age.
Many thanks ....Frank

[Edward Huggins]

The switch off from the motor can create an arc, so some decks (not BSR) used a "Switch Click Suppressor" to cure this. The other Popping might be a slow capacitor discharge as the amp dies away. None of this is liable to cause harm.

[frankw678]

Many Thanks Edward,
I have just been reading a previous post about this which mentions a 'quencharc' and suchlike suppressors . I think I shall try a capacitor across the primary of the main transformer first , then perhaps a capacitor across the deck auto/off switch.
I'm sure i will get it in the end, thanks for letting me know it is not a serious issue.
Frank

[paulsherwin]

You may have some dirty/worn switch contacts which will make the problem worse.

As Edward says, unlikely to burn down your house.

[Bookman]

To put you at further ease my Quad 405 does the same thing when switched off at the mains albeit its not a pop but more like a 750 millisec electronic skyrocket going off. It's not particularly loud so it's left as is.

[Edward Huggins]

Originally Posted by frankw678

Many Thanks Edward,
I have just been reading a previous post about this which mentions a 'quencharc' and suchlike suppressors . I think I shall try a capacitor across the primary of the main transformer first , then perhaps a capacitor across the deck auto/off switch.
I'm sure i will get it in the end, thanks for letting me know it is not a serious issue.
Frank

You need to put the suppressor across the turntable mains on/off contact switch on the underside of the deck. Use a well rated capacitor c.350V.

[julie_m]

There are two things going on.

When the power to the amplifier is switched on, it takes some time for all the various capacitors to charge up; the biggest being the loudspeaker coupling capacitors, which have to charge up to half the amplifier's supply voltage. As this capacitor charges, it forces the speaker voice coil suddenly to one end of its travel; then it returns gently to its resting position. This creates a loud thump. If the DC conditions in the rest of the amplifier still haven't stabilised, all sorts of strange noises may be heard until they do. When the amplifier is turned off, it continues to run for awhile on the energy stored in the power supply capacitors and again, strange effects can sometimes manifest as the voltage gets too low for things to work.

The other effect is the motor switching. As the current is interrupted, the magnetic field collapses, trying to maintain the current. This creates a high voltage which eventually jumps the gap as a spark, releasing a burst of broadband radio-frequency noise which is practically certain to interfere with something. Most probably the tonearm wiring, even although it is deliberately mounted far away from the switch.

You can't do much about the first, except that replacing capacitors may reduce any tendency for instability at switch-off. It's only for a moment at a time anyway. For the second, you need to wire a 100 ohm, 0.6W resistor and a 100nF class X capacitor in series, and then connect the free ends of the resistor and capacitor to the switch terminals.

EDIT: Crossed with Edward.

[mark_in_manc]

I recently repaired one similar to this for a friend, which I think uses the same plastic chassis. Hang on to your smoothing cap (or, indeed, fit it) - apparently this cap failing is a common fault on these, and indeed on my mate's one the cap had failed, bulged and pulled itself off the board. The fault then is very substantial (!!) humming through the speakers, as the DC supply to everything ends up... somewhat ripply...

[frankw678]

Thank you all for the replies, I really appreciate the help.

I await delivery of several values of suppressor and intend to try suppressing the gram motor switch first as I have noticed that if I turn from gram to radio before switching off then no pop is made. This would surely point to the gram motor field collapsing and causing the surge. I will try a 0.1uF class X cap across the contacts of the deck switch first and then from the out to neutral if that does not work. I will be in touch as soon as I have results.

Frank.

[frankw678]

Final report

Many many thanks to all replies, especially Julie_m .
I first tried 10nF across primary of the mains transformer but no difference,
Next made up the 'snubber' as Julie advised..
100 ohm, 0.6W resistor and a 100nF class X capacitor across the switch contacts, with perfect results, NO POP!

Once again, best wishes and thank you to all.
Frank

[Techman]

Good news, and well done. It looks very nice stood there, with the bottom of its legs in those proper 'cups' to protect the floor.

[frankw678]

Disaster!

just as everything was going well,after a week of perfect playing,
the right channel has stopped working,
severe disappointment ......

tried swopping the right speaker connection ( in multi plug) over to left channel and it works fine so right speaker and cable fine therefore, I think,it must be an amplifier fault.

I have noted down all the capacitor values whist chassis was out and am now considering replacing all caps, but I am having great difficulty finding the correct values, e.g. all 18v 750mf, 8mf, 400mf 150mf.

How close does the replacement have to be to the original value
I have found on the web 1000mf, 10mf 450mf 200mf will they do to replace ?
how about voltage as some are 250v and some are 20v does it matter, as long as they are above the originals?
any advice would be greatly appreciated , also if there anyone in the Midlands area repairs electronics?

[Station X]

This player is transistorized and I would suggest that wholesale replacement of capacitors won't cure your fault and may even make things worse.

I'd start by taking voltage readings and comparing the results with the working channel and the service sheet.

[Grubhead]

Station X is right, it's more likely to be transistor fault.
But if you do need to replace capacitors, you can replace capacitors with higher MF than the originals. Again the same with the voltages. In most cases it's space to fit them, when they get bigger, but modern capacitors are generally smaller than the originals.

[frankw678]

Thanks for the quick reply chums, It looks like professional attention is required as I don't know where to begin checking voltages. The chassis 50 has a considerable number of components (see picture) including radio and stereo amp. I will remove the chassis again and get it on the bench with my multimeter and try. but it looks like I need a technician!

many thanks

Frank

[Grubhead]

You just need to identify the right hand channel, as the left is identical. I would start with resistance tests on the four transistors mounted on the heat sink. If the all get roughly the same readings then they are OK. However if you can identify one or more on one side with odd readings, then they could be faulty.
Set the Ohm meter to 1K with no power applied to the unit to do the test.
And since the fault is in the right channel, you can ignore the components on left side of the board. The fault should be near those mounted on the heat sink.

I would have thought many repair shops would turn the repair down, saying they couldn't the parts, or wanting an arm or a leg for a transistor that might cost £1 to buy.

[Edward Huggins]

The regretable fact is that most, if not all, repair shops are unlikely to take this on. Also a bulky Radiogram will certainly deter. You can seek out help, and save the risk of being ripped off, by posting for help here via this Forum.

[frankw678]

Thank you Grubhead for the advice I will follow it and try the resistance of what I suppose are the power transistors, On the circuit diagram they are just marked as VT12, VT14 or OP3A and OP3B I have identified that the right hand channel is the bottom most components on the board have been looking on the web for modern equivalents and see that it is a minefield for the beginner but first step first ..I will try the resistance test first and report back asap..
I would hate to loose the gram now i have got used to it being on all day
best wishes and thanks again
frank

[Grubhead]

The codes OP 3A and OP3B are the BRC/Thorn Company codes for Transistors. They are found in a Volume of Radio and Television Servicing from the 70's. I can't say which, but they are on pages 840 to 844. I photocopied them at my local library.
OP3A are AC142K
OP3B are AC141K
If you can't find the pages I can scan my old ones.

[frankw678]

Good morning Grubhead , thank you for the info, I am just testing the transistors now,
but keep getting different reading, I am using an old GPO multimeter which has a 2K setting which I am using, The transistors are all in line so assuming it is base in the centre of the three terminals do I use the positive probe on the centre terminal and then check the resistance each side of that with the negative probe or do I check across the outer two terminals, sorry this is probably basic knowledge but I thought I had better check with you.
I have discovered which channel is which
I have sent an image of the board as seen from the back
hope you can help

thanks
Frank