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zippysticks
May 4th, 2018, 01:05 AM
Can anyone recommend a replacement transformer for the Northstar Horizon please ?

Mine seems to be producing around 20V and 10V rather than 16V and 8V, regulators on both memory boards are uncomfortable to touch (hot!) and it appears that over time the heat has discoloured the PCB around the heatsink mount (perhaps this is normal?) so I'm looking to try and get things back closer to spec.

I did look at the article below where another owner has done similar but the spec of that transformer does not look correct.

http://www.connect.gi/northstar.htm

Chuck(G)
May 4th, 2018, 08:57 AM
Transformers for S100 systems of that genre were usually custom jobs (not expensive back then if you wanted to order 100 or 1000 of them).

Instead of replacing the transformer, consider adding a small one in the primary circuit as a "buck" source. To elaborate: you want to reduce the output voltage by 20%, so reduce the input voltage by 20%, which would be 0.2 x 120V = 24V. So power a 24VAC output transformer from the line and hook the secondary in series with the main power transformer primary, so it "bucks" the AC input. Note that the output current rating only needs to match the primary current rating of your main transformer.

Just an idea...

MikeS
May 4th, 2018, 09:21 AM
What do you think is wrong with the specs?

I assume the transformer doesn't have a multi-tap primary?

I've got a few transformers from scrapped systems but the shipping would be a killer.

Chuck's idea sounds like the way to go, although it looks like you'd need a somewhat harder to find 48V transformer (assuming we're talking 240V); if nothing else connects to the primary (8" rives etc.) you could probably even put it in an external box.

m

zippysticks
May 4th, 2018, 09:24 AM
Its a good idea Chuck - thanks. Not sure I have the physical space to fit another transformer in the case.
I presume you meant find a transformer with 120V-(120*0.2) = 96V output in your example or 192V for me as I'm in the UK using a 240V supply ?

EDIT: Found this - seems to explain the 'buck' approach more clearly to me:

http://sound.whsites.net/articles/buck-xfmr.htm

This was quite an interesting note I found :

http://www.retrotechnology.com/herbs_stuff/s_power.html

Seems to imply my situation is quite common with (lightly loaded) Horizons and suggests a few other ways to reduce the voltages including variac or 'buck' mechanism

zippysticks
May 4th, 2018, 09:28 AM
Hi Mike - really just that they are 20% over spec - although my system is lightly loaded with just CPU, memory, floppy controller and twin floppy drives.
Despite already lasting 40 years, I don't like the regulators running that hot on the memory cards.

Chuck(G)
May 4th, 2018, 10:39 AM
There are a couple more ways to skin this cat. You could simply insert a fixed voltage drop with a couple of power rectifiers in series (0.7V per diode) or use an LDO regulator (e.g. LM1084-ADJ, which is rated at 5A). Power transformers are usually designed to deliver the nameplate voltage at a specified current. For example, I've got a couple of "control" transformers here that are rated for 12.6V RMS at 4A. No-load on these is closer to 15VRMS. No matter what you use, absent some sort of switching regulator, any llinear approach that drops the DC voltage directly will dissipate heat. So, to drop 2V at 5A, you're going to need to get rid of 10W of heat.

The other issue is that with a simple capacitor-input filter, the no-load output approaches the peak-to-peak AC voltage output of the transformer, which is about 1.414 x the nameplate RMS voltage.

Another approach would be to replace the 78xx linear regulators with 3-terminal drop-in switching regulators. There, the input voltage doesn't make as much of a difference, since the regulator only draws what's needed to maintain the output. But that would entail replacing the on-board regulator(s).

MikeS
May 4th, 2018, 02:24 PM
Hi Mike - really just that they are 20% over spec - although my system is lightly loaded with just CPU, memory, floppy controller and twin floppy drives.
...

Ah, OK; I thought you meant the replacement: 8.5 and +- 17.5 sounds reasonable.

