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Old 8th Aug 2020, 11:23 pm   #1
Uncle Bulgaria
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Default Uher Miniline 220V mains drop

Having had it pointed out to me that my strange overvoltages in the Uher system were due to a 245V mains supply on a 220V unit, I looked up means to drop this voltage. The Uher Z140 power amplifier is running extremely hot, and I don't see a need to overstress it. I checked with my variac and its power meter and it suggested I was having to dissipate an extra 10W in a case that's already made of heatsinks.

I've seen possibilities involving new transformers or dropper resistors, both of which are more suited to valve radios with rather larger chassis. This 1978 block runs an SMPS and has power transistors bolted to the finned sides of the case which are the heatsinks. As you can see from the pictures, there isn't room to put in another transformer, and I'm concerned about adding another source of heat to the case by using a resistor. I used it to supply sound for a film instead of the TV's built in 'speakers and by the end it was hard to touch the sides of the Z140 case as they were so hot.

Would dropping the 20V and 10W or so be noticeable in terms of heat dissipated? Would the components sing me a thousand thank yous if they could speak?

Dropping the mains before the SMPS would bring all the subsequent voltages into line, but I can't see a convenient way to do this in the existing circuit, unless L802 can be replaced with a transformer.

Is it worth it, or am I being silly?
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Old 9th Aug 2020, 7:23 am   #2
wd40addict
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Default Re: Uher Miniline 220V mains drop

Are you sure the switch mode supply is working correctly? Do its output voltages (+/-32v) change when you wind the input down from 245 to 220v?

With a properly working switch mode there should be no change.
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Old 9th Aug 2020, 7:27 am   #3
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Default Re: Uher Miniline 220V mains drop

I have been rescuing a Uher 263 recently and alarmed at the temperature that the motor reaches. As they is a reasonabl e amount of space I added a 200 ohm TV type mains dropper section strategically placed to allow the heat to rise to such vents as there are. This drops 40v (in series with the mains supply) and has dramatically reduced the motor temperature but not affected performance in any way
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Old 9th Aug 2020, 8:10 am   #4
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Default Re: Uher Miniline 220V mains drop

The safest way of reducing the mains voltage is by using a bucking transformer.
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Old 9th Aug 2020, 8:54 am   #5
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Default Re: Uher Miniline 220V mains drop

My initial thought was similar to PJL's, except I would make it an auto-transformer rather than buck transformer.

Then you say running it at 220 volts will drop 10 Watts? So the question is how much power does it use in normal operation at 220 volts? Not idle but in real operation. What percentage of that power is 10 watts?

Run it for an hour or two at 220 volts on the variac, then see how hot the heatsinks are. If they are much cooler there is a benefit in dropping the volts. If there is little or no difference in temperature, that is how it is...

Somewhere I have read if you can touch the case for five seconds the temperature is 65 degrees C or less.
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Old 9th Aug 2020, 1:10 pm   #6
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Default Re: Uher Miniline 220V mains drop

The power the variac reports in use is 55W with VG 840 amplifier and EG 740 Tuner (which takes its power from the Z140 via the VG 840) both on at 240V, with the CR 240 cassette deck playing through the amplifier. This drops to 45W at 220V. So it's a hefty chunk. With the tuner off, it's 30W at 220V and 40W at 240V.

I had it on all day yesterday for TMS on MW, so had all three boxes on at once (on 220V). The difference between the burning after watching the film, and the rather more palatable warmth during the cricket was palpable.

In my original thread on getting the VG 840 working, I was worried about the high +20V rail, looking for problems before Lawrence pointed out that the Z140 was meant for 220V operation. Winding it down brought it back to the manual's ratings. The other rails are regulated in the 'stabiliserung' section so were close to their rated operation anyway, but presumably having to dissipate the excess somewhere.
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Old 9th Aug 2020, 1:23 pm   #7
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Default Re: Uher Miniline 220V mains drop

You can of course drop some volts with an external light bulb in series with the mains, just as with a lamp limiter. You could also fit a resistor in an external box.
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Old 9th Aug 2020, 1:47 pm   #8
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Default Re: Uher Miniline 220V mains drop

Oh sure, I was just wondering whether there was some way to keep the trimness of the original casings, as they've made such an effort to fit them into their matching cases! It seems a bit inelegant to have to make up external boxes (and as mentioned in another thread about mains dropping, without strapping it to the unit it can get separated).

