Well I do have some fairly hefty 50W power resistors in 22 Ohm and 11 Ohm (I think they are) varieties. I could run 1 of each in series to give me near 600mA current. I haven't got much else in the lower ohm range.

I do have plenty of fuses though so I'm not bothered about blowing a few of them.

I will scrape away the wax tomorrow and take a look at those caps.

 

Many of the suggestions above are good and valid ones. They are assuming a worst case scenario.

What is the cost of the replacement fuse and two replacement transistors?

I would replace the two transistors and the fuse and hope for the best.

 

About £2 although I bought 6 transistors and have more than 10 fuses so I have actualy spent more like £8 and a few hours of time so far, but I've enjoyed it and appreciate all the advice.

I'm hoping it's just that NPN too and will be trying it out when the transistors arrive. If they don't arrive tomorrow I will look at those caps though, just for experience and to learn about the layout of this high voltage stuff and I will have the opportunity to ensure that there is no arcing going on at the 9kv end, this is apparently an issue with some OS3000.

 

Finally got some BFY50 today so as I had just a few minutes this evening so I soldered two of them in place. The little collar around the one that had blown was actually a little bit melted from the heat.

Turned on. Blown fuse. Hmm. I was hoping it would just be that but a part of me knew it wouldn't be.

Replaced with a 800mA fuse (original was 500mA) and tried again.

Fuse is not blowing so clearly something is still amiss but it's not producing a dead short this time.

Still a dead screen. No change to apparent brightness in dark room on switch on or switch off.

What I did notice on one power on was the soft light from the back of the CRT which normally glows quite dim started off bright and went dim after a few seconds. This only happened on one power on. That's probably unrelated as I think that is the heater and it comes from a separate secondary on the main transformer.

Will pull the oscillator board out again tomorrow evening and just double check those BFY50 and their connections as the board didn't respond well to being worked upon so maybe I pulled a trace and didn't make the connection good after or something. Was in a bit of a rush.

 

Kind of figured that wasn't going to be your problem. Sucks, but now back to hunting uphill

 

Yep. Time to get rid of all that wax and get some readings off those 4700pf HV caps. I obviously can't get ESR off them but maybe there's a low resistance path or something.

I was sure I also heard a high pitch noise coming from the EHT box at one point but after listening again I couldn't hear anything.

Will pull the scope back in the kitchen tomorrow away from all that streey noise and relisten tomorrow.

 

you really need an ESR meter

 

Something is overloading your BFY50s. They shouldn't blow straight away when replaced. That they seem to suggests they are not the primary fault.

 

I don't know if the BFY50 are actually blown this time as I haven't checked. Only had time to solder and quickly check for operation this evening.

I thought ESR meters were for electrolytic caps of greater than 1uF.

 

I don't know if the BFY50 are actually blown this time as I haven't checked. Only had time to solder and quickly check for operation this evening.

I thought ESR meters were for electrolytic caps of greater than 1uF.

On ceramic caps, the ESR is probably less than the resolution of almost any meter commonly available. Leakage or HV breakdown is more likely to be the problem - but faulty tripler diodes is even more likely than that.

BTW: 2N3053 is pretty much the same as a BFY50, just in case you see anywhere offering them cheaper.

 

Negative Ghost Rider. I know there are meters out there that can test capacitance down to 1pF and .01R of resolution.

 

Negative Ghost Rider. I know there are meters out there that can test capacitance down to 1pF and .01R of resolution.

How much do those cost? - the last one I saw anywhere near that resolution derived ESR from direct loss factor/phase angle measurement.

 

I cleared the majority of the wax from the multiplier board.

It was actually fairly simple. Mostly it flaked off with a little gentle prising.

Early in circuit indications are that none of the diodes anything near a short. I've tested with two meters and they appear as open circuit in both directions. The caps all seem to measure 0.1nF, again in circuit. Using my DMMs I can detect no short across any of the multiplier caps. The resistor, tested in circuit, measures a shade over 1MOhm as it is supposed to.

Thanks Ian. I've got 6 BFY50 but if I manage to blow them all I will certainly look at that alternative as it seems more available than the old BFY50.

The BFY50 I put in yesterday are still intact, there is no short across C-E. Tested in circuit. I was a bit concerned about my soldering of the new BFY50s but they look absolutely fine and even if the track is lifted slightly it is still making connections to where it needs to.

Now on to the Grid and Cathode supply before I attempt any out of circuit stuff.

 

Grid and Cathode all looks fine, in circuit tested.

The diodes show open circuit when measured with my meter. The caps show open circuit, or very high resistance and the measure of capacitance is close to nominal. The resistor shows a resistance as per schematic.

I checked the wiring back to the bright up board from the Grid and Cathode outputs and they make a connection. Not sure what happens to the voltage once it gets there but at least it's got a nice low resistance path to wherever it is meant to go.

