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Robotrontechnik-Forum » Technische Diskussionen » PC 1715 fast rattling noise from power supply » Themenansicht

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000
12.12.2022, 04:12 Uhr
milan



So I had problem that during power on, two fuses on back of the power supply unit (PSU) of PC 1715 were blown up repeatedly. The culprit was SU179 power transistor (the first one, from the power input side). It was damaged, shorted out. So I replaced him.

Now I can start PC and measure voltages next to lighting red LED diodes on motherboard. Measured values are for 12V and -12V rail: (11.33V, -11.79V) and for 5V and -5V rail: (4.90V, -4.90V) respectively. Computer is not booting form floppies, sometimes floppies just turn on motors and keep spinning. But this problem I would like to solve later hopefully.

Currently I hear fast rattling sound (which I think was still there but I thought that it was coming from computer fan, but I unplugged the fan and sound is still there). This sound comes I think from that already replaced SU179 transistor. I think it is not normal. You can listen to the record of that sound here: http://sndup.net/x4tk (I recorded that with my phone, so sound has a bit unstable volume as I moved a bit my hand left to right like 10 centimerers above PSU, but in reality that rattling sound is of constant unpleasant volume.) I would even say it is comming from back cooling plate of transistor, maybe it is vibrating, but why?

I hear this sound when PC mainboard is connected, but also when I put resistors (3ohm and 33ohm) as fake load in place of of motherboard (on 5V and 12V rails). So I presume problem should be located on PSU board.

The advice I got from Rudiger (Admin) on this forum was:
"Ticking sounds come usually, when device is switching off and restarting cyclical because too high current, possibly because a secondary capacitor or rectifier having a short or crowbar circuit is igniting.

Power off the unit and feed it backward using a laboratory power supply with appropriate +5V or +12V.

If you find golden capacitors from producer "Roe", it will be likely the reason."

I have to say I have not find golden Roe capacitor. From current picture of my PSU here https://imgur.com/a/j40cJo6 (you can even download that picture and see the details when magnified) , you can maybe see little bit suspicious blue Tesla 220uF/25V capacitor second next to the SU179 transistor. I removed it and measured it, it reads 228uF and 0.05 ESR so this sems OK. No other problematic like leaking or burn out cappacitors are detected by my eyes. The termistor (black rectangular plate next to thegiant silver capacitor) coating looks also suspicious, maybe I should measure voltage drop there too.

I did not backfeeded motherboard with laboratory power supply as I do not have one, yet. But I think failure is inside PSU itself as I explained earlier.

My only idea now is to take all capacitors from main PSU board out and measure them one after another, also rectifying diodes (the very first diodes from voltage input side were ok) ... Also I think I will measure stability of output voltages with osciloscope, because I wonder about those possible cyclical restarts which Rudiger mentioned, but I did not noticed them when measuring by voltmeter (and as I wrote here https://www.robotrontechnik.de/html/forum/thwb/showtopic.php?threadid=20150 , I was even able to get picture on my LCD TV, so I do not think there are restarts of the whole device.)

Do you have any other idea how to diagnose it easier?

P.S. I'm not skilfull electrician, just trying my best to get this old PC working again.

Dieser Beitrag wurde am 12.12.2022 um 05:14 Uhr von milan editiert.
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001
12.12.2022, 09:52 Uhr
kaiOr

Avatar von kaiOr

Hi,

please check the two big diodes SY185, their screw connections and especially their contact to the circuit board (cold solder joints). I think a phase is missing and the control tries to compensate, which leads to higher peak currents and reverse induction in coil.

regards
Kai
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002
12.12.2022, 11:36 Uhr
Rüdiger
Administrator



Zitat:
milan schrieb
I did not backfeeded motherboard with laboratory power supply as I do not have one, yet. But I think failure is inside PSU itself as I explained earlier.



I did not wrote you should back-feed the motherboard, but you should back-feed the exits of the PSU to check that secondary capacitors and diodes have no damage.
Of course the PSU must be switched off before and the motherboard should be disconnected therefore.

The sound also comes without plugged screen?

