Issue 4a docs needed
Re: Issue 4a docs needed
I'll hoike out the coil next, then
- 1024MAK
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Re: Issue 4a docs needed
A coil with shorted turns tends to kill TR4 (as in cooks it until it goes short circuit)...
What does TR4 show when tested? Or have you binned it?
One other test before removing the coil. Is your current limited supply (is it a bench adjustable supply? If you have already said, sorry, please remind me...) capable of 12V with a current limit of between 100mA and 166mA? If yes, what happens if you use it to power up the +12V rail ONLY. That is, don’t power up the +9V or the +5V. Obviously the 4116 DRAM chips should not be fitted.
What current is drawn from your bench PSU when connected as described?
Also test the diodes in the +12V rail and the -5V.
Mark
What does TR4 show when tested? Or have you binned it?
One other test before removing the coil. Is your current limited supply (is it a bench adjustable supply? If you have already said, sorry, please remind me...) capable of 12V with a current limit of between 100mA and 166mA? If yes, what happens if you use it to power up the +12V rail ONLY. That is, don’t power up the +9V or the +5V. Obviously the 4116 DRAM chips should not be fitted.
What current is drawn from your bench PSU when connected as described?
Also test the diodes in the +12V rail and the -5V.
Mark
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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Re: Issue 4a docs needed
Hi Mark,
Haven't had a chance to look into this further, but TR4 shows fine when tested.
The bench power supply allows me to set voltage and current limits - is there anywhere you suggest I inject 12v, and should I remove anything first from DC-DC converter stage to avoid issues?
The only diode that I tested in circuit that looks funky is D15, so before i go injecting anything, i'll unsolder one end and retest that.
What is the logic of the 12v/100mA test? Presumably if it trips the current limiter, then it's something on the 12V line shorting to ground?
Haven't had a chance to look into this further, but TR4 shows fine when tested.
The bench power supply allows me to set voltage and current limits - is there anywhere you suggest I inject 12v, and should I remove anything first from DC-DC converter stage to avoid issues?
The only diode that I tested in circuit that looks funky is D15, so before i go injecting anything, i'll unsolder one end and retest that.
What is the logic of the 12v/100mA test? Presumably if it trips the current limiter, then it's something on the 12V line shorting to ground?
- 1024MAK
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Re: Issue 4a docs needed
This:
Mark
Yes, to see if there is something else shorting out the +12V supply.
Mark
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ZX81 Chip Pin-outs
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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Re: Issue 4a docs needed
I've checked all my diodes, and they're good.
Where would you reccommend injecting 12V? Positive side of C44 goes to the +12V DRAM supply, Positive side of C45 goes to +12VA(Analogue supply) (or either side of R62)
Now to try and find some spare banana plugs to make up a jumper cable...
Where would you reccommend injecting 12V? Positive side of C44 goes to the +12V DRAM supply, Positive side of C45 goes to +12VA(Analogue supply) (or either side of R62)
Now to try and find some spare banana plugs to make up a jumper cable...
Re: Issue 4a docs needed
OK, went +ve to positive of C44, and ground to negative of C45
Both at 100mA and 130mA, the current limiter of the bench supply is tripped.
So it would appear then I have a short on the 12V line
Looking at the schematics, the stuff I ought to be looking at are:
IC6-13 (already removed, currently have sockets)
C1-C8
Then starting with IC14 (LM1889), C48, R42... dunno how much further past there i need to go
I seem to recall someone on Youtube suggesting if all else fails, removing components in such a way to split the remaining circuit in half to see if the short remains
Thoughts?
Both at 100mA and 130mA, the current limiter of the bench supply is tripped.
So it would appear then I have a short on the 12V line
Looking at the schematics, the stuff I ought to be looking at are:
IC6-13 (already removed, currently have sockets)
C1-C8
Then starting with IC14 (LM1889), C48, R42... dunno how much further past there i need to go
I seem to recall someone on Youtube suggesting if all else fails, removing components in such a way to split the remaining circuit in half to see if the short remains
Thoughts?
- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
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Re: Issue 4a docs needed
Lifting one leg of R62 will split the +12VA (analogue) supply from the main +12V supply.
C1 to C4 are for the -5V supply, C5 to C8 are on the +12V supply.
Mark
C1 to C4 are for the -5V supply, C5 to C8 are on the +12V supply.
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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ZX81 Chip Pin-outs
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
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Re: Issue 4a docs needed
Having cocking the leg of R62 (and confirming +12VA was now seperated), I tried injecting as before +ve to +ve of C44 and gnd to -ve of C45
12v at 130mA still tripped the supply
Would I be right in thinking that if I were to lift R59 and the -ve lead of C44, this will completely isolate the DC-DC converter from the rest of the board supplied by 12V?
Would this be a sensible next step to seeing where the short is?
12v at 130mA still tripped the supply
Would I be right in thinking that if I were to lift R59 and the -ve lead of C44, this will completely isolate the DC-DC converter from the rest of the board supplied by 12V?
Would this be a sensible next step to seeing where the short is?
- 1024MAK
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- Joined: Mon Sep 26, 2011 10:56 am
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Re: Issue 4a docs needed
No.
If R59 is okay (1.8kΩ, although an in circuit test will produce a lower result) then that will only let through less than 7mA at most.
C44 negative (-) lead being connected or disconnected (unless C44 is short circuit itself) will make no difference to a DC current flow.
It now looks like either one of C5 to C8 is faulty, or there is a short circuit between one of the +12V tracks and a 0V/GND track. So I suggest a good visual examination of both top and bottom of the board using a good light and a magnifying glass. Also have a good look at C5, C6, C7 and C8. If any of them look to have any visible damage, remove the damaged one(s).
If you still have the fault, then remove C5, C6, C7 and C8 one at a time.
Note: all of the above assumes that diode D15 is good and not short circuit, that none of the 4116 chips are fitted, the modification that is shown here has been carried out correctly and that resistor R62 still has one leg lifted.
Mark
ZX81 Variations
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ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Issue 4a docs needed
(Reminder TR4 is still out of circuit)
Removed C5-C8 - short remained
Disconnected R59 and C44 - current draw went to zero
Replaced C5-C8, current draw still zero
Suggests to me the issue still lay in the DC-DC converted board somewhere....
Removed C5-C8 - short remained
Disconnected R59 and C44 - current draw went to zero
Replaced C5-C8, current draw still zero
Suggests to me the issue still lay in the DC-DC converted board somewhere....