marconi 264 caps and PSU
 

I'v managed to get a service sheet for my marconi radio.

I'v kind of poked about in a few places. The main 8uf smoother cap reformed, but the 4nd HT smoothing cap is very leaky. So now i think it's time to change some caps.

However...i'm pretty puzzled by the PSU. I'v looked at the schematic, and from that i could make out that the psu looks like this.

http://img.photobucket.com/albums/v4...ok/qmrkpsu.gif

This is really strange. I dont understand why there is a choke between shassis earth and the center tap on the transformer.

Before starting to take the radio appart and replacing caps, i would like to at least understand abit how it works. Can anyone explain the psu?

Re: marconi 264 caps and PSU
 

Hi Adi,

The transformer has a centre tap because the rectifier valve is Full Wave and is 2 diodes rectifying both cycles of the output of the transformer.

250
|---------->|----------------| +
| | |
|----|0v ----
| | | -----
|-----|---->|-- |
250 |____________________| -

Sorry about the diagram (tricky doing circuits like this !!). On the plus rail you would normally find the choke and another cap, however if the chassis is above 0v for biasing purposes it is easier to put the choke in the -v rail (the effect is the same as far as power supply smoothing is concerned).:o
I maust admit to being fooled by this type of layout before and assuming that the choke is in the +ve rail and it turned out to be -ve (very warm and angry smoothing caps !!! check carefully these can be very messy and smelly and violent if they go bang !!).:angry:

All the best,
Alan.

P.S. Hope this makes some sense.:thumbsup:

Re: marconi 264 caps and PSU
 

Sorry , just seen my wonderful diagram totally rejigged and definately now makes no sense !!!!!>((
Check out a full wave rectifier using 2 diodes and a centre tapped transformer and compare it with you diagram Adi.

Alan.:-/

Re: marconi 264 caps and PSU
 

Hi Adi, If I remember correctly, L20 is actually the energising coil of the loudspeaker in these sets, (No permanent magnet), The HT current passes through this coil which also acts a a smoothing choke. The -ve grid bias is obtained from the voltage drop across the coil, so the output valve cathode goes to chassis without the usual bias resistor. :) .

Regards, Mick.

Re: marconi 264 caps and PSU
 

Adi,

Here's my re-drawing of the circuit.

It's standard full-wave rectification using a centre-tapped transformer - modern equipment would use the same basic circuit, but with 2 silicon diodes instead of the valve.

Nick.

Re: marconi 264 caps and PSU
 

This is quite a common sort of circuit on older radio's Adi. If the electrolytics are of the aluminium can type, you'll probably find that one of them (C22) is insulated from chassis. Make sure it remains insulated from chassis or there will be no grid bias developed across the two resistors :o.

Dont be tempted to increase the value of the reservoir capacitor (C21) or it could place undue stress on the rectifier.



Rich.

Re: marconi 264 caps and PSU
 

oooh..riight.

So it's like a normal ht circuit..but up side down...the smoothing choke is on the -ive rail. Wierd. But i guess that makes sence.

Ok... now i'v got that figured out...i need to do somethgin about the caps.

So far i can see several ways of dealing with the cap situation.

The caps are in a block of tar under the main circuit (which holds resistors).

I can tag every wire with masking tape and a number, and desolder them from the resistor holder. Then drill out the rivits on the resistor holder and take it off. Then remove all the caps from the tar block (and maybe rip out all the tar?) and install new caps underneath the resistor holder.

BUT...i'm wandering if i can do it another way. Modern caps are tiny compared to 30's caps. maybe i can just leave the circuit...and the tar block...in place, and just cut all the connections to the tar block. And just add modern capacitors to the circuit?

any ideas?

BTW...how did they manage with so little smoothing?

Re: marconi 264 caps and PSU
 

Either way is fine, but it might be difficult to connect all the caps neatly without using the old block to house them.

As you've said, you must disconnect the old ones, even if you leave them in place.

I think heating the tar would allow you to remove everything without resorting to brute force... maybe boiling water would be hot enough???

Nick.

Re: marconi 264 caps and PSU
 

Not much HT current. For a given ripple voltage the capacitor you need is roughly proportional to the HT current. Also a choke and 2nd capacitor make a very good smoothing network.

Re: marconi 264 caps and PSU
 

I'm sure that melting out some tar would not be a big challenge to Adi. Rememeber he also collects blow lamps:) If boiling water isn't enough then a hot air gun should be OK. The domestic oven is not recommended, more for reasons of family peace than practicality.

Re: marconi 264 caps and PSU
 

These pictures show the route that I took for replacing the caps:)

Roger

Re: marconi 264 caps and PSU
 

Hi,

My 264 arrived this morning :D. My first job is going to be replacing the eleven capacitors in the condenser block.

The Trader Sheet (No 577) shows their values, but it doesn't tell you the voltage ratings? Also, with values from 1 to 4uf, I was wondering if I can I replace any of them with electrolytics?

Any advice gladly accepted;)

Many Thanks

David

Re: marconi 264 caps and PSU
 

Hi Dave,
If you do use electrolytics, be sure that the polarisation is correct. One, if I remember needs to be upside down (Chassis positive). However, there is ample room to fit mylar or polyester types in the can and space for 400 to 600 volt items, so who cares about the working voltage?
Nigel

Re: marconi 264 caps and PSU
 

For those who are wondering what all the fuss is about with these block capacitors, here's a picture of my chassis with around 45 disconnected connections!

This was just to remove the, combined, resistor and cap block. Another dozen or so connections had to be disconnected to separate that:o... I hope none of those labels fall off :-)

David

Re: marconi 264 caps and PSU
 

I've done a few of these in various EMI models which used this multi-cap block. The domestic oven on gas mark 4 for about 30 mins does the trick beautifully, but use gloves (gardening or 'rigger' gloves are best) to handle as that tar is HOT and can burn. I then make up the caps on tag strip which is soldered to the base of the tin case, and flying leads to make the connections. I don't usually refill with tar, but cover the tags etc with a piece of paxolin with small holes drilled in the appropriate places for the leads to poke through, then araldited into place

It is a messy, vile job but these chassis work extremely well after the operation..!.....Steve