Vic 20 startup time

[eslapion]

I have not seen a Vic-20 take 15 seconds to start.

I have. 2 Prong PAL VICs take an unusually long time to startup. I have one and it does the same thing.

This was discussed with Shaughn Bebbington on Lemon64.

Try changing these:

Most probable culprit is the big grey one (4,700uf)
Changing little blue one (100uf), is not a bad idea while you have soldering iron out.

I disagree.

With a scope you can clearly see the reset line held low. This can only be caused by the 555 timer. It has nothing to do with capacitor charging time.

Unlike HiFi audio amplifiers, the VIC is not equipped with a delay system that waits for capacitors to charge before activating the system.

[6502dude]

I disagree.

Everyone is entitled to their own opinion.

With a scope you can clearly see the reset line held low. This can only be caused by the 555 timer. It has nothing to do with capacitor charging time.

This statement makes no sense at all.
The 555 is controlled by an RC time constant based on R9 (for trigger) and R14, & C13 (threshold/discharge). All 555 timers are based on capacitor charging (and discharging for astable mode).

If a reset line is being held low by 555 timer for 15 seconds, then either C13 is out of tolerance or VCC has poor regulation.

I disagree.

Again, you are entitled to your opinion

Unlike HiFi audio amplifiers, the VIC is not equipped with a delay system that waits for capacitors to charge before activating the system.

This is another statement that makes no sense.
The purpose of the 555 timer in a Vic-20 is to provide short delay (to allow VCC to stabilize on power up). The 6502 needs about 6 clock cycles of reset being held low before it fetchs cold start reset vector address.

BTW, how the heck did audio amplifiers come into the discussion?
Some amps will use a time delay (often with 555 timer and a relay) to mute input to power amp section during power up to prevent turn on thump.
However, audio amplifiers are extraneous to Vic-20 discussion.

[eslapion]

I disagree.

Everyone is entitled to their own opinion.

My scope has no opinion... it only gives actual objective facts...

This statement makes no sense at all.
The 555 is controlled by an RC time constant based on R9 (for trigger) and R14, & C13 (threshold/discharge). All 555 timers are based on capacitor charging (and discharging for astable mode).

If a reset line is being held low by 555 timer for 15 seconds, then either C13 is out of tolerance or VCC has poor regulation.

The capacitor used to make the RC constant on PAL VICs is not the same as the one used on NTSC VICs. The problematic capacitor is C22.

On 2 prong VICs, C13 is related to paddles...

[6502dude]

My scope has no opinion... it only gives actual objective facts...

No one needs a scope to find a level transistion that takes 15 seconds to occur

The capacitor used to make the RC constant on PAL VICs is not the same as the one used on NTSC VICs. The problematic capacitor is C22.
On 2 prong VICs, C13 is related to paddles...

I used schematic that I had on hand for NTSC model. I'm sure there are a few variations for PAL units.
I still suspect the 4,700uf capacitor is more likely cause of the the problem.

Perhaps you can fix your unit with 15 second turn on delay and post results.

[RiSXC]

Ok guys I am still waiting for the new caps to arrive so I went to my pal and I had the caps taken out of circuit and tested
they are withing tolerence

then while I was checking the line voltage on the board reaching the chips
(I was testing the voltage during the 15 second startup interlude)

interesting thing
as soon as I touched the bottom pin of the 555 next to C13
the system came on I tried it a few times evertime i do that
the system comes on

so I thought the 555 must be faulty or something
so I replaced it
tested still took a long time to start
then I replaced C13 is it a 1uf capacitor
same result

then for the hell of it I started it up without c13
I came on immediately

so at the moment c13 is not in curcuit and the startup in instant
I loaded a few games to check the system and it seems fine (played with joystick)

so this is as it stands now

do you guys recon I can leave c13 out ?


what does c13 do anyways ?

[PaulQ]

so I thought the 555 must be faulty or something
so I replaced it
tested still took a long time to start
then I replaced C13 is it a 1uf capacitor
same result

then for the hell of it I started it up without c13
I came on immediately

so at the moment c13 is not in curcuit and the startup in instant
I loaded a few games to check the system and it seems fine (played with joystick)

so this is as it stands now

do you guys recon I can leave c13 out ?


what does c13 do anyways ?

I have version E of the main logic schematic, and I see c13 as being rated at 0.001uF, 50V and not 1uF. It appears that it is being used as a low-pass filter on POT Y of the 6560, which is connected to POT Y of the controller (joystick) port.

I would try replacing it with a 0.001uF capacitor and see if that fixes the problem. I expect that it will. I would not leave it out; it will work, but it's supposed to prevent higher frequencies from causing problems.

[Leeeeee]

what does c13 do anyways ?

