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Old 22nd Nov 2016, 12:23 pm   #21
ms660
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Default Re: BC-348-0 Help Please

When folks modified these it was usual to rewire the valve heaters for parallel operation and power them from a 6.3 volt winding on the mains transformer that they would have fitted, is that the case with your receiver?

If not and they are still series parallel connected to operate off 28 volts then the value of the shunt resistor 501-B would have to have been lowered in order to supply sufficient current for the EL84, the value of 501-A would have to have been lowered as well in order to maintain the correct voltage across that particular series heater string.

Pins 2&7 are the heater pins for all the octal valves.

Pins 1&7 are the heater pins for the VT70 valve.

Pins 4&5 are the heater pins for the EL84 valve.

So, need to confirm, are all the heaters parallel connected or as original?

Lawrence.
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Old 22nd Nov 2016, 1:45 pm   #22
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Default Re: BC-348-0 Help Please

The 6k7 has the metal case connected to pin 1. As these are the rf amps I suspect that link is connecting pin 1 to gnd along with one side of the filament.

The "A" on the top right (from board 274) connects to the "A" at bottom left. Pin 9 is the source of the 6.3V.
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Old 22nd Nov 2016, 8:23 pm   #23
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Default Re: BC-348-0 Help Please

I have one of these that was already partially and badly rewired for parallel filament when I got it. As someone has already mentioned, the micamould capacitors are truly awful. Wax capacitors tend to leak a bit but these go near s/c with age so they are not even safe in decoupling positions.
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Old 23rd Nov 2016, 11:31 am   #24
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Default Re: BC-348-0 Help Please

Hi everyone,

It appears they were rewired for 6.3v operation, the transformer has a 6.3v winding also that was wired up. I have heard there are a number of different ways to rewire for 6.3v operation?. I have placed an order for new capacitors yesterday to replace some of the micas. On the weekend, thanks to Lawrence providing the schematic for the EL84, I mapped out the connections on the EL84, which corresponded to those on the 41, and numbered them for reference for connecting them to the resistor boards etc..., I definitely feel like I am getting somewhere. Most of the interior of the radio has been messed around with prior to acquiring it, with lots of different types of wires used as replacements, I am going to replace these wires with modern wire. The valve shelf, I have removed a lot of the post war rewiring that was done by the original modifier, and attempted to rewire it for 6.3v operation, using the diagram attached on the previous page. I will have to get more photos for you all of the interior.

Many thanks

Will
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Old 23rd Nov 2016, 6:40 pm   #25
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Default Re: BC-348-0 Help Please

Quote:
I have placed an order for new capacitors yesterday to replace some of the micas.
You may know this already, but for the benefit of future readers:

"Micamold" was a trade name and the Micamold capacitors that normally need replacing are paper dielectric types (not Mica), usually in metal cans, that fail due to insulation breakdown after 50 years or so of service.

Mica dielectric capacitors are usually reliable and long-lived, and unlikely to need replacement. The few failures I have encountered have been due to reduced capacitance, presumably caused by corroded internal contacts.
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Old 25th Nov 2016, 7:59 pm   #26
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Default Re: BC-348-0 Help Please

To add to the above, Micamold branded capacitors in the BC-348 are in black Bakelite cases similar in appearance to a mica "domino" capacitor, except they are paper-wax inside and marked in color dots "103." They are all .01 mfd and they will all be bad by now. In addition to becoming electrically very leaky, if you place a high impedance voltmeter across them you will find that many have mutated into a battery and are producing up to a Volt!

There are two basic series to the BC-348 -- the H/K/L/R and E/M/O/P/S models go in the first generation series, while the J/N/Q go in the later "cheaper to manufacture" series. J/N/Q do not have the Micamolds but I am finding that the "John Fast" paper tubulars they have instead are usually quite leaky by now. The H/K/L/R sets have mica .01 mf capacitors inisde the RF and OSC boxes, while at least the P sets (Stromberg Carlson) have the paper Micamolds in those boxes as well, making a lot more work for you.

The Micamold paper capacitors were prolific during this period and even into the early 1950's in USA made military and civilian gear. They are generally regarded as one of the poorest quality components of the era.
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Old 27th Nov 2016, 11:19 am   #27
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Default Re: BC-348-0 Help Please

Hi Geoff,

I have a BC348 which I assumed was a Q (missing its ID plate as so many seem to). However, I seem to remember that it has Micamold caps. Is there a way that I can verify the model from other clues?
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Old 27th Nov 2016, 12:14 pm   #28
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Default Re: BC-348-0 Help Please

So far as I know the J,N,Q series was fitted with valves with no top caps, I used to have a Q version, all original including the shock mount and dynamotor, there were no top cap valves in it.

I'm not sure what the exact difference is between the J, N & Q versions but looking at the copy of a manual I have there appears to be not a lot in it about that.