To exclude a short circuit at the mainboard, you could plug it off and connect lamps with matching voltages and power at the PSU.

Btw: Do I see it right, the varistor in the lower left corner is burnt down?
But it is not the reason of the sound...
--
Kernel panic: Out of swap space.

Dieser Beitrag wurde am 12.12.2022 um 13:22 Uhr von Rüdiger editiert.
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003
12.12.2022, 20:27 Uhr
Ordoban




Zitat:
milan schrieb
The advice I got from Rudiger (Admin) on this forum was:
"...If you find golden capacitors from producer "Roe", it will be likely the reason."


The "Roe" ones also comes in black. At first I thought the statement about the "Roe" as the main evil as exaggerated, but now I know its true. Just replace them, even if they measure fine. It is possible they start leaking current if the voltage is higher than the voltage of the measuring device.
--
Gruß
Stefan
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004
15.12.2022, 01:15 Uhr
milan



When measured the output PSU voltage with oscilloscope, there was saw teeth pattern instead of linear DC output.The problem was in biggest giant metal 200uF smoothing capacitor located at the voltage input side (when I measured the capacitor itself, it had only tens of nF). Upon closer inspection rattling noise indeed came from the first transformator (its coil?) located next to this giant capacitor. After replacing this large capacitor, there is sill much smaller noise (probably from 12V transformer located in the opposite corner of PSU, next to the three blue Frolyt capacitors). This noise is influenced by floppy drives when they are seeking for floppies. You can listen to it here: https://sndup.net/y8mg/
This noise is not so loud but still possible to hear and record. You can compare its volume level with loudiness of power switch click, at the end of the recording.

* Question 1: I presume PC1715 power supply (excluding fan) shoul be totaly silent? Maybe I should check rest of the capacitors...

My biggest problem to use this PC1715 now, is the ventilator (Style Fan UP12D22-T) is noisy like a turbine, you could listen to it here:
https://sndup.net/f6ws/ (measured 55-60 decibels with opened case, from 10-20 cm distance). I wonder if the probably burn out (as noted by Rudiger) termistor NTC111/32 is not used as a speed regulator for the ventilator, causing probaby too high rotational speed and therefore noise of the fan.

* Question 2: Is you fan so loud like mine? Should it have variable speeds provided by NTC111/32 termistor?

All PSU capacitors except few Teslas seem to be Frolyt brand, I found a few capacitors of unknown brand to me, with the following sign - brand signature on it: https://imgur.com/a/JzdBHPc

* Question 3: Do you know that capacitor brand?

I also checked power diodes, you can see their picture here: https://imgur.com/a/fqeM3hQ
It looks like they were tin connected on the bottom of mainboard, but I wonder why they would do that, because their rectangular housing acting as a cathode has nice firm tinned junction connection to the mainboard in all of its four corners.

I was finally for first time able to boot from floppy and got quite stable text on the screen, see here:
https://imgur.com/a/YGox0tX
I used the following schematic https://imgur.com/a/3Wj5hZg

* Question 4: Any idea why screen is moved to the left? How to correct it, if LCD TV does not offer any horizontal/vertical corrections in component input (H/V positioning works on VGA input, but not on composite input, at least on this LCD TV).

Dieser Beitrag wurde am 15.12.2022 um 09:58 Uhr von milan editiert.
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005
15.12.2022, 09:32 Uhr
MarioG77

Avatar von MarioG77

Hi Milan,

just 2 weeks ago, I repaired my fan with the replacement of the ball bearing (684ZZ 4x9x4mm).
It was making some noise, but not too loud. It takes now much longer, to stop, than before.

If you have a Papst fan, then...
There are 2 small screws under the label, which will release the fan blades.
Then you have to remove carefully the retaining ring, to release the coil.

There is a spring inside, be prepared, when opening the retaining ring.

That is much cheaper, than a new fan. They are still available, but expensive.
I replaced my fan for one 1715, so the fan was waiting for repair since a few months.

In the end - the air flow will still make some noise.

