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Old 6th May 2018, 10:48 am   #41
evingar
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Default Re: 1980s Pansonic microwave - repairable?

Quote:
our old sharp was the same, used a slightly smaller than SES base, in ~2005 managed to get a replacement bulb ordered from our local repair shop (Cockings in Weymouth ) and after me describing the bulb knew exactly what it was. Even ended up with a genuine sharp replacement!
This is what you loose when everything shifts from the high-street to on-line. We used to have a wonderful place in Newbury (Olive of Newbury) that held loads of white goods spares going back to the ark !
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Old 6th May 2018, 11:09 am   #42
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Default Re: 1980's Pansonic microwave - repairable?

For personal use, I'd have considered grafting a suitable lamp onto the defunct base. I keep old CFL / LED lamps for just this reason, as I never seem to have the base I need.
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Old 6th May 2018, 1:59 pm   #43
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Default Re: 1980's Pansonic microwave - repairable?

Some Panasonics use a bulb with an E17 base. It's bigger than an SES (E14).

I've ordered an LED version from China.
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Old 6th May 2018, 11:50 pm   #44
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Default Re: 1980's Pansonic microwave - repairable?

Surely it would be best to just patch another general purpose fuse in and see what happens before going out and ordering replacement parts that may not be needed, such as the main capacitor.

I remember back in the 1990s when I had an old microwave that had originally come out of a skip. It had given me a few years of good service when one day I returned to the kitchen having left it cooking something like a jacket potato only to find a nasty smell and the room full of smoke and an un-cooked spud. The transformer had burnt out. A pal of mine who worked with domestic appliances at the time found me a random transformer out of a scrap oven that looked about the same size and I bunged it in and it worked for years after that without problems.
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Old 7th May 2018, 1:16 am   #45
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Default Re: 1980's Pansonic microwave - repairable?

I just remembered one of the s/c failure modes of a magnetron. The insulation on the heater/cathode connectors sometimes fails. In that case some carbon tracking can be seen after lifting the square cover. This can actually be repaired by removing the feed through and using some other form of insulation.
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Old 7th May 2018, 1:12 pm   #46
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Default Re: 1980s Pansonic microwave - repairable?

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Originally Posted by ex 2 Base View Post
....incidentally the shock from it is no worse than that from a discharge light fitting.
This is misleading - why would you say that? - I really hope this comment gets taken down! These threads get seen by many, many people, including complete newbies.

The transformer of a microwave oven has a short-circuit capacity of several amps at 2,300 volts. ☠

The capacitor, if holding residual charge, can source over 2.5 Joules.

The doubler circuit is prevalent in ovens of this generation, and off-load (say the magnetron is kaput), the pulsed DC voltage is some way over 5,600V. It can be sustained under load at close to 1A for a long time before the transformer gets cooked. ☠☠

Operating around the power supply of a microwave oven, apparently working or not, has the capacity to be one-touch lethal. Simples.

I'm not saying this for the OP's benefit - I know the OP knows this, but out of context, your comment could embolden someone who should be leaving well alone.
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Old 7th May 2018, 4:19 pm   #47
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Default Re: 1980's Pansonic microwave - repairable?

Warning note added to the thread, also please heed Al's warning.
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Old 7th May 2018, 6:56 pm   #48
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Default Re: 1980's Pansonic microwave - repairable?

I will repeat, there is never a need to work on a powered up oven. We repaired and tested thousands.
In my business to do so was an instant dismissal offence.
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Old 7th May 2018, 10:27 pm   #49
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Default Re: 1980's Pansonic microwave - repairable?

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Warning note added to the thread, also please heed Al's warning.
I remember bumping into a friend's brother heading out of one of the electronics shops in Liverpool as I was going in. I hadn’t seen him in years and after a bit of chitchat I asked him why he was there (he wasn’t a techy type person at all). He had gone in for a fuse for his microwave oven as “it lights up and spins around but the food doesn’t warm up”. Alarm bells started ringing and I advised him in no uncertain terms to either get it looked at professionally (I’m a coward when it comes to microwaves) or bin it and buy a new one. I was very concerned about this encounter and unfortunately he and my friend weren’t on speaking terms so at the time I had no idea of the outcome.

