Bush TV125C

Interestingly I never had a problem with Suflex caps in the Bush TV105/115/125 series although I know their reputation. I did have problems with polystyrene caps in other sets.

There was a long thread on UKVRRR about this subject a few years ago and one comment was that there were a few makers of theses capacitors and according to Suflex theirs didn’t suffer problems. Not sure who made that post, bit embarrassing if it was me and I have forgotten.

Unfortunatley whether or not it is true, polystyrene caps have a good reputation for stability of their capacity with temperature variations  but not for shorts or going O/C.

Posted by: Nuvistor

I never could understand why they put a 300ma thermistor in series with both the heater string and HT line. The thermistors did fall apart with the excessive load ...

I never checked the specification but, as its cross sectional area was several times greater than that of the (virtually trouble free) CZ1 used in 300mA heater chains a decade earlier, I would have expected it to be higher. Also, these thermistors did not 'fall apart' - they just cracked in half, as if some great external force had gripped both ends and snapped them in two. 

Posted by: Katie Bush

I can see how the system switching could lead to unreliability ... It's a mucky looking switch with almost every single contact looking sooty black, and yet there is little evidence of smoke ingress - either from a coal fire, or from cigarettes.

It may well be the environment in which it was used, Marion. I grew up and worked on the north bank of the Thames, surrounded by heavy industry of all types and saw many Bush slide switches - mostly in VHF tuners - with jet black contacts. We were forever cleaning them!

I remember one day being with a colleague in a customer's house and, despite all our attempts, couldn't get the switches to reliably make contact. The customer asked what the problem was. After we'd explained, his wife disappeared and returned a minute or so later and presented us with a tin of Silvo! 

Now we'd both seen the results when some idiot had resorted to Brasso - as soon as you saw those naked brass contacts you new you were on a hiding to nothing - but this was something different. I didn't even know it existed!

We dipped the wrong end of a match into the Silvo and cautiously applied it to one set of contacts, applying slight pressure as we moved the contacts back and forth and Hey Presto! - silver appeared!

When we finished all of them we used good old fashioned  RS switch cleaner (carbon tet and oil) to wash off the surplus Silvo and were greeted with a beautiful gleam for every slider.

A tin of Silvo subsequently took its place on our workshop shelf!

However, when I was at college, another chap there worked for a firm that were scrapping ex-rental TV85s and brought in a tuner and IF module for anyone who wanted them. I took the bottom off the tuner and was amazed - the contacts looked brand new! But, of course, these contacts had spent their life free of pollution on the edge of Epping Forest. It was quite an eye opener.

The contacts went black and tarnished in South Lancs, another heavy industrial area at that time, we used Electrolube, there were various types, not sure sure which one it was now. Did the job.

Terry, the thermistor could well have been a greater rating than 300ma, I always thought it was, CZ19 from STC, cannot find any specifications for it. I had various failure modes from broken in two, wires off the end etc. Whatever the problem was the mod cured it.

CZ1 300mA; CZ19 800mA according to this data sheet

I knew I'd written about the thermistor mod on here before and now I've found it - it was 5 years ago!

You can find it here.

Posted by: Terry

CZ1 300mA; CZ19 800mA according to this data sheet

The official, period Brimistor catalogue covering the characteristics of the entire CZ range, can be found in the component section of the data library

That’s a lot larger than I thought so why did it fail? The replacement was fitted out of the way of the dropper resistor so the heat from that would not have helped but 800ma is well within its rating.

Well, it's certainly had 'something' done in the power department. There are three 'high watters' above the main dropper - see picture below. One would appear to be a simple repair to a dead section, but the other two emerge from behind chassis.

The following picture was taken on Saturday, so despite my lofty idea of going out and having another look at it, I haven't touched it today.

So whether that's a factory variant, or just several bridged dropper sections, at this point I can't really say. As yet, I have seen the thermistor!

>>> EDIT <<<

Looking over LLJ's article, referenced by Chris, above, the component physical layout diagram shows the two upper wire-wounds as factory fitted, yet looking at Trevor's photos of the set he had, those two resistors aren't there, so were there two variations of the power section?

Posted by: Nuvistor

That’s a lot larger than I thought so why did it fail? The replacement was fitted out of the way of the dropper resistor so the heat from that would not have helped but 800ma is well within its rating.

 

According to LLJ, the heater fuse was rated at 500mA instead of 350mA, so presumably this heater chain was a tad more power hungry?

From that photo I would say one bridged dropper section and instead of doing the mod just replaced the thermistor with a resistor, unless of course the is a thermistor on the other side of the heat shield.

