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Electronics => Repair => Topic started by: JS_NY1 on July 04, 2018, 01:19:29 am

Title: Schematic for Tenma 72-705
Post by: JS_NY1 on July 04, 2018, 01:19:29 am

I recently unearthed a Tenma 72-705 dual laboratory power supply.  One channel works beautifully, the other channel outputs the maximum voltage with no control.  I have a very poor quality schematic for this unit.  Does anyone have a good, clean schematic they could send me?  Thanks, Jim KC2ZVK

Title: Re: Schematic for Tenma 72-705
Post by: Chris56000 on July 04, 2018, 06:28:52 pm

Hi!

Would you mind posting the poor–quality diagram you do have please?

I might be able to redraw it for you if it really isn't too indecipherable!

Chris Williams

Title: Re: Schematic for Tenma 72-705
Post by: bitseeker on July 06, 2018, 02:22:14 am

Hi JS_NY1,

Welcome to the forum. If that's a linear power supply (I didn't see much with that model number), it sounds like the power transistor on that stuck channel is shorted. Remove it from the circuit and check. If it is shorted, then replace with a similar working transistor. If not, then do comparisons between the good and bad channels.

Although schematics can be helpful, you have the benefit of a working example as reference. You'll probably find the culprit with a little sleuthing.