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Old 25th Jun 2019, 2:47 pm   #1
bobbyd123
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Default Bush DAC 10 Re-build

Hello
I am new to valve radio I have so far rewired the perished cables
And about to order replacement capacitors and out of tolerance resistors
The one I am stuck on is C27 it is directly across the mains supply switch
The existing capacitor is 0.1uf 500v DC Can I use a 630v AC Capacitor
I have uploaded some photos (Not sure how they will or won't work)

I found that when the radio arrived from a dealer, who said it was plugged in
and crackled So I have not risked switching on The aerial coil has come off the
Plastic drum (Is it possible to carefully wind back on) or best left alone and search for a donor unit

I have removed the tuner unit so I can get to the capacitors resistors etc
I am new to all of this Hope you can help thank you
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Old 25th Jun 2019, 3:11 pm   #2
Herald1360
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Default Re: Bush DAC 10 Re-build

Technically the mains filter cap could be replaced by a 630Vac part but the correct part nowadays will be an X-type capacitor. This type is intended specifically for connection directly across the mains from line to neutral.


Here's an example:


https://cpc.farnell.com/vishay/bfc23...vac/dp/CA06025


You may be able to find something less physically "different" though possibly not any axial type.

You'll need to replace most of the wax/paper capacitors on the tagboard, too. One I did recently only kept the V1 RF cathode bypass on the basis that it would need to drop to less than a kilohm or so to upset the circuit. All the other ones were leaky (hundreds of kilohms or less) when snipped out and tested with a megger.

If you're not lucky, chances are the UL41 will have grid current problems too, even when the grid coupling capacitor has been replaced.
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Last edited by Herald1360; 25th Jun 2019 at 3:24 pm.
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Old 25th Jun 2019, 3:17 pm   #3
TrevorG3VLF
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Default Re: Bush DAC 10 Re-build

C27
This should be an X class capacitor as it is across the mains 630V or even 1000V would be ideal. It is for RF suppresion and will not affect the operation of the set if there is no noise present. Hope springs eternal in the human breast.

The aerial coil does not look as if it needs a rewind. Replace the coils carefully and lash out on some new sticky. This is an ideal DIY job.
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Old 25th Jun 2019, 4:07 pm   #4
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Default Re: Bush DAC 10 Re-build

I have one of these. Really a Deluxe DAC90A. Works nicely. I used 6.3V panel bulbs, but the same CURRENT as the originals.
I had trouble with one preset not working. It was a snapped (no excess, too tight) wire on a coil. I added a little enamel wire.
Other than that it was the usual capacitors and a leaky g2 on the UL41 when it warmed up. Cathode current increased. VCM163 tester and substitution proved it was a "leaky" valve. Around the pins on the inside looked 'grungy'.

No resistors needed changing.
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Old 26th Jun 2019, 10:00 pm   #5
bobbyd123
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Default Re: Bush DAC 10 Re-build

okay thank's for your help
I can now order the replacement capacitors
I am interested in checking these old ones and maybe the new ones
I have a metrohm megger and an Avo 8
but not sure how you test a capacitor I have a couple out so I can try those
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Old 27th Jun 2019, 12:07 am   #6
paulsherwin
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Default Re: Bush DAC 10 Re-build

You can't really test waxed paper capacitors reliably. You need to measure the leakage current when the actual circuit working voltage is applied, which is possible but difficult. Most people either change the lot or look for specific faults in the radio's operation. All the wax caps are likely to be leaky now, but that doesn't always matter and some people like to retain as many original components as possible.

The suspect caps will all have values between 0.1uF and 0.001uF.
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Old 27th Jun 2019, 8:06 am   #7
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Default Re: Bush DAC 10 Re-build

A megger with multiple test voltages is good for a quick leakage check on old paper capacitors. Use the voltage closest to the capacitor's rated one. Anything less than around 50megohm is suspect. Whether it matters depends on circuit application, most critical is where it can upset the output valve control grid voltage by more than a few tens of millivolts.

