Re: No start 7854 Power supply

george edmonds


 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:

Thanks Harvey, 

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. 


-----Original Message-----
From: [] 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 when
its removed from the board, but megohms when in circuit?

It's not the only resistor I saw with this odd behaviour.
Is there any voltage across this resistor?  I've seen that really bug

And darn it why can't I buy a 180k ceramic or carbon composition 1W
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.


I have some value flexibility I think as the 7104 supply used a 300k part!


-----Original Message-----
From: [] On Behalf Of george
edmonds via Groups.Io
Sent: 14 January 2019 17:49
Subject: Re: [TekScopes] No start 7854 Power supply

Hi Dave
R10 is the AC start up series resistor, replace it, it is going voltage OC.
Better 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

    On Monday, January 14, 2019 5:34 PM, David C. Partridge
<@perdrix> wrote:

Hah! Not for long - it stopped running after about two or three minutes.

Both the new inverter transistors still check OK in circuit.

Interestingly the start circuit is *not* firing any more.  The voltage
across C42 is around .45V so something is pulling it down or not feeding it
volts.  Strangely when I measured the resistance of R10 in circuit to see
that had gone open it measured in megohms, but when I removed it, it read
180k which is spot-on.  After reinstalling, it reads in the megohms

Any ideas?

-----Original Message-----
From: [] On Behalf Of David
Sent: 14 January 2019 14:26
Subject: Re: [TekScopes] No start 7854 Power supply

Good news! The inverter is now running OK, but the supply is ticking at
about 5Hz or so!  I suspect over-voltage shutdown circuit.


-----Original Message-----
From: [] On Behalf Of David
Sent: 11 January 2019 16:26
Subject: Re: [TekScopes] No start 7854 Power supply

Both Q34 and Q40 (151-0632-00) were totally deceased - one completely fell
apart once I unsoldered the leads, and the other lost a lead and the case
cracked!  Clearly both suffered massive over-current and burnt out.

So, before I solder in some new MJE13007s to replace them what is likely to
have caused that to happen - I can see the replacements blowing up too
unless the root cause is fixed!


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