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Robotrontechnik-Forum » Technische Diskussionen » Robotron 1715 video output intensity (voltage) too high? » Themenansicht

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000
30.08.2022, 00:24 Uhr
milan

Hello,

I wanted to connect Robotron 1715 to composite video input on my old LCD TV, so I applied this scheme (the one with capacitor): https://www.sax.de/~zander/pc1715/1715_tft.pdf.

Before connecting AV cable to the TV (just yellow cinch), I have measured with voltmeter voltage between ground (the ring on the cinch) and video signal output (middle pole of the cinch). To my surprise it was 7.2V. Then I Measured video pin 5A on Robotron 1715, no voltage, but on intensity pin 1A there is 7.7V (all measurements made with multimeter switched to measure AC voltage).

I think this is just too high voltage on intensity pin 1A (I read somewhere that composite video signal should not be more than 1Vpp).

Any hint what could be wrong...? Is this common problem?

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001
30.08.2022, 01:19 Uhr
milan

I would like to add, that I do not have any original Robotron monitor. This is for the first time I'm trying to connect any video output to this machine.
Before turning on the computer, I measured its power supply output and it looked OK, and all red LED diodes on the mainboard emit light (so all voltages from the power supply to the motherboard should be OK, I suppose)...

Dieser Beitrag wurde am 30.08.2022 um 01:22 Uhr von milan editiert.

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002
30.08.2022, 07:16 Uhr
Bert

With which tool did you measure? A voltmeter or an oscilloscope?

regards,
Bert

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003
30.08.2022, 09:51 Uhr
milan

I used just voltmeter, for quick checking. I can try to use old analog oscilloscope if needed.
I also did comparative measurement with voltmeter on Plasystation2 composite video output, and there was zero voltage (probably because voltage there is fluctuating to no more than 0.7V-1V). But on PC1715 I constantly get such high voltage as described.

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004
30.08.2022, 10:20 Uhr
milan

I can say, that Robotron seems to try to boot up, as it tries to access floppy drives (first drive, then second drive in a loop). Regarding this boot, the strange thing I noticed is that usually on the first power-on, I do not see this floppy drive accessing pattern. I have to power-off and power-on again, sometimes also hitting reset button and then I see this floppy loop pattern...

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005
30.08.2022, 11:23 Uhr
Ordoban

Digital Voltmeters can get confused by digital signals.
So if you can use an oszilloscope (even an old analoge one) you should do.
Of coarse you should verify your oszi is working correctly before.
--
Gruß
Stefan

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006
02.09.2022, 22:18 Uhr
milan

Well, Stefan was right, I should have tried with oscilloscope, there voltage levels looked OK.

Finally I got picture on LCD screen. Background was grey-white and flowing from top to bottom in waves (unstable), but at least I was able to see stable SCP text output,. Unfortunately there was error (retry, abort, ignore) in reading disk further. So I turned off computer and attached its keyboard. But I had no luck booting second time from that floppy. The disk which "worked" just once was DS DD 96 TPI floppy, formatted on linux with programs "fdparm" and "fdformat" (capacity 800kB, 1024B per sector, 80 tracks, 10 sect. per track) and then I used linux "dd" program to transfer this floppy image: https://xepb.org/robotron/downloads.html (the one with label "[1999] This is boot image for R1715 with SCP 2 0.05 (?)".). That downloaded floppy image was in copyqm format, so I converted it to the raw image format with the following program: http://www.pofo.de/P8000/misc/sources/tools/read_cqm.c (which compiled under Debian 11 just fine, after including correct C language header files).

So during this floppy disco process I turned off Robotron many times, cause reset seems not work all the time, but what happened is that during one power-off switch I heard slightly different sound, and sure the two fuses on the back of power supply are blown-up. The fuse inside power supply is intact. I discovered that blown-up fuses were T1.6 Amps and not F1.6 Amps as engraved on the power supply case (so somebody already has changed Fast fuses for Time delay fuses...).

So now I should probably replace those fuses and do this testing again: https://www.robotrontechnik.de/html/forum/thwb/showtopic.php?threadid=18299
(unfortunately I do not have 500W heater as a load, so I can test only with resistors as load again).

If anyone has better advice, how to proceed, what to check, it would be nice to hear from you.

Dieser Beitrag wurde am 03.09.2022 um 05:32 Uhr von milan editiert.

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007
05.09.2022, 01:56 Uhr
milan

With google translation of robotrontechnik.de pages I found interesting info, which explained why I was not able to boot from floppy B: The SCP is forcing reading from drive A:, even when booting starts from another drive.

I found also PC1715 schemes. So fuses type were OK, and so I put new T1.6A fuses back, and as a load I put resistors and fuses were blown-up again, so problem is in power supply.

I also found on your pages with google translate, that on PC1715, power switch was causing problems to lot of users back in time, so users were using power switches on cables.
Question 1): Could be this my case, that power switch caused havoc? Because the short happened when I was powering off the PC.

Question 2): Also I'm wondering if ticking sound which I heard from time to time as PC was powered on, was not from drive heads (I have not seen any light from floppies at that time), but from power supply? Also on the screen I saw little tearing when that sound clicked...

Dieser Beitrag wurde am 05.09.2022 um 01:57 Uhr von milan editiert.

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008
05.09.2022, 16:39 Uhr
Ordoban

1) A power switch making a short? That sounds very unlikely. You also would have heard if a short is happen in the switch while switching.

2) Ticking sound? Uhhhh, i had this experience too. On my CM1910 i had a ticking sound now and then, and some heat radiation even if the main switch was off. I found out, it was the Netzfilter (how to translate? "Net filter"?), who was failing. Seems the isolation in this things go bad after some years.

The issue with this things where discussed several times here, if you can or can not just throw it out and make a direct connection. My CM1910 runs fine without. Cooked birds falling from the sky? Noooo, that is not the radiation I emit...
--
Gruß
Stefan

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009
05.09.2022, 16:56 Uhr
Rüdiger
Administrator

Zitat:

milan schrieb
Question 2): Also I'm wondering if ticking sound which I heard from time to time as PC was powered on

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.
--
Kernel panic: Out of swap space.

Dieser Beitrag wurde am 05.09.2022 um 17:05 Uhr von Rüdiger editiert.

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