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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 8th Dec 2023, 5:45 pm   #21
Richardgr
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Default Re: Old Philco A3756 hi-fi amp safety question.

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Originally Posted by 1valve2many View Post
is that to do with there plugs?, i read that they can be put into a wall socket either way, so with no earth, the power can reverse . very dangerous

thank you for your reply
Not ideal, but always safer to have double pole on-off switch because a lot of older items or houses might have had bad wiring anyway, and Euro plugs are earthed but can be fitted each way.

I think the original reason for the cap was to protect the on-off switch a bit from arcing. Nowadays there are X and Y rated capacitors that fail predictably and safely.
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Old 9th Dec 2023, 9:21 am   #22
Edward Huggins
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Default Re: Old Philco A3756 hi-fi amp safety question.

I can understand the OP being concerned over safety, but in working with mains powered valve technology this is an ever-present danger. This is where experience comes in. But in an attempt to offer re-assurance, if a cap has not been in use for some time, as in an old record-player, the chances of it still holding any charge is unlikely.
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Old 17th Dec 2023, 4:49 am   #23
1valve2many
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Default Re: Old Philco A3756 hi-fi amp safety question.

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Originally Posted by Edward Huggins View Post
I can understand the OP being concerned over safety, but in working with mains powered valve technology this is an ever-present danger. This is where experience comes in. But in an attempt to offer re-assurance, if a cap has not been in use for some time, as in an old record-player, the chances of it still holding any charge is unlikely.
many thanx to all for your input, it is much apreciated

can i ask, how long is long enough, its not on my list of to do things. i wish to gain more experiance in electronics first, and im certainly not turning it on if i have made anything untill an electrician has tested it. everything i have baught so far is still in the box it arrived in. i just had a spurt of buying stuff as the price was so low. many thanx tony
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Old 17th Dec 2023, 4:49 am   #24
1valve2many
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Default Re: Old Philco A3756 hi-fi amp safety question.

Quote:
Originally Posted by Richardgr View Post
Quote:
Originally Posted by 1valve2many View Post
is that to do with there plugs?, i read that they can be put into a wall socket either way, so with no earth, the power can reverse . very dangerous

thank you for your reply
Not ideal, but always safer to have double pole on-off switch because a lot of older items or houses might have had bad wiring anyway, and Euro plugs are earthed but can be fitted each way.

I think the original reason for the cap was to protect the on-off switch a bit from arcing. Nowadays there are X and Y rated capacitors that fail predictably and safely.

many thanx to all for your input, it is much apreciated tony
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Old 18th Dec 2023, 11:34 pm   #25
knobtwiddler
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Default Re: Old Philco A3756 hi-fi amp safety question.

I would classify converting an old record deck into a guitar amp as an activity best undertaken by someone with intermediate / advanced knowledge of electronics. My advice to someone starting out would be to avoid high voltage altogether, and take on something such as a Velleman solid-state amp. You may even be able to power it from a wall-wort. You could put it into the cabinet of the Philco, and when you've had more experience, replace it with a valve amp.
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Old 19th Dec 2023, 1:45 pm   #26
Kala_12
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Default Re: Old Philco A3756 hi-fi amp safety question.

Do you have a voltmeter? With regard to capacitors, It seems logical to measure the voltage across the suspect capacitors, starting by using the highest range on the meter.
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Old 20th Dec 2023, 2:52 pm   #27
Richardgr
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Default Re: Old Philco A3756 hi-fi amp safety question.

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Originally Posted by knobtwiddler View Post
I would classify converting an old record deck into a guitar amp as an activity best undertaken by someone with intermediate / advanced knowledge of electronics. ....
Hi Knobtwiddler,
We were debating on another discussion thread why there is a dearth of interest in older electronics.
I am not sure what your age is, but would you have appreciated that advice you gave when you were in your 20's?
The important thing is that we remind people doing these conversions that they do have the ability to deliver a fatal punch if basic safety controls are not in place, and if the device is a complete mystery so the dangerous parts cannot be identified.

For the original poster, you should really be looking for an amplifier with a proper transformer for power so that it is isolated from the mains.
Then it helps if you have some pictures or plans you can share. Usually preparing that background information can help solve the problem anyway.
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Old 20th Dec 2023, 3:06 pm   #28
Richardgr
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Default Re: Old Philco A3756 hi-fi amp safety question.

With electrolytic capacitors, which can hold a charge for a very long time, I have a 10K 5W resistor that plugs into the positive and negative test leads on a digital multimeter. Then I set it to the max DC range, and carefully probe the connections on the cap, watching the voltage reduce as it is bled by the 10k resistor.

Ideally only one probe is used, the other attached to the ground, and then you only have one hand near the high voltages. That is a potential life saver because the dangerous situation is when the voltage goes across your chest, affecting the beating of your heart.

When you adapt the device then add a bleeder resistor as a matter of course, so dangerous potentials are quickly bled away.

One person I saw a comment from even adds a LED to the PSU caps so it is lit when they are charged.
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