Author Topic: VFD unwanted segments glowing  (Read 1932 times)

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Offline AndyC_772

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VFD unwanted segments glowing
« on: January 20, 2017, 09:48:48 am »
Hi all,

I recently picked up an old Sony DAT recorder: Here.

It's in pretty good shape generally and works fine, except for a slight problem with the display. Some segments, particularly (but not exclusively) in the upper left corner, glow dimly when other segments are switched on, and they get brighter the more other segments are enabled. When the affected segments are themselves enabled, they light up OK, but not quite as brightly as they should.

You can see the effect in the photos in the ad; it's not just that the photos are a bit overexposed, the display really does look like that.

There's a -25V supply to the front panel PCB which looks fairly clean, and there are no electrolytic caps on the front panel to replace.

The machine has clearly been owned by a smoker, and there's a thin layer of gunk on all the PCBs which I'll need to clean off when I get the time to fully strip the machine apart - but in the meantime, I've cleaned the accessible side of the front panel board and it's made no difference at all. Unfortunately the design of the PCB is such that there are many components underneath the VFD itself, and getting at them will be a major de-soldering job that I'd prefer not to undertake as it risks damaging the VFD.

With the front panel PCB removed from the case, I did notice a dark blob in the top left corner of the VFD which has that 'magic smoke' look, but there's a virtually identical blob on this unit which suggests it may be a manufacturing artefact and not actually a sign of damage.

Does anyone please have any suggestions as to what the likely cause of leakage or crosstalk on a VFD might be?

Thanks
Andy.
 

Offline bitseeker

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Re: VFD unwanted segments glowing
« Reply #1 on: January 20, 2017, 10:19:45 am »
Instances I've seen of VFD segment ghosting were due to a failing display driver. If cleaning all the gunk off doesn't fix it, it may be the driver chip(s).
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Online Ian.M

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Re: VFD unwanted segments glowing
« Reply #2 on: January 20, 2017, 10:33:57 am »
The blob is normal *IF* its silvery or dark metallic in appearance, with edges that fade gently to clear glass.  Its the getter.  If the getter spot develops a light mother of pearl iridescence it may indicate the VFD is loosing vacuum or worse if the VFD stops working and the getter spot has gone matte whitish or greyish, that would be an indication that the tube has lost vacuum.   Unfortunately changes in the spot's appearance aren't immediate, but if you ever think you've mishandled a VFD or other tube with a getter, check it several days later to see whether or not you've screwed the pooch.

First try to find a copy of the service manual.  You'll need it to check voltages round the display. There are a couple of things you can check before you commit to desoldering the VFD.   First would be DC bias on the filament.  Better quality VFD drive circuits typically bias it between 5V and 10V more positive than the negative rail that the VFD driver pulls grids and anodes down to when the digit or segment is off. This guarantees enough negative grid bias with respect to the cathode (filament) to make sure the digit is cut off so segments that are at +B due to the multiplexing don't glow.  You'll need to check the datasheet for the correct bias voltage and the circuit that provides it, which from experience with other Sony kit may actually be on the main board, usually an oscillator + transformer to provide the -25V rail and the filament supply. Its also worth checking the filament supply waveform is as expected.  If it has become very asymmetrical with short -ve going pulses, its possible that the bias on the peaks of the pulses at one end of the filament is ineffective to keep the digit cutoff.  This could be due to bad caps in the filament supply circuit.

However its more likely that the buildup of crud has caused a leakage path that is upsetting the grid bias for that digit - check waveforms, specifically the drive voltage levels.  If the service manual isn't specific enough, compare with the least affected digit (and check which one that is in a darkened room).   If that's the case, cleaning the board thoroughly with flux cleaner and a toothbrush (preferably electric) is the most likely cure.  With luck, you may not have to desolder the VFD for access.  However be aware that Sony front panel tactile switches of that vintage are *NOTORIOUS* for failing across much of their domestic and professional product range, usually either failing to return from clicking down so the button jams on. or failing to register button presses, usually due to elastomer failure and solvent exposure may be the final straw for them.   If you do have to remove the VFD, it usually isn't a problem as long as you make sure each pin is fully desoldered and loose in its hole before you attempt to move the whole VFD, then have a suitable padded box to put it in to keep it safe without risking bending a pin or chipping the glass.
 
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Offline AndyC_772

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Re: VFD unwanted segments glowing
« Reply #3 on: January 20, 2017, 07:01:51 pm »
Thanks for the comprehensive info!

The service manual is available here.

The display is driven by a set of transistors which are located under the VFD. There are 100k resistor networks on the bases, and I can easily believe that a layer of crud has a comparable impedance.

I can probably get a spray nozzle under the display to clean under there, so that's probably a good next step.
 

