Marconi 2018 problems

[Malcolm T]

Thanks for the replies , will check out the link. Something just occurred to me , even though the caps are 4 pins the circuit diagram won't necessarily show this because the pins are connected on the PCB as if just a normal cap, am I right or wrong?

[MrBungle]

Originally Posted by Malcolm T

Thanks for the replies , will check out the link. Something just occurred to me , even though the caps are 4 pins the circuit diagram wont necessarily show this because the pins are connected on the PCB as if just a normal cap , am i right or wrong ?.

That is correct. Lots of circa 1980s devices used that method. I refurbed a philips PM3217 this year and came up with this as a solution which will show you the pin utilisation.



Edit: just realised I’m “necroposting” here. Oops.

[Radio1950]

I have just replaced all Power Supply electro capacitors in my Marconi 2018 Sig Gen.

While I was working on the PSU PCB, I replaced D1 with a RS604 bridge.

And I installed a Crowbar Circuit to protect the components fed from +5 V DC. See Separate Thread.

Capacitors

DC line voltages and ripples measured OK before work started.
Most caps checked OK for ESR, but replaced anyway.

The large cap C1 (originally a single15,000 uF) was replaced by 3 x 4,700 uF 35V.
As this is not seen, visual aspects can be ignored a little.
I used cable ties to hold the trio together, and added a small piece of 2 mm lexan, with lead holes, as an insulating spacer between cap and PCB.
I used two small dabs of hot melt glue to stop cap movement and possible lead fracture due vibration.

The large cap C6 was replaced by a single 4,700 uF 50V.

For MalcolmT -

On these Philips capacitors, pin1 is positive and pin 5 is negative, pins 2,3,4 are not connected to any other pin, and used as cap physical supports only. Photo.

All other caps are easily sourced (although not as axials probably) and replaced.


PSU PCB Diode Bridge D1

In a previous post in this thread, I mentioned that I had “remoted” D1 to the rear panel, as it gets quite hot.
When working on the PSU, I noted that I had made a mistake.
I thought the PCB D1 was the bridge supplying +9V UNREG, but ... there are two D1 bridges.

“Main chassis D1”, and supplying +9V UNREG (and then +5V) is located under the PSU PCB, out of sight, and is bolted to the main steel chassis bottom.
“PSU PCB D1” is originally a 2KBB20R device (2A, 200V) supplying +20 UNREG, and which sometimes discolours the PCB with heat.
I remoted this D1 in error.
I have reversed my error.

I now use an RS604 bridge (6A, 400V) for PCB D1, and mounted on the PCB, after very gingerly enlarging the PCB holes slightly.
I left the legs quite long to assist with the thermal issues.
If you drill the holes, then solder both PCB sides.

For interest “PCB D1” supplies approx 800 mA “DC”, and “Main Chassis D1” supplies approx 2.05A, both currents as read on an averaging DC VOM (like an AVO8).