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Old 21st Jun 2018, 1:18 pm   #21
Ed_Dinning
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Default Re: Bestone model 20?

Hi Gents, note with class X caps in any suitable circuit. Their failure mode is to go short circuit for a very brief time and the dielectric then self heals. In my experience this has never caused and problems in AC/ DC radios. I fit a fuse and this has never blown.

However, when the dielectric self heals it isolates part of the capacitor and reduces in value. This can cause a reduction of current through the set. Not a problem as the set will simply get quieter and not be damaged.

This effect is also often seen on SMPS. They generate more noise as the X caps get lower in value and in need of replacement.

X caps are convenient and small sized. the other alternative is motor start/ run caps. Larger and more expensive but work just as well.

With all these caps it is best to read and understand the data sheets for the particular caps you intend to use.

If you want to do the maths you will see that the mixture of AC and Dc currents drawn by a set with a cap dropper alter its operating conditions only slightly. The do produce some distorting of the mains current waveform, but miniscule if considered in proportion to that produced by SMPS etc.

Ed
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Old 21st Jun 2018, 1:39 pm   #22
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Default Re: Bestone model 20?

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Originally Posted by PJL View Post
Let's have the circuit as some USA midget sets put the field coil across the incoming mains.
This is nonsense, I meant from HT to earth and not in series as is common in UK sets.

Ed, I think the issue here is that the capacitor calculation is based on a resistive load. If the rectified HT is taken from this then the calculator will no longer be accurate. A true RMS meter would be needed to check the voltage on the heater chain. The other concern with this arrangement is if the HT current drops because of a poor connection to a valve the heater chain current increases.
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Old 21st Jun 2018, 7:49 pm   #23
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Default Re: Bestone model 20?

Hi PJL, true, but this is engineering and most resistors are 20%, heater ratings are +/- 10% etc.
Close enough for all practical purposes in the field of miniature radios; not obviously used with high accuracy test etc equipment.

Ed
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Old 21st Jun 2018, 8:52 pm   #24
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Default Re: Bestone model 20?

The original resistive dropper would not really be any more benign, if the set stopped drawing anode current, than a capacitive one.

Wiring the loudspeaker field coil across the HT probably was done to avoid wasting too much voltage, since there will only be at most 170V DC available from a 120V AC mains supply. It means more turns of thinner wire; so check it for continuity before you do much else.

The common "building site" transformers give out 110V, with an Earthed centre-tap; this means the highest voltage to which you will be exposed is 55V AC. This is not enough to give you a shock, which makes it safe for use outdoors. They have been used continuously in factories to power USA or Japanese-spec computers controlling machine tools, so you should not have any problem running a radio from one. Probably best to unplug it when the radio is not in use, though, as the magnetising current is significant.
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Old 25th Jun 2018, 9:18 pm   #25
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Default Re: Bestone model 20?

Finally some photos of this little gem (well I like it!). The wooden case looks like it may promise to clean up nicely just with a little bees wax as I don't mind the patina. I've removed the valves and plugin dropper to clean the chassis. Evidence of old repairs - some white pvc cable and a Radiospares cap. I found the output transformer primary to be open circuit but the field winding of speaker measures 2.9K so hoping that's ok. Note the dreaded 'curtain burner' resistive mains cable complete with 2 pin 'Nettle' UK 2 pin plug! To be removed and disposed of safely. Excuse my ignorance but from what can be seen here is this a superhet or TRF? I see no IF transformers. The thing that looks like a small transformer near the speaker has only 2 wires so I think it may be a choke? I must trace the circuit out as no chance of a circuit anywhere.
Paul.
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Old 25th Jun 2018, 10:34 pm   #26
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Default Re: Bestone model 20?

I should add the valve lineup is not original. One of the 6 pin sockets has been replaced by an 8 pin socket that can be seen bolted in behind the speaker in the photo this contained a 25A6 pentode. All the other sockets are engraved with valve types and I note the rectifier should be a 25Z5 but had a 1D6 in situ. Interesting - as the former is a dual rectifier, so I think this set may have been got at big time.
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Old 25th Jun 2018, 11:06 pm   #27
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Default Re: Bestone model 20?

With the valve lineup described it can only be a TRF.

Dual anode rectifiers are simply used with both halves in parallel if fitted to a half wave rectified set like this one, though the 25Z5 is actually two independent diodes and can be used as a voltage doubler. If used half wave, the 1D6 is a drop in replacement.

The set likely had a type 43 output originally. The 25A6 is the same valve but on an octal rather than UX6 base, so again a reasonable substitute.

It sounds like whoever got at the set at least understood what they were doing!
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Old 25th Jun 2018, 11:44 pm   #28
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Thanks for the info. I won't expect much performance from a TRF but will be nice to have one in my collection. I do hope I can get it working - it looks quite a simple circuit if I can trace it out. I thought TRF sets had a reaction control, or is that just early sets? This set has just volume, tuning and band switch.
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Old 26th Jun 2018, 9:27 am   #29
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Default Re: Bestone model 20?

Not all TRFs had reaction, particularly later ones. If the set has enough gain/selectivity without it, Joe Public is happier with the easier operation!
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Old 26th Jun 2018, 1:55 pm   #30
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Default Re: Bestone model 20?