According to the diagrams I have C13 is the POTY capacitor. My two prong Vic is in storage so I can't check this.

interesting thing as soon as I touched the bottom pin of the 555 next to C13 the system came on I tried it a few times evertime i do that the system comes on

From your description the capacitor you've found is marked as C22 in the diagram, a 1uF 16V electrolytic that is part of the reset circuit along with the 555 timer chip. This capacitor along with R24, 1.5Mohms (brown green green), should give a reset time of about 1.5 seconds.

do you guys recon I can leave c13 out ?

Leaving this 1uF capacitor out may mean the Vic will fail to reset on power on but otherwise will do no harm.

There probably is still a fault with the reset circuit that removing the capacitor is now masking.

Lee

[PaulQ]

There probably is still a fault with the reset circuit that removing the capacitor is now masking.

Lee

Good call; I'd check R24 to see if it's still in spec. C20 and R25 also appear to be on the same circuit.

[eslapion]

I used schematic that I had on hand for NTSC model.

The picture above indicates this is a PAL rev. B

You were right, the cap at fault is C13. I used the Rev. A schematics of the Programmer's reference guide. In there the same cap is labelled C22.

AFAIK, the problem with this cap is that it develops a huge ESR and so can't do its job properly. Replace it with a monolithic (ceramic) capacitor and you'll fix the problem for good.

See:
http://www.zimmers.net/anonftp/pub/cbm/ ... 4001_2.gif

I still suspect the 4,700uf capacitor is more likely cause of the the problem.

If that capacitor was faulty, then the voltage regulator could no longer do its job and more than likely the VIC-20 would not turn on at all. Or the capacitor would blow up.

Please read Leeeee's post.

Perhaps you can fix your unit with 15 second turn on delay and post results.

Did that last year.

[Leeeeee]

It appears that it is being used as a low-pass filter on POT Y of the 6560

This capacitor, along with C14 on that diagram, are the timing capacitors for the POT inputs.

bottom pin of the 555 next to C13

I now have the diagram that does label that capacitor as C13, the resistor that goes with it is labeled R14, 1.5M.

AFAIK, the problem with this cap is that it develops a huge ESR and so can't do its job properly.

ESR is very unlikely to be a problem here. The circuit arrangement is such that an increasing ESR will actually decrease the reset time as has been demonstrated by the removal of the capacitor, which effectively is the same as a capacitor with an infinite ESR, 'fixing' the problem.

Lee.

[eslapion]

ESR is very unlikely to be a problem here. The circuit arrangement is such that an increasing ESR will actually decrease the reset time as has been demonstrated by the removal of the capacitor, which effectively is the same as a capacitor with an infinite ESR, 'fixing' the problem.

Lee.

Wouldn't an increasing ESR make the capacitor take more time to charge?

I guess removing the capacitor makes the remaining parasitic capacitance an incredibly small capacitor that charges instantly.

In truth, I just removed the electrolithic cap and put a ceramic one in its place and that did it for me.

What was wrong with it exactly, I did not check.

Sometimes old electrolithic capacitors develop resistive short circuits (could we call this an EPR?) that leaks some current and, with a 1.5MOhm resistor, this would make it take an incredibly long time to charge to the threshold level.

What I do find surprising is the fact that both Rev A and Rev C VIC-20s also employ a 1uF electrolithic cap for the same function and never suffered this problem (that I've heard of).

These are labelleled C22 for the Rev A and C42 in the rev C.

Rev. B PAL VICs, on the other hand, I've heard of dozen of cases like that and my own was affected.

In PAL VIC-20s, these caps are orange. In Rev C they are black. Different brand, I guess.

[Leeeeee]

Wouldn't an increasing ESR make the capacitor take more time to charge?

Yes but the ESR would now form a potential divider along with the charging resistor that would raise the effective voltage above that of where it would have been with a zero ESR.

For example if the ESR were to equal the charging resistance then the voltage at time=0 would be..

Code: Select all

Vth = Vcc - Vcc * Rchg/Rchg+Resr
    = Vcc - Vcc * Rchg/Rchg+Rchg, Rchg = Resr
    = Vcc - Vcc * 1/2
    = 1/2 Vcc

.. effectively starting from a half charged point.

Lee.

[6502dude]

I still suspect the 4,700uf capacitor is more likely cause of the the problem.

If that capacitor was faulty, then the voltage regulator could no longer do its job and more than likely the VIC-20 would not turn on at all. Or the capacitor would blow up.

An aging (defective) 4,700uf capacitor can quite easily delay the start up of Vic-20, resulting from insufficient current being available for 7805 to draw when switched on.

If you doubt this, I suggest that you try a defective 4,700 capacitor in a Vic-20 and observe the result.

[eslapion]

.. effectively starting from a half charged point.

Yes, it makes perfect sense.

I suggest that you try a defective 4,700 capacitor in a Vic-20 and observe the result.

Electrolithic capacitors can be defective in myriad of different ways, some of which can severely damage their peripheral components...

Forgive me for not following this advice...