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Old 15th May 2018, 5:17 pm   #21
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Default Re: Yet another Bush DAC90A

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switching to the ohms function I am reading 72.5Ω across the heater pins for my UL41 valve.
Sounds about right for a cold filament. You could be in luck.
 
Old 15th May 2018, 6:46 pm   #22
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Default Re: Yet another Bush DAC90A

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The valve heater was checked with the continuity function and there was no audible output - switching to the ohms function I am reading 72.5Ω across the heater pins for my UL41 valve.
As MM said, that would be about right.

As a rough guide for future reference most valves will have a cold heater filament resistance of between approx. 1/5th and 1/8th of their hot resistance, the normal hot resistance can be determined by the voltage and current rating of the valve given in the valve data specs by using the ohms law formula eg: UL41 heater data is given as 45 volts at 0.1 amp so when hot the resistance will be V/I...45/0.1 which = 450 ohms, 450 ohms divided by your measurement of 72.5 ohms = 6.2 which falls within the 1/5th to 1/8th remit given above.

The trick is to remember that the filament is non ohmic:
https://www.youtube.com/watch?v=MaBSzmxmb3s
hence a much lower resistance figure when cold compared to when hot, it has a positive temperature coefficient (PTC) in other words the resistance of the filament increases as the filament temperature increases.

A resistance that decreases with an increase in temperature has a negative temperature coefficient (NTC) in other words the resistance decreases as the temperature increases, a practical example of that would be the type of thermistor used in some series heater chains that you may find in some radios.

The continuity function on your meter should be used for expected very low resistance circuits such as fuses, switches, a length of wire or soldered or screwed connections, very low resistance coils/transformer windings etc. But for the real deal, low ohms range is best.

I hope this helps.

Lawrence.

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Old 15th May 2018, 7:02 pm   #23
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Default Re: Yet another Bush DAC90A

The confusion he's having over the use of his test meter for continuity and resistance readings is making me wonder how the output transformer primary was determined to be open circuit. Having said that, they often are o/c the same as the ones in those Vidor sets, hence everybody asking for them in the wanted section. I would advise re-checking that transformer with a meter that you know how to interpret the results from.
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Old 15th May 2018, 8:46 pm   #24
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Default Re: Yet another Bush DAC90A

As stated, don't use the 'continuity' function for checking valve heaters or the primary of an output transformer....you'll get the wrong result.
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Old 15th May 2018, 10:34 pm   #25
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Default Re: Yet another Bush DAC90A

Well, it depends on what you mean by "continuity" .....

Anything that isn't an open circuit is continuous, strictly speaking. But as far as most test meters are concerned, "continuity" means a short circuit; less than a few ohms, at any rate. Which is good if you are testing something that should be a short circuit, such as from the mains plug to the on/off switch, or even something that should not be, such as between the line and neutral pins of the mains plug.

Moral: When measuring an unknown resistance, if the meter shows "overload", switch to a higher range, and only declare it open circuit if it still reads so on the highest resistance range.
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Old 16th May 2018, 10:45 am   #26
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Default Re: Yet another Bush DAC90A

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I would advise re-checking that transformer with a meter that you know how to interpret the results from.
Ouch ...!
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Old 16th May 2018, 11:57 am   #27
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Default Re: Yet another Bush DAC90A

Hi Techman - I'd forgotten your comment about me having a " ... 'painting-by-numbers' approach to capacitor changing" during my Bush AC91 restoration - that was an Ouch! as well.

Not sure if it really helps me but you are entitled to your opinion.
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Old 16th May 2018, 1:32 pm   #28
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Default Re: Yet another Bush DAC90A

Hi Donald, I can't actually remember saying that to you, but you're probably right as I think I've said it on a few occasions on different threads, so you're certainly not the only one that has done it, and those that should know better should stop encouraging it. This changing all the caps in one go without knowing why your changing them is just very wrong and likely to lead to disaster if you make a mistake along the way, also you learn nothing. Anyway, I don't think you've started on the caps yet in this set, so you already know better. However, with this particular set, if you were to do a blanket change of components, the layout is so simple that you'd probably get away with re-tracing your steps and finding the mistake - which you wouldn't be able to do in some more complicated sets, some of which are like a rats nest inside. Correct me if I'm wrong, but I think it was you that said you didn't want to do a 'painting by numbers job' in your recent record player thread? I was quite impressed by that statement, but I'd forgotten until you just said it that I had said this to you once before in a radio thread, so you're certainly doing things right, you just need to gain a bit more knowledge and experience, which you're doing all the time.

