Yet another Bush DAC90A

[DonaldStott]

Thanks - I understand that this is the answer to the question "why do you measure with no signal input" - but my "daft boy" question was "how do you measure with no signal input?"

A bit basic I know but we all need to learn somehow!

[ms660]

Originally Posted by DonaldStott

Of course I need to ask another "daft boy" question - how do you measure with no signal input?

Tune to the quietest part of the band where there's no transmission, then do the measurements.

Voltages/currents in most service data are usually specified in most cases under no signal conditions, it avoids the effect a transmission would have on the valve voltages/currents if the AGC was responding to it.

The AGC voltage is fed to g1 of the IF and mixer valves and being -ve going above a certain signal level it will affect the current flowing through the valves and thus the voltages.

EDIT: Post crossed.

Lawrence.

[DonaldStott]

Tune to the quietest part of the band where there's no transmission, then do the measurements.

Got it now, thanks - I had visions of having to disconnect something!

Resistor replacements and more voltage measurements on the Agenda for this evening.

[mark2collection]

It's also worth remembering the environment these sets live in today, what with Ethernet over mains, switch-mode power supplies, florescent lighting etc.

I spent a good 15 minutes quizzing a set which made awful noises between stations ... turned out to be my PC's flat-screen monitor, the set must have been 'picking up' the backlight(?)

Lesson learned, I now trial sets with modern tech off and where possible, away from the house (luckily the workshop is detached & away from most things )

I'm trying to remember where I read about a mod on these, where you connect 1nF or possibly a 2.2nF cap from anode to cathode on the rectifier. Apparently to reduce modulation on stronger stations, which I gather comes through as a loud(ish) hum/buzz(?)

Sounds like you're making good progress Donald

Mark

[HamishBoxer]

I get noise off my Belkin anti surge sockets.That lead me a merry chase!

[DonaldStott]

I have a goose neck LED lamp that I use when doing any restoration work and is especially useful when working under the chassis.

It does, however, make any set scream/screech when the radio is powered up - the lamp is always switched off at the socket at that point.

[DonaldStott]

Originally Posted by TrevorG3VLF

UBC41I would look at the cathode resistor/capacitor.

Looking at the Circuit Diagram to me that would be R8 and C15 which are connected to Pin 7 (Cathode) of V3 (UBC41)??

[kalee20]

Originally Posted by DonaldStott

4. Apart from the Pin 4 modifications on the UL41 I've not tried any of the other changes mentioned in various Threads over the years - are there any that I should really consider before putting the chassis back in the case?

I'd certainly suggest disconnecting the 0.003μF capacitor from UBC41 pin 2 (it's physically on the tagboard) and replacing with 390pF or 470pF (directly on the valveholder. You will find the sound brighter and less muffled!

Easy to try - unsolder one end of the capacitor and lift it. Then tack the 470pF capacitor from pin 2 to a chassis tag. Switch on and listen. If you don't like it, untack the capacitor and resolder the lead on the original. But if you do like it - make it permanent!

[DonaldStott]

Courtesy of Forum member Northyorks1 (Ian) I now have a very nice replacement Dropper Panel - see Post #57 above.

When you consider I have cleaned up the chassis, Dropper and associated nuts, bolts and washers, fitted a replacement/rewound Output Transformer, new wiring to the mains input, bulbs and loudspeaker, a replacement for C22 the Mains filter capacitor and C20 Fixed tone corrector it would almost seem discourteous not to replace the "shabby" wire wound resistors - see picture in Post #30 above.

If I were to change these then what would be suitable replacements - the Service Instructions say: -

R13 - 10kΩ : 2 Watts

R16 - 250Ω : 6 Watts

R17 - 150Ω : 4 Watts (on later receivers R17 is 250Ω)

What I have fitted is: -

R16 - 75Ω

R17 - 250Ω



I acknowledge that Lawrence has already partly answered this query in Post #31 above.

