| Astec power supply AA11040B ticking problem when cold [message #338416] |
Sat, 25 February 2017 09:55  |
retrogear
Messages: 245
Registered: November 2012
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This one has me stumped. I have replaced all caps and still have the same symptom. When I power on with just a bare II+ or IIe (no cards) it will just tick for awhile (maybe a few seconds)then beep and boot ok. The more load I put on by say adding a memory card, the longer it has to warm up before it will boot ok. When it finishes booting, the outputs measures a rock solid +5.0V, -5.0V, +12.2V, -12.2V and varying the AC120V up and down with a variac +- 5V or so all the output voltages remain solid. There is a shutdown SCR monitoring the +12V line which Sams shows as 11.7V so maybe 12.2V is a little high? I'm pretty sure by using my scope that the SCR is firing causing the tick cycle. Any thoughts? My only thought at the moment is the type of caps I put in? I remember years ago in electronics class learning about the power factor of a capacitor. Would our modern day 'teeny' caps have a weak power factor?
Larry G
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| Re: Astec power supply AA11040B ticking problem when cold [message #338598 is a reply to message #338565] |
Tue, 28 February 2017 07:00  |
retrogear
Messages: 245
Registered: November 2012
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> did you also try looking for cold solder joints
Yes, I resoldered the entire board which surprisingly improved it but stlll could load it to where it ticked again. Worst case was an Apple 2+ board which draws twice the power of a 2e at 32W, then add a 1meg populated RamFactor card, an 80 column card, a z80 card and disk controller then it would tick all the time and never boot. The tick mode was caused by the overvoltage scr tripping on the 12V line which means the 5V sample could not get up to spec. My 4 caps tied to the 5V line (C12 C13 C14 C19) were new Nichicon brand so decided to replace with Panasonic brand I had. As I replaced each of those 4 caps again, things started improving. With all 4 now Panasonic, it would always boot with all that load but made an audible "singing" noise for the first 20 seconds or so when it was cold. I see in the schematic there is a pi filter arrangement on the 5V line with 2 1000uf caps paralleled (C13 C14), coil then another 1000uf (C19). Why did they parallel 2 instead of a single 2200uf ? Maybe because of physical size back then? So I added another 1000uf across the single one and it removed all noise, no ticking, rock solid !!! Why, I have no clue unless compensating for other component(s) out of spec. I'm ordering a 2200uf replacement for C19. I still think these modern caps are missing something in design.
Larry G
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| Re: Astec power supply AA11040B ticking problem when cold [message #338611 is a reply to message #338598] |
Tue, 28 February 2017 11:23 |
Michael J. Mahon
Messages: 1767
Registered: October 2012
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retrogear <larrygr510@gmail.com> wrote:
>> did you also try looking for cold solder joints
>
> Yes, I resoldered the entire board which surprisingly improved it but
> stlll could load it to where it ticked again. Worst case was an Apple 2+
> board which draws twice the power of a 2e at 32W, then add a 1meg
> populated RamFactor card, an 80 column card, a z80 card and disk
> controller then it would tick all the time and never boot. The tick mode
> was caused by the overvoltage scr tripping on the 12V line which means
> the 5V sample could not get up to spec. My 4 caps tied to the 5V line
> (C12 C13 C14 C19) were new Nichicon brand so decided to replace with
> Panasonic brand I had. As I replaced each of those 4 caps again, things
> started improving. With all 4 now Panasonic, it would always boot with
> all that load but made an audible "singing" noise for the first 20
> seconds or so when it was cold. I see in the schematic there is a pi
> filter arrangement on the 5V line with 2 1000uf caps paralleled (C13
> C14), coil then another 1000uf (C19). Why did they parallel 2 instead of
> a single 2200uf ? Maybe because of physical size back then? So I added
> another 1000uf across the single one and it removed all noise, no
> ticking, rock solid !!! Why, I have no clue unless compensating for other
> component(s) out of spec. I'm ordering a 2200uf replacement for C19. I
> still think these modern caps are missing something in design.
>
> Larry G
>
Two capacitors in parallel will have about half the Equivalent Series
Resistance of a single cap. That's a pretty big advantage over a single.