Regarding reading the digital meter - I think that everyone should have an Avo type analogue meter as well as a digital meter. I often use an old Avo for doing the sort of tests that you're doing. Actually seeing the needle and where it is indicating on different parts of the scale can often be much more informative that a DVM readout, and I really hate these auto ranging things and wouldn't use one. Once you get an analogue reading, you can always go to your digital meter for an exact confirmation. With a meter such as yours I feel that you need to be more experienced in its usage as I can see you're going to come unstuck interpreting some of the readings you're getting with it, although having said that, I think you've already learned quite a lesson with this last valve heater continuity test.

Why not put a request in the wanted section for a cheap Avo. They sometimes even get given away or offered for a fiver plus postage. It's much better to have one go to a forum member who's going to put it to good use rather than it just sit on a shelf somewhere, unless of course it's a particularly rare and mint example. If it's just a bog standard Avo with a few dings and scratches then it's much better on the work bench being used. You just have to appreciate how an Avo loads a circuit more than a digital meter will, but it's always good to have both and would have saved you some of the confusion that you've just had.
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Old 16th May 2018, 1:49 pm   #29
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Default Re: Yet another Bush DAC90A

Donald, have a look at this thread, looks like some Avos have already been given away and others available free if you're quick!
https://www.vintage-radio.net/forum/...=141490&page=2

Send a PM to Phil and ask him if he has one going spare.

Edit: Actually not free, but I see that Phil says reasonable prices. However they may have now been promised to a 'collector'.

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Old 21st May 2018, 8:49 pm   #30
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Default Re: Yet another Bush DAC90A

Here are some resistor values I have measured with my digital multi-meter: -

R13 - H.T. Line Smoothing : Should be 10,000 ohms, I'm getting 11,100 ohms

R16 - Scale lamps shunt : Should be 250 ohms, I'm getting 71 ohms

R17 - V5 Surge limiter : Should be 150 ohms, but I'm getting 241 ohms

(There is a Note in the Bush Service Instructions that on later receivers R17 is 250 ohms)

The picture below shows: -

R13 Ω and a bit worse for wear at the bottom end?

R16 75Ω +-10%

R17 250Ω +-20%

What would be the modern day equivalents if I needed to replace any of these??
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Old 21st May 2018, 9:31 pm   #31
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Default Re: Yet another Bush DAC90A

Those three resistors look original, the two wire wound resistors measure ok, no need to replace them, I would only replace the 10k if its present appearance was a bother, a 2 watt job would be ok if that were the case.

Lawrence.
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Old 21st May 2018, 11:35 pm   #32
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Default Re: Yet another Bush DAC90A

Thanks Lawrence - I'm not concerned about changing the 10k for appearances sake just a little concerned about its condition and what could have caused the "bubbling" at the bottom end - is that due to overheating?
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Old 22nd May 2018, 9:08 am   #33
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Default Re: Yet another Bush DAC90A

It could be due to it's proximity to the dropper resistors, it could also be due to excessive current that's flowed through the resistor at some time, if it starts bubbling/smoking when power is applied to the receiver then there is a problem, if not I wouldn't be bothered to much about it.

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Old 22nd May 2018, 8:37 pm   #34
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Default Re: Yet another Bush DAC90A

I'd be more concerned about the charred paxolin holding the dropper connections.
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Old 21st Jun 2019, 6:52 pm   #35
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Default Re: Yet another Bush DAC90A

Thread reopened at OP's request.
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Old 21st Jun 2019, 7:36 pm   #36
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Default Re: Yet another Bush DAC90A

Thanks to Graham (Station X) for reopening this Thread - it became dormant due to a period of inactivity on my part. The Bush DAC90A is now back on the bench, well table ...

Some good news is that I have a working DAC90A - did all the pre-turn on checks and replaced all the above chassis wiring around the bulbs, dropper, on/off switch, loudspeaker and Output Transformer which was rewound by Ed Dinning. All new wiring courtesy of Phil Marrison. Didn't bother with replacing C22, the mains filter capacitor - the consensus appears to be that the set doesn't really need a Class X capacitor here?

Underneath the chassis C18 (the audio coupling capacitor to V4) has been replaced of course, along with C14 (the coupling to V3) and R9. Good squirt of De-Oxit into the wave change switch and the volume control on/off switch. Powered up via my lamp limiter and watched it cycle through the valve heaters warm up stage where the lamps starts bright, go dim and then brighten again as the radio comes to life - deep joy and satisfaction!

Good reception on both wave ranges although the usual hum, crackles and pops when tuning. Now I can slowly work my way along the tag board underneath the chassis and replace anything which is out of tolerance - noting any improvements as I go. I'll also have look at doing the Pin 4 rewiring on the UL41 although there is no real obvious hum problems?

Question - how hot should the dropper get?! A distinct smell of something getting too hot and some smoke but not sure if that was just several years of dust and stuff burning off? Definitely too hot to touch so I turned it off...

This reminded me of the picture I attached to Post #30 above that showed "bubbling" at the bottom end of R13 and charred Paxolin holding the dropper connections.

