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Need help troubleshooting oscilloscope power supply

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R_C

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For my first learning oscilloscope I bought a Hitachi V-1585 on eBay. It works but it has a common problem with its Selector switch. Following the suggestions in this thread, I tried cleaning/lubricating the switch. When I reassembled and powered it back on it blew the 2A fuse. I checked everything again and it blew a second fuse. I ordered replacement 2A slow blow fuses and now the scope will not power up but it no longer blows the fuse.

With the attached schematic I started looking for the problem but I'm a novice. There is mains AC power to the RBV-406 bridge rectifier and I measure 109 VDC at the rectifier's pos and neg output. I don't understand the purpose of the DTA10E triac I've circled in red. I removed it from the board and tested it on a breadboard by applying about 2.5 VDC at 40 mA and it worked. Any ideas what other components I can test and how? According to the schematic, what is a good DC ground point to use when testing DC voltages? Is T1501 a DC step down transformer? Thanks.

Hitachi v-1585-65-60-v-695_om_sch_74.jpg
 

R_C

Member
The 3W 10 ohm resistor labeled R1501 is bad. I've got one on order. Maybe that will solve the problem.
 

dr pepper

Well-Known Member
Most Helpful Member
I'd say the triac is part of a soft start circuit to reduce current inrush at turn on.
This being shorted could blow the fuse, however if it works I suspect something else is up.
You could check the diac, the device inline with the triacs gate, with a led and 10k all in series with it, the device should switch on at about 20v or so, meter it first if its shorted that could be yer problem.
A lot of times on old 'scopes the ht supply to the tube is the issue, dampness gets in and messes things up.
 

R_C

Member
I only see diode D1521 and resistor R1521 inline with the triac's gate. Both the diode and resistor check out OK. I'll know more when I replace the bad 3W resistor R1501.
 

dr pepper

Well-Known Member
Most Helpful Member
I cant read the schem that well, however yes d1521 is a diac, a diac is kind of diode, sort of a back to back zener, the thing should only turn on at a certain voltage, 20v is common for use with triacs, if it reads short then the triac will fire too soon possibly blowing the fuse.
 

R_C

Member
I'm confused. Diode D1521 is of type AU01, which a search shows as a zener. If it really is a a diac and it turns on at 20v, how would that affect the triac's gate? According to the datasheet for the triac, it has a 2.5v max gate trigger voltage.
 

chemelec

Well-Known Member
The 3W 10 ohm resistor labeled R1501 is bad. I've got one on order. Maybe that will solve the problem.
If that resistor is burned out, Probably there is a Problem after it.
 

R_C

Member
If that resistor is burned out, Probably there is a Problem after it.
I am guessing the second time I blew the 2A fuse at the mains input may have fried the 3W resistor. If it blows another fuse after I replace the 3W resistor, I will know the problem still exists after the resistor. Since everything except the front panel's Selector switch worked before I went after that switch, the problem is something I caused.

When I blew the first fuse after reassembly, I found a piece of vinyl that went between the STK7308 (TV switching regulator) and a metal shield cage. I think that is just some kind of thermal transfer vinyl to dissipate heat from the back of the STK7308 so that shouldn't be the problem. But removing the power supply board did require some tugging against the STK7308. Although its pins are all still intact, maybe that's where the problem lies. I have one of those on order too.
 

dr pepper

Well-Known Member
Most Helpful Member
Diacs are often used with a triac to trigger correctly, but if its listed as a zener then its not a diac.
If the vinyl piece you mention is grey with a woven finish then that could be a thermal pad.
 
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