Proper voltage for ZX81 AC adapter

Discussions about Sinclair ZX80 and ZX81 Hardware
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povvercrazy
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Re: Proper voltage for ZX81 AC adapter

Post by povvercrazy »

Good points Mark, a lot of people stuff there power supplies down the back or hidden in a corner and then wonder why they get hot!
Or even coil cables up while on!
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msellan
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Re: Proper voltage for ZX81 AC adapter

Post by msellan »

Hi Mark,

Thanks for the explanation! I knew that without load the voltage from the supply would be higher (but not by how much, exactly). What I didn't know (I was once a programmer not an engineer ;) ) was about calculating AC RMS peak and the multiplier. I love learning new stuff on here. :D

So for equally spec'd wall bricks (both 9.75V and 600ma) with different outputs of 12.xx volts vs. 14.xx volts is just due to the variance/tolerances of the transformers in the two units?

I have been thinking about replacing my 7805 regulators in at least a couple of my 'driver' zeddys just because I lost a ULA in one of them last week. :cry: From what I've read on here, that's the best thing to do to help prolong the life of a ULA by reducing current and therefore reducing heat.

The price of the regulator replacements seems higher than I would have thought @~ $7.00 each (although way cheaper than buying a 'parts' zeddy to steal the ULA - note: that makes me much sadder than tearing out an RF modulator ;) ) Does that price seem about right?

Best,

-mark
3 ZX80s (1 UK issue 2, 1 US issue 2 & 1 US issue 1), 5 ZX81s, 1 TS1000, 2 Memotech 16k RAM,
2 ZX81 16k RAM, 2 TS 16k RAM, 1 ZX81 unbuilt kit, 1 TS2040 printer, 2+ ZX8-CCB and counting
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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

msellan wrote: Thu May 11, 2017 9:31 pmSo for equally spec'd wall bricks (both 9.75V and 600ma) with different outputs of 12.xx volts vs. 14.xx volts is just due to the variance/tolerances of the transformers in the two units?
Yes. Keep in mind that in the U.K. Sinclair used at least four different (likely more) suppliers for the UK700 PSUs. It's likely the same story with the other PSUs.

The price for the DC-DC switching type regulators varies a fair bit. So shop around. See this post of mine in another thread in answer to someone in the U.K. asking about the same subject.

Mark
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msellan
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Re: Proper voltage for ZX81 AC adapter

Post by msellan »

Thanks Mark - getting the actual part numbers from the other thread was helpful. I've got an order to put in with Mouser so I'll pick up a couple.

I have another question about power adapters :? - one of my "Sinclair" labeled power supplies makes a pretty loud buzzing noise. It's not the typical high frequency hum but more like a vibrating buzz. Is there anything to do to fix it?

The person who originally owned the zeddy clipped the plug off of the supply and soldered the wires directly to the zeddy pcb. I was thinking that I could remove it from the zeddy and then pry it apart and just replace the whole cable (I know that's not going to fix the buzz). But I've never tried to open a power supply before and wasn't sure how feasible that is.

Best,

-mark
3 ZX80s (1 UK issue 2, 1 US issue 2 & 1 US issue 1), 5 ZX81s, 1 TS1000, 2 Memotech 16k RAM,
2 ZX81 16k RAM, 2 TS 16k RAM, 1 ZX81 unbuilt kit, 1 TS2040 printer, 2+ ZX8-CCB and counting
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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

The Sinclair UK700, UK1200, UK1400 (for ZX Spectrum) all are held together with just three screws in the bottom.

This video is about ZX Spectrum PSUs, but shows a UK700 PSU as well.

If yours are different, post up some pictures ;)

Mark
gozzo
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Re: Proper voltage for ZX81 AC adapter

Post by gozzo »

many 50/60hz transformer based psus 'buzz' , especially if the laminations are a bit loose, normal,. but, if it gets warm/hot even with no load, could have shorted turns ... loud buzz on load could indicate problem with one or more of the diodes gone open circuit, making it half wave instead of full wave rectifying, ,, this may make it overheat....
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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

msellan wrote: Fri May 12, 2017 4:57 amOne of my "Sinclair" labeled power supplies makes a pretty loud buzzing noise. It's not the typical high frequency hum but more like a vibrating buzz. Is there anything to do to fix it?
As Gozo says, it's the iron laminations vibrating at mains frequency. Sometimes varnish dripped on to the iron laminations can help.

Mark
gozzo
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Re: Proper voltage for ZX81 AC adapter

Post by gozzo »

..thin the varnish down a bit to help it seep in all the spaces..... you can get proper 'transformer varnish', but i've used normal wood varnish before, maybe not advisable really as i think the proper stuff is less flammable in case of gross overheating....but the thermal fuse should pop long before it gets to that state!
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msellan
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Re: Proper voltage for ZX81 AC adapter

Post by msellan »

@gozzo and @1024MAK thanks!! Your expert advice is always helpful! The real trick it seems now is to figure out how to open these things up. All I have are the 'wall wart' bricks like this:

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IMG_0782a.jpg
IMG_0782a.jpg (272.54 KiB) Viewed 2809 times
IMG_0783a.jpg
IMG_0783a.jpg (214.75 KiB) Viewed 2809 times
They seem really brittle. With laptop cases it's common to find hidden clips that with the right pressure at certain spots will pop them open. I wonder if these have a similar setup?

Best,

-mark
3 ZX80s (1 UK issue 2, 1 US issue 2 & 1 US issue 1), 5 ZX81s, 1 TS1000, 2 Memotech 16k RAM,
2 ZX81 16k RAM, 2 TS 16k RAM, 1 ZX81 unbuilt kit, 1 TS2040 printer, 2+ ZX8-CCB and counting
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1024MAK
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Re: Proper voltage for ZX81 AC adapter

Post by 1024MAK »

The PSU with the cord that you want to replace, I have an alternative idea.
Instead buy a black table lamp switch (like this for example - ebay listing and use it to join the cable from the PSU with a new lead complete with plug suitable for the Zeddy ;)

Plastic cases with no fixing screws are held together by one of three methods:
  1. hidden clips (as you have said)
  2. glue
  3. plastic welds
Alas, unless you can see anything, it's anyone’s guess which method yours are held together with.

Mark
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