Posted by: Katie Bush

Looking over LLJ's article, referenced by Chris, above, the component physical layout diagram shows the two upper wire-wounds as factory fitted, yet looking at Trevor's photos of the set he had, those two resistors aren't there, so were there two variations of the power section?

No, Marion, there was only one but LLJ's article shows the original Bush circuit from the Service Manual and, although he refers to the mod, he just says "3R58 changed to 10Ω and transposed with 3TH2" but what is missing is the instruction to place them behind [1] the metal heat shield. This keeps the thermistor and resistor away from the heat of the dropper. As originally built, the thermistor was directly over the dropper, so was being 'cooked' from both inside and out: perhaps this is the reason that there have been reports of the connecting wires unsoldering themselves? I saw lots of these sets but never that problem - probably because they'd been modified before the problem had a chance to reveal itself or because the original cracked in two and the mod was done when it was replaced.

Perhaps some companies couldn't be bothered to carry out the mod and left the thermistors that remained cooking in perpetuity? 

Yours doesn't even have a thermistor! I would suggest that, before you do anything else, you find out the values of all three WW resistors and tell us which go where in the circuit. I don't know if any kind soul has a CZ19 lurking about - if not, we'll have to put our collective minds together to find a solution for the future

[1] Strictly it should be in front of but it depends on whether you call the back the front when it's facing you or not! Perhaps it would be better to say 'the other side'! 

Okay, today I took pot luck on what looks like a 'fat CZ1' - hoping it will turn out to be a CZ19 ? . Even the seller couldn't offer any real wisdom, but for a couple of quid, I was willing to take a punt.

I did have a large heap thermistors in my sights, but I reckon I must have underestimated their worth - I was outbid in the dying seconds. ? 

Hopefully, I'll get to see what's actually in the set before too much longer - it still resides in the V70! I need to get it out of there, perhaps into the back of the trailer, and get the compressor fired up then blow off all the dust, cobwebs, dead spiders, dead cowboys and Indians, and get the wood bugs terminated.

After that, I'll contemplate the application of power to the set and see how well it wakes up.

Marion, if you carry out the thermistor mod, it will only be carrying the same 300mA heater current that the CZ1 was designed for so, if you have a CZ1 handy, I'd put it in.

It looks as if it will have a higher resistance than the CZ19 but the data sheet I've looked at only gives the resistance for a current of 800mA and it would never have carried that much in this set even when handling the combined heater and HT current.

I suggest you measure the  mains voltage and the crt heater voltage and, if your mains differs from 240V, scale the reading up or down as appropriate.

If there is an error, adjust the value of a resistor in the heater chain. If you are lucky, you might find it is close enough anyway

Aside from the telly still being firmly rooted in the back of the car, I now have what looks like a Brimistor of the correct (or very close) physical dimensions, and a cold (ambient room temperature) resistance of 93Ω. So once the set is out of the car, I can at least look forward to approaching the task with some hope of sorting out the power supply, if it proves necessary.

Posted by: Katie Bush

 I now have what looks like a Brimistor of the correct (or very close) physical dimensions, and a cold (ambient room temperature) resistance of 93Ω. 

The Brimar datasheet in the Data Library says 800Ω at 20°C for a CZ19, so it can't be one of those.

Nearest looks to be a CZ12 - 120Ω at 20°C but subject to unspecified 'manufacturing tolerances'.

Difficult to estimate the hot resistance because it is only given at the maximum rated current  (1.5A for the CZ19 and 2.5A for the CZ12) although there is an extra figure for the CZ19 which shows a nearly 50% increase by dropping the current to 1.25A, so it's anybody's guess what it will be at 300mA!

There is more detailed data available but it only goes up to the CZ12, where a graph suggest about 20Ω at 300mA.

So, I suggest that you carry out the modification - you don't want the new thermistor cracking in two! - and monitor heater voltages when it has stabilised and work from there.

Of course, your first priority is to shake off the flu bug - I hope it won't be long now.

In that case, I wonder what it actually is.... I'm only going on what the seller quoted for its cold resistance on a slip of paper, and one thing I shall do, is to check that for myself just make doubly sure.

However, and not wishing to jump too far ahead, we haven't yet examined the physical circuit in the set - You never know, it may have already been modified.

From what I saw of the set on the day I collected it, it would seem to have been maintained to a high standard - there are no obvious signs of bodgery in this one.

As Norman Wisdom would say. Stop mucking about.... Fit a cap dropper to the heaters.