Crackling Inside Beovision 7802

[bobbyball]

"PTC" usually stands for "positive temperature coefficient". PTC devices are essentially resistors that change value depending on how hot they get. Often used in degaussing circuits (eg on your B&O set) where the coils are in series with them. When cold, the PTC has a low reistance but as it heats up, this rapidly increases. The effect with the degaussing coils is a rapidly deminishing maganetic field which demagnetises the tube. PTC (and NTC - "negative ...") devices are also used as temperature compensation across scan coils, possibly not in this set though, would have to look at the circuit.

The hum on audio sounds like it could be a supply rail smoothing issue. Check the service manual to see which capacitors smooth the supply to the audio circuits and try replacing them.

[Okto1984]

That's interesting. It looks like it does go to the degaussing coil, but my schematic reading abilities are not great. There's a good few things going on I don't recognise, but much like learning another language the words/parts I do know are kind of jumping out when I look over it.

[Michael Maurice]

The PTC does indeed go to the degausing coils. It used to be connected across the mains on the majority of sets of that era so that when you switched on from the mains from cold, it would degauss the screen.

B&O took this a stage further, it connected the PTC via a Triac so that it was activated when the set was brought out of standby.

I've had loads of trouble with the capacitors on the scan board.

If you want I've got a complete 20" working version of this set, though you'll have to collect it, it could be a usefull source of spares.

[Okto1984]

Hi Michael, that's a very generous offer, thank you. Unfortunately, I live in a small house and can't think of anywhere suitable that I could store a spare TV. However, these are good TVs, I'm sure you'll have no trouble finding someone who could use it - particularly as it's in good working order.

Fortunately, my TV appears to be in pretty good shape too, so I'm hoping to solve the last couple of things without needing many more parts. So far, the new EHT cable has been tried out for about 5 hours and it looks like everything is fine now, and the focus adjustments were definitely worth the trouble too as the picture seems just right now.

However, replacing the power supply capacitors is turning out to be more confusing than I expected. I haven't yet found out what the original's ESR or ripple current would have been (may never know) and I'm not sure just how critical it is to match these. As the service manual doesn't mention either value for any capacitors I'm tempted to assume not too critical - but I don't want to get it wrong and create a new problem. Meanwhile, the total range of capacitors available is vast and nothing is standing out as the right choice yet. Am I worrying too much about the details for this step?

[Okto1984]

Ah, I've had a capacitor break through. Looks like the high voltage capacitors (currently looking for 385v, for example) are mostly snap in types, and some of these clearly state low ESR and high ripple current, and that they're good for power supply filtering. I had been looking only at leaded capacitors as that's what I think are currently there. I presume snap in, as long as the lead spacing matches, will be compatible with my pcb?

[Michael Maurice]

The main electrolytic wont give you any problems, leave it alone. The only ones that may cause you problems are the small capacitors.

[Okto1984]

You're right, I'm making a total guess what (if any) capacitors may be dried out. It's a long shot to fix my sound right now. I'll worry about it later and in the meantime, I guess if any decide to really go bad they'll let me know!

Anyway, I'm mostly interested in fixing the buzzing sound - and I have a new clue. I kept thinking to myself the sound is good, but not like my beovision 9000. Well, the set has six speakers, three on each side of the screen, and I had a closer listen to them.

I found that the top speakers hiss faintly but emit no audio, the middle ones buzz softly and emit the audio, while the ones at the bottom make no sounds and emit no audio. So, only getting the audio from the middle speakers on each side. I'm also finding that the bass and treble dials have little or no noticeable effect on the sound. I've tried a couple of devices through both the scart and the coax and everything remained the same with the sound. I'm really puzzled and not too sure where to start with this just yet.

[bobbyball]

Just to say the 7802 only has two speakers per channel. The grilles with a circular pattern in them imply three speakers, but it is only two. The "bass" speakers are in a housing of their own, either side - an acoustic chamber in fact, whereas the treble speakers are behind the top grilles. The bass and treble should have some effect. The 7802/8902 sets have rather more complex (and better sounding) audio stages, with a separate output IC per channel, something like TDA2030 or something similar, can't remember exactly without finding the circuit. If the "buzz" is equal on both channels it does possibly imply a common supply smoothing problem as these audio stages are usually very good on the stereo sets. There is also a pseudo "expand" facility, shown by an indicator LED behind the left hand panel where the standby one is on the right hand panel.

[Okto1984]

That's good to know. The third speakers not actually existing certainly explain why they don't appear to be working That's good news too, it suggests as all four speakers are doing something.

I've been trying to reason what might be going on. The lack of bass and treble adjustments may be connected with the buzzing, or not. I don't know, but I do know at least that the buzzing is coming from the tv speakers equally - and any headphones I connect, so I currently doubt it's the speakers. I've also tried with the AV socket and coax with no difference. That leaves me with the amplifiers and power supply.

Anyway, as far as I understand things, there's basically two amplifiers. It's not impossible, but I guess fairly unlikely both have developed the exact same fault. So, I'm wondering what parts they share in common. With my current understanding that gives me the following suspects - the power supply and the circuits that control volume, bass, treble and balance.

I only suspect right now the power supply, or the bass/treble controls, because the volume and balance appear to work as expected.

Not too sure where I'm going to go from this, but hopefully I'm on a reasonable track so far to narrow down where to investigate.

Also, I have found the expand facility - and it seems to work pretty well. It's interesting, but I don't know how it works yet.

[Michael Maurice]

The bass and treble controls are separate for each amplifier using a 'dual' control. The only common thing is the power supply.

[Okto1984]

Thank you, that helps clear that up as I was finding that part of the schematic confusing. The suspect list is down to the power supply now.

