Utter novice looking for answers to easy questions:

[shnaggletooth]

These should be easy answers, but believe it or not, tons of electronics pages on the Web doesn't seem to provide them (or at least, provide them readily enough).

Question one:

From this early part of the Tube Faq:

"When you open up an amp, you need to find a way to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground."

"Ground" in this case meaning what? What's the ground - the chassis itself? Some specific part of the amp? "How-to" articles often neglect to explain exactly what they mean by "attaching to ground".

Question two:

From an Instructables article on discharging caps, written for absolute beginners:

"THEN,
--Take a screwdriver or a jumper and short the capacitors leads.
--OR jumper the power amp tube plate pin to GND for a minute or so (Class A, single power tube only.)
--OR jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here..."

Wait! Rewind. HOW does one "short" something using a screwdriver? What touches what? What doesn't touch what? How do you know the short was successful?

Question three:

Pictures of some equipment I've recently picked up. What are the rectangular components? (I've actually watched a whole bunch of Youtube vids of people displaying their tube amps, pointing out the tubes, caps, and large transformers, but then completely neglect to explain or even acknowledge the very rectangular things below I was hoping they'd explain!)




Thanks in advance from a frustrated novice

[Craig405]

Hi,

Question one, chassis is often called earth and is connected to the mains earth for safety should the casing raise in potential.

Ground is 0V reference for signals in the equipment and is often but not always connected to earth.

To discharge equipment:
You are best taking a resistor of say 1Kohms @ 2 watt or so and holding it using a pair of insulated pliers short ( short = connect the two terminals togeather) of a power supply reservoir capacitor using the two legs of the resistor.

The capacitor will discharge through the resistor and the resistor will get hot.

Use a Voltmeter to carefully check that the capacitors are discharged before playing about.

I dont think shorting capacitors with a screwdriver is a good idea at all really, so dont do it.

The metal boxes i believe are variable capacitors, looks like radio chassis or somthing.

Hope ive been fairly clear here, but probably there is some ambiguity somewhere.

P.S. I dont think you should fiddle around with valves unless you have a good idea of safety and basic power supply knowledge, they can be dangerous.

[Jeb-D.]

I dont think shorting capacitors with a screwdriver is a good idea at all really, so dont do it.

Yeah, I tied discharging a 3,300uF cap with a 200V charge that way. It made a very loud pop and took a big chunk out of the screwdriver. Never again.

I use a 10W bathtub resistor now. The body is big enough so that I can easily hold it by the casing, without accidentally touching the leads.

[Brit01]

To discharge equipment: You are best taking a resistor of say 1Kohms @ 2 watt or so and holding it using a pair of insulated pliers short ( short = connect the two terminals togeather) of a power supply reservoir capacitor using the two legs of the resistor.

I use a light bulb for this. Seems to work effectively and you can see the discharge as the light fades.

[EC8010]

Those rectangular cans are IF (Intermediate Frequency) transformers. What you have there is a superheterodyne receiver that beats a local oscillator against the incoming signal to produce sum and difference frequencies. Because the local oscillator is adjustable by the tuning control, we can arrange for any selected radio frequency to produce a required (and constant) difference frequency, which is known as the intermediate frequency. Because the IF is constant, we can make a special amplifier that amplifies that frequency with very high gain and rejects all other frequencies. Doing that makes our receiver able to pick out the radio station we choose and reject all others. To make the IF amplifier so selective, we need lots of tuned transformers, and those are what you see. The larger cans that aren't in a row with the valves might be the stereo decoder, whereas the ones in a row are the IF amplifier - often called an IF strip because of the mechanical arrangement.

[SY]

Dumb question- why are standard IFs what they are? For example, most AM broadcast band radios have it at 455kHz, FM at 10.7 MHz. Why not 450kHz and 11 MHz?

[EC8010]

I vaguely remember that it's to do with images and broadcast spacing. Remember that the mixer is deliberately nonlinear, so the trick is to make it produce sum and difference frequencies, but not all the other possible intermodulation products. You don't want any of the unwanted images to coincide with a broadcast frequency, so you choose your IF carefully w.r.t. broadcast spacing. So, did WARC** that changed spacings to multiples of 9kHz suddenly make all old receivers duff? Probably not, probably receivers were good enough by then.

I expect a radio ham would be able to give a better answer.

[nigelwright7557]

From what you have said so far I would seriously suggest you dont even open the amp up.

Get a qualified professional to fix it for you......

Valve amplifiers kill !

[Craig405]

Its an old radio receiver, not an amp EC8010 knows more than I do

I think everyones got to start somewhere, as long as he reads up on safety it will be a good learning experience to do somthing with this thing.

Looks like a nice chassis with good parts to me

Have a look at the HV safety thread:
Safety Practices, General and Ultra-High Voltage

[Brit01]

I think everyones got to start somewhere, as long as he reads up on safety it will be a good learning experience to do somthing with this thing.

Totally agree as long as you have some common sense and follow safety practices.

hey I started from nowhere and now I have 4 projects on the go.