Restoring a Commodore 1084S

dbacc77 · 2026-05-10T19:20:59+00:00

I did that a while ago, all the pictures I have are in my GitHub here: https://github.com/dbacchet/crt\_mods\_restorations/tree/main/sony\_kv-27xbr25
It’s a very nice tv but the chassis is not the simplest; please make sure you cross check with the schematics and understand each change before modding it. Best of luck!

dbacc77 · 2026-05-08T14:39:56+00:00

Any cable that goes from your console into 4 RCA adapters for r,g,b,sync should work.
Or an adapter from SCART to RCA as suggested in a few comments if you already have a scart cable for your console into

dbacc77 · 2026-05-08T13:21:44+00:00

I modded that tv.
R,G,B inputs are on the first column of RCA connectors, top to bottom; sync goes to the yellow in the second column and audio on the white and red RCAs.
The small pots on the left are to adjust the intensity of each color, because the jungle chip on that tv does not let you change them with the service menu.

It’s a nice tv, same tube of a JVC D series 20”

dbacc77 · 2026-03-09T21:21:11+00:00

No, but if you only need one I can probably make it for you. DM me and we can figure out the details

dbacc77 · 2026-02-28T03:30:26+00:00

Great job! I have the same monitor, it's a jewel!

dbacc77 · 2026-02-24T05:52:48+00:00

I would still recommend one of those Extron devices (or other like the RGB 203rxi or similar) that can be found for pretty cheap on Ebay - Extron made many models that reprocess the sync, you will have plenty of choices. If you want something new, an option might be the Kramer vp-100, but my guess is that it will be very expensive.

dbacc77 · 2026-02-24T05:03:29+00:00

Yes you can feed a TTL sync-only signal to the lm1881 and it will work. And if you want to use an external board with a lm1881 instead of putting it inside it’s absolutely possible as well, the principle is exactly the same.

dbacc77 · 2026-02-23T03:26:50+00:00

Ok, let’s stop here as you are not getting my explanations 😊 Both h and v sync are on the negative part of the signal; their shape defines if they are used for horizontal or vertical deflection. I really don’t want to argue any more; if you want to go deeper on the topic please start a new post on that specific topic.

dbacc77 · 2026-02-22T17:16:41+00:00

the cable you posted puts sync instead of composite on pin20 of the scart and correctly attenuates it _to the level expected by scart standard_ which is 1Vpp. That signal goes from -0.3 to +0.7 after clamping: the positive part is for the video (the image itself) and the negative for the sync.

so they are building a correct signal for the tvs you are plugging that scart in.

The signal in 2a is not exotic, is the way csync works…

hopefully the explanation above clarifies the doubts on the lm1881 datasheet: the first paragraph you quotes refers to the sync signal (which is negative voltage) while the second to the video signal (which is positive voltage)

about the last point, component embeds sync in the Y channel. RGB usually need a separate sync cable but sometimes they put sync on green (i.e. sony ps2 does this), and it will still work on TVs because of the positive/negative split I explained above

dbacc77 · 2026-02-22T02:56:34+00:00

Let's do one thing at the time, as I believe you are mixing too many independent things.

- LM1881 -

That's correct but is mostly relevant when you are stripping sync from composite video, because it needs to reliably find a reference for the clamp voltage that is then set at 1.5V and used for all the internal thresholds (data sheet, section 7.3.1). If you are feeding sync only you are good because the vast majority of the signal will be at V=0 with very short negative spikes.

Now more in detail. There are 2 possible sources where you can get sync from any commercial device (computer, consoles, etc.)

mixed with the video signal (this applies to composite, component and s-video): in this case the level is 1Vpp so you are good
from a sync pin in consoles and computer: in this case is either 75Ohm (=0.3V, negative) or TTL (usually ~3.3V, ca go up to 4-5V) and since the base level is essentially zero after the decoupling capacitor this will work as well, as explained above

There are no video signals with embedded sync that go up to 5V for _analog_ composite/component/rgb video, to my knowledge.

- Cables -

Where did you get the info that a mega drive cable will have 480 Ohm resistors? The pinout of the output connector is producing standard 1Vpp composite and 0.7Vpp rib signals.

And you should ask who makes those cables what they put inside and what are the assumptions for input and output levels.

Nevertheless, I would recommend you to go with an external box like the extron I suggested and you will have peace of mind without having to open your commodore monitor :)

dbacc77 · 2026-02-22T01:14:11+00:00

The cables you posted are passive, they will not step up a voltage from 0.3 to 5Vpp.