Wonder where he got it; does he mention it anywhere?

m

hjohnson
May 7th, 2018, 02:01 PM
Thanks for the compliments about my notes on that S-100 Web page. I ran into similar problems on an H-89 I was restoring: DC voltages too high due to AC primary voltage too high; and marginal-voltage tantalum caps. I've added a link from that S-100 Web page, to my H-89 notes at:

http://www.retrotechnology.com/restore/h89_4th.html

The general problem with excessive unregulated DC voltages on vintage 70's computers, seems to be that modern AC line voltage is higher, near or above 120V AC, than AC line voltage decades ago. Solutions have been discussed in this vcfed thread; my two Web page go through some of the details. I suppose I should explain a bucking transformer configuration, the details aren't obvious, Lee Hart explained them to me.

Herb Johnson

1980s_john
June 7th, 2018, 02:06 PM
Its a good idea Chuck - thanks. Not sure I have the physical space to fit another transformer in the case.

EDIT: Found this - seems to explain the 'buck' approach more clearly to me:

http://sound.whsites.net/articles/buck-xfmr.htm


Wow thanks for the link, figure 4 looks to be the way to go. A 230V to 18V (9V + 9V) or 24V (12V + 12V) transformer should be easily available. I would suggest mounting this in a separate box with a fuse on the input, and feed the output to a chassis socket. The only caveat I would have is a lot of transformers are not marked with a dot, so you would need to power it on with nothing connected and check the output voltage is lower than 230V. If wired incorrectly the output would be higher.

Regards,
John

GiGaBiTe
June 8th, 2018, 11:54 AM
The general problem with excessive unregulated DC voltages on vintage 70's computers, seems to be that modern AC line voltage is higher, near or above 120V AC, than AC line voltage decades ago.

While the mains voltage being higher is a factor, it's not the primary cause of transformer voltage drift.

The windings start shorting over time as the varnish on the copper migrates or deteriorates, which can be accelerated by excessive heat. The single core wound types where the primary/secondary are wound on top of each other tend to have worse problems with it, while the dual bobbin type have less since they dissipate heat better and aren't generally wound as tightly. The shorts can happen on both the primary side and the secondary side, I've pulled dozens of transformers apart which had primary, secondary or both which had shorts. When you start unwinding the copper wire, the varnish will be irregular in glops or waves with often many exposed section of copper.

This shorting process can happen very quickly, I've had transformers not a year old measure 20% out of spec and it only gets worse with time. The worst transformer drift case I had was a 9v for a game console that drifted upwards in excess of 24v, itself being about 27 years old. This transformer hadn't been in use most of that time either, which shows it doesn't need to be used to drift.

It's always a good idea to check linear transformers before you use them so they don't nuke something with too much voltage.

glitch
June 8th, 2018, 01:07 PM
Fact of life with lightly loaded S-100 systems using a regular transformer. Later power supplies compensated by using a CVT (constant voltage transformer, uses a separate winding and a capacitor to keep the transformer saturated). Of course the final evolution in S-100 was regulated switchmode supplies.

You can fill the system up with boards to drop more voltage, increase cooling air flow (especially when you have the cover off!), run a variac on the line input, etc. I use the variac approach if I'm going to be using my IMSAI for a while with very few boards and the top cover off. If you don't mind modifying your boards, there are new drop-in replacement regulators that are little switchers designed in a TO220 footprint. They run cool despite the higher +8V rail.

GiGaBiTe
June 8th, 2018, 08:46 PM
there are new drop-in replacement regulators that are little switchers designed in a TO220 footprint. They run cool despite the higher +8V rail.

The problem with these retrofit switching regulators is that they have super noisy output. It may not cause problems with the machine running properly, but it can introduce noise in audio circuitry and cause distortions on video output, especially analog like CGA/EGA/VGA or composite.