The Portadyne Princess and look alike 'Noble', both of which had a very low component count (about 7 resistors and ten caps), had no separate reaction control - the volume control doubled as reaction. For such simple TRF sets they perform remarkably well. It's worth noting that the volume control is an inverse log law track though that isn't apparent from the sparse service data. I don't know if that's typical of other similar TRF sets with no separate reaction control such as the Bestone, but it's worth bearing in mind should a replacement become necessary.
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Old 1st Jul 2018, 8:58 pm   #31
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Default Re: Bestone model 20?

The mix of octal and UX6 is very common in late 30's USA sets. The valves in your set are almost identical to the ones in this https://www.vintage-radio.net/forum/...d.php?t=147484. The circuit can be found here: http://www.nostalgiaair.org/pagesbym...4/M0004854.pdf

A little detective work and you may be able to work out who made the bestone before it was adapted for the UK market.
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Old 2nd Jul 2018, 11:24 am   #32
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Default Re: Bestone model 20?

Thanks very much. Yes it does indeed look very similar to my Bestone. It will be a great help as I am trying to trace out the circuit. I've never done it before and finding it hard, even though it's a relatively simple circuit. I think it's a case of visualising the connections and familiarisation with circuits. I'd never come across a field coil speaker so just read up on that bit of history.
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Old 13th Jul 2018, 4:28 pm   #33
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Back on the Bestone I have just about finished tracing out the circuit and will upload it here asap. Meanwhile I have a few queries on some of the components and I would value comments.
(I have attached photos of each component I'm referring to)
The output transformer mounted on the speaker is open circuit, what are my options?
There is a choke in the HT line measuring 266 ohms so seems intact. Looks like a transformer so is quite substantial I wonder what value it is and is this usual?
There are 2 MES scale lamps, an OSRAM 6.2V and RYDEX 21.5V both 0.3A they have continuity but the 21.5V seems most unusual? I can't find anything like that online?
The speaker field coil seems intact and measures 2.9K. It is connected directly across the H.T.
There are some tubular resistors and I interpret the colour dots on one as 560 but the other I can't understand - seems to start with black colour?
There is an axial capacitor marked FELGO 20MF 50V but it seems physically small for a 20 uF of the day (late 1930s I think?)

P.S Someone has connected a huge Radiospares 16 uF 450V across the HT in addition to the original 12 uF 275V. The data on the 1D6 rectifier says max. capacity is 16uF so this doesn't look good.
Any comments on my musings would be appreciated! Thanks Paul.
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Old 13th Jul 2018, 4:42 pm   #34
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Default Re: Bestone model 20?

Body (1st number)..Tip (2nd number)...Dot (multiplier)

650 ohms.

500 kohms.

Lawrence.
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Old 13th Jul 2018, 5:30 pm   #35
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Default Re: Bestone model 20?

red body, black tip, green spot = 1M ohm

The 20MF cap is an oddity, looks not to be an electrolytic and too small as you say, what value does it read if you measure it? Where in the circuit is it?

Lamps in series with the filaments? The valves are 0.3 A heaters. What does the chain add up to?

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Old 13th Jul 2018, 6:05 pm   #36
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Default Re: Bestone model 20?

How about a complete picture of the underside and topside of the chassis.

Looks very clean!
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Old 13th Jul 2018, 6:32 pm   #37
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Default Re: Bestone model 20?

There are a couple in #25. It is clean for the age - you can see some dust in some of the close ups I took in #33 (I haven't cleaned it thoroughly yet), but all in all very good nick for a 1930s set. In #25 attachments you can see evidence of later mods. a large grey Radiospares smoothing cap. (16uF 450V) and the original large beige cap. (12uF 275V) both are across the HT and I can't understand why as this overloads the rectifier.
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Old 13th Jul 2018, 9:31 pm   #38
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Default Re: Bestone model 20?

If an HT choke is fitted there would normally be another capacitor (usually an electrolytic) connected between the cathode of the rectifier and chassis so as to make a CLC (Pi) filter arrangement, eg: Reservoir capacitor...choke inductor...filter capacitor (aka smoothing capacitor) the filter capacitor can be a larger value than the reservoir capacitor.

So far as I can make out the 650 ohm resistor is the cathode bias resistor for the 25L6, it's bypassed by an electrolytic (Micamold ?) The 500 kohm resistor is the detector valve's anode load and the 2 megohm resistor is the detector valve's screen grid feed, the 470 kohm resistor is probably the 25L6's grid resistor, the detector looks to be of the anode bend type, can't quite make out the value of it's cathode resistor (can't see the colour dot) but it might be 25 kohm, it appears to be bypassed by the 20uF (if that's what it is) capacitor.

Most single ended output transformers would work as a replacement for the one that's open circuit, typical impedance (not DC resistance) being 5 kohm as presented to the anode.

Lawrence.

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Old 13th Jul 2018, 9:52 pm   #39
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Quote:
Originally Posted by Boater Sam View Post
red body, black tip, green spot = 1M ohm
2M ohm, perhaps?
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Old 13th Jul 2018, 10:12 pm   #40
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Default Re: Bestone model 20?

Hello,
According to this http://www.radioremembered.org/ballast.htm for the L55C ballast tube the L means a 6.3v 250ma dial lamp, the 55 is the voltage drop at 0.3A (for use with two 6.3V tubes and two 25V tubes).
The C gives the base connections as per the diagram showing where to connect the dial lamp etc.
I have a Bestone 23 which appears to have a similar circuit (but with different physical arrangement) with a 43 output valve 25Z5 rectifier valve and 185R8 (glass) ballast tube.
Yours, Richard
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