So: -

1. Has overheating been an ongoing problem with this set and if so how do we resolve it?

2. Would it make a difference switching round the live and neutral connections as there appears to be some evidence of a "phantom tinkerer" having done some rewiring?
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Old 21st Jun 2019, 7:53 pm   #37
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Default Re: Yet another Bush DAC90A

I agree with Sideband. That Paxolin dropper panel looks like a fire risk. I would fabricate a new one making the selector screws clean and tight.
Bubbling on dog and bone resistors is quite normal. It's probably been like that for six decades. For safety's sake, please deal with that panel... Regards, John.
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Old 21st Jun 2019, 8:46 pm   #38
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Default Re: Yet another Bush DAC90A

Those dog and bone resistors do bubble a bit when they are fully run up. If I was you I would make up a new tag board and solder in the resistor with some high melting temp solder the same as you would use for soldering dropper resistors. And some new BA nuts and screws.

Regards, Robin.
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Old 21st Jun 2019, 9:21 pm   #39
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Default Re: Yet another Bush DAC90A

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Question - how hot should the dropper get?! A distinct smell of something getting too hot and some smoke but not sure if that was just several years of dust and stuff burning off? Definitely too hot to touch so I turned it off...
Droppers get hot...usually too hot to touch. I could work out the power dissipated if I had all the information but at a guess it's going to be somewhere between 20 and 25 watts.....that's as hot as a soldering iron..... After many years of non use, there will be a 'dust' smell as it burns off together with any grease from fingers etc during handling. Nothing to worry about.

However at some time in the past, there appears to have been considerable heat build-up on the paxolin panel connected to the dropper which I already mentioned. Quite likely this was caused by the set being used with leaky capacitors. Now that you have replaced several of the common ones including the audio coupling capacitor, chances are it will run at correct temperature. If the voltages are about right then there is nothing untoward going on so just run the set but keep an eye on things just to make sure. The odour from the dropper will stop after a short while.


Quote:
2. Would it make a difference switching round the live and neutral connections as there appears to be some evidence of a "phantom tinkerer" having done some rewiring?
Well only if they are incorrect now. It won't make any difference to the operation of the radio but you should ensure that the live and neutral are wired correctly. The circuit diagram will show which is correct but if you are still not sure, the neutral side should go (via the on/off switch) to the scale lamps and shunt.
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Old 21st Jun 2019, 9:56 pm   #40
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Default Re: Yet another Bush DAC90A

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Originally Posted by Heatercathodeshort View Post
I agree with Sideband. That Paxolin dropper panel looks like a fire risk. I would fabricate a new one making the selector screws clean and tight.
Bubbling on dog and bone resistors is quite normal. It's probably been like that for six decades. For safety's sake, please deal with that panel... Regards, John.
Thanks John - any clues as to where I can source a small piece of Paxolin to fabricate a new dropper panel? Only ones I can find are industrial size sheets!

Quote:
Originally Posted by robin coleman View Post
Those dog and bone resistors do bubble a bit when they are fully run up. If I was you I would make up a new tag board and solder in the resistor with some high melting temp solder the same as you would use for soldering dropper resistors. And some new BA nuts and screws.
Thanks Robin - a source please for high meting point solder as I only have standard 60/40.

Good news is that I'm well stocked up with BA nuts, screws and washers.

Quote:
Originally Posted by Sideband View Post
Droppers get hot...usually too hot to touch. I could work out the power dissipated if I had all the information but at a guess it's going to be somewhere between 20 and 25 watts.....that's as hot as a soldering iron..... After many years of non use, there will be a 'dust' smell as it burns off together with any grease from fingers etc during handling. Nothing to worry about.

However at some time in the past, there appears to have been considerable heat build-up on the paxolin panel connected to the dropper which I already mentioned. Quite likely this was caused by the set being used with leaky capacitors. Now that you have replaced several of the common ones including the audio coupling capacitor, chances are it will run at correct temperature. If the voltages are about right then there is nothing untoward going on so just run the set but keep an eye on things just to make sure. The odour from the dropper will stop after a short while.

Well only if they are incorrect now. It won't make any difference to the operation of the radio but you should ensure that the live and neutral are wired correctly. The circuit diagram will show which is correct but if you are still not sure, the neutral side should go (via the on/off switch) to the scale lamps and shunt.
Thanks Sideband - just out of interest, what information would I need to work out the power dissipated?

Your detailed response has reassured me that I may be worrying too much about this - but better to be safe so it shouldn't be too difficult (hopefully) to fabricate a new dropper panel as suggested by John and Robin above. I think my main concern was that it appeared to be more than just the "normal" dust burning smell with which I am familiar and there was a small amount of smoke, although this did decrease after a short time. What is the composition of the insulation material around the dropper?

After I've checked that all the voltages are correct I'll look again at the live and neutral wiring and cross check with the circuit diagram as you suggest.
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