Regards Mario
--
Gruss Mario

Betriebsbereit: KC85/3, 2x [KC85/4, D004+Floppy], PPC512, PC1512, 2xEC1834, Soemtron 286, 3x PC1715, picoAC1
Zu restaurieren: 1x D004
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006
15.12.2022, 09:57 Uhr
milan



Hi Mario,

thanks for the response and information. My fan is following:
Brand: Style Fan
Model: UP12D22-T

Milan
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007
15.12.2022, 11:39 Uhr
Rüdiger
Administrator



Zitat:
milan schrieb
* Question 1: I presume PC1715 power supply (excluding fan) shoul be totaly silent? Maybe I should check rest of the capacitors...



I don't believe more than one capacitor ist bad in your PSU.
If computer works, let fingers from PSU.



Zitat:
I wonder if the probably burn out (as noted by Rudiger) termistor NTC111/32 is not used as a speed regulator for the ventilator, causing probaby too high rotational speed and therefore noise of the fan.



The varistor protects your primary diodes from high current caused by 200µF capacitor when switching on.
If you don't fix it, these diodes will die one day.



Zitat:
My biggest problem to use this PC1715 now, is the ventilator (Style Fan UP12D22-T) is noisy like a turbine, you could listen to it here:

* Question 2: Is you fan so loud like mine?



Give its ball bearing a drop of oil.
It is normal that the fan ist not noiseless.



Zitat:
Should it have variable speeds provided by NTC111/32 termistor?



No.



Zitat:
* Question 4: Any idea why screen is moved to the left?



Because the computer was not made to run with a 30 years younger flat screen.
--
Kernel panic: Out of swap space.
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008
18.12.2022, 10:49 Uhr
DL




Zitat:
milan schrieb
* Question 4: Any idea why screen is moved to the left? How to correct it, if LCD TV does not offer any horizontal/vertical corrections in component input (H/V positioning works on VGA input, but not on composite input, at least on this LCD TV).



Laut Initialisierung des RTC ist das schon relativ weit rechts, man kann versuchen den HRTC-Wert noch auf 5E zu erhöhen, aber eventuell synchronisiert das dann nicht mehr !??
19 00 18 4F 18 57 18 BB 18 5D 19 20

Dieser Beitrag wurde am 18.12.2022 um 12:06 Uhr von DL editiert.
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009
20.12.2022, 05:10 Uhr
milan



Thanks a lot, this gave me a perfect 80 columns per line. I will post photo later. But I noticed that the last line is missing, hidden under bottom of the screen. Would it be somehow possible to fit all rows vertically into the visible area of the monitor?
Dieser Beitrag wurde am 20.12.2022 um 05:12 Uhr von milan editiert.
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010
20.12.2022, 11:22 Uhr
DL



Könnte man nur mal versuchen den VRTC-Wert zu verändern, siehe Datenblatt vom i8275. Die Initialisierung in Beitrag 008 setzt sich aus Adresse,Wert,Adresse,Wert... zusammen und kann in der Tabelle links oben nachvollzogen werden
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011
29.01.2023, 16:58 Uhr
milan



Here are the results of my experiments:

Changing VRTC value is equal to adding 1 to 4 empty character lines to the top of the screen, so the effect is that you push screen further down.

It looks like top and bottom area, which would be on Commodore64 or other contemporary 8 bit computers considered as overscan, is used by Robotron PC 1715 (I know Robotron should have proprietary monitor which I do not have).
Then the problem wiith modern LCD is that they are mostly strictly against any overscan on the top or bottom.
On the old CRT Sony TV I can vertically very tightly fit all 24 character rows on the screen, having first row a bit distorted at the very top.

Advantage of having LCD is sharpness of the characters, which is very bad on coloured CRT TV, probably because of the screen mask, which is made for RGB and not for the Black and White...

What could ideally help is to squeeze font height. This would be achieved by changing the parameter "Number of Lines per Character Row" as seen in the document you enclosed (programming Intel 8275 CRT video chip). But reducing the character height did not help me (it looked like font is scaled to be same height on the screen as previously, this scaling is probably done by i8275 itself, not by TV ...)

Dieser Beitrag wurde am 29.01.2023 um 17:03 Uhr von milan editiert.
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