I don't understand the purpose of L802 in the circuit. It doesn't have the same symbol as the other chokes or transformers, but must be an inductor of some kind. As it's straight off the mains, I wondered whether that would be a position to include some kind of internal dropper.

As you say, I can always make up an external unit which wouldn't have to be too big - perhaps I could even box it up as a 'Uher UK Adaptor', paint it black and have a go at Germanic industrial design labelling!
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Old 9th Aug 2020, 2:00 pm   #9
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Default Re: Uher Miniline 220V mains drop

An external bucking transformer should work and will reduce heat.
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Old 9th Aug 2020, 4:26 pm   #10
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Default Re: Uher Miniline 220V mains drop

Quote:
Originally Posted by Uncle Bulgaria View Post
... I don't understand the purpose of L802 in the circuit. It doesn't have the same symbol as the other chokes or transformers, but must be an inductor of some kind. As it's straight off the mains, I wondered whether that would be a position to include some kind of internal dropper.
That's a common mode choke. It is there to stop the noise from the oscillator section of the power supply getting back to the mains. The oscillator wave form is square wave so the hash it creates needs filtering. There are another two in there if you follow the diagram. One no number and one L807.
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Old 9th Aug 2020, 8:27 pm   #11
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Default Re: Uher Miniline 220V mains drop

Hi Folks, an auto transformer will drop the required voltage with very little heating and should also be much more compact than a bucking transformer (which it effectively replicates

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Old 24th Nov 2020, 6:20 pm   #12
Uncle Bulgaria
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Default Re: Uher Miniline 220V mains drop

Thanks Ed - I'm finally back on to looking at this. Now all the Uhers are back together it's time to wean them off the variac.

How would the autotransformer be more compact? Can't the same transformer be wired as a bucking and as an autotransformer? Can you give me an example of the sort of transformer I could use? I calculate I don't need more than 5VA for dropping 20V at 0.25A, which is the current the variac's display reports during use.
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Old 24th Nov 2020, 8:46 pm   #13
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Default Re: Uher Miniline 220V mains drop

in an auto transformer the primary section current is offset by the secondary section current so smaller diameter wire can be used than on a conventional transformer and a smaller lam set can be used.

That is a simplified explanation, for full details you need to look up the theory.
For maximum size reduction a 2:1 ratio is best

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Old 24th Nov 2020, 8:59 pm   #14
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Default Re: Uher Miniline 220V mains drop

I was recently looking at purchasing Uher Minilines and read that they have a potential design issue with the Z140 Power amp capacitors failing in the DC supply circuitry so disabling the DC outputs that are used by the EG 740 Tuner and VG 840 Pre-amp units.

Any knowledge of this, could it be related to running at 240 volts ?

David
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Old 24th Nov 2020, 9:10 pm   #15
Uncle Bulgaria
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Default Re: Uher Miniline 220V mains drop

Thanks Ed, yes I understand that as I was reading up on autotransformers for my capacitor reformer. I wondered whether there was any difference practically, as I've not managed to find autotransformers of this type available. If that's the case, then, as I understand it, all I can do is have the option of wiring the same transformer as either a bucking or an autotransformer.

David - I've read that online as well. I can't say I understand why that might be, or which capacitors. Mine hasn't blown up yet, except for one electrolytic I put in backwards last night while putting it back together tired. All the originals I took out tested fine for leakage and capacitance, including the giant 470F 350V. Surely one would just replace them with modern ones more suitable if they blew, as with any other equipment. I think there are some 35V ones on the 33V line.
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Old 24th Nov 2020, 9:23 pm   #16
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Default Re: Uher Miniline 220V mains drop

Yes I was not easily able to find out which capacitors were the so called ones that fail or what the actual issue (s) with them were. I think I also read that Uher subsequently released a separate PSU unit to provide the external DC supplies.

As mentioned in a couple of other older threads sometimes capacitors used by Uher are extremely close in their voltage ratings compared to the actual voltages they are used on.

David
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