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Old 17th Jul 2018, 2:16 pm   #61
ms660
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Default Re: Bestone model 20?

The way C9 is connected is a mystery to me, is it possible to photo the 6D6 valve socket showing the connections so we're absolutely sure that the suppressor grid is floating in DC terms as shown in your schematic. The wave change arrangements still look wrong if it's a MW and LW receiver.

Lawrence.

Last edited by ms660; 17th Jul 2018 at 2:31 pm.
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Old 17th Jul 2018, 2:52 pm   #62
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Many thanks. Actually the circuit of mine is very similar to the Emerson BA201 you provided the link for me in your earlier post, but mine has the speaker field coil across the H.T and is higher impedance. I have drawn out the circuit and attached it here. There could be a few errors so would value your opinion - does it look right? Surprised to see the volume control on the RF stage. How does the detector work, is there some background reading you know of? The component count is even lower than the Emerson and this looks a very simple set.
Paul
Hi Paul, what software do you use to draw the schematic?

Tam.
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Old 17th Jul 2018, 3:18 pm   #63
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Thanks! I think that's it, I have corrected the schematic. Looking forward I need to find a way to get 120V into the set, first step, I plan on testing the rectifier & heater chain in isolation and checking the HT. I had thought I could use an external dropper as a temp. measure but estimating the current as around 400mA ( 60mA field coil + 300mA heaters + 50mA HT?) gives around 50 Watts for a dropper which is a bit large isn't it?
Hi, I have a solution for you.. Get two AC mains transformers (T1,T2) that have a centre tap on their primaries. Then connect half of the secondary of T1 to the full width of the secondary of T2. That should give you the equivalent of a 240 step down to 120v transformer. (Or 240 to 480 step up if you go the other way) The power rating of the overall transformer would be half of T1, as you're only using half the secondary winding.

Shoddy sketch attached...

PS. I've never tried this, but I'm pretty sure it'll work ok.

Tam
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Old 17th Jul 2018, 3:53 pm   #64
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The way C9 is connected is a mystery to me, is it possible to photo the 6D6 valve socket showing the connections so we're absolutely sure that the suppressor grid is floating in DC terms as shown in your schematic. The wave change arrangements still look wrong if it's a MW and LW receiver.
I will try but I only have a phone camera. If it doesn't work I will rewire that bit as per the Emerson that is a very similar and uses the same valves. I'll be interested to see what happens when I power up the RF & detector stages as they are... will keep you posted.

The band switch seems to be the same as the link in #53 kindly posted by your good self but this set doesn't have the second switch gang on the coils feeding the detector.

Paul
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Old 17th Jul 2018, 3:55 pm   #65
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Hi, I have a solution for you.. Get two AC mains transformers (T1,T2) that have a centre tap on their primaries. Then connect half of the secondary of T1 to the full width of the secondary of T2. That should give you the equivalent of a 240 step down to 120v transformer. (Or 240 to 480 step up if you go the other way) The power rating of the overall transformer would be half of T1, as you're only using half the secondary winding.
Thank you, certainly worth a try but I don't have any transformers that would suit. As I already made a lamp limiter I will see if what voltages I get with a 50W bulb as a start....
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Old 17th Jul 2018, 3:56 pm   #66
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Default Re: Bestone model 20?

Which is the detector coil, the one underneath the chassis or the one on top of the chassis?