I suppose next I have to get some forward voltage into those BY187 diodes. I have a centre tapped 15v/15v regulated power supply I made for an audio circuit which I think I will utilise, rather than make something up specially.

So my question is this...Is it safe/ok to leave the diodes in circuit to perform the diode test? I will add the 30v to the anode and a 10k resistor (I'm thinking 3mA should be fine to allow through the diode) to the cathode and link it to the power supply and measure the voltage drop across the diode. Spec says <26v drop so there must be near 30 diode junctions in each package. What would happen if a few junctions had failed?

Or should I take the diodes out? It doesn't look easy to do this. If needed which end is best to lift, or both?

If no fault is here is this pointing towards the oscillator or the CRT being dead?

The bleeding resistors, 2 x 1M in series are intact and measuring ok.



Regarding the oscillator. I still don't quite understand where the oscillation comes from as I've not studied oscillators so wouldn't know where to start troubleshooting. I'm thinking I could grab a PCB Scope, a Picaxe thing for £15 to check that 30kHz oscillation. It sounds like a little fun device and for £15 is almost disposable. Would be better than a DSO Nano I'd imagine and might be useful as a portable quick analyser moving forward.

 

Managed to get the three EHT diodes off the board without too much difficulty.

Diode 1 measured 15.88 forward voltage drop when measured at 30v.

Diode 2 measured 16.31 forward voltage drop.

Diode 3 measured 16.20 forward voltage drop.

I also wired the diode up backwards, with the anode off the 0v and the cathode off the 30v and had a resistor in series with the anode. I measured across the resistor to see if it was dropping anything across itself. Across all three diodes I couldn't get any voltage reading. If the diode were short in the reverse direction or leaky I would surely get a volt or two dropping across the resistor.

So it's not the HV diodes on the EHT, or at least not at 30v anyway.

 

Grid and Cathode all looks fine, in circuit tested.

The diodes show open circuit when measured with my meter. The caps show open circuit, or very high resistance .

Any high voltage cap that shows high resistance is leaky!

Of course electrolytics take a significant time to charge from the meter's test current - so you get an initial kick that sinks back toward infinity.

In theory - the tripler diodes should show low resistance one way round, and infinity the other.

In practice - your EHT diodes could be stacked junctions whose combined Vfs exceed the resistance meter's test voltage.

In ye olde days - EHT rectifiers were thin paxolin tubes containing hundreds of stacked selenium rectifier pellets.

 

The only caps I got anything approaching a reading were in circuit tests on the grid and cathode area. All EHT caps are open circuit. All EHT diodes are silicon stacks from what data I can find. 16V drop at 4mA current and no current when reversed.

 

I tried measuring AC on the the transistor bolted to the case, the TR813. I got a sub 1v reading. Measuring Dc varied but was around the 12v range with DC. According to the manual it is meant to be 15v, but that could be peak-peak. My DMM is orely lacking here. I have ordered a replacement that can do Hz and duty cycle among other things. I've also ordered a PCBScope, a Picaxe thing which if nothing else will be a fun toy.

Here's some closer images of the oscillator board.

Of note for me are the two black plastic transistors. These are not the ones featured on the schematic. Schematic calls for BC107 and 2N3905. The board holds BC147 and a BC22 something.

I've ordered a couple of replacements for the 4 on the board and have a couple of subs in my parts bin that should work ok.

I've replaced the 33uF and the small 10uF electrolytic caps today but this large capacitor has me puzzled. It is much bigger than the other 10uF and is written twice. It is referred to as a Mullard 12uF 121-16129 in the schematic so it's strange how 10uF is written upon it.

Also of note is the brown marking on the inductors. In the image above there is clearly some heating, when compared with this image I took a few days ago. I also see some marking at the bottom of the 390 Ohm resistor, but I have no prior image of that, although when I was poking about I don't remember noticing it before.

I guess this is indicative of a near direct current flowing through the inductor rather than an oscillating one.

I also noted that while powered up and with case mounted TR813 collector connected, the TR815 I replaced (I may have thought this was TR814 but I think I mixed them up) got very hot. The little plastic cradle it sits upon started to melt and give off a little smoke. The transistor is still intact but there is clearly too much current going through it, or it's not being shared between the two transistors.

So I think this on the side of the transformer that would normally get the positive side current.

 

I've replaced the 33uF and the small 10uF electrolytic caps today but this large capacitor has me puzzled. It is much bigger than the other 10uF and is written twice. It is referred to as a Mullard 12uF 121-16129 in the schematic so it's strange how 10uF is written upon it.

Is it my imagination or does that larger 10μF capacitor look a little bulged to anyone else?

 

Its not got a nice square edge that's for sure.

I don't know but I'm guessing it's some sort of exotic material electrolytic.

I have now changed it for a radial 10uF 50v cap I had salvaged from something. The cap itself is tiny compared with that monster I took out. I had to use a bit of wire to bridge the gap.