Also, the bass and treble do have an effect, I just needed to try them with some music playing to hear it. The adjustments in general on this set seem modest in range compared to others, but they're sufficient.

To my ears however, the sound still seems not as good as my beovision 9000, it sounds weaker and I have to turn it up much higher to get the same volume.

[bobbyball]

The Beovision 9000 has a different audio amplifier and I have found the mono models to have a "thinner" sound than the stereo ones, so yout 7802 still has some way to go to sound as it should. Even though the "bass" speaker is only 3" in diameter it is fitted into a pressure chamber enclosure and sounds much better than such a small speaker "should" do!

[Okto1984]

The beovision 9000 is certainly the best sounding TV I've heard - so I hope I can get the 7802 working as it should to hear that properly.

Unfortunately, I don't have any test equipment to directly investigate, so I need to decide if I should get some test equipment to narrow down the bad capacitor(s), or just replace the old capacitors under the assumption they're past their typical life span already. On one hand, if one hasn't gone bad yet it possibly never will, while on the other by the time I've taken it out to test I suspect I might as well replace with a good quality new one while I'm there as it's about the same amount of work. It seems at least for Hi-Fi equipment, replacing all the old electrolytic capacitors is a common thing to do and people are reporting good results.

[bobbyball]

Just replace the capacitors with nearest like values, never fit one with a lower voltage rating. It's not worth the time, effort and indeed expense of test equipment. The parts you will need will not be expensive, just the ones on the various rails on the secondary side of the power supply - and in particular here the audio stages.

[julie_m]

If you do replace a capacitor -- especially an electrolytic one -- with a higher-voltage one, make a note of the fact somewhere. Otherwise you could get on a treadmill where you keep installing higher and higher voltage caps and eventually, install one rated for such a high voltage that it won't form at the actual application voltage.

[Okto1984]

Thank you, those are good points. Test equipment is costly as I have little use for it, and as for replacements, I'm planning to keep to the existing capacitance and voltages, as I don't want to change anything more than needed.

I've had a good look inside the set earlier to make a capacitor shopping list, and did some cross referencing with the schematic. Most of this is new to me - so please excuse any odd bits. It looks to me that just about everything is on the secondary side of the power supply, with only one big electrolytic (filter) in the primary side and a few plastic/ceramics.

The stereo amplifier power supply (board 47) was easy enough to find, it's a little panel plugged in to the secondary side of the power supply, where it appears to be tapped off transformer 5. I was surprised by the design of this little power supply board - it really doesn't have much going on. It seems to be connected to the transformer for power, and being a transformer it must be producing AC, then it goes through one diode and has a 470uF/50V capacitor, a 10 uH coil and a little resistor I forget the value of. So, using what I've learned so far, I guess it only gets half the waveform from the mains and the capacitor has to sustain it in between? There's a wire leading out of it marked as -40V, which I traced off to the stereo amplifier on another page. I couldn't see any more filtering on the amplifier board, so I assume everything that is going on with smoothing happens back on board 47. If what I think is correct, it's not hard to imagine this capacitor has had a rough life.

Anyway, I'm now tempted to assume this little power supply board's single capacitor is my bad capacitor and at first just replace this one to see if it solves the problem.

Also, I noticed a polymer capacitor, C9 on board 46 (2.7nF +/- 1% 63V) which has cracked. It appears to be attaching something called the oscillator (found on IC1) to ground. I can't see any obvious connection with the audio, but I'm trying to figure out why it's cracked and what effects this might have.

[bobbyball]

I can't yet locate my service manual for these sets so I can't tell what significance your cracked 2.7nF capacitor will have in the greater picture though it isn't likely to be affecting the sound. Change it anway if it is damaged.

You are correct in your analysis of the "secondary" rails in the power supply. Since the power supply circuit is of the "chopper" or "switched mode" type it is running at many KHz and the need for "full wave" rectification of these supply lines is unnecessary. What is needed is good quality capacitors of the "Low ESR" type as these work hard in these circuits and are the cause of many and varied faults in anything that uses this type of circuit - most power supplies these days in fact, from "wall wort" type mains adaptors to flat screen LCD etc TV sets.

Whether or not the particular capacitor in your set will cure the hum or not remains to be seen but the part needed to prove this should not cost a fortune...

[Okto1984]

I've ordered some capacitors (mainly Panasonic FR range) from Farnell, and will see how things go when they're here. I did also track down a 2.7nF +/-1% polystyrene capacitor (which I believe the current one is), but had to get the 160V version as the 63V just couldn't be found anywhere.

Despite my earlier plans, I've had to deviate a little on voltages to the next highest available. I also found the service manual suggests some odd ranges, like -10+50% or even -10+100% in many places. As most the capacitors I found seem to be +/-20% I had to up the capacitance a little to be certain the replacement actually falls within the range suggested.

As for switch mode power supplies, that's interesting and explains better what I'm looking at - although there are still mystery parts throughout. I've looked up some more about how they work.

[bobbyball]

You should be fine with the capacitors you have ordered. As you have seen, with electrolytics there is a very wide tolerance spread anyway.

[Okto1984]

I replace my first capacitor today. Not just first in this set, first capacitor ever. And, it works!

I replaced C1 on the stereo power supply (board 47) with a Panasonic EEUFR1H561. It's 560 uF, in place of a 470 uF - but taking in to account the new one's much smaller range, it's still comfortably within the range specified in the service manual. Well, the buzzing hasn't gone, but it's quieter. Meanwhile, the sound is much better. I think that when the volume of the program goes up it responds much better now. Although I could turn it up very loud before, it didn't seem to have a great range of volume at whatever setting.

Seems great so far, but I'm going to leave it a while before replacing anything else. Make sure all is good here before I do anything else. However, I am feeling more confident to replace further capacitors.