[psg]

For the techies out there ....


When I increased the refresh rate on my HTPC to near the top end of the specs for my Sony 1292 (I don't remember the rate, but it is within the 1292's range), it displays it for a while, then one of the six horizontal output damping diodes fails shorted, and the projector shuts itself down.


The first time, it was one of the smaller ones, D23, a DFC15TR (there are five of these in parallel).


After I replaced it with an NTE506, the projector ran for hours, displaying 1080i from my DTC-100.


Tonight, I set her back onto the computer's output, muted the picture, and went away for an hour. When I came back, she was shut down again, with the same old error #02, "Horizontal Stop". This time, it was D1, the sole DD82RC, that had shorted.


All six of these diodes are in parallel. Does anyone know of any other component whose failure could cause these diodes to poop out like this?


On the other hand, would it make sense to just put more of them in parallel to better handle the load?


Here's a portion of the schematic:



These are on the 'E' board, hanging off the collectors of Q7 and Q8, the main horizontal output transistors. The schematic doesn't show it, but IMHO the other end of all the diodes *has* to be going to ground.


Puzzled,

Paul

http://www.cooldog.com/images/1292eBrd4.jpg

 

[mp20748]

Paul,

this is a tough one. If the diodes are in parallel, and one is different (NTE506) and it is the same one that keeps going out. I would suspect that it's not a good substitute for the original.

I would also think that all of the diodes in that circuit SHOULD be of the same type. The higher the scan rate the faster the diode.


That circuit is tuned by the assocated components, and the diagram should show the parts on the collector of the output transistors in a *Shaded area*, which normally means that those parts are critical safety components, and that they should be replaced with the original. I would use a substitute but they don't always work in every sweep circuit, or you should match them out with all NTE506's. It is also possible that the NTE506's cannot handle the high scan rate that the other diodes are handeling, because they are seeing and operating at whatever scan frequency you sync to.


But whatever you do, DON"T ADD anything to that circuit. This just sounds like the projector is working fine at the higher scan rate, but the diode is overheating and breaking down.


well, I hope this helps. I don't have a manual for that set, and it has been over four years since I have worked on one.


My 2 cents.



Mike

 

[psg]

Mike,


Thanks for the reply. I've updated the post with an image of the schematic from the service manual. As I said, I'm fairly certain that the other end of the diodes goes to ground; the scematic must be slightly wrong.


It wasn't the NTE506 that failed this time, it was D1, the DD82RC. Also, they don't seem to be spec'd as safety components ... although I will be getting an exact replacement for D23 soon anyway.


Normally, I'd not even ask about "adding more diodes in parallel", but looking at this circuit, I can't see any reason not to do it ... but that doesn't necessarily mean there isn't one.


When I replace the DD82RC, I'll use a DD82SC instead --- it is rated for 100V higher PIV, but I haven't found any compatible (but beefier) replacement for the DFC15TR's.


Paul

 

[mp20748]

Paul,

I would replace D1 as it is (82RC) instead of changing it. It appears to be a SAFETY diode (clamping). I'm not sure here but I would think that it is there to protect the circuit from exceeding a certain limit. It is hard for me to see the rest of the diagram, but the two horizontal transisitors seem to be operating from two different sources, and this is sometimes necessary, because there are two different bands (high and low) that most CRT projectors internally switch between when the input sync signal exceeds 60 or 70 Khz. How Sony is performing their switch I'm not sure, but it is usually combined (parallel) or separate (one or the other) depending on the frequency of the sync signal. If D1 failed I would suspect a problem with that circuit, I would also replace all of the diodes with either the NTE506 or the originals to include C13/C14, the D1 I would use the same number for protection. Though the circuit does not indicate it being a Critical Safety Component, trust me, it is!


Yes, it does appear as if there is a mistake in the diagram, look at the emitter on Q7 with the resistor going to base. It should be on the same ground as the other output transistor and diodes, or used in another circuit for pincussion and geometry, pedending on the design of the circuit. test with a meter (ohms) to be sure.


Walk slow, and carry a little stick...

 

[Curt Palme]

Mike is right, change C 13 and 14 as well. Order directly from Sony.


curt

 

[psg]

Guys ...


While I do appreciate your input, what I was looking for was an explanation of what those diodes did, how they did it, and why they might be blowing.


I found some info on horizontal deflection circuits here: http://an.hitchcock.org/repairfaq/RE...ml#DEFLFAQ_029


My interpretation of this is that the deflection yoke, C13 and C14 make a tuned LC circuit, and so any changes to C13 and C14 would be insane.


Further, the purpose of the damping diodes is to simply dump all the current from the back-EMF from the collapsing field in the yoke during the horizontal retrace to ground, as quickly and as completely as possible.


This implies that the critical parameters of the damping diodes are simply that the peak reverse voltage and forward current ratings be high enough (1500V and 8A) and that recovery speed be fast enough (1.5uS forward and 200nS reverse).


It also implies that there would be nothing wrong at all in popping another DD82RC and one or two more DFC15TR's in there to help share the load, and/or substituting diodes with better specs, like the DD82SC, with a 100V higher reverse voltage rating.


What make me nervous is that I don't understand why there are one DD82RC and five DFC15TR's in parallel. My first theory was that the switching times were faster on the DFC15TR's, and that they carried the current until the larger DD82RC could kick in and take the bulk of it, but according to the specs, the switching times are pretty much the same.


As to why they are failing in the first place, I think Curt may have had the best idea --- replace C13 and C14, since if they aren't just right, that could result in higher voltages appearing across the diodes.


Anybody have a different analysis of this? Any EE's out there with a little deflection circuit info?


While at one time I had that title, I only ever did digital design, so I'm a bit out of my depth here .....


