Re: No start 7854 Power supply
HiDave.toggle quoted messageShow quoted text
Ihave seen such behaviour from resistors before, for example last year Irepaired some HP 8920A SMPSU’s where I knew from voltage measurements that thetickle up resistors that were dropping the high DC voltage from the mainreservoir capacitors were open circuit under load, but, they checked OK with amultimeter using its low voltage ohms range. Strange things can happen with resistors when they are subject to arelatively high voltages across them. Itwas, and probably still is, a common design mistake with SMPSU’s designers to nottake note of the maximum voltage rating of resistors in the pre switchingstages. You must take heed of the factthat quite a number of the designers are living in a 115 VAC mains world notour nominally 230VAC mains country.
Ido not think that the actual maximum value of R10 is critical as 180k wasselected to provide sufficient start up voltage/current from 115 VAC not our230 VAC.
Ifyou let me know the maximum dimensions that R10 can be I will try to provideyou with a suitable replacement by post. I still have a reasonable stock of 1W resistors of various values and types.
Fromyour previous posting I suspect that you still have a fault downstream from theswitching stage.
On Tuesday, January 15, 2019 4:47 AM, David C. Partridge <@perdrix> wrote:
I think I have probably located the problem with the PSU not starting - one
of the leads to T30 which is part of the oscillator base drive circuit had
corroded. A bit of current through was left of the copper soon finished it
off as the supply was running for the first minute or so.
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Harvey
Sent: 14 January 2019 23:37
Subject: Re: [TekScopes] No start 7854 Power supply
On Mon, 14 Jan 2019 21:08:36 -0000, you wrote:
Hi George, perhaps you could explain to me why the ohmmeter reads 180k whenits removed from the board, but megohms when in circuit?
Is there any voltage across this resistor? I've seen that really bug
resistor - Ohmite OY 2W resistors are too large and not readily available in
UK - drat it.
If you can't find one, I think I could.
edmonds via Groups.Io
Sent: 14 January 2019 17:49Better replace is wit two resistors in series to obtain a high working
voltage, or you can find on eBay 500 V working resistors .
73 George G6HIG<@perdrix> wrote:
that had gone open it measured in megohms, but when I removed it, it readC.
cracked! Clearly both suffered massive over-current and burnt out.