Offline bitseeker

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Re: VFD unwanted segments glowing
« Reply #4 on: January 21, 2017, 09:28:07 am »
Nice service manual! It's good that the drivers are discrete. If there is an issue with any of them at least you don't have to go hunting for an obscure or custom replacement driver IC. Hopefully, a good cleaning will do the trick.
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Offline AndyC_772

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Re: VFD unwanted segments glowing
« Reply #5 on: January 21, 2017, 10:16:49 am »
I have another unit to compare against now too - this one, which arrived this morning. It display is dimmer, but uniform, consistent with age.

The BioLogic unit is interesting; it appears to be a modified version of the Sony PCM-2300, which is the professional version of the DTC-57ES consumer deck. Since it's a bit obscure, I was able to save it from the skip for the cost of a nice dinner.

The front panel PCB looks identical, so swapping parts between them should be straightforward. It's a shame the VFD isn't nice and bright, or I could have simply exchanged the whole board.

I'm also curious now as to what the differences are between the consumer and professional main PCBs. Maybe there's a little project coming up to build a single machine with the best features of both?
 

Offline AndyC_772

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Re: VFD unwanted segments glowing
« Reply #6 on: January 23, 2017, 07:25:50 am »
[Update]: it looks as though the fault isn't on the display board after all. Swap it into the other unit and it's bright and clear with no problems  ^-^

It looks instead to be a fault with the power supply. On the working unit, the supply voltage to the VFD is -25V, and the filament voltage is -20V with 8.4v pk-pk ac superimposed. On the faulty one, the supply voltage and the filament DC bias are both -23V.

There's a shorted zener in the PSU which looks to be the culprit, and a couple of other parts in the -25V regulator which are getting oddly warm. They're all generic parts, so it's easy enough to swap them for new ones, and since the whole machine is 25 years old, I'll re-cap the PSU while I've got it out.

Hopefully I've found the root cause now, and there won't be something else loading the negative rails and causing the PSU to die...
 

Offline bitseeker

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Re: VFD unwanted segments glowing
« Reply #7 on: January 23, 2017, 12:52:51 pm »
Very interesting. I haven't heard of ghost segments from a power supply being a couple of volts off. Regardless, it's good you found that issue. One step at a time. :-/O
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Online Ian.M

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Re: VFD unwanted segments glowing
« Reply #8 on: January 23, 2017, 01:03:06 pm »
Its not the fact the PSU is a few volts off, its that its lost its negative grid bias required for a complete cutoff because the filament's DC potential has dropped to the negative rail because of the failed Zener.  I mentioned the grid bias issue in the second paragraph of Reply #2.
 

Offline bitseeker

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Re: VFD unwanted segments glowing
« Reply #9 on: January 23, 2017, 01:07:57 pm »
Thanks, Ian. I missed that.
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Offline AndyC_772

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Re: VFD unwanted segments glowing
« Reply #10 on: January 26, 2017, 07:10:22 am »
The PSU is now fixed and the display is bright, clear and ghost-free now  :D

As well as the dead zener, one of the PSU caps (a 6800uF 35V Nichicon) had sh*t itself too. Good job I was going to replace it anyway.

I did a final check of the whole unit with a thermal camera before putting the lid back on, and found a tiny resistor getting very hot whenever the unit had a tape in. I didn't recall seeing that during my 'before' check, so I had another look at the service manual...

This resistor (R380, for anyone wanting to take a look) is part of a discrete transistor circuit that drives the mechanism. All the current that flows through it ends up in the base of a small transistor, so there's no way it needs to be passing enough current to get it hot... is there?

The value of the resistor is 330R, and sees the whole of the "+10V" supply across it normally, which would result in a dissipation of 300mW. However, the 10V rail is unregulated and comes straight from the mains transformer, and since this is an old EU model that I'm using in the UK (240V vs 220V), its voltage is significantly higher - about 13.6V. This means about twice the designed-for dissipation, and a glowing white spot on my thermal image.

(It's exactly the same in the BioLogic unit too, so I'm fairly sure it's a design feature rather than another fault as such).

It looks to be part of a simple logic circuit that turns a solenoid on and off, and I can't for the life of me think of any reason why it needs to pass so much current into the base of a high gain transistor, so I've swapped it for 1k8. Now it dissipates 100mW and is warm, but not worryingly so. I could probably increase the value further without issue.

I also checked it against the PCM-2300 service manual. In this version, there's a little '!' warning next to it on the schematic, but the value is the same. Maybe someone at Sony spotted that it got hot, but didn't have the authority to actually change it?

Either way, with the PSU fixed and R380 changed, the unit is back together and working perfectly. Result  ;D
 

Offline bitseeker

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Re: VFD unwanted segments glowing
« Reply #11 on: January 26, 2017, 09:44:17 am »
Looks great, congrats! I'm not sure about that resistor. It does seem unusually overdriven.
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