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Old 9th Jun 2019, 11:18 am   #1
Malcolm T
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Default Marconi 2018 problems

I have a Marconi 2018 and experienced a problem with the unit over a year ago so most detail i have forgotten !
I would have to toggle the on off switch several times before it would display , granted the switch feels dodgy but then the unit just went flat dead.
I investigated the PSU board and checked around the rectifier blocks 2KBB20R
of which there are three, the suspect one here is D2.
D2 had a brown pcb heat stain under it like its been ongoing for some time also on one side of D2 rectifier has kind of bubbled up almost likes its been cooked, could this be the cause.
Are there any known problems with the PSU board documented on the forum please.
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Old 9th Jun 2019, 11:32 am   #2
MrBungle
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Default Re: Marconi 2018 problems

Yes I lost a 2019A entirely due to a power supply failure. It went in/out short and blew everything digital in the entire unit. I would check the regulators and capacitors carefully!
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Old 9th Jun 2019, 12:34 pm   #3
Malcolm T
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Default Re: Marconi 2018 problems

You mean everything got unregulated high voltage and took out the ICs ! ?
This was a freebee piece of kit but I'd hate to see it go South!
',m not having much luck lately with equipment!
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Old 9th Jun 2019, 1:12 pm   #4
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Default Re: Marconi 2018 problems

Yes unfortunately. I had to go for a brisk walk when that happened. Must have walked about 10 miles by the time I had calmed down!
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Old 9th Jun 2019, 8:40 pm   #5
Malcolm T
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Default Re: Marconi 2018 problems

Thanks again for reply
HMMM , that doesnt sound too good then, i dont fancy walking 10 miles here LOL.
Not much i can check then before the PSU board is repaired and hope for the best, got any bright ideas then ?.

The 15,000uf off the board may have been bust , i cant find it so may have threw it already . It is a four pin type , BUT i do have the same value in two pin style with a higher voltage rating , not sure if i fully understand the 4 pin deal though ? The ones i have here are slightly physically larger so may be possible to fit to the board with some PCB drilling or firmly secure and insulate and run jump leads.
How do i fit a 2 pin Electrolytic to a 4 pin Electrolytic PCB application ?.
As for rectifiers i have some , will have to check values or go for four diodes .
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Old 10th Jun 2019, 7:55 am   #6
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Default Re: Marconi 2018 problems

Assuming the electrolytic is a single unit not two capacitors in one can, the extra two pins are for mechanically fixing the capacitor to the pcb. They should probably be left o/c.
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Old 10th Jun 2019, 9:41 am   #7
Malcolm T
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Default Re: Marconi 2018 problems

Right thanks , will check the circuit diagram.
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Old 11th Jun 2019, 3:27 am   #8
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Default Re: Marconi 2018 problems

Hi Malcolm
I had a little PCB overheating around D1 on my 2018 PSU PCB, which I believe is a common finding with these units.
I used a replacement bridge type KBU604 and remotely wired it to the rear panel, to save the PCB, and to act as a partial heatsink, and I screw mounted it with a dab of thermal paste.
Applicable to the other diode bridges if heat affected.

Refer also to the Yahoo Groups Marconi Forum for much other info.

On the balance of comments by Mr Bungle, I may revisit the unit and rethink some added form of PSU protections.
Walking is no problem, but I need the sig gen for years to come.

Good luck, these are good sig gens.
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Old 11th Jun 2019, 7:21 am   #9
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Default Re: Marconi 2018 problems

Yes if I owned another one I’d probably stick a crowbar in it!

After consulting with someone at the time it appears that the failure in mine was likely caused by either the LM350K giving up or the setting divider. The precise root cause would have been nice to find but I didn’t look too deeply into it. The 9v unregulated line was at 12 volts and was present on the 5V line which is what killed it all. My excessively high mains voltage here probably was the contributing cause. I’d expect a piece of equipment with that much expensive stuff in it to have a crowbar on the power supply though. A 8 regulator can cost a 500 signal generator.

A TL431, SCR and a fuse (consult art of electronics 3e chapter 9) would be an inexpensive safety precaution.

My only consolation was I only paid 10 for the 2019A off someone on gumtree and it went on to help someone who had a pretty beaten up one with a burned out attenuator because some numpty had decided to transmit into it!
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Old 11th Jun 2019, 8:31 am   #10
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Default Re: Marconi 2018 problems

Hi Mr Bungle
Thanks for suggestions.
For the want of a nail ....

