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Old 23rd Jul 2022, 6:39 pm   #1
dmowziz
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Default Broken Tenma supply

Hi... This power supply stopped working. It made the kind of noise from being on for long then it suddenly shut off

Guessing the problem will be one of the transistors connected to the heat sink but I can't check without taking out the board


After checking the transistors: If it is not the problem, how should I proceed?


Thankss

Already checked the fuses
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Old 23rd Jul 2022, 8:31 pm   #2
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Default Re: Broken Tenma supply

Apart from mistakenly touching the cap leads together, is there any other "danger" ?
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Old 24th Jul 2022, 5:17 pm   #3
Chrispy57
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Default Re: Broken Tenma supply

Don't like to see a post with zero responses, although I'm not familiar with this unit, perhaps a common-sense approach may take you a bit further on your fault-finding journey. The photo shows a dead unit, any led or fan activity when powered on?

"Already checked the fuses" - is there one in the primary of the mains transformer, thermal fuse, Worth a resistance check of the primary first? The transformer looks like it has two identical sets of secondaries, you could unplug them and check and compare resistance readings? Apply mains and check secondary voltages with plugs disconnected from pcb. Reconnect plugs, any voltages on the rectifier diodes and smoothing capacitors

Looks like a few relays in there, check coil resistances. Could be the output transistors as you suggest, but all blowing at once seems a bit unlikely, sounds like something common to both channels involving the above components.

Good luck
Chris
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Old 24th Jul 2022, 7:32 pm   #4
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Default Re: Broken Tenma supply

I've repaired a few of these before and it was either open circuit 3W resistors, rotted tracks due to that nasty brown glue they sometimes use, and blown output transistors. In the last case i could not get the right ones so fitted some equivalent audio transistors as i figured they were designed for series pass operation (this is a linear supply with stepped tap-changing relays)
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Old 24th Jul 2022, 7:57 pm   #5
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Default Re: Broken Tenma supply

What test equipment do you have?

Start with the transformer. Measure the resistance of the primary winding from the mains plug.
Measure the secondary windings after unplugging the leads.

Is there any voltage on the main smoothing capacitors?

What is on the small PCB next to the fan?
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Old 25th Jul 2022, 9:02 pm   #6
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Default Re: Broken Tenma supply

Thanks Chris! and McMurdo and Silicon Valley

I checked the primary resistance winding. They are 2
I was getting like 1.5 ohms 0.3 ohms.

When I return home today I will give the right numbers for the resistances... Hopefully bold enough to apply mains and check the voltages. Hopefully

@McMurdo, thanks... I remember checking the diodes (all good) ,resistors all correct values... I checked the transistor, the only unusual thing was collector at ground (I havent taken the board out yet to trace path)...

How I checked the transistor : using diode mode on multimeter, BE drops more voltage than BC... Also, no short except the collector that is a short
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Old 26th Jul 2022, 5:14 am   #7
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Default Re: Broken Tenma supply

I suspect the small board top right with the 8-pin IC is a fan speed controller with a temp sensor on the heatsink somewhere. It may have it's own separate winding on the transformer.

Quote:
I checked the transistor, the only unusual thing was collector at ground
What do you mean by 'ground'. Looking at the front panel I would say that none of the inside bits have any connection to the chassis ground, only to the positive or negative output terminals. (some years ago I had a long telephone discussion with a older guy who didn't understand why the panel voltmeter said 13.8volts but he couldn't measure any voltage between "ground" and positive.)

THE transistor - what transistor was that? I don't like testing transistors using the diode function on a DMM - old faithful multimeter on 'ohms' can tell you far more - even give a hint at the gain.
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Old 26th Jul 2022, 7:22 am   #8
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Default Re: Broken Tenma supply

If it's not even powering up with the displays blank I would switch on and listen for the little buzz or 'boing' noise you get when one of these powers up, this will tell you if there's power to the transformer. Then if not it's fairly simple to trace it back through the board, mains switch and back through the rfi filtering and to the rear inlet.
If you have a meter you should be able to measure the low resistance of the primary at the mains terminals.
If there's no power then I suspect the mains switch followed by dry joints. I assume you've already checked the obvious things like fuses.

I did once have one where someone had fed mains into the output terminals, this had fried half the components on the front display board and one of the low voltage capacitors had gone short, dragging down the low voltage regulator for the LEDs. The transformer still powered up ok though.
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Old 26th Jul 2022, 9:33 am   #9
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Default Re: Broken Tenma supply

Quote:
Originally Posted by majoconz View Post

THE transistor - what transistor was that? I don't like testing transistors using the diode function on a DMM - old faithful multimeter on 'ohms' can tell you far more - even give a hint at the gain.
Thankss... For ground, I assume the heatsink is at "Ground"



Quote:
Originally Posted by McMurdo View Post
If there's no power then I suspect the mains switch followed by dry joints. I assume you've already checked the obvious things like fuses.
Yeah It's not powering up, blank display
Thanks I'll check but....

What are the ways one can get an electric shock from working on this?
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Old 26th Jul 2022, 12:27 pm   #10
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Default Re: Broken Tenma supply

Hi - avoiding electric shock is really about avoiding placing youself between earth (ground) and the earth-seeking mains voltages present within the equipment. So, unplug it and you ain't gonna get shocked while taking any measurements with your DVM. If you need to make live voltage/current measurements then connect up your DVM before plugging the equipment in, and switch off then unplug again before handling the DVM or equipment. Keeping one hand in your pocket can minimise the chance of accidental connection of both hands to live circuit bits if you work on live equipment and get distracted.

