Restoring the BT Cheetah 85
Verfasst: So 3. Nov 2019, 15:57
Some of you are probably sick of hearing about the mad happenings in my radio shack, but for those of you that are slightly interested, please see some of the ramblings from me regarding the ongoing process of restoring the BT Cheetah 85 Teleprinter.
I must firstly thank Rainer, Henning, and Werner for their help so far-each of you have played a part in getting this far.
So, to set the scene.
The British Telecom Cheetah 85 and 87 Teleprinters are both very similar electrically, they were a joint venture with SEL Lorenz, and the Lo3000 is almost exactly the same machine in a slightly different case. This machine appeared on eBay earlier this year, and was a fairly good price (£75GBP) It was located in the North West of England, so required a 3 hour drive to collect it - At this point it is fair to say that my long suffering Wife accompanied me for this trip, and she does not share my enthusiasm for all things old and electrical.
My original plan was to use this machine as a source of parts to repair my other Cheetah - Henning and Werner had been kind enough to supply me with a spare set of Lo3000 boards to try to get this machine working, but this was not proving to be a good solution. I decided that before I disturbed anything, I would try a short power up test, to see if there was any life at all - the machine had sat in my workshop for about 5 months, so should have dried out from it's previous agricultural storage. Power was applied, and one finger was left on the mains switch just in case the power supply decided to turn into a smoke grenade.
Much to my disappointment the power supply behaved correctly, and the Cheetah started making encouraging bleeps, indicating that it would like a fresh roll of paper - for some reason the reset button would not shut the beeping up, so I had a look at the screen.
It reported, quite correctly that the battery was faulty, and all options had been lost.
So, it seems much fairer at this point to allow this new machine to live.
Happy that this proved the machine was at least partly viable, I then shut everything down and removed the as yet un-detonated smoke grenade - first order of business would be to prevent any magic smoke from escaping, as Lo3000 owners will know, the power supply module is not the most reliable thing after some 15+ years of service, and then a 25 year retirement period.
Here in the UK we are well versed in the activities of the RIFA class X and Class Y supression capacitors, and have considerable experience of their behaviour when woken from an extended sleeping period. As much as I enjoy the excitement of a smoke filled room, these components were the first thing to be removed from the PSU.
Based on the experience of Patrick, and my previous knowledge of Switch Mode Power Supplies, I have also ordered a full set of replacement electrolytic capacitors - Most of the components in this PSU are still available, but with the advance in technology, they are all a lot smaller! Wound components are all obsolete, so prevention is a major part of this area of restoration - Where safety or reliability are a concern, originality is sacrificed, and modern components are used.
The PSU is different from the Lo3000 in that all the components are on the one PCB - this module appears to be a replacement that made it's way into the machine right at the end of the lifespan of Telex in the UK
I must firstly thank Rainer, Henning, and Werner for their help so far-each of you have played a part in getting this far.
So, to set the scene.
The British Telecom Cheetah 85 and 87 Teleprinters are both very similar electrically, they were a joint venture with SEL Lorenz, and the Lo3000 is almost exactly the same machine in a slightly different case. This machine appeared on eBay earlier this year, and was a fairly good price (£75GBP) It was located in the North West of England, so required a 3 hour drive to collect it - At this point it is fair to say that my long suffering Wife accompanied me for this trip, and she does not share my enthusiasm for all things old and electrical.
My original plan was to use this machine as a source of parts to repair my other Cheetah - Henning and Werner had been kind enough to supply me with a spare set of Lo3000 boards to try to get this machine working, but this was not proving to be a good solution. I decided that before I disturbed anything, I would try a short power up test, to see if there was any life at all - the machine had sat in my workshop for about 5 months, so should have dried out from it's previous agricultural storage. Power was applied, and one finger was left on the mains switch just in case the power supply decided to turn into a smoke grenade.
Much to my disappointment the power supply behaved correctly, and the Cheetah started making encouraging bleeps, indicating that it would like a fresh roll of paper - for some reason the reset button would not shut the beeping up, so I had a look at the screen.
It reported, quite correctly that the battery was faulty, and all options had been lost.
So, it seems much fairer at this point to allow this new machine to live.
Happy that this proved the machine was at least partly viable, I then shut everything down and removed the as yet un-detonated smoke grenade - first order of business would be to prevent any magic smoke from escaping, as Lo3000 owners will know, the power supply module is not the most reliable thing after some 15+ years of service, and then a 25 year retirement period.
Here in the UK we are well versed in the activities of the RIFA class X and Class Y supression capacitors, and have considerable experience of their behaviour when woken from an extended sleeping period. As much as I enjoy the excitement of a smoke filled room, these components were the first thing to be removed from the PSU.
Based on the experience of Patrick, and my previous knowledge of Switch Mode Power Supplies, I have also ordered a full set of replacement electrolytic capacitors - Most of the components in this PSU are still available, but with the advance in technology, they are all a lot smaller! Wound components are all obsolete, so prevention is a major part of this area of restoration - Where safety or reliability are a concern, originality is sacrificed, and modern components are used.
The PSU is different from the Lo3000 in that all the components are on the one PCB - this module appears to be a replacement that made it's way into the machine right at the end of the lifespan of Telex in the UK