Also, have you replaced the filter caps on the mains side? These have a
lot to do with the ability of the supply to stay in regulation under load.
They are the 175-250v capacitors near the bridge rectifier.
--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
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| Re: Astec power supply AA11040B ticking problem when cold [message #338636 is a reply to message #338611] |
Tue, 28 February 2017 17:22 |
retrogear
Messages: 245
Registered: November 2012
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> Two capacitors in parallel will have about half the Equivalent Series
> Resistance of a single cap. That's a pretty big advantage over a single.
Interesting, I didn't know that. So what is the benefit of low ESR ? Does
that increase the caps power factor? I still remember that as being important.
So I might just stick with two caps paralleled then. I'll run it a few hours and check for any unusual heat buildup.
> Also, have you replaced the filter caps on the mains side? These have a
> lot to do with the ability of the supply to stay in regulation under load.
> They are the 175-250v capacitors near the bridge rectifier.
Yes all 4 with 47mfd 250V.
Larry G
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| Re: Astec power supply AA11040B ticking problem when cold [message #338661 is a reply to message #338636] |
Wed, 01 March 2017 10:41 |
Michael J. Mahon
Messages: 1767
Registered: October 2012
Karma: 0
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retrogear <larrygr510@gmail.com> wrote:
>
>> Two capacitors in parallel will have about half the Equivalent Series
>> Resistance of a single cap. That's a pretty big advantage over a single.
>
> Interesting, I didn't know that. So what is the benefit of low ESR ? Does
> that increase the caps power factor? I still remember that as being important.
"Power factor" usually refers to the actual power in an AC circuit divided
by the "reactive power", which is RMS voltage times RMS current. If the
instantaneous voltage and current are in phase, the power factor is one,
and if out of phase, the power factor is less than one.
For example, if an ideal capacitor is connected across the AC line, the
voltage is unchanged and current flows, but the power factor is zero
because the current is 90 degrees ahead of the voltage.
ESR is a measure of a capacitor's deviation from "ideal", along with
electrical leakage, inductance, etc.
Since a filter capacitor partially charges and discharges during each cycle
of the applied pulsating DC, usually at current levels comparable to the
supply's load current, any equivalent series resistance results in voltage
drop that decreases the effectiveness of the filter capacitor. In other
terms, it adds to the AC output impedance of the supply (which would be
zero in an ideal supply).
> So I might just stick with two caps paralleled then. I'll run it a few
> hours and check for any unusual heat buildup.
Although it may work with a single cap, a lower ESR will result in less
ripple under load with multiple parallel capacitors.
>> Also, have you replaced the filter caps on the mains side? These have a
>> lot to do with the ability of the supply to stay in regulation under load.
>> They are the 175-250v capacitors near the bridge rectifier.
>
> Yes all 4 with 47mfd 250V.
Good. There may be other issues in the primary or "line side" supply, like
a failed diode in the bridge (which would turn it into a halfwave rectifier
rather than a fullwave). This would reduce the ability of the supply to
stay in regulation under load.
But if the crowbar is triggering (the ticking), then the problem may be in
the crowbar circuit. Maybe it's detecting overvoltage when there is no
overvoltage.
I'd check all the voltages in the crowbar circuit, preferably with an
oscilloscope. It helps to have a working supply of the same design for
comparison.
Switch Mode Power Supplies are fairly complex in operation, with high
voltages, switching transients, and feedback, which is why component
swapping is such a common method of repair.
> Larry G
>
--
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com
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| Re: Astec power supply AA11040B ticking problem when cold [message #339130 is a reply to message #338661] |
Sun, 12 March 2017 12:25 |
retrogear
Messages: 245
Registered: November 2012
Karma: 0
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Senior Member |
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As a follow up on this subject, I recapped another Astec power supply for tick mode under heavy load. It worked fine except audible hissing noise with heavier load. So I special ordered some 1000uf 16V caps specified as low ESR. Replacing C19 again with just a single one of these low ESR caps removed the noise and it works fine even with heavy load.
Larry G
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