Fortunately I did see him again a few years later so he had either took my advice or was damned lucky.
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Old 8th May 2018, 3:15 pm   #50
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Default Re: 1980's Pansonic microwave - repairable?

I agree with Sam, i used to fix microwave ovens as part of my job, all faults on the high voltage side of the transformer can be diagnosed without mains applied, a minute with a multimeter will show most faults,
When testing the mains side of the oven live i always used to first unplug one of the spade terminals from the input of the transformer that way the H/V side could never be accidentally energised

We were taught on the Sharp microwave course the 3 D's
Disconnect (unplug from mains)
Door (leave open so if some other idiot plugs it back in it wont start up )
Discharge ( short circuit H/V capacitor with insulated screwdriver)
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Old 8th May 2018, 8:15 pm   #51
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Default Re: 1980's Pansonic microwave - repairable?

The diode is cheap and so is the correct fuse,all on Ebay where I bought mine from.

Then having repaired it because I could,er I then recycled it!

So,we no longer have a microwave.

PS I remember a long time back one of the various door switches used to fail o/c on Panasonic
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Old 8th May 2018, 8:33 pm   #52
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Default Re: 1980's Pansonic microwave - repairable?

Out of interest, I've run some numbers (Daddy? What's arithmomania?) based on the wiring diagram in post #16.

2400V RMS from the transformer gives 3394V peak. Assuming no loss in the diode, the energy stored in the capacitor will be given by
Code:
E = 0.5 * C * V ** 2
E = 0.5 * 1e-6 * 11520000 = 0.5 * 1e-6 * 1.152e+7 = 0.5 * 11.52 = 5.76J.

I've personally experienced the energy in a 470nF capacitor charged to peak mains, through a step-up pulse transformer (in a gas ignition sequence controller, in case you were interested), a few times more than I cared to. So that's 339.4V peak:

E = 0.5 * 4.7e-7 * 115200 = 0.0271J.

And I can tell you, that hurt for the rest of the day. There is over two hundred times that amount of energy in the microwave oven capacitor .....

For comparison, a penny (weighing 3.5g.) dropped onto a person's foot from a height of 79cm. would impart a quantity of energy equal to
Code:
E = m * g * h
E = 0.0035 * 9.81 * 0.79 = 0.0271J. The dose may make the poison, but the route of administration also influences it heavily .....
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Old 9th May 2018, 12:06 am   #53
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Default Re: 1980's Pansonic microwave - repairable?

I also have a no-heating microwave awaiting my attentions. This is a fairly modern affair (10 years or less I'd say) and had signs of dirt/arcing inside near the mica plate.

I was surprised to see that there was no fuse - there was the usual CL01-12 diode to chassis and also a 2X062H bidirectional diode. I just changed both but the thing still refuses to heat. Magnetron has continuity of a few ohms (I think - was a month or more ago now). So it'll probably end up skipped. Anyway, reason for posting all this is curiosity - How common is the bidirectional diode setup?
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Old 9th May 2018, 10:15 am   #54
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Default Re: 1980's Pansonic microwave - repairable?

Hey Ben, the bidirectional diode is a component that was added to protect the rectifier diode in the event that the cap in the doubler cap goes leaky, for example .

If you order a replacements cap, you’ll often find it with lugs attaching it to a new rectifier diode as well as one of these bidirectional diodes. That’s because when this blows, the rectifier is also blown.

When the peak inverse voltage of the bidirectional diode is exceeded, the device goes short circuit. This places the rectifier diode in overload condition and it goes short. This shorts the transformer, blowing the fuse instantly and protecting the expensive transformer .
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Old 10th May 2018, 6:50 pm   #55
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Default Re: 1980's Pansonic microwave - repairable?

it's called a short protector
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Old 11th May 2018, 6:23 pm   #56
Al (astral highway)
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Default Re: 1980's Pansonic microwave - repairable?

