Bush T86C IF Strip Info

Hi Tim.

The 20page R&TS service data for this suite of sets, is to be found in the library. You already have an account, so can login to download

Hi Tim,

welcome aboard

Couple of things: I assume you've tried the tuning (sounds obvious I know, but!), and another possible cause is AGC. If the RATS info isn't of any help, I do have Bush service manuals and I'm pretty certain that this receiver section is amongst them. I'll have a look later.

Thanks both for your replies!

I've downloaded from the library and looking through that data now. According to what Iv'e read so far the fringe IF strip is the same as the T76.

I've also been reading some of the LLJ notes on Bush sets of the era, he suggests the vision detector diode is commonly faulty. If this has failed wouldn't I get nothing at all? It looks more like a severe gain fault to me.

The answer is... not necessarily. Quite often if the diode fails there's enough capacity to pass some HF content through it. So you'll see edges of captions and the like.

Your best friend at this point is a 'scope.

I'd be looking at getting the brightness under control first, myself...

The OA70 vision detector is inside the final IF can but if you remove one of the connections - to L29 or the earthy end - you can then measure the forward and reverse resistances of the diode* without removing the can to see if it looks OK - or not!

Saves a lot of trouble removing and dismantling the can only to find nothing wrong!

* Forward = Low; Reverse = High.

Based on the forward voltage and reverse current figures in the data sheet, forward resistance should be of the order of 1Ω - 2.5Ω and the reverse resistance 50kΩ - 1.5MΩ

Thanks Terrykc and Cathovisor.

I'll check that over the weekend if I get a chance. Will also check the tube voltages for the brightness problem. From memory, that last time I worked on it and checked, the grid voltage seemed normal. Does any one know if the faults I've come across are common for this chassis?

Thanks

Hi Tim,

There's a Practical Television article on these sets (or their immediate forebears) in the library IIRC - well worth a download (look it up in the indexes kindly provided) . Of course, we cannot forget that Anno Domini will have played a part too.

Yes been reading those articles, which made we wonder about the tube which it mentions. So did some checks- pin 2 varies from 35v- 110v (should be 0-195v) when the brightness is turned end to end, pin 11 varies from 55v- 115v when brightness turned, instead of 120- 150v. Does that point to the possible cathode/grid short the article mentions on these sets?

Hmmm, forget the last post, it's a bit more fundamental than that. I've realised the HT is low, very low at 126v instead of around 190v.

I understand this is probably down to the metal rectifier. I doubt direct replacements are still available so a modern silicon one is in order, but I remember reading somewhere that this will raise the HT. What's the recommendation- silicon with an additional resistor, or hunt down an original?

It's been a long time since I did TV repair, I'm learning all over again!

Thanks!

Hi Tim,

Silicon with an extra resistor. A 1N5407 will be adequate, and fit something like a 7W or 11W wirewound in series. Adjust the value to give the correct HT.

A useful place to measure the HT is at the anode of the PY81 efficiency diode which, according to the Trader sheet, should be 190V

Temporarily connect the silicon rectifier, measure the voltage and calculate the difference. A typical value for the series resistor is 1Ω/volt. Find the nearest value you have to that and connect it in series with the output of the new rectifier and measure the new voltage. This should enable you to calculate the actual value required more accurately. Note that the voltage obtained will vary slightly from set to set and is particularly dependent on the Mains voltage input, so there is no point in aiming for absolute precision!

Note that the voltages shown in the RTS service data seem to relate to the TV79 (page 309 which, presumably, contains the correct table for the other models is missing). The Trader sheet is for the non-fringe TV85 so some licence will be needed when comparing voltages with either of these tables.

Thanks Terrykc and Cathovisor again for your input.

Right I've changed the metal rectifier for a silicon one. HT is now about 198v measured at C68. I changed the mains voltage tap to 250v to get it close while I wait for a high wattage resistor to arrive.

The symptom I get now is snow on screen when out of tune, when in tune to a station (Aurora set to CH3) the screen goes bright white with flyback lines. With brightness turned right down nothing is visible but bright raster and flyback, but no video content. When slightly off tune, peak white edges are visible.

Tube voltages are pin 2: 0- 177v depending on brightness setting, pin 11: 48- 170v again depending on brightness control and pin 10 is about 370v.

I've scoped the output of the vision detector and there seems to be video. The IF probably needs some re-aligning as I can see some audio on it too. There's HT on pins 1 & 6 of the PCF80, V7.

So over to the experienced experts for advice please!!

My main concern is that the cathode of the tube seems to follow the grid when you vary the brightness - they should be independent. It'd be worth trying to borrow a CRT tester just to check there aren't any grid-cathode leaks.

Here's the circuit from the provisional service data:

Thanks, that's a great help.

I suspect the tube too. With the cathode disconnected there's still around 50v on in at the lowest brightness setting, up to about 180v at the highest.

Is there a way to clear such a short?

Thank you.

I'm not sure that doing that is a valid test - electrodes don't like to be left floating... it did occur to me that you may get a variation due to grid current flowing at the settings of the brightness control where the grid potential exceeds that of the cathode, but I did wonder if R138 (270k) is in good order - if it was o/c or very high in value it might lead to the symptoms you're seeing.

The real test of course is to check the potentials at both pin 11 of the CRT and pin 8 of V207 (the cathode follower bit of the PCF80). Any major differences there will show up what's going on, both with the tube and R138.

CRT testers often have a "clear leak" function.

Yes, just connect a meter between the CRT cathode (pin 11) and the cathode of the PCF80 (pin 8) and check for continuity. This should give a reading of 270kΩ and will also check for continuity between the Rx unit and the main chassis at the same time - a dodgy connector or disconnected wire will show up as an open circuit, even if the 270kΩ resistor is OK.

The voltage on pin 8 of V7 when disconnected is 123v. When reconnected it's measures 320k from pin 8 to thin 11 of the CRT. It's not that high to make a difference is it?

When connected to the CRT, pin 8 of V7 goes from about 120v down to 31v as I tune into the test card. At the same time pin 11 on the CRT goes from 92v to 49v. Time to look for a replacement tube?

I'm not convinced - the tube voltages seem to be following the PCF80 voltages correctly - which they wouldn't if you had a grid/cathode leak. Check the voltage at the CRT to see that it remains reasonably stable as you tune into the test card.

For an increase in brightness, the cathode potential has to drop so the voltage changes you are seeing are in line with the change from black level/no signal (maximum) to full signal with lots of white (minimum).

Can you take pictures of the oscilloscope waveforms of the rest card at the detector and output of the video amplifier?

Remember to adjust the scope's Y deflection to suit the changes in level and keep the trace large on the screen. Make sure the timebase is locked to the incoming video - the speed should be 10µS/Div - and don't forget that the sync reverses polarity from detector to output, so you'll have to adjust the trigger too.

If you are not familiar with scope usage, you may find this thread useful ... viewtopic.php?f=15&t=5927