[DonaldStott]

So what's this noise between stations and how do I fix it - this is Medium Wave and starts off with a good strong signal on BBC Radio Scotland. Long Wave is not much better and picks up BBC Radio 4, but ... : -

https://youtu.be/TVLFN8w3lxA

[McMurdo]

Have you eradicated any external interference, it sounds like some modern equipment radiating something

[Graham G3ZVT]

Wait till the "management" goes shopping, then go round the house and unplug everything until the interference stops.

It could be a neighbour though.

It won't be the radio itself making that noise.

[Ian - G4JQT]

I've heard something very similar before on a Youtube radio interference clip. I can't remember the cause and can't find the clip, but yours has a very specific sound so someone should be able to help identify exactly what it is.

Homeplug Powerline adaptors seems very likely. It could be switched-mode power supply, but sounds far too regular for that.

To confirm it's nothing in your home, UNPLUG everything with a power supply. No point just switching off as many things actually stay in standby mode.

Thinking as I type, there is a fridge electricity save device that causes interference - not sure if they are still available. Do you or a neighbour have one - maybe fitted years ago and forgotten?

If it's not something in your house it's probably a near neighbour. I hope you're on good terms with them so you can ask them to switch off their mains (and maybe help them reprogram all sorts of devices!) or ask them to switch things off one at a time.

Good luck and please keep us informed.

Ian

[ms660]

Son of Woodpecker

I would first try another radio in the same location as the one in the video.

Lawrence.

[DonaldStott]

Thanks for all the suggestions so far.

I can confirm, however, that it's not any of those nasty little Homeplug Powerline adaptors - I have the whole house wired up as a Cat 6 network all connected back to a Patch Panel and 24-port managed switch (these are on a different floor and in a different part of the house).

Systematic unplugging will commence tonight ...

This seemed like a promising resource at first but I've listened to all the audio samples of noise from known sources: -

http://www.arrl.org/sounds-of-rfi

Is it time to fire up the oscilloscope, if so, how would we detect and display this noise??

Or are we too far down the Rabbit hole ...!

[Mike. Watterson]

A ferrite rod and 3m wire with 10 turns to earth. Tuned with a cap. Feed to scope on 1:1. Optionally add 1N60 or similar in series and 2nF across scope.

Or connect scope to RF tuning cap on the radio.

Note a wire picks up FAR more local electrical noise than a loop or ferrite rod, which will pick up less noise from your house. A loop or ferrite can even have a nearby earthed aluminium plate or foil as long as there is no complete electrical circuit in parallel to the rod or loop turns. Some valve portable radios have an alloy case with insulation and isolated catch splitting the bottom and the loop at one of the plastic or Bakelite end caps. RCA, Philips, Pye, Murphy, Braun all had such models 1947 to early 1950s.
Some transistor HiFi sets have the ferrite in a split alloy tube for E field shielding.
It's possible even to have a short whip and feed at a suitable phase & level to the ferrite rod aerial to cancel local interference. One French AM/FM valve table model has a switch to do this using the internal FM foil aerial to the drum with MW & LW loop aerials. Other AM/FM table models use the wide foil FM dipole to electrically shield the Ferrite rod. Esp German sets, though I've seen such an arrangement on at least one UK model.

[DonaldStott]

Or connect scope to RF tuning cap on the radio.

Thanks Mike - sounds like much the simpler solution but where would that be on the Bush DAC90A?

I'm assuming that such a connection wouldn't need the additional components listed in your first paragraph?

[DonaldStott]

Poorly worded by myself - of course I know where the RF tuning capacitor is on the DAC90A.

Should have asked where is the best location to make a connection to my 'scope?

[Mike. Watterson]

The "signal" side of the tuning cap. Fixed vanes terminal likely has a wire to mixer valve. The moving ones are 0V/Chassis. If you don't have an isolation transformer, then use 1kV 100pF to scope probe and 1nF 1kV from scope probe earth to chassis. The 1:1 shouldn't detune to much, but switch to 10:1 if there is enough signal. Short out the L.O. tuning cap if there is too much L.O. leakage to see the RF!

[DonaldStott]

Thanks Mike, very informative - I managed to understand most of that but fell at the final hurdle: -

"Short out the L.O. tuning cap if there is too much L.O. leakage to see the RF! "


How would I do that?