Quote:
Originally Posted by julie_m View Post
For comparison, a penny (weighing 3.5g.) dropped onto a person's foot from a height of 79cm. would impart a quantity of energy equal to
Code:
E = m * g * h
E = 0.0035 * 9.81 * 0.79 = 0.0271J. The dose may make the poison, but the route of administration also influences it heavily .....
Hi Julie, I love this analogy, only I'm curious, why 79 cm?! (Actually, don't answer that ;-) I did see that you found a figure analagous to your electric shock experience, but if you skim over it, it seems like an esoteric reference!)

I've had between 70 and 200 Joules directly into my heart, but that was under general anaesthetic and timed with the peak of the R wave in the QRS complex, to cure a persistent arrythmia probably caused (back then) by overtraining. It didn't do so permanently, but they had a go.

The thing about the timing of this bang energy is that it is applied at an exact spot found by computer analysis and administered by it, and is limited to an excursion during the positive to peak part of the ventricular contraction, (the 'R' part of the heart's QRS complex waveform). The whole of this complex lasts between 60-120mS, and this 'R' component is only a small fragment of it, so the window is very small indeed. Luckily, I didn't know all this at the time!

This is routine for cardiologists/ anaesthetists working together and under and only under these conditions is 'safe', but at all other parts of the waveform it would cause instant ventricular fibrillation or death. OT but the problem was later cured by vaporising tissue around the roots of the pulmonary veins - not the same procedure at all.

Your illustration is really useful for the situation in which the oven is powered down and the bleeder resistor is o/c.

But back to the transformer: I've seen the resistance of the human body cited as between 300R and 1K typically. The doubler circuit in a microwave oven sources 5,600v pulsed DC, even under the load it's designed for, the magnetron.

So even assuming an optimistic 1K load imposed by the human body, that's a possible 5.6A sunk into the human body if the microwave oven's power supply is doing its thing. The short circuit capacity/ endurance of the transformer is extremely high, as witnessed by rather frightening Jacob's ladder demos using them, out there on the internet.
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Old 14th May 2018, 12:01 pm   #57
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Default Re: 1980's Pansonic microwave - repairable?

I made up a discharge lead for working on microwaves - two insulated handle screwdrivers with a lead and 1k 5 watt wirewound resistor connecting the two used for discharging the cap and NEVER worked on it live (cabinet cover interlock m/switch should stop you but cannot reply on it). Tested for all the door switches, front panel and diode - if these OK but still no heat then junked it. Problem for most parts is that time you've tracked it down to a supplier and paid the postage etc over here in NZ it's probably cheaper to buy a new one.
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Old 14th May 2018, 12:09 pm   #58
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Default Re: 1980's Pansonic microwave - repairable?

Same applies in the UK re. cost (hence the demise of big spares suppliers like AWI), except if your labour is free and/or the machine is a high quality one with stainless steel interior etc.
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Old 14th May 2018, 7:36 pm   #59
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Default Re: 1980's Pansonic microwave - repairable?

Quote:
Originally Posted by avocollector View Post
I made up a discharge lead for working on microwaves - two insulated handle screwdrivers with a lead and 1k 5 watt wirewound resistor connecting the two used for discharging the cap and NEVER worked on it live (cabinet cover interlock m/switch should stop you but cannot reply on it). Tested for all the door switches, front panel and diode - if these OK but still no heat then junked it. Problem for most parts is that time you've tracked it down to a supplier and paid the postage etc over here in NZ it's probably cheaper to buy a new one.
The cover interlock device must be a NZ thing, and a good idea too.

They are not mandatory here in the UK, and I have never seen one fitted.
I have repaired hundreds of microwave ovens.
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Old 17th May 2018, 8:09 pm   #60
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Default Re: 1980's Pansonic microwave - repairable?

My old 1980's Thorn Tricity microwave oven has an additional safety door interlock micro switch which is in the form of a short circuit across the primary of the transformer when the door is open. Thus if the door latches are broken or the door forced whilst in operation it causes a deliberate short across the primary of the transformer and blows the mains fuse before much microwave radiation has Escaped the oven cavity.

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