Lawrence.
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Old 27th Nov 2016, 1:22 pm   #29
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Default Re: BC-348-0 Help Please

The single-ended valve variants also have different aerial coupling arrangements, the obvious sign being no aerial trimmer control on the front panel lower RHS.
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Old 27th Nov 2016, 8:45 pm   #30
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Default Re: BC-348-0 Help Please

It should be pretty easy to tell whether your model is a late (J/N/Q) or early one (everything else.)

J/N/Q are the same other than being different contracts. The suffix identifies the contract series. That being said, there were some very slight component differences depending on wartime component shortages.

The J/N/Q have no antenna trimmer and there is a rotary switch for the CW Osc while the others use a toggle switch. The others use aluminum castings for the chassis pieces while the J/N/Q use plated steel panels, often coated with anti-fungus varnish. The J/N/Q use point to point wiring while the others make use of terminal boards. The J/N/Q do not have the terminal board with the Micamolds on it behind the front panel "tube shelf" cover.

The manuals should all be available on the web as decent scans, and if you rummage in Google Images you should find photos of the different models and identify it that way. As mentioned above, a major difference is that the J/N/Q have single-ended valves, i.e. no grid caps on top.

All that being said, I have an early series 348 missing the nameplate that defies identification. There are some wiring differences that do not match any manual, either BC-348 or BC-224, and that look original. I think it's an "R" or an "H" but in the end it will be a study left for the archaeologists...
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Old 28th Nov 2016, 10:12 am   #31
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Default Re: BC-348-0 Help Please

Geoff,
Thanks for that. Mine is definitely not a J/N/Q then, so that's narrowed it down a bit. I'd like to identify the variant so that I can replicate the nameplate. I guess I also need to look at replacing the Micamolds, although it seems to be working pretty well at the moment. Incidentally, one useful tweak I found was to detune the crystal filter phasing such that it is usable on AM.
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Old 28th Nov 2016, 11:59 am   #32
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Default Re: BC-348-0 Help Please

Hi Everyone, Just an update on work completed on the radio this weekend. I replaced all of the Bakelite Micas. Many had seen better days, I have also replaced a lot of the frayed and damaged wiring. I may have went a bit overboard with the cable ties, but its nice and tidy enough to access the front of the resistor boards and the valve wiring. I forgot to add, when I acquired the radio, vt70 a 6F7 was missing, in the manual it refers to cw osc and 2nd if, for its purpose. Will this cause any problems, i.e., within the circuit?, I tried applying power over the weekend, the three valves which are located behind the osc can, ant can etc.. which have a small bulb connected, the bulb lit up, but no power to the dial lights, which I attached to pin no.9, the 6v line. There did not seem to be any power to the valve shelf, the EL84 did not light up, nor any sound through the headphones. Would the missing valve cause this sort of a problem?. I have a few pics to follow. Also two of the resistors burnt out, eep. 42-5 on resistor board no. 273, and I think 42-3, on resistor board no. 274.

Many thanks

Will
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Old 28th Nov 2016, 4:22 pm   #33
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Default Re: BC-348-0 Help Please

Will wrote: "when I acquired the radio, vt70 a 6F7 was missing, in the manual it refers to cw osc and 2nd if, for its purpose. Will this cause any problems, i.e., within the circuit?"

Yes, the receiver will not work without that valve.

"I tried applying power over the weekend, the three valves which are located behind the osc can, ant can etc.. which have a small bulb connected, the bulb lit up, but no power to the dial lights which I attached to pin no.9, the 6v line"

Check all the heater wiring, it's a simple parallel circuit, most folks would wire it up with just one feed wire from one end of the 6.3 volt winding to one heater pin on each valve in turn and connect other end of the heater winding to chassis and the other heater pins to chassis, the chassis acts as the other feed wire.

The two dial bulbs should be wired in parallel if using 6.3 volt bulbs and then connected in series with the bulb dimmer rheostat if you want to retain that feature, that series string should then be connected in parallel with the valve heaters.

If using 3.15 volt bulbs they should be connected in series and then one end of that series chain should be connected in series with the bulb dimmer rheostat if you want to retain that feature, that series chain would also be connected in parallel with the valve heaters, in either case the resistor designated 503 (60 ohm) which is in series with the bulb dimmer rheostat should be replaced with a wire link.

"Would the missing valve cause this sort of a problem?"

The missing valve will cause the receiver not to function, it should not prevent any of the valve heaters lighting up.

"Also two of the resistors burnt out, eep. 42-5 on resistor board no. 273, and I think 42-3, on resistor board no. 274."

Resistor 42-5 feeds the anode supply for the 1st IF valve, the most likely cause for that burn up would be capacitor C9-9 having excessive leakage.

Resistor 42-3 feeds the anode supply for the 1st detector (mixer) the most likely cause for that burn up would be capacitor 9-6 having excessive leakage.

So....what I would suggest next is to make sure that all the heater circuit wiring/connections are correct for parallel operation, all the original valves or there direct equivalents are fitted (EL84 excepted) Make sure that those two burned up resistors are replaced along with their decoupling capacitors as mention above.

There are various ways to power up an unknown quantity such as your receiver, power up via a lamp limiter, power up via a Variac or just switch on and hope for the best, I'll leave that to you.

