Help with repair

[Retrocosm]

With regards to the two electrolytic capacitors pictured would someone kindly point me in the direction of some suitable replacements?

Is it simply 1 axial, 1 radial 4700uf 16v, is it worth getting 25v?

Charlie

[Dave]

Charlie,

see here

http://www.primrosebank.net/computers/m ... eplace.htm

The page gives the required values (C & V) and a pointer to Rapid-on-line where DKW sourced his replacements

regards
Dave

[1024MAK]

The capacitors fitted in most machines are rated for 85°C and the one for the 12V supply (C51) is rated at 16V, but should be rated at a minimum of 25V. I recommend using types rated at 105°C where you can get them. If not, buy 85°C type that are rated for at least 2000 hours at 85°C. It has to be said, large axial types rated at 105°C are difficult to find Because of this, I recommend using a capacitor rated at 25V instead of 16V, it should last a little longer

Note that modern replacements are normally smaller that the ones from 30 years ago.

C51
I used: 4700uf 25v 105°C Radial Electrolytic Capacitor Rapid 11-3748 but Rapid have discontinued this type.
They do have these:
4700uf 25v 105°C Radial Electrolytic Capacitor Rapid 11-3531
OR
4700uf 35v 105°C Radial Electrolytic Capacitor Rapid 11-3770

C56
4700uf 25v 85°C Axial Electrolytic Capacitor Rapid 49-2154
OR
4700uf 25v 85°C Axial Electrolytic Capacitor Rapid 11-0160

The 4700uf 25v 105°C radial and the 4700uf 35v 105°C radial types have the correct pitch to fit the PCB.

I have some of this 11-3748 type in stock, but I currently don't have any spare axial types at the moment.

Hope this helps

Edited because Rapid have discontinued type 11-3748. Post edited to list suitable alternatives

Mark

[Retrocosm]

Thanks for this chaps and thanks very much for the links Mark. I think I'll just replace the capacitors as it's relatively easy and then run the tests you outlined and see what happens.

Thanks again,

Charlie

[Retrocosm]

Can I also pick your brains regarding the power output from the transformer? Can you advise how to go about putting a meter on the connector to check it's working correctly.

Charlie

[1024MAK]

Can I also pick your brains regarding the power output from the transformer? Can you advise how to go about putting a meter on the connector to check it's working correctly.

Yes
First, unless you have a suitable DIN socket, you need a steady hand (well, three really!) to hold the DIN plug, and two meter probes...
The transformer outputs are not protected by any fuses on the secondary side, so be careful and don't short anything out.

I originally posted the transformer and pin-out of the DIN plug on a ZX80/ZX81 forum. Dave has since put some "pages" on his web site (along with a better computer generated diagram compared to my scanned in hand drawn diagram).
See here (scroll to the pictures of the PSU and click on the links) and here.

Use the AC voltage range (e.g. the 200V AC or 20V AC depending on which voltage you are trying to measure). This multi-tapped transformer does not have a common as such. Test the voltage between whichever windings you want until you are happy that all the transformer windings are working.

Also note that the off-load (that is with no output connected) voltages will be higher than when a load (the MTX) is connected.

Any further questions?

Mark

[Retrocosm]

That's great, thanks for this Mark.

Charlie

[Retrocosm]

'Test the left hand pin of the 7805 regulator with the red meter lead / probe. If the fuse (FS1, 3.15A) is intact and the electrolytic smoothing capacitor is connected and at least partly working, the resistance should start low and go upwards in value. Resistance should be >500 ohms.'

I've replaced both capacitors and this seems to be the only part of the list of tests you kindly posted that is not responding as expected.

Rather than starting low and going up the reading just stays static?

Any ideas?

Charlie

[1024MAK]

'Test the left hand pin of the 7805 regulator with the red meter lead / probe. If the fuse (FS1, 3.15A) is intact and the electrolytic smoothing capacitor is connected and at least partly working, the resistance should start low and go upwards in value. Resistance should be >500 ohms.'

I've replaced both capacitors and this seems to be the only part of the list of tests you kindly posted that is not responding as expected.

Rather than starting low and going up the reading just stays static?

Any ideas?

Charlie

Well, the readings you get do depend somewhat on the meter that is being used.

When switched to resistance, multimeters output a small voltage on the meter leads. So when you connect the meter leads to a component (like a resistor) or to a circuit, a small current flows. The voltage and the current are internally limited by the meter. From the current and the actual voltage (which may be different once the meter is connected to a component/circuit) the meter can "work out" the resistance of the component or circuit under test. This is nice and simple for a resistor, or a circuit made of just resistors. The resistance is the voltage applied divided by the current flow ( V / I = R, ohms law).

When testing a circuit that contains capacitors, the current from the meter causes an electrical charge to build up on the "plates" inside the capacitor. As capacitors have insulation between the plates, the DC current from the meter cannot flow through the capacitor. As the charge level on the plates increases, the voltage across the capacitor rises. This reduces the voltage difference between the circuit in the meter and the capacitor, so the current reduces. Soon the current is too small to measure. With small capacitors, they charge up so quickly, that it is too quick for the meter to show it, and too quick for a human to see even if the meter was quick enough. However, with large value electrolytic capacitors, it takes a few seconds (there are a number of factors, so it could be a quicker or a slightly longer time) for the capacitor to charge up. The meter will show this as lowish values that very quickly change to a higher value, then often an over-range indication.

If you don't see this effect, don't worry too much. If you meter has manual ranging (more than two resistance switch positions ), try using a higher range. If your meter is auto-ranging, it may not be possible to do this test at all.

I hope this helps

Mark

[Retrocosm]

Ok, thanks for that clarification Mark.

Charlie