Sencore LC102 Display Problems

[Seekonk]

This has been an interesting thread.  I assume your display is back to the way it was, still blooming.  I will have to try that frequency trick some time.  Buying old equipment with LCD displays always scares me.

Not to hijack the thread, but I have a LC101 with the plastic bezel on the front missing.  This now seems unavailable.  Was there another product they used this same bezel I might find on ebay cheap?

[Johnny10]

I was thinking how to implement the frequency change last night.
Rereading the comment on that and searching out the oscillator data.
Signal comes from the 7556 chip on the display board. The 7556 IC contains two 555 circuits, one for the backplane oscillator and one for flashing the LED's.
The formulas are the same as for the 555 timer IC.
Astable mode 1.44/((R1+R2+R2)(C). Calculates to 45Hz 52.4% duty cycle.
I measure 46.4Hz 49% duty cycle.

Display is still blooming but I have found that lowering the current or the voltage also cleans up the blooming.
Problem is the circuit shuts down with either lower current or voltage as soon as a test is performed.

I purchased a display board from another member and have yet to install it in my unit until I am sure everything is working properly.

[rf+tech]

Johhny10,

If you have a few minutes to try this - temporarily change the timing capacitor to 1/10 the existing value. This should result in the backplane running about 460 Hz. If this looks promising, then try decreasing C again by a factor of five.

I've yet to come across another LCD with this blooming problem to try this on. I would appreciate knowing how your display responds.

Very interesting how your display responds to lowered current or voltage. Would you mind sharing the details as to how you did this?

Thanks,

RF+ Tech

[Johnny10]

Glad you responded! rf-tech
I haven't forgotten your message.

I have added an image of the circuit controlling backplane oscillator.
The right side controls timing with the 10K, 100K resistors and .15uF cap.

[Johnny10]

I changed the cap C4 to .01uF.
Making for 709 frequency, sort of the middle of your change.
Good results

[Johnny10]

Remember before image?

[glicos]

Excellent thread!!!

[rf+tech]

Most Excellent, Johnny10!

This just made my day, made me laugh and smile. Thanks for posting your results!

Anybody have some theories about why raising the backplane frequency works to correct the blooming, or what causes the blooming?

RF+ Tech

[Johnny10]

RF+Tech  Thanks for the help
I did put a note in to a fellow that was making his own LCD screens to see if he understood the problem of frequency.

I guess I am going to go back in there and change the capacitor again but I have been debating on an adjustable trimmer cap?
1nF - 10nF or similar.

Now I have another problem with LC102 and that is I get a short every time I try and zero out the probe.
This didn't happen before the unit shorted out. I cleaned the connectors but it keeps showing a short while holding the "open" switch.
Which keeps the capacitor value function from working.

I am also wondering if I should end  this thread with a" fixed "notation and start a new thread on the circuits used to zero probes?

[glicos]

I am also wondering if I should end  this thread with a" fixed "notation and start a new thread on the circuits used to zero probes?

Better be unless "zeroing probes" problem is caused by your recent modification on the LCD

[rf+tech]

glicos,

Please carefully review this thread and do note that the "probe zeroing" problem was reported by Johnny10 *after* replacing the microprocessor and *before* changing the backplane timing capacitor.

Additionally, referencing the schematic, one will observe that there is no connection between the backplane oscillator and the probe zeroing circuitry. Uninformed speculation provides no benefit. My apologies if I sound a bit strong.

Johnny10,

Yes I think it may be better to start another thread, with a title that reflects this additional problem.

RF+ Tech

[Johnny10]

RF+TECH That was quite a fix !

I changed the cap again to a .0076 that I found a my local surplus shop.
Not much of a change that I could see visually. It changed the frequency to 912 Hz.
Also I had a bit of a problem keeping waveform steady on the oscope. All for another day.

To finalize this repair I will order a variable cap and make just a few more tests.

This has certainly been an adventure I never imagined and neither did most, that this display was salvageable.
Your post was always in the back of my mind as an interesting solution even though I purchased a replacement screen
I felt it was a curiosity that had to be followed.
Thanks again!

I guess this is what makes for the fun!!

[rf+tech]

Johnny10,

I've posted this additional information to this thread for the benefit of others as it adds a degree of insight and credibility.

This quote from the Racal RA6790/GM receiver technical manual, page 4-43:

LCD displays operate from low frequency square wave pulse trains. The contrast of the display being a function of the frequency.

put me onto the path of discovering the fix, back in February 2010. Since this receiver didn't have a "contrast adjustment", like you, I felt it was a challenge to be followed. Not to mention the near unobtanium of replacement displays.

I will also mention that raising the backplane frequency higher still, will result in the segments looking faded. The new optimal frequency is not sharply defined, just whatever looks good to the eye. Installing a trimmer isn't necessary as one will find that the range of what looks good is very broad.

Call it fixed and have a beer on me. 

RF+ Tech

[Johnny10]