Issue 3b board

caver99 · March 2016

diode ID5 and cap C47 are missing from my board will these have any effect on system powering up.

I have checked the voltages on the 7508 and its good, the ram chips have +12, +5 and -5 but still strange blocks when start up I am now sure its a bad lower ram chip unless the above components could have something to do with it.

caver99 · March 2016

and I will post pic of board and screen as soon as I can

1024MAK · March 2016

For C47, read the section in this link http://www.1000bit.it/support/manuali/sinclair/zxspectrum/sm/supp1.html#intro

Not sure which one you mean by "ID5"?

D5 is part of the keyboard circuit.

Mark

1024MAK · March 2016

Schematic here http://www.worldofspectrum.org/pub/sinclair/technical-docs/ZXSpectrumIssue3b-Schematics.gif

Mark

caver99 · March 2016

its D15 sorry but the link you sent its part of the DC-DC converter mod so that's ok but I have found a resistor across the 7805 in and out that's burned but measures 332 ohms which from the schematics is does not have one would I be best to replace it or remove it and replace the 7805 as well.

caver99 · March 2016

I have picked up some 27c256s and going to make a diagnostics rom to fit in place or the basic rom chip and see what that tells me

1024MAK · March 2016

A resistor connected between the input and output pins of the 7805 5V voltage regulator? I can only think that someone added this modification to reduce the heat from the regulator. But a 330 ohm resistor will not help much, so I can't see the point :-S

Mark

caver99 · March 2016

so is it best to remove it or add a bigger one all voyages are ok but the resistor does get very hot

1024MAK · March 2016

If it gets very hot, it's not a 330 ohm resistor :-O

It may of course be a lower value, that will then pass more current, and so get hot...

Maybe post a picture...

Mark

caver99 · March 2016

I will take one today at some point, I desoldered one leg and measured it and yes the multimeter is a crap one but definitely a 330 on the meter

1024MAK · March 2016

If the 330 ohm resistor is connected between the input and output terminals of the 7805 regulator, the input should be between about 10.5V to 12.5V. So let's say it's 12V.
The regulator output should be 5V.
So 12 - 5 = 7 volts across the resistor. 7V / 330 Ohms = 21.21mA
Power dissipated by the resistor is 7V x 21.21mA = 148mW. Most small wire ended resistors are 1/4W rated. So should have no trouble with this power level.

Mark

caver99 · March 2016

thanks 1024Mak for that I wasn't able to get to my other office today but will be there tomorrow so will take some pics of board

caver99 · March 2016

the resistor to the right is the one I am on about

DEATH · March 2016

You know it's not supposed to be there, right?

:)

caver99 · March 2016

yes and I have asked should I remove it, all the voltages are correct she the 7805 is working right

will I do any harm leaving it or removing it

1024MAK · March 2016

Remove it.

Also, it may now read 330 ohms, but it may have started as a lower value.

The whole point of the 7805 and the attached heatsink is to correctly produce nice smooth noise and ripple free DC +5V supply. Adding a resistor like someone has done, is not really helping one bit.

Mark

caver99 · March 2016

no I thought something like that, I will disconnect it and check the voltages again to just make sure all is as it should be

DEATH · March 2016

Take off all the bodger components, and re-build the circuit as per the circuit diagram. That way you don't have to guess. When you try to repair something that a bodger has had a go at, you have no reference circuit to work from.

caver99 · March 2016

as far as I can see that's the only one the other ones on the top of the board are from the factory

1024MAK · March 2016

Still wise to check. Also print out a copy of the schematic. Then you can work your way through the circuit, marking each component as you find it.

Just be aware that parts of the circuit that form the switching regulator [for the +12V, the -5V and the "~12V" (a AC voltage)] have changed as Sinclair improved it. Your board may, or may not have been partly, or fully updated. So watch for this.

Mark

DEATH · March 2016

caver99 wrote: »

as far as I can see that's the only one the other ones on the top of the board are from the factory

That exactly is my point "As far as I can see". You're still having to guess! Remove the burned resistor across the coil and that 100R one on the bottom too, then check again.

caver99 · March 2016

ah right I see I didn't think of that other one on the bottom of the board thanks I will have a look on Monday

caver99 · March 2016

1024MAK wrote: »

Still wise to check. Also print out a copy of the schematic. Then you can work your way through the circuit, marking each component as you find it.

I have checked with the schematics and the mod on the top is part of there upgrade with the cap moved and diode removed

[quite=]Just be aware that parts of the circuit that form the switching regulator [for the +12V, the -5V and the "~12V" (a AC voltage)] have changed as Sinclair improved it. Your board may, or may not have been partly, or fully updated. So watch for this.

Mark[/quote]

when I remove the resistors under the board I will check all the voltages again to make sure they are correct and see why the have fitted the 100r on the bottom and boron the top as the components should be

caver99 · March 2016

I have removed the resistor and all voltage's are there
-5v - -4.97v
+5v - +4.49v
+12v - +12.11v
+9v - +10.53

not sure if the +9v is 10 high.

DEATH · March 2016

that 5V rail is a bit low, take out Vreg pin 3 and test the output directly from the Vreg. If still 4.49 volts, change it out. If it reads 5V (plus or minus a few percent) then you may have a near short circuit somewhere, so measure the resistance of each RAM IC (you'll need to remove the chip to test) Measure between the +5V pin and GND, anything suspiciously low, change it out. Do this for the upper and lower RAM chips. If all are ok, get back to us.

1024MAK · March 2016

The +9V is a nominal, unregulated supply, so it varies with different load currents. It's typically between 10V and 12V.

I agree with Death, investigate the low +5V line. A 7805 is rated at 1A, but a normal 48k Spectrum typically uses around 530mA to 620mA on the +9V input (with no expansions connected). Note that there is a slight variation in this current depending on the board issue / the exact chips fitted. For example:
Issue 1 16k byte RAM uses 490mA
Unspecified 48k+ RAM uses 562mA (keyboard fault, but otherwise works okay, so in my repair pile)
Unspecified 48k+ RAM uses 603mA (keyboard fault, but otherwise works okay, so in my repair pile)
Issue 6A 48k+ RAM uses 537mA

Mark

caver99 · March 2016

ops that's my typing sorry the +5v rail is 4.95v not 4.49v sorry

1024MAK · March 2016

No problem :D
4.95V is fine :-)

caver99 · March 2016

I have a feeling its one of the lower ram chips that is faulty, I will get some sockets and some replacements, I was going to put an eprom in with the diagnostic rom on it but I might just buy the smart cart interface now as I would like to keep the original rom in place for authenticity sake

Issue 3b board

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