Help: C128DCR transformer broken, need replacement - Page 1
Forum section: electronics / repair

author: Bicurico
author_rank: Super Contributor
timestamp: November 23, 2020, 11:47:47 pm
content: Hello,I own a Commodore C128DCR, which looks pretty good and works well, except the 1530 Datasette, which does not get any power. The Datasette works on another C64 without problems.I traced the problem down to the transformer T102 in this schematic:http://cbm.ko2000.nu/schematics/computers/c128/power/ps-128dcr-usa.gifWhen powered on, I read 0.2VAC instead of 9VAC. Testing continuity on both output ports of the transformer will give OK, but not on both input ports.My question is: how do I find a suitable replacement? And what does "138] || [1.5" in the schematic mean? Does it mean that the side under 220VAC has 138 windings and the 9VAC side only 1.5 windings? There is shop in town that repairs transformers, rewinding them. I am not so sure if it is worth the effort and if it isn't cheaper to just get a suitable replacement transformer.Because I never dealt with this, I am not sure what specs I should take into consideration. Any help would be welcome.The power supply is exactly like this one:https://www.worthpoint.com/worthopedia/oem-commodore-128d-128dcr-power-1832939761The transformer T102 is the one in the back with two red wires (220VAC) going in and two black wires (9VAC) coming out.Regards,Vitor

author: bob91343
author_rank: Super Contributor
timestamp: November 24, 2020, 01:04:28 am
content: Simply replace it with a transformer of similar size and weight with 9 V output. The power handling of transformers is well established and varies with the weight of the unit if the construction is similar.The numbers marked on the diagram might be winding resistances in Ohms.If you have an open circuited primary it's probably not worth it to rewind. Just find another unit.Alternatively, often an open is a break where the external leads are connected so you could peel away some insulation to see.

author: james_s
author_rank: Super Contributor
timestamp: November 24, 2020, 02:29:32 am
content: I have fixed a few transformers that had thermal fuses in the primary blown by nearby lightning strikes. In those cases the fuses were on top of the top layer of winding under just a few layers of tape. I agree though that finding a suitable replacement transformer ought to be fairly easy.

author: Bicurico
author_rank: Super Contributor
timestamp: November 24, 2020, 02:48:25 pm
content: Thanks!This is by no means urgent, as I don't really use the Datasette. However, I would like to know that my C128DCR is 100% functional.I have seen a 220V/9V AC transformer of similar size and shape at our local electronics store, so next time I go there I will get one. Interestingly the price is around 8 Euro, while AliExpress charges around 7 Euro with shipping (take more than a month). So, no need to go the China route. At this price it is not worth trying to have the existing transformer rewinded...Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: November 24, 2020, 08:42:18 pm
content: Update:Could not resist and went to the electronics store.They only had a 220V-9V dual output but it was cheaper.Replaced the broken one and now have cassette working!Cheers,Vitor

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 25, 2020, 03:09:30 am
content: Wow! This is the first time I hear about the 9VAC transformer blowing in a 128DCR! Good job on the repair. I also have a 128DCR. Must admit I never used a Datasette on it. Is it useful in 128 mode?

author: Bicurico
author_rank: Super Contributor
timestamp: November 25, 2020, 08:39:09 am
content: Yes, it was a strange one. It seems that it is much more frequent that the fuse for the 9VAC blows. But indeed the transformer had its primary coil broken.The Datassette is not a very useful device, as it manages to be even slower than the 1541 disk drive without any fast loader.Recordings made in C64 or C128 mode are interchangeable: you can record a C64 BASIC program on tape and load it back in C128 mode. If it is not using PEEK&POKE it will even run.The same is true in opposite direction, as long as only BASIC V2 commands are used in C128 mode.I did have a hickup: initially I was not able to load files in C128 mode. My first fear was that the repair was not 100% successful. The loading process in C128 mode would show "Found file...", but then, after pressing SPACE, it would load forever, much past the end of the file. I tried this on another C128 I have and due to finding the same symptoms, I concluded that I must have been doing something wrong.Turns out that in C128 mode, tape operation is a little enhanced compraded to the C64: pressing SPACE will NOT load the found file, but instead keep searching for the next file (which I did not have on my test tape). In order to load the found file, the "C=" key needs to be pressed!Conclusion:1) Strange repair - did not find any reference to the 9V transformer being broken.2) Datassette is not particularily useful at all, especially when you have a real 1571 (as is the case in my C128DCR) and/or a PI1541.3) But it definitely gives you a big retro-feeling! I am considering storing some big programs on tape, just to be able to load them from tape: the nostalgia of having a loading process take 5-10 minutes is something REALLY retro in our modern fast paced world, where we need high speed SSD in combination with smart OS that actually loads files BEFORE we ask them to, so they instantly open. No wonder people are having mental breakdowns, with all the speed that is expected from us!Regards,Vitor

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 25, 2020, 04:11:25 pm
content: Congrats on the repair! It's indeed a nice retro feeling with the datasette; but also nice to know, that things have improved since that time. Unfortunatelly I don't remember anymore where I went with my C64 after liquidating our junior chemistry lab where it stood when our teenage ways splitted somewhere in the late 90'. It's a shame!