Lawrence.
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Old 17th Jul 2018, 4:06 pm   #67
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I'm working towards powering up as next stage. Removed C1 for now may replace with X type later. Rewired mains connections as I don't want a live chassis - even when switch is off! Switching live through the single pole pot. switch to anode and permanently connected neutral to chassis. Next I think I need to check both smoothing caps. and reform if I can or replace.
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Old 17th Jul 2018, 6:46 pm   #68
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Hi Paul, what software do you use to draw the schematic?
It's just a Photoshop like clone (Gimp) and I copy and pasted some symbols for valve and inductors from an existing schematic as a starting point, that's why they are a bit fuzzy. I am refining them as I go on... probably not the best way but it works for me.
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Old 17th Jul 2018, 7:29 pm   #69
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Which is the detector coil, the one underneath the chassis or the one on top of the chassis?
Detector coil is underneath.
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Old 19th Jul 2018, 8:16 am   #70
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Have now powered up for the first time on a lamp limiter. Valves in situ but HT disconnected after smoother C3. Using my lamp limiter in place of the sets dropper cable, a 110W bulb gives 130V to the set, a little higher than I'd like but don't have a 100W rated bulb to hand and having a problem finding any. The heaters all glow nicely and the ballast tube gets hot. HT measured on smoothing caps (DC): C2 139V , C3 132V. Heater voltages correct 24.4V and 6.2V as expected, HT lower than expected and C2 running warm so quickly switched off - C3 still cold. I propose to change the smoothing caps as I don't have reforming capability. Pleased the heater chain looks good. No scale lamps, one is blown (measures 1.2M ohm weird!). Can I use an oscilloscope to look at my rectified waveforms? I ask because the probe screen is earthed. Maybe I can disconnect the earth to the instrument temporarily. I guess I need an isolation transformer but just wondered if I can disconnect the earth to the instrument as a stop gap to take a peek at the waveforms. As always - any advice welcomed. Thanks - Paul

https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu

Last edited by egerton; 19th Jul 2018 at 8:33 am.
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Old 19th Jul 2018, 9:04 am   #71
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Default Re: Bestone model 20?

Change the electrolytics. You can restuff them to look original. Wouldn't worry about the waveforms.

Not sure your use of a bulb to reduce 240V to 120V is a good idea. Bulbs have a positive temperature coefficient so you will be hitting the heater chain with a very large surge on switch on.
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Old 19th Jul 2018, 11:39 am   #72
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I have ordered replacement caps. I can't afford a variac at present and I plan to use a lamp limiter to measure the current drawn by the set when working properly. Knowing this current I hope to calculate a dropper capacitor value and see if I can get it working that way as a permanent solution. I am thinking of shorting out the set mains at start up with the lamp limiter in and then removing the short when the bulb is fully lit, hence avoiding the start up surge.
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Old 19th Jul 2018, 5:57 pm   #73
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Default Re: Bestone model 20?

Taking earths off 'scopes is a bit lairy.... but will work. Be absolutely certain that chassis is neutral connected and definitely use an RCD!

Don't try this at home, as they say. Best to have someone else around as well.

A battery powered or otherwise properly designed floating 'scope is better.

Something like this:

https://uk.rs-online.com/web/p/din-r...rmers/2787074/

won't break the bank or you could get lucky (I did) with something like this:

https://www.ebay.co.uk/itm/Carroll-M...AAAOSw3ChbSHvx

Whilst on the subject, battered 110V site transformers are often fairly cheap, two back to back will give a choice of 110 or 240 isolated, though for isolated 110 you'll probably need to remove the secondary centre tap earths. On site supplies are usually 55-0-55 with 0 earthed.
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Old 20th Jul 2018, 8:18 am   #74
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Default Re: Bestone model 20?

Thanks for the info. and ideas. I will have a look online and also locally in sites like gumtree, where I've found a few old radios being sold - got the Bestone from there.
Meanwhile I can report that shorting the mains of the set when starting up on a lamp limiter and removing the short when the bulb in up to temp. seems to work well, the surge is avoided and I will adopt this as my normal start up procedure for test power ups.
Paul
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Old 20th Jul 2018, 11:01 am   #75
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Default Re: Bestone model 20?

That startup shorting trick is a neat one.


Adding another switch to a lamp limiter unit would do it tidily- just don't close both switches at the same time!
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Old 25th Jul 2018, 12:17 pm   #76
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Default Re: Bestone model 20?

Continued here: https://www.vintage-radio.net/forum/...98#post1061298
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