hot transformer

[glenn.f...@gmail.com]

On my “gaming” H8 setup (same one I demo’ed at VCF East last fall) I have a problem with the power transformer – it gets extremely hot.

 

This machine has the original Heath power supply but all new boards, including:

                Rev 3.1 Z80 CPU

                H37/67 combo board (no H17 piggyback installed)

                New HA-8-3 clone board

                New front panel board

 

All of the boards have buck regulators and run very cool. The only thing generating heat is the transformer.

 

I haven’t made any measurements, nor have I removed the power supply cover to visually inspect.

 

Thoughts?  My initial thought is it must be one or more power supply components, possibly very high leakage current in the large filter capacitor? Could the transformer be damaged?  Any experience with something like this?

 

I guess one way to test my theory is to run it with no boards connected and see if it still gets hot?

 

Any advice on how to diagnose/repair would be appreciated!

 

• Glenn

 

[Joseph Travis]

Glenn,

How about checking it as a minimal setup (I.e. just the CPU, memory, H17) and compare your findings?  If it still runs hot, it could be something in the power supply itself.

Regards,

Joe Travis n6ypc

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[Steven Feinsmith]

You need to measure current with DMM to ensure it is not excessive per design. Remove all boards except for CPU and memory. The transformer became hotter due the current was abnormally high. It is possible the electrolytic capacitors leaked or a bad regulator(s). Also, measure the voltage to ensure they stay the specification.

Steven

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[Mike Ladwig]

I had an old guitar amp power transformer fail just after I noticed it getting unusually hot. I think that one was just age and internal resistance.

For the H8 I'd start with the power supply checkout procedure in the manual -- I think it diagnoses issues with all the various filter caps. If that checks out OK, then measure temperature, remove all the boards, measure temperature again. If it's still hot the transformer, if not add boards back one at a time.

[glenn.f...@gmail.com]

Since I have an available chassis I switched the “Gaming” boards into that and I am running the “hot transformer” chassis with just  a single Z80 board.  Will monitor this configuration for a bit and see what I can learn.

 

I realize to go any further I’ll have to do some disassembly and measurements but for now I’m taking simple steps first…

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[glenn.f...@gmail.com]

After ½ hour or so the transformer in the “hot” chassis was getting quite warm, even though all it was powering was a CPU board.  The 3-board “gaming” configuration, now in a different chassis, is running nice and cool

 

So this would seem to point to a likely power supply issue on the “hot” chassis.  Will investigate further…

 

• Glenn

[Glenn Roberts]

Wow three months later I finally got to look at this overheating H8.  Via a process of elimination, I believe the issue was the transformer itself!.   Perhaps an internal short in the windings?  I replaced everything else a bit at a time and the problem persisted.  I was saving the transformer for last because it’s a pain to remove.  But when I replaced it (I had a spare from an old rescue system) the problem is solved – no more overheating transformer.  I also cleaned up some of the connections along the way – some of the solder joints were not up to snuff… (I used a high wattage iron on these power supply connections).

 

Curious of anyone else ever had this issue… a first for me…

 

So that’s one more thing off my Heathkit “to do” list…

[Joseph Travis]

Over the hundreds of Heathkit computers that I've serviced, I've never replaced a power transformer.

On a related but different subject, are you using a fan to cool your H8s?  In my original H8, I used two 4" fans installed into the left side panel and connected them to a motor speed controller to reduce the noise level.  Nowadays I have a 3" fan from a H89 that I glued 3 mini super magnets to and place it inside the H8 (without any additional cutouts) to circulate the air.  I find this works very well. 

Joe

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[glenn.f...@gmail.com]

Joe: I feel the same way.  And still kind of doubt that there’s anything wrong with the transformer, but I’m struggling to figure out what else could have been the culprit.  Anyway things are working fine now.

 

No I don’t have fans in any of my H8s.  the ones I use day today mostly have buck regulators  on the boards, which keeps things quite cool.  I also have two systems that use PC ATX style power supplies  and that *really* keeps things cool.

 

There certainly were boards that notoriously ran hot in the old days.  Memory boards were real culprits, I’m thinking of the Trionyx and Godbout RAM boards.  I think the Trionyx had four 7805s mounted on the heat sink!?  Also the original  HA-8-3 graphics board runs pretty hot.  So I can imagine that if you stuffed your machine with multiple such boards there was a real issue back then…

 

But I’ve always stuck pretty religiously with the Heath design philosophy of convective cooling and, maybe I’m just lucky, but never had a problem with heat in the chassis…  I am always careful to attach the heat sink bar across the top of the boards (but, admittedly, rarely secure the boards with the bottom screws 😊  )….

 

• Glenn

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[Lee Hart]

Also check the rectifier diodes in the power supply. I've had cases where one shorted, which overloaded the transformer, yet you still got DC voltage (halfwave rectified instead of fullwave). The fuse should blow if this happens; but sometimes someone has replaced it with a higher current one!

Lee

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[glenn.f...@gmail.com]

I did check the diodes and they are OK.  I didn’t think to check the fuse but you make an excellent point.  It would be easy for a previous owner to stick the wrong fuse in there.  Just checked and it’s a 1A slo-blow so it’s good.  Looks to me like it’s probably the original fuse!, so I replaced it with a new one just to be safe.

 

The machine is running nice and cool now so problem solved.  the only other thing that I did was in replacing the transformer I basically fully rewired everything, following the original assembly instructions.  There were quite a few poorly-done connections in there… perhaps there was one bad enough to introduce some resistance?  Dunno.  FYI the machine was running fine otherwise – I saw no other side effects of the issue, just the very hot transformer…

 

Anyway all’s good now… thanks

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