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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 18th Sep 2020, 6:29 pm   #1
Davewantsone
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Location: Wigan, Greater Manchester, UK.
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Default Rusty BUSH DAC10

I recently obtained a DAC 10. The case is in good condition but it must of been stored in a damp area. The tuning mechanism was stuck with the pointer in the middle of the scale.
On removing the chassis from the case I examined the tuning capacitor which seemed to be seized. I applied a little oil to the bearings but it still would not move. A short while later I realised the variable capacitor was not responsible.
The scale pointer moves along a metal support and had rusted to it! Using a thin blade I parted them, then using sandpaper removed the rust and applied a light coating of oil. The tuning now moved freely.
I checked the unusual inside out loudspeaker, On the meter it read OK but I could not get any crackle when I appled the diode check (around 2V) from the multimeter. It seemed the coil must be jammed! I manually pressed the cone, but still no crackle. I then tried a DC power supply 6V managed to get it moving and after several reversals of the supply it appeared to work OK.
I now proceeded to the electronic repair. Both bulbs had blown. I replaced all those nasty waxy capacitors, everyone was faulty! I also replaced a couple of high value resistors that had gone more than 30% higher than stated value.
I connected a variac via a rectifier to the smoothing capacitors and over an hour brought the voltage up to the DC operating value..
I then via the variac switched the radio on. A little steam from the dropper resistor as it warmed up and all the valves lit up. I was rewarded with nothing! No crackle when wave change buttons pressed. No hum from wiper of volume control when touched with meter probe. The HT was nearly right but some of the voltages on the valves where way off!
I removed all the valves and cleaned the valve holders. I cleaned the valve pins and bent them slightly to help with connection in the valve holders.
On replacing the valves and switch on the radio came to life! The output was distorted and rattled alot. I replaced the loudspeaker with another type and the rattle went! The sound was still distorted , then I noticed if I tuned to a weak station the distortion went. So I presumed this was caused by overload.
On examining the circuit diagram valve V3 provides negative feedback from its second diode via C19 and a centre tap of the primary of the IF transformer. This then goes to V1 and V2 to limit the gain. On checking the primary of the second IF can I disovered the output pin to C19 was open circuit. (dreaded green spot decay?) I decided not to go into the can to investigate
So my solution was to do what I have seen on quite a few valve radios. Negative feedback is taken from the top side of the volume control so via a 200k ohm resistor I connected the top of the volume control (at C18) to top end of R14, switched on and got a result! strong stations attenuated whilst weak stations remain the same I cleaned the three presets (set to RTE1,R5 and Gold). Then tweeked the IFs and tuning and Bobs your uncle it now works! I had to fit a slightly smaller loudspeaker and I still need 2, 3.5v 150mA bulbs. I have some 300mA but they do not even light up! Funny thing 12V 200mA do glow dim!
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Old 18th Sep 2020, 6:39 pm   #2
HamishBoxer
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Default Re: Rusty BUSH DAC10

Should be able to sort that IF out,by the sound of it could be dry where it goes to the pin on the IF ,other wise the chances are if the IF was o/circuit then you would get no signals.

Wont hurt to have a look.
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Old 18th Sep 2020, 6:59 pm   #3
Davewantsone
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Default Re: Rusty BUSH DAC10

The fault is inside the can. It could be just the wire broken off inside but with my luck I could do more damage so I will leave as is. If its good enough for Murphy (same valve line up) and Pye to do this it will do for me!
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Old 20th Sep 2020, 6:11 pm   #4
Davewantsone
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Default Re: Rusty BUSH DAC10

As a post-script to my repair. I noticed the radio on no or low volume had a noticeable 50Hz mains hum. The typical cause is a leaky UL41. I had read in other threads about "zapping" with high voltage the pins of the valve to reduce or remove the inter-electrode leaks. Not having a EHT source I used the gas stove igniter. I shorted out all the pins except pin 7. I connected the shorted pins to cooker chassis. I removed the gas burner (on my stove just lifts off), then via a shorting wire one end connected to pin 7 I placed the other end around 5mm from the sparker. I pressed the igniter button and sparks jumped across into the wire going to pin 7. The igniter gives pulses at around 3 a second.
I did this for around 20 seconds a couple of times I got a spark inside the valve.
I then disconnected the shorts and connected the valve back in the radio. Then on switch on silence-no mains hum!
It must be understood any one trying this must note the valve could be destroyed. For safety do not in any way touch the valve or wires when sparking.
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