Re: HP 1955 Inverter Problems
Thanks for your quick rpely.
The probes were at 1X, switching to 10x produced similar results.
I have attached a picture of what I am seeing - the top trace is Y1 and is taken from the FET gate, Y2 is Pin 7 of TL1451.
Digital cameras - giving every 'scope a digital storage option!...
1lostsock
New Member
Last Activity: 11-01-2012, 11:33 PM
Joined: 09-07-2012
Location: Christchurch
-
Re: HP 1955 Inverter Problems
OK. It has been a while and I did find the venerable oscilloscope and wired it in for a peek.
The hot Q812 is because the switching waveform is not a square wave.
The frequency seems to be about 80 kHz and the duty cycle is about 5-10% on (= voltage high) from pin 7 of TL1451.
When pin 7 goes to ground (internal transistor conducting) the waveforms on the scope at pin 7 and the output of Q810/Q811 both cut off nicely.
However when Pin 7 stops conducting the voltage waveform does not abruptly step up,...Leave a comment:
-
Re: HP 1955 Inverter Problems
I have largely left the original working inverter alone. On the borken channel the .22 capacitor has been changed for a new part.
Q812 was also replaced but I will try and find a moment to squeeze my ancient oscilloscope on the the gate (I might have to solder a test lead to run with the CFLs attached).
C801 & 802 show no physical signs of damage so I have left them alone. I only have basic caps (i.e. not much better) around me to replace them with.
Since the circuit seems to run well with the FU9024N bypassed I will...Leave a comment:
-
Re: HP 1955 Inverter Problems
OK installed a 2N3904 where it should be (Q810).
Operational test successful - the light turns on for more than two seconds and stays off when off.
Unfortunately Q812 (FU9024N) gets quite hot (+30 Celsius degrees in ~10 seconds) so I turn it off quite quickly when I can smell it.
As the screen turns on there is a quiet squeal that climbs in frequency (sort of sounds like heavy loading in some components while the voltage is slowly topped up, lasts ~5 seconds until stopping or passing the edge of audiblilty).
...Leave a comment:
-
Re: HP 1955 Inverter Problems
OK.
As luck would have it I had some 2N3906's - they were even SOT-23.
One was clumsily soldered in but no change - 2 seconds to black.
I started looking at components on the feedback path but came back to the driver circuit for Q812 (FU9024N) where I had the original Q811 (2N3906) collector-emitter short (1 Ohm).
I found that Q810 (2N3904) also had a collector-emitter fault (30 Ohm 'short').
Why I had not seen this earlier I don't know, excessive stupidity or worse, a new failure perhaps.
...Leave a comment:
-
Re: HP 1955 Inverter Problems
That is a good idea - I am pretty sure that the oscillator runs as I have had the screen running recently but at what heat I have not checked. I will get some part temperatures (via thermocouple, too many volts for my fingers) next time I have it up for more than a few seconds.
I have some more of the 0.22 caps so I could just straight out replace them.Leave a comment:
-
Re: HP 1955 Inverter Problems
Hi Budm,
Thanks very much for the schematic - it is a much better match that the previous one I was using and shows Q811 as a 2N3906 rather than a 2SA733.
The two are superficially similar though so I am not sure if replacing my T0-92 hack with the right part will help. For the cost of 2N3906 is is probably worth a go. I will track one down in the next few days.
My apologies, I believe the short was a collector-emitter short on the Q811 which makes sense given the original path of silicon destruction.
This would...Leave a comment:
-
Re: HP 1955 Inverter Problems
Hi,
The markings are illegible without a microscope but it's identical part on the other inverter channel is marked:
W0452
with the 52 sideways
Hope that helps, my limited SMD experience was unable to make much sense of this marking.
Thanks for your interest.Leave a comment:
-
Re: HP 1955 Inverter Problems
Hello All,
Hijack alert! My problem seems to share a lot of background with this thread so I thought I should tack this on the end.
Let me know if a new thread would be better and if you have a few suggestions please help me save this back-burner project from the landfill.
I have a dead HP 1955 monitor with a Benq 48.L1A02.A31 inverter board (a stirling piece of work). I have repaired 3 others of the same type with the same failure in almost identical ways and apart from the HF noise when powered down I am very happy...Leave a comment:
No activity results to display
Leave a comment: