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Old 22nd Jul 2010, 8:56 pm   #1
JoshWard
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Default Bush BAC31 motorboating

Hi all, I have my BAC31 on the bench at the moment for a recap. Anyway, with all the leaky old waxies and plastic Hunts' gone I expected it to work ok...

This was sadly not to be, and it produces no signal at all sitting there, instead having some healthy crackle and when the wavechange switch is turned another healthy crackle.

One odd thing I did notice though, is with the wavechange switch between the two settings sometimes it will start to motorboat and bring in stations loud and clear. The noise will fade before the station coming in fine (after actually moving the switch to the band position) before the noise coming back and the signal being lost.

I've tried cleaning the wavechange switch but this hasn't done anything....

Any suggestions on getting a permanent signal on this?

Thanks, Josh.
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Old 22nd Jul 2010, 9:31 pm   #2
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Default Re: Bush BAC31 motorboating

Josh, is this the version with the built-in power supply? It has an EZ40 rectifier. If the HT is slightly low, it could be stopping the oscillator. The LT for the valves is derived from the HT via a dropping resistor and various ballasts. Typically these little battery valves are rather critical on their supplies and it may be that the filaments are glowing just sufficient to keep the audio active but not the mixer/oscillator. Measure the HT on the output transformer. If it's much below 80 volts then probably a new reccy is required. All the filaments are in series and add up to 7 volts (the DL94 is a double filament with the sections in series so it's 2.8 volts rather than 1.4 like the others). So if you measure across the filament chain you should read somewhere near 7 volts if everything is OK. Be careful when measuring. The valve pins are rather close and it's easy for the meter prod to slip...popped filaments.....!


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Old 22nd Jul 2010, 9:37 pm   #3
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Default Re: Bush BAC31 motorboating

Are the signals in the right places on the dial. If so then press the contacts together and see if that helps. Otherwise resolder everything and make sure nothing is touching where it shouldn't or not touching where it should.

As far as the Motorboating is concerned, check C2.

Cheers,

Steve P.
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Old 23rd Jul 2010, 6:42 pm   #4
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Default Re: Bush BAC31 motorboating

Right, I can't get it to motorboat anymore and bring in stations but I changed C2 anyway (more out of hope than anything else!)

On the DL94 I get about 2.4v and on the DK92 and DAF91 all of 1 volt. On the output transformer I get about 70 volts so perhaps it's the reccy. I can't get to the DF91 to measure it (tuning capacitor in the way).

It's definitely capable of picking up signals and amplifying them as it picked up the signal of an incoming text message.

Josh.
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Old 23rd Jul 2010, 7:07 pm   #5
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Default Re: Bush BAC31 motorboating

A mobile phone signal will probably crash through at I.F if it's close enough. That doesn't mean the oscillator is working. That annoying bleep that comes through just as a text message arrives just means that the aerial circuit is probably ok. If you have another DK91 you could try it. You could also try the old trick of picking up the oscillator on another radio. Just get a working MW radio and tune it to a quiet spot then place the BAC31 next to it and tune the BAC31 across the dial. At some point you should hear a whistle on the working radio. If so, the oscillator is working, if not, there's your problem. Try the DK first. If no luck then it's probably a low reccy.

Sorry..haven't got a spare EZ40 or you would be welcome to it. The volts do seem a bit low and it doesn't take much to stop these valves from working. They are probably OK down to a filament voltage of about 1.2. Much below that and they struggle, especially if they are a bit below par anyway.

SB.
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Old 23rd Jul 2010, 7:49 pm   #6
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Default Re: Bush BAC31 motorboating

Hello josh, it might pay to check the resistor that is across the smoothing can, if it goes too high, it pulls the ht down, it was a problem on mine.

Regards

Paul
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Old 23rd Jul 2010, 9:43 pm   #7
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Default Re: Bush BAC31 motorboating

OK, tomorrow I'll try another radio and try the DK (I think I have another somewhere).

Thanks for that tip Paul, I think I already checked it and it was ok but will check again!

Josh.
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Old 25th Jul 2010, 5:44 pm   #8
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Default Re: Bush BAC31 motorboating

I had a hunt through my spare valves and other radios but I don't have another DK91, the nearest I have is a DK96 (why oh why did I sell most of my battery portables )

I can't pick it up on another radio either (TR82).

What's all the more annoying is it was working fine at one stage, yet I can't find any faults with the work I've done to the set. Remember, it has been working now too when it started motorboating although I can't get it to do that anymore.

The resistor across the smoothers measures 2.9k so well within tolerance.

I'm guessing then it's either the DK91 that's given up or the rectifier getting rather tired thus making the set not function correctly?

Josh.
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Old 25th Jul 2010, 7:40 pm   #9
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Default Re: Bush BAC31 motorboating

Quote:
Originally Posted by JoshWard View Post
I'm guessing then it's either the DK91 that's given up or the rectifier getting rather tired thus making the set not function correctly?

Josh.
One of those (possibly both). If you are only getting about a volt across the filaments, then it's likely the reccy is low. Try a request in the 'parts wanted' forum


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Old 25th Jul 2010, 7:57 pm   #10
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Default Re: Bush BAC31 motorboating

I'll have a look around some other sets tomorrow, might have an EZ40 in one.

Josh.
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Old 25th Jul 2010, 8:07 pm   #11
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Default Re: Bush BAC31 motorboating

If you have a set with a good EZ41, you can try it but don't run it for too long. The EZ40 has a 600mA heater (I think) and EZ41 is 1 amp so you don't want to overrun the heater winding on the transformer. I think it's fairly conservatively rated.



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Old 25th Jul 2010, 11:50 pm   #12
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Default Re: Bush BAC31 motorboating

Hi, The rectifier valve on the P 42 power supply is quoted as an EZ 41, anode, pin 2 is 125volts, anode, pin 6 is 125 volts, cathode, pin 7 is 106 volts, H.t is 80 volts, @ 9ma L.T. is 6.5 volts @ 47ma. Regards Dick.
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Old 25th Jul 2010, 11:55 pm   #13
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Default Re: Bush BAC31 motorboating

Hi

If you have the chassis out on the bench ensure the two halves(chassis and PSU) are connected together-either bolted or with a lead, as there is no such connection through the umbilicals. They are of course bolted together when the PSU and chassis are installed in the case.
If low HT might be a cause, try it for a SHORT time on the 220 volt setting..................(<<<<<<<<<<<<BODGE ALERT>>>>>>>>>>>>>>)
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Old 26th Jul 2010, 8:42 am   #14
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Default Re: Bush BAC31 motorboating

Quote:
Originally Posted by Dick Glennon View Post
Hi, The rectifier valve on the P 42 power supply is quoted as an EZ 41,
Oops!! I got the bottles the wrong way round! Yes the EZ41 is used in this set and that is the lower rated valve. You could try an EZ40 briefly if you have one just to prove the point.

SB

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Old 26th Jul 2010, 10:18 am   #15
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Default Re: Bush BAC31 motorboating

That's good an EZ41 will probably be easier to find! All the markings have worn off the one in the set, that's my excuse
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Old 26th Jul 2010, 2:02 pm   #16
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Default Re: Bush BAC31 motorboating

I was wrong about that.... Haven't got a single EZ41. I've found an EZ40 if that will be ok to use and might work....

Josh.
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Old 26th Jul 2010, 5:39 pm   #17
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Default Re: Bush BAC31 motorboating

Try it. The heater current is higher which is why you shouldn't run it for too long. You don't want to end up with a duff mains transformer as well! I suppose you could fit a solid state reccy like a 1n4007 but you will have to be very very carefull when doing this because of the increased HT available which will almost certainly be much higher than the 80V required. If you want to go down this route, there is a way of doing it without causing damage to the valves which I can go through with you. At the moment try the EZ40. A new EZ41 would not be expensive anyway and it's hardly worth the effort of fitting a solid state version while there are plenty of the thermionic variety around.


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Old 26th Jul 2010, 7:26 pm   #18
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Default Re: Bush BAC31 motorboating

Why on earth would I want to take a valve out of a circuit? I'm rebelling against my generation by not using solid state devices!

I will bung an EZ40 in it after tea and report back.

Josh.
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Old 26th Jul 2010, 9:53 pm   #19
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Default Re: Bush BAC31 motorboating

With the EZ40 in it still makes the same crackles and wavechange switch pops. No signs of any signals though.

Tomorrow I might have a go at resoldering all my joints just to make doubly sure (it's the school holidays so I would only be eating digestive biscuits anyway.....)

Josh.
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Old 26th Jul 2010, 11:24 pm   #20
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Default Re: Bush BAC31 motorboating

It's worth checking the HT and LT with the alternative reccy just to see if there is any difference. If you are still only getting 70V, it's probably not high enough. Of course, it could just be the DK92......!

It was only a suggestion about the 1N4007. No harm in carrying out a temporary mod just to prove a point. I've currently got two of these in a Philips radio temporarily replacing an AZ31 while I work on the rest of the set. It'll get a new AZ31 and output valve when it's finished.


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