Regards,

Paul

 

[KBK]

The only way you are going to get into overheating of the circuit, if itis indeed a tuned LC circuit, is if the C part of the LC network is off spec. The mistunig will cause more energy to have to be shunted by the diodes. I would look very closely at the idea of changing out the Cpart of the circuit with 'measured value' replacements. if they re film caps, it is very unliely that they have drifted in value. if this is a very early (older) 1292, then it is entirely possible that a capacitor (if it is electrolytic can type) has drastically shifted in value. if you are using the circuit consistently at one end of it's effective range (specifically the top, which would be higher scan rates) then the capacitor is the major compioent, and too mcuh energy may be 'bouncing' off of it's diminished, or aged characteristics. Shunting to the diodes in this case would be of a nature that it may be popping them with too much current flow. The circuit actually sounds critical, and in that case, premium replacement parts may be the key,and actually give you a more stable and correct image.

 

[psg]

Ken,


The caps are .0032 uF 1.6KV film --- and as I said, I would not even think of using anything but Sony's parts for them. They look fine, but I will replace them anyway, just in case they are somehow off spec.


Any thoughts on adding a couple of extra diodes? Would you agree that there should not be a problem with that?


Paul

 

[Brett]

Hi Paul,


To pump some water under Ken's bridge, I'd take a look at your manufacturing date and the hours logged on your unit. I use quite high refresh rates up to 144Hz on my 1292Q and have not had such problems. Mine was built in 1996 and has between 500 and 600 hours. KBK's cap theory could be worthwhile investigating.


Cheers,

 

[wm]

Since D1 is a Zener, I would expect it to operate differently from the other diodes it is in parallel with. I would expect to find the anodes all tied to the emitter of Q7 (missing in the schematic).


I don't have a 1292 manual, but I took a look at my G90 manual to see how they did it there - the G90 employs a network of 6 diodes, 3 in parallel in series with 3 more in parallel, and a pair of zener diodes in parallel with that as well. Also 2 transistors operating in parallel, with the parallel diode array tied to the collector one of these, and the zeners, hv caps, and some other components on the other, and both collectors tied to the h yokes...


As has already been mentioned, this circuit has to handle a very wide range of frequencies.


William

 

[mp20748]

Guys,

C13 and C14 are SPECIAL capacitors, they should be replaced ONLY with the originals. What makes this circuit a tuned circuit is the flyback/yoke transformer (coils) and C13/C14 (caps). The diodes (D21 thru D25) are for damping. D1 is a SPECIAL diode, I'm sure it's purpose is to protect that circuit from exceeding a predetermined limit, if that level is exceeded you could have a higher anode voltage than is needed for the CRT's, that could also raise the X-radiation level. When it shorts, that is an indication that the circuit has exceeded its predermined limit.


Using different caps could cause radiation problems as well as forcing the output transistors to run hotter than normal, this would cause unstable operation and output transistor failure. And in some cases: damaged CRTs.


This is not a circuit to get CAP HAPPY with. Don’t trip on ESR here. These caps are not cheap, therefore they MUST have a very low ESR, and that would have to be true for this circuit, because low ESR is very critical for this stage. A different cap might have a higher ESR rating, regardless of cost.


SONY is a master designer of their sweep circuits, they are VERY well designed and built. Please don't attempt to improve on their design, unless you intend to use this projector for experimentation, and if that is the case, get yourself one of those special vest that they use in the X-ray room of a hospital and go for it. Make sure you have plenty output transistors on hand.


Replace C13/C14, check the damper diodes for leaks, if problem still exist suspect flyback transformer.


The 1292 would probably have this problem in their database by now, give SONY tech support a call.

 

[psg]

William,


Despite the symbol on the schematic, D1 is not a zener. It is just another fast-recovery diode; the specs on it are almost exactly those of D21 - D25, except that it has an 8A average and 80A surge current rating, in contrast to the 1.5A average rating on D21 - 25. Its also in a TO-3PML package, clipped to the large heatsink along with the horizontal output transistors, where the others are axial-leaded diodes, mounted directly to the board.


On the G90, what are type diodes are used?


Given the configuration, it seems obvious to me that they are using the series/parallel arrangement to increase both the voltage and current capabiliites of the damping circuit, which is EXACTLY what I have in mind to do (as well as replacing c13 & c14 with factory replacements).


Paul

(getting less puzzled)

 

[psg]

OK EVERYBODY --- LISTEN UP !!!!


FOR ABOUT THE FOURTH TIME: I am planning to replace C13 & C14 with FACTORY replacements!!!!!!!


The ONLY design change that I'm contemplating is apparently one that Sony did in going from the 1292 to the G90.


Thank you all for your concern, but *please*, read the posts when replying .... its starting to get downright comical


Paul

 

[KBK]

Now that i know the value and type of cap.. it seems that it is a very unlikely source. As for the diodes, they should all be replaced with uints from the same manufacturing run.. soa s to keep current flow inthe circuit even. if the circuit has a rf component or characteristic to it at all, then layout is a critical factor,as well as solder junctions and traces. All these items rear their ugly head in standard electroncs audio design,except that engineers generally are too dense to think 'out of the box' when it comes to design, forgetting that the sum of the parts equals the whole. (those that do, aren't talking, so we don't even know that they exist)


So, replacing one diode is probably not an option. Replacing them all as a group, is. Many times I have to do this with an audio amplifier, as the pressures on the circuit, dictate that the output transistors have to have IDENTICAL characteristics. A similar type, or even exact replacement will not do. A full whack of matched transistors from the same lot is all that will do. Otherwise, failure of the amp WILL occur again, and it will be the exact same spot or transistor.