Dwight Elvey
June 9th, 2018, 06:17 AM
Do note that, for a buck/boots, it is only the secondary current rating that you need to worry about. Do check the ripple voltage on your power supply. If it drops below the 2 volt head room that the regulators need, you'll have issues.
I recently worked on a unit that looked to have fine voltage with a meter but it had one half of the bridge circuit blown out. The ripple was dropping below 1 volt headroom for the regulators. This cause erratic operation.
I've seen cases were the design of the boards did not account for all the power needed by the board. Resistors were placed in parallel to remove some of the power being dissipated in the regulators. Like was said, to doesn't change the total heat, but it does move some way from the regulator. A shorted regulator can do a lot of damage!
Dwight

glitch
June 9th, 2018, 07:03 AM
The problem with these retrofit switching regulators is that they have super noisy output. It may not cause problems with the machine running properly, but it can introduce noise in audio circuitry and cause distortions on video output, especially analog like CGA/EGA/VGA or composite.

This is true. Especially if you go with the cheapest available models.

As I mentioned, I just plug in the variac if I'm going to be running the IMSAI with nearly no load and the cover off. Also, if I've got a board up on an extender, out of the normal chassis airflow, I'll aim a small desk fan at it.

jimwatt
June 11th, 2018, 05:57 AM
Can anyone recommend a replacement transformer for the Northstar Horizon please ?

Mine seems to be producing around 20V and 10V rather than 16V and 8V, regulators on both memory boards are uncomfortable to touch (hot!) and it appears that over time the heat has discoloured the PCB around the heatsink mount (perhaps this is normal?) so I'm looking to try and get things back closer to spec.

I did look at the article below where another owner has done similar but the spec of that transformer does not look correct.

http://www.connect.gi/northstar.htm

A lot of thought and testing went into that, the transformer was custom made as a result. I calculated the
voltages in excel and built a test power supply before ordering.

If I made another I might include more voltage taps on the primary to allow for 220, 230 and 240v
but certainly for my Horizon it works nicely and the regulators on the boards no longer cook.

The machine is an early Horizon and NorthStar issued a transformer upgrade later to allow for powering
eight UP-8 slave processor cards fitted and a 5.25 HDD.

However if you are in the UK I can strongly recommend Eastern Transformers Ltd as being responsive and
producing a custom transformer at a very sensible price. I asked another company to quote and got no answer
but the transformer made was exactly the right size mechanically. It looks better than the original.

AND it works.

zippysticks
June 12th, 2018, 10:54 AM
I'm using Eastern Transformers but have taken the variac route with them - they do quite a nice unit HERE (https://www.ebay.co.uk/itm/Enclosed-Variable-Autotransformer-Variac-for-Bench-Mounting-1ph-10A/332174704857)

jimwatt
June 29th, 2018, 06:27 AM
I did not include the transformer spec on my website but have now;

SE250 1PH 220VA double wound transformer made to order
Primary: 0/240Vac centre tap
Secondary 1: 0/12Vac @ 60VA
Secondary 2: 0/12Vac @ 60VA
Secondary 3: 0/6 Vac @ 50VA
Secondary 4: 0/6 Vac @ 50VA

Remember the voltages quoted are RMS, not peak and there is a voltage drop on the diode.

With that transformer the voltages on the motherboard with a basic set of cards is
8.5v 17.5v and -17.5v and everything runs cool.

The transformer is an exact physical replacement for the NS one.
Cosmetically better looking with screw connections rather than flying leads.

Otherwise its ... a transformer.

zippysticks
July 5th, 2018, 11:16 AM
Thanks Jim - I might get one of those too.

jimwatt
July 10th, 2018, 02:49 PM
Incidentally my input voltage is 245v yours may be 230v so you need to consider primary taps;

I did a centre tap to allow running a 240v fan at 120v for reduced speed as it cuts the noise down dramatically. The airflow is adequate.

Having got this far and with two decent half height floppies working I need to spend some time finishing the rebuild.