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Old 14th Jan 2021, 8:21 am   #41
Diabolical Artificer
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Default Re: Selmer Bassmaster 50 - mains transformer

Good to hear your making progress. Noticed a mistake, the red wire (HT?) off the valve base, top left is going to what looks like a mains fuse,this then goes straight on to a big cap. 1) if it is 400v ish HT it should go to the bottom tag on the valve holder,anyone changing a fuse could get a belt. 2) What's the point in the wire/connection then going off to the cap? It's not fused.

Also noticed what looks like a zener,or resistor, top of tag board flopping about. Sorry,there's your,your caps in the smoothing section have long leads and as Dave Jones says are "flapping around in the breeze". Big electrolytic aps have mass and as such should be secured, every bump or knock will stress the solder joint leading to potential failure.

Looking again at the fuse I can see another fuse holder, looks a bodge of two fuse holders, hard to see what's going on, there's wires all over the shop, keep wires as short as poss,same colour for same part of circuit, EG red for HT, black for ground etc & keep your work neat and tidy. Remember,someone else will probably work on this after you, if they're a good tech & your work is good, it makes it easier for them & you'll get praise & not a roasting. Hope this comes over as constructive criticism rather than a personality execution.

Andy
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Old 15th Jan 2021, 4:29 pm   #42
Stretcher
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Default Re: Selmer Bassmaster 50 - mains transformer

Thanks Andy, I think I might survive! The issue with the red wire is an optical illusion, actually it is coming off the HT fuse above the mains fuse, going to the big capacitor, so fortunately no danger there. The amp originally had no HT fuse. I put it there so that I wouldn’t have to make a new hole in the chassis, and therefore keep things more original. However, you are right that it doesn’t look very neat, and also you would have to pull out the chassis to change it, so I am going to move it to a new holder in the back panel. The “flopping about” resistor you identify is me experimenting with values to get the bias for the output valves to the correct value, so will be replaced on a more permanent basis.

I got hold of a new meter, which pretty much agrees with my old meter within 2%. This shows that our mains voltage is around 230v, which might give some explanation why the heater voltages are a bit low. With the correct biasing, the HT voltage is now showing at 362v. The voltage at the secondary is showing as 281v AC. The open circuit secondary voltage with the valves out was 300v – is this a reasonable drop under load?
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Old 15th Jan 2021, 4:52 pm   #43
snowman_al
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Default Re: Selmer Bassmaster 50 - mains transformer

Quote:
Originally Posted by Stretcher View Post
With the correct biasing, the HT voltage is now showing at 362v. The voltage at the secondary is showing as 281v AC. The open circuit secondary voltage with the valves out was 300v – is this a reasonable drop under load?
Without knowing the spec of the replacement transformer that is difficult to say, but it is only a 5% drop so more than likely fine. Only way to tell is run it for an hour and monitor the transformer temperature. As long as you can keep your hand on it all should be well. Then do 2 hours and repeat. If it is the same temperature and not too hot to touch, job done?

PS. Glad you have it going again.
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Old 15th Jan 2021, 11:35 pm   #44
ITAM805
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Default Re: Selmer Bassmaster 50 - mains transformer

Glad you got it sorted. I've serviced a couple of Treble and Bass, nice amps and they're much more appreciated these days.

I'm puzzled by the your 'low' mains voltage though, I'm not doubting your meter readings but when I lived in London (south) I dont remember it ever being lower than 240/245V? It's 250V down here, sometimes creeping up to 253V?
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Old 16th Jan 2021, 6:27 pm   #45
Stretcher
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Default Re: Selmer Bassmaster 50 - mains transformer

Thanks Alan, I tried the test in my admittedly cold shed, with the chassis open on the bench, and the transformer got no more than warm after over an hour, so I am reassured.

Itam, I guess the voltage one gets at the socket is mainly dependent on the equipment in the local substation. I am usually getting around 231, up to 234v. Maybe our substation is equipped with the 230v standard in mind, or is usually overloaded!
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Old 21st Jan 2021, 4:40 pm   #46
Richardgr
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Default Re: Selmer Bassmaster 50 - mains transformer

Quote:
Originally Posted by ITAM805 View Post
I'm puzzled by the your 'low' mains voltage though, I'm not doubting your meter readings but when I lived in London (south) I dont remember it ever being lower than 240/245V? It's 250V down here, sometimes creeping up to 253V?
230VDC is the correct UK standard since 2009, when the UK harmonised with Europe.

Manufacturers usually allow a further margin of safety and, if the voltage does occasionally fall outside these limits, there should be no adverse effect on your appliances. In the UK, the declared voltage and tolerance for an electricity supply is 230 volts -6%, +10%. This gives an allowed voltage range of 216.2 volts to 253.0 volts.

Voltage changes

So your 253 is a bit 'hot'.
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