What you need to see at least is all the valve heaters working and an HT voltage, If the power supply that was fitted was built correctly the HT rail should be approx 250 volts DC if the receivers valves are drawing current, if any of the valves are not drawing current, particularly the EL84 then the HT will probably measure higher than 250 volts.

Hope this helps.

Lawrence.

Last edited by ms660; 28th Nov 2016 at 4:32 pm.
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Old 16th Dec 2016, 2:25 am   #34
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Default Re: BC-348-0 Help Please

There were several changes between model numbers of the BC348 series.
Some also had built in power supplies, and NO FUSES!

The one I have had a note tucked inside that said: "This is what happens when you don't use a fuse";and the inside was all charred.

Quite possibly Electric Radio had some schematics of the "O" version in an article. But memory is hazy on that.

Yes, the bathtub caps that had oil likely used PCB type.

ANY MicaMold paper cap should be pulled and tossed in the rubbish bin. The paper is breaking down, and even if they test good still, they WILL fail. Just replace them with a good silver mica type, preferably epoxy dipped. I had a box of NOS MicaMold ones and they refused to work in ckt, even though they were new. Replacement with a new Epoxy dipped silver mica types fixed the problem I had fought for over a week. (The NOS MicaMold ones tested just fine capacitance and leakage wise, but were crook.) I have that Tee Shirt.
You can replace the 500V types with 600V or higher just fine. I typically use 600 or 630V ones.
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Old 17th Dec 2016, 11:19 am   #35
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Default Re: BC-348-0 Help Please

Hi everyone, apologies for my late reply. Over the last few weekends I have been working on the radio. I have replaced the Mullard valve with the correct VT41, and a VT70. And made sure that the heater wiring is series parallel, I've had success when applying 6.3 volts the dial lights and two valves light up. Where do I connect the negative lead of the power supply, I have read that it should not be directly connected to the chassis. Thank you all for your help and input, without I would not have been able to get this far.

Will
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Old 17th Dec 2016, 12:01 pm   #36
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Default Re: BC-348-0 Help Please

Do you mean a 41? In the schematic I'm looking at the original designation was a VT48, aka a 41.

When the original valve was fitted the receiver's power supply was from the dynamotor, the -ve output from the dynamotor was connected to chassis via a choke, that choke was part of the original sound output transformer assembly, the voltage developed across that choke was used for the output valves grid bias.

As I understand it the original dynamotor in your receiver was removed and a purpose built power supply was installed, the original sound output transformer assembly had been removed and a separate sound output transformer fitted along with an EL84 valve to replace the original VT48, if that was so then unless the modifier replaced the original choke with something similar (choke or resistor) he would have probably connected the new HT supply -ve direct to the chassis and provided the EL84's grid bias by the inclusion of a resistor and capacitor in the EL84's cathode circuit (we call that cathode or automatic bias)

Whichever, the sound output valves grid needs to be biased correctly, if it isn't the valve can draw excessive current and damage to it and other components can result.

You mention series parallel for the heaters, usually when these receivers were converted from dynamotor to a separate mains power supply the valve heater circuit would have been changed from series parallel to parallel only.

Lawrence.
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Old 17th Dec 2016, 12:33 pm   #37
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Default Re: BC-348-0 Help Please

Hi Lawrence, it's a VT-48 type 41, I have also acquired a 6F7 for VT70. If I can remember correctly, there was a capacitor with two resistors in parallel attached, which went to chassis ground, I think it might have went to the avc mvc switch when I acquired the radio. I think the resistor values were 300 ohms each and the capacitor 50 mfds, 25v DC. I think it went to the grounding post just above VT-86 before I took it out to start from scratch. Sorry Lawrence I meant parallel only, I've connected one feed wire to one side of the heaters in parallel, and then the other side of the heaters to the chassis.
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Old 17th Dec 2016, 12:51 pm   #38
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Default Re: BC-348-0 Help Please

Pic of capacitor and resistors attached.
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Old 17th Dec 2016, 1:15 pm   #39
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Default Re: BC-348-0 Help Please

Quote:
Originally Posted by wlindsay1984 View Post
Pic of capacitor and resistors attached.
Those resistors look about the right value for an EL84, too low though for the 41 valve, I'd suggest somewhere around 500 ohms for starters, from the 41 valve data sheets the cathode voltage should be around 16 to 18 volts.

The 41 valve data sheets can be found here:

http://www.shinjo.info/frank/sheets4.html

There should also be an effective grid return resistance fitted, probably was done by the original modifier, quick way to check is (with power off) to measure the resistance between the 41's valve socket control grid tag and chassis, should be a high value typically above 200k ohms.

Lawrence.
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Old 17th Dec 2016, 1:33 pm   #40
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Default Re: BC-348-0 Help Please

Hi Lawrence, many thanks for the speedy reply. I have fitted the b+ lead to tab number 12 on resistor board 271, would that be correct?., I will refit the cap and resistor to the el84, and connect the negative lead directly to the chassis.
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