The other hazard could come from a charged capacitor (as you have mentioned) and this can be prevented by temporarily connecting a suitable resistance (tens or hundreds of kilohms) across the capacitor via insulated leads, to discharge it slowly and safely, again once the equipment has been switched off and unplugged. In well designed equipment like this Tenma unit there will likely be some resistance provided to do this automatically, but don't take it for granted. Finally, measure the capacitor voltage to confirm it is zero before handling it.

Good luck with the testing - but don't forget to check the "zero ohms" reality of your DVM by shorting the test leads and noting this reading then subtract that from the measured resistance of such low resistance items as the transformer windings.

Cheers
Chris
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Old 26th Jul 2022, 1:51 pm   #11
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Default Re: Broken Tenma supply

Quote:
Originally Posted by Chrispy57 View Post
Keeping one hand in your pocket can minimise the chance of accidental connection of both hands to live circuit bits if you work on live equipment and get distracted.


Cheers
Chris
Thanks Chris... This gives me confidence.. Will work on it today

Quote:
Originally Posted by Chrispy57 View Post
If you need to make live voltage/current measurements then connect up your DVM before plugging the equipment in, and switch off then unplug again before handling the DVM or equipment.

Cheers
Chris
I don't understand this sentence but I think I'll be very fine with one hand in pocket
Thank you
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Old 26th Jul 2022, 2:24 pm   #12
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Default Re: Broken Tenma supply

Just ensure it's unplugged before you stick your fingers in and you'll be ok.
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Old 27th Jul 2022, 12:48 pm   #13
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Default Re: Broken Tenma supply

Sorry if something got lost in the translation from English to Canadian! I tried to imply the following sequence of events:-
Switch off and unplug the equipment.
Connect your meter leads to the required test points.
Plug the equipment back in to the mains supply and switch on.
Note the meter reading.
Switch off and unplug the equipment.
Remove the meter leads.

Basically what Kevin said much more succinctly above!

Good luck with the tests.
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Old 27th Jul 2022, 8:37 pm   #14
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Default Re: Broken Tenma supply

What's up with the power switch on the front panel? Has it just lost it's cap or something more serious. Some equipment has a power switch with an integral fuse holder so I was thinking that popping the cap off may be an indication of something.....
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Old 28th Jul 2022, 12:09 pm   #15
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Default Re: Broken Tenma supply

Quote:
Originally Posted by majoconz View Post
What's up with the power switch on the front panel? Has it just lost it's cap or something more serious. Some equipment has a power switch with an integral fuse holder so I was thinking that popping the cap off may be an indication of something.....
It looks like the switch itself is near the back panel, with a long rod to actuate the switch.
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Old 28th Jul 2022, 12:13 pm   #16
Chrispy57
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Default Re: Broken Tenma supply

Inspired by Martin's suggestion above regarding the dodgy looking mains switch, perhaps you could repeat your transformer primary resistance checks, but with the meter connected between the unplugged mains plug's live and neutral pins and see if you get a similar low resistance value?

If not, then check the continuity of the switch. If it shows no continuity, then try operating what remains of the mains switch by manually gripping and pushing the rod that Duncan has highlighted above or pushing a suitably sized insulated tool or plastic pen down the hole where the missing push button should be to actuate the switch manually, just in case the switch has popped out to the off position. Did the switch used to have a push button when you swiched it on just prior to this fault occuring?

Good luck
Chris
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Old 16th Sep 2022, 2:05 pm   #17
dmowziz
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Default Re: Broken Tenma supply

I'm sorry (sorry for myself) I didn't provide any feedback

I need a variable supply "badly" right now. No money to spend for new or repair
Please kindly guide me

1. I measured the live voltage of the capacitor (one side of multimeter on the diode) and other side on heat sink : There is no voltage

2. There is no buzz sound from transformer

3. Is the mains switch that which is showing in the picture below? (The switch is ok and working fine).

4. The primary winding of transformer (6 wires), 3 each... Low resistances, no short or open

I think I've followed all the suggestions, maybe not.


Thank you Chris, Duncan, majoconz, Mcmurdy, Silicon. Thank youu
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Old 16th Sep 2022, 2:20 pm   #18
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Default Re: Broken Tenma supply

It's still not absolutely clear whether you have made one measurement which you were asked to earlier.

Unplug the PSU's mains plug from the mains, turn the power switch on the front of the PSU -ON- and measure the resistance between the live pin of the mains plug and the neutral pin of the mains plug.

What resistance do you see? (Make sure you are not measuring your own body resistance when you do this - keep fingers away from the metal parts of the probes / pins).
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Old 16th Sep 2022, 2:40 pm   #19
dmowziz
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Default Re: Broken Tenma supply

Hi... Thankss

I did that and measured : It's open
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Old 16th Sep 2022, 2:57 pm   #20
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Default Re: Broken Tenma supply

You have already measured the transformer primaries and found that you have low resistances there. As you are in Canada I am assuming there is no fuse in your mains plug, but if there is one, hopefully you have already checked it.

The next thing you are going to need to check is the mains switch. It looks as though it is soldered into the printed circuit board.

Can you unmount the PCB so that you can swing it upwards and outwards a little bit so that you can see the soldered connections to the mains switch underneath the PCB? You will probably need to pull the button push rod off the end of the switch in order to be able to move the PCB around.

If the solder joints are the only thing keeping the switch attached to the PCB there is a chance that the constant push on / push off action of the switch may have weakened and cracked the solder joints on the pins of the switch, so that is another reason we need to have a look in that area.
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