I have been curious about your Forum Name for quite a while.
Care to enlarge, or best left in the crypt?

Malcolm,
replace that suspect Power Switch while you have the covers off.
Don't disturb those LCD displays; now unabled to be sourced anywhere.
Rocking Horse poo.

R50
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Old 11th Jun 2019, 8:52 am   #11
Malcolm T
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Default Re: Marconi 2018 problems

Thanks fellas.I'm going to do what you suggested and hope for the best.

And on the capacitor side, the circuit diagram shows schematics symbols for a single can not a double electrolytic 4700uF, I have just noticed some writing on the PCB, pin 1 is + and pin 5 is unmarked but there are only four pin on the cap??

The other cap has writing on the can, 1=15000uf and 5=- (minus sign). Is pin 5 the can?
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Old 11th Jun 2019, 9:01 am   #12
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Default Re: Marconi 2018 problems

On “MrBungle”; The character bungle was on children’s TV programme Rainbow in the 70s. He was a mostly incompetent bear who managed to wallpaper over a door at one point and was generally quite a grumpy character. I got the nick name in real life a number of years ago from my other half after getting grumpy over putting way too much of a scientific effort into installing a carpet and ending up with a rug instead

I’m not so sure about those LCDs being impossible to replace. You can get basic plain LCD modules these days with that right number of digits. I’m sure some work in a PCB CAD package may end up with a fruitful replacement. One of the good things about the 2019A is there isn’t much that isn’t replaceable!
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Old 11th Jun 2019, 9:22 am   #13
Malcolm T
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Default Re: Marconi 2018 problems

You're showing your age now Bungle. I like the carpet story LOL. I had my other half a couple years ago sorting out capacitors and resistors and labelling up with values in Jiffy bags on rainy days, I kept reinforcing the idea it was moving things forward toward a tidier house and for tidying up my DIY stores and it worked, but she has flat refused to learn basic electronics and soldering, but she's good in the kitchen so I can't grumble !!!!!.
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Old 11th Jun 2019, 9:48 am   #14
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Default Re: Marconi 2018 problems

My staff apparently had a nickname for me ..."Sandpaper".
"A bit rough with people".
They only told me on my last day at work.
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Old 23rd Jun 2019, 11:34 am   #15
Malcolm T
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Default 4 pole electrolytic in Marconi PSU

Well back on the task of problem solving here . I'm trying to trace the PCB circuit of the Marconi 2018 sig gen duff PSU board .

Can someone explain to me the reasoning behind a 4 pole cap on this board when the circuit diagram shows a normal symbol cap please ? and three of the pins are connected together to the + supply of the rectifier bridge o/p, the other pin I assume negative looks like it goes to the LM317 heatsink tab, bit confused here, any help appreciated.
Can i replace the 4 pole with a 2 pole cap.

Many thanks
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Old 23rd Jun 2019, 2:12 pm   #16
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Default Re: Marconi 2018 problems

It's just a normal electrolytic, but I'm assuming the multiple pins are for security of mounting (ie support).
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Old 23rd Jun 2019, 2:31 pm   #17
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Default Re: Marconi 2018 problems

Yes that is the case in these if I remember correctly.

Note that if you replace it you need to make sure all the pins are bussed together properly as a lot of the PCB layouts, Tek especially, use the capacitor case to bridge chunks of the circuit.
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Old 23rd Jun 2019, 3:12 pm   #18
Malcolm T
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Default Re: Marconi 2018 problems

Oh no it just gets worse , looks like it was a clever idea then but today no way , probably a dumb design. Looks like its back to paper and pencil and sketch it through then .
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Old 23rd Jun 2019, 4:36 pm   #19
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Default Re: Marconi 2018 problems

Quote:
Originally Posted by MrBungle View Post
as a lot of the PCB layouts, Tek especially, use the capacitor case to bridge chunks of the circuit.
Just a trick to avoid some common impedance in the charge and discharge loops and so reduce the ripple voltage hitting things downstream a little.

There were special 4-terminal capacitors with two sets of connections into each foil as a step further down this path. Also 4-terminal resistors for precision applications.

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Old 23rd Jun 2019, 5:37 pm   #20
chriswood1900
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Post Re: Marconi 2018 problems

The info you need should be here:-
http://bama.edebris.com/download/mar...Schematics.pdf
The psu is pretty straight forward and for safety build a crowbar with a suitable fuse to protect the digital circuits. One thing I have found on several occasions with these units is the plugs and sockets on the boards need cleaning and reseating and this has removed problems with the display and booting.
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