It won't help with measuring actual capacitance, though, an open circuit cap will look same as a good one though there will be no initial needle kick as the cap charges.
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Old 27th Jun 2019, 8:20 am   #8
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Default Re: Bush DAC 10 Re-build

The Avo 8 will tell you if a paper capacitor is definitely bad but it won't necessarily confirm that it is good. The way to do it is to disconnect the radio from the mains and unsolder one end of the capacitor. Then switch the Avo to the x10k Ohms range and connect the meter probes across the capacitor. If the capacitor shows a leakage resistance <5MOhms then it is bad and needs to be replaced. Any leakage at all is unacceptable for capacitors connected between pre-amp anode and grid of output valve. For some other applications 5MOhms leakage might be tolerable (cathode bypass capacitors for example - always leave them as-is unless completely short circuit). However, the Avo is only testing at 15v and so as Paul says the acid test is to leave one end of the capacitor connected to the high voltage HT and set the Avo on the 500v voltage range (or at least 300v range if HT is less than that) then apply power to the radio and measure the voltage between the other end of the capacitor and chassis. For a good capacitor the meter needle should kick up initially then reduce to zero as the capacitor charges up. Any residual voltage means the capacitor is leaking and needs to be replaced. Failure to do so can cause distortion, instability and spurious oscillation, overheating of HT dropper resistors and early failure of the rectifier. It also puts an unnecessarily high current load on the mains transformer. I have seen info about reforming paper capacitors by applying high voltage through a dropper resistor for a period of time. The most typical failure mode of this type of device is absorption of moisture into the paper dielectric material which causes a conductive path through the dielectric. It is virtually unavoidable if the radio has been stored for years without power applied. The idea of the reforming process is to gradually heat up the capacitor and evaporate the moisture. While this might work very occasionally, I don't recommend it - the high subsequent failure rate means it just isn't worth the effort. Test and replace! Good luck, Jerry
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Old 27th Jun 2019, 10:07 am   #9
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Default Re: Bush DAC 10 Re-build

If the radio has parallel heaters, then remove any valve which has a grid which is fed from a high voltage. Turn the set on and measure the voltage on the gtid pin. Any measured voltage means the capacitor is faulty.
This can not be done with series heaters since the rectifier will not work.
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Old 27th Jun 2019, 11:26 am   #10
bobbyd123
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Default Re: Bush DAC 10 Re-build

Thanks My Avo 8 is Mark III and as such there is no x10k
I presume you need to use the x100 setting this i have done and reads above 50 meg ohms on a capacitor out of circuit This is just for interest as I will be fitting new capacitors
I have not done any power on testing as I am building a test unit using an Isolating transformer The only one I can find at the moment is a 220v-220v 50 hz 100watt this is from china. Will I need to fit a dropping resistor as Measured mains voltage is 244 v RCD Protected since I am not using an earth connection I presume the RCD Will not trip

Many thanks for all your help so far in my aim to try and get this radio working
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Old 27th Jun 2019, 3:59 pm   #11
Mike. Watterson
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Default Re: Bush DAC 10 Re-build

Resistance testing is nearly useless. You need Insulation testing. I use a modified pcb from a single use camera. I removed the xenon flash tube and the 300 uF cap, replacing it with 1uF plastic type. Then 2 x 1M to + test terminal and a neon with 100nF across it from - test terminal.
Good = Neon only flashes once as cap charges.
Poor = Neon flashes slowly (flash rate can measure 100M to 10G Ohms)
Rubbish = rapid flash / fast flicker / looks solid on.

The rubbish ones are often fine at 20V or on a DVM, so I don't automatically replace old paper dielectric caps in transistor sets. (1) The voltage is 6, 9 or 18. (2) The input and cicuit impedances can be 1/10th to 1/1000th of the valve circuits.
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Old 27th Jun 2019, 4:06 pm   #12
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Default Re: Bush DAC 10 Re-build

A 220V isolation TX ought to be OK at 244V, about 10% high, but if its design is marginal it may overheat. It won't do this terribly quickly, though, so may be usable for short term testing anyway.


Best voltage reduction methods are either to feed the TX from a variable transformer or else use the 20Vish secondary of a 20VA or so low voltage isolating transformer in series opposition with the live feed to the 220V transformer. You can check that you have the 20V connections the right way round by measuring the output of the 20V transformer intended for the 220V transformer. If it's the wrong way round you'll see 260V+!
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Old 29th Jun 2019, 12:25 pm   #13
bobbyd123
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Default Re: Bush DAC 10 Re-build

Managed to find a RS 230/230 1 amp transformer so now on to build my Safety Isolated supply Thanks for your help
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Old 30th Jun 2019, 9:43 am   #14
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Default Re: Bush DAC 10 Re-build

I have a spare aerial coil if you want it, its in good cond pm me if you want it
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