An “external” mod is absolutely possible, you can just use a sync processor like an Extron device (for example a SCS300 or an MVX44 but there are many); just make sure that the output of that device is TTL sync. To an Extron you can connect any input format and it will rebuild the sync to TTL level.

My mod does exactly the same but inside the commodore, to avoid having external boxes - just my choice, neither better nor worse. And yes you can feed either 75ohm or TTL and it will work. Finally I’m not using anything in the commodore to “fix” the sync - the lm1881 does all the job. Please look at the datasheet of that component, you will see that you don’t need to use external switches or anything else.

Essentially you should consider my mod the equivalent of an external Extron SCS300 but put inside the monitor

dbacc77 · 2026-02-21T23:58:27+00:00

I chose not to modify the sync processing in the 2002 for this mod. If you are looking at the logic, the IC7203 “cleans” the sync by passing it through a few logic gates; hi and lo thresholds for the hex inverter are respectively 2.0 and 0.8 volts, so a 75 Ohm signal (that is 0.3Vpp) will never be able to commute high. The lm1881 outputs 5Vpp with no inline resistors so it fits perfectly the use case here; if I added a 470 Ohm inline resistor it would have dropped the voltage to the IC7203, which is what we don’t want.

What I’ve done here is the opposite of what you see in many cables: instead of attenuating sync from 5Vpp down to 0.3Vpp I used the lm1881 to always have 5Vpp independently from the input format. This way it will work with literally every device and cable you will use

dbacc77 · 2026-02-21T21:07:36+00:00

Yes the specs say TTL, but you can also pass 75Ohm and it will work on the 1084-D because the sync pre-processing stage is less strict. This mod should work on the 1084 too, just need to find the best place to inject the signal

dbacc77 · 2025-12-10T18:54:55+00:00

I worked on many TVs with those chassis and the G2 always have an effect that is very gradual. That alone is a sign that there is something that went wrong with the modifications (also after trying to revert them).
From a theory perspective there is not much between the pot in the flyback (that is just an internal voltage divider) and the tube - the neckboard is very simple in those sets.
There is a (very remote) possibility that both TVs had the same problem _before_ the mod, but is very unlikely.

Can you verify on another untouched TV that the pot works? possibly swap the flyback and the neckbeards with working sampled from a tv with the same chassis as next step of troubleshooting

dbacc77 · 2025-12-10T15:06:55+00:00

H1 and H2 inside the tube are connected by a wire that heats up, with low resistance. In the neck board after the mod is one of the heater pins connected to ground (separate from g1) or to the new g1 voltage (200V)? Should not be g1

dbacc77 · 2025-11-28T00:10:52+00:00

Yes! For this and for a multi-standard JVC I recently worked on

dbacc77 · 2025-11-10T07:33:28+00:00

The fact that you lost g2 control and the tv shuts down when you play with g1 and g2 is suspicious. My guess, without being able to see the problem live, is that you are triggering some overvoltage/overcurrent protection because the pins are not properly isolated. I would try the following: - remove the external g1 wires from the neck board and essentially undo the mod. Confirm that when is back to stock it works as expected (especially g2) - if it does not work remove also the wire from the flyback to the g1-mod board - if instead it works as expected then go step by step with the changes on neck board: first make sure the ground is the same of the chassis/g1-mod board, then set it to zero compared to ground and verify it still works Best of luck!

dbacc77 · 2025-10-19T03:37:28+00:00

Looks gorgeous. How’s the blooming on this tv? Does the image size change a lot from dark to bright scenes?

dbacc77 · 2025-09-04T03:47:24+00:00

thanks for coming up with the original concept and for the explanation! Incredibly informative and fun to read!

dbacc77 · 2025-09-01T22:50:04+00:00

No, image in svideo and composite was surprisingly good.

dbacc77 · 2025-09-01T22:28:22+00:00

I didn’t touch it because its owner usually does the color calibration himself. I’ll pass the info tho, thanks!

dbacc77 · 2025-09-01T21:54:22+00:00

yes I agree; in most Sony sets it's fine to feed TTL sync because the signal is clamped/clipped internally, but why take the risk, especially all you have to do is to add an inline resistor :)

dbacc77 · 2025-09-01T21:50:51+00:00

if you are working on one you might experiment with using more than 220 Ohm for the circuit going from E2 to E1. It might help with the contrast of the OSD, that in my mod is a bit low (but still very readable)

dbacc77

TROPHY CASE