Lafayette HE-40

[ORAWA01]

Hi All

Just got this old receiver, and it has some list of serious problems.

1 of the electrolytic caps exploded and burnt, and 2x of them were well out of value so replaced 3x of them.

Also the 12AV6 double diode is not lighting up.
So, I must find out, if the valve is bad or any of the parts around it, is bad.

The set powers on, but has loud hum, no sound.

And there are 1-2 resistors on the BFO pot look burnt and bad too, but that will be looked at later.

But firstly how do you check if the 12AV6 is OK, or is bad? I read somewhere continuity between the Cathode and Heater could be checked. I did that, and there is no continuity between pin 2(Cathode) and Pin 3, and 4(Heater).

Pin 3 and 4 has continuity.

Thanks.

[Station X]

You say the 12AV6 isn't lighting up. Are you sure? Some valves can only be seen to be lit if viewed at the correct angle in a darkened room.

Check that with the set powered up you have the correct heater voltage on the 12AV6's valve holder pins 3 and 4.

If it's OK, the problem is down to dirty valve pins, dirty valve pin sockets or loose valve pin sockets which need nipping up.

[Station X]

Circuit in this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=54873

[Nuvistor]

The circuit appears to be for 100-120 volts AC input, is your version 230 volt version, if it’s 120 volts that could be the reason for the damage.

[ORAWA01]

The set was powered on with the cover open, and I put the light off in the dark room at night, and yes the 12AV6 is totally dead. There is no even a spot of dim light in the tube.

The other 3x tubes are lighting OK.

The set is 240V version. The mains transformer had been wired for 240V and must had been in use for some time by the previous owner.

[ORAWA01]

Originally Posted by Station X

Circuit in this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=54873

Thank you for the circuit. Much appreciated.

[Station X]

If you pull out the suspect valve do the others stay alight?

[ORAWA01]

Originally Posted by Station X

If you pull out the suspect valve do the others stay alight?

I will test on that, and will get back to you.

[ORAWA01]

Originally Posted by Station X

If you pull out the suspect valve do the others stay alight?

When I took the 12AV6 out, and powered on, nothing came on. Not even the dial lamps.
When the 12AV6 is put back on, then the set powers on - bulbs light up and all 3x other valves light up.
Maybe the 12AV6 is OK, in that case?

Yesterday, a resistor on the top of the variable BFO pot went on fire when powered on. I am assuming it is a resistor, but cannot tell if it is a capacitor due to burns on the part. I have cut it off from the circuit to read the value, and it is OL in ohms reading.

I wonder if the 12AV6 is not coming on because that part is no longer in line in the circuit.

[ORAWA01]

Which one would be the Variable BFO pot in the schematic?
The part either resistor or capacitor was on the BFO pot, and connected to one of its 3x outputs.

[img]https://***********************/65535/49669773797_c0c1b2df6f_b.jpg[/img]

Will get the photo of the BFO and the cut out burnt resistor or capacitor soon.

[Station X]

Originally Posted by ORAWA01

When I took the 12AV6 out, and powered on, nothing came on. Not even the dial lamps.

Maybe the 12AV6 is OK, in that case?

Two possibilities there. Either the valve is lighting up and you can't see it, or the heater is short-circuit, which I think unlikely.

With the set powered up and the valve plugged in measure the AC voltage between pins 3 and 4. If zero the heater is short-circuit. If around 12.6V the heater is OK.

[ORAWA01]

Originally Posted by Station X

Two possibilities there. Either the valve is lighting up and you can't see it, or the heater is short-circuit, which I think unlikely.

With the set powered up and the valve plugged in measure the AC voltage between pins 3 and 4. If zero the heater is short-circuit. If around 12.6V the heater is OK.

When checked with DMM, the 12AV6, pin 3 and 4 (both are heater pins according to the data sheet) have continuity. Pin2 (cathode) have no continuity with pin 3 and pin 4. Will check the voltages later on.

[ORAWA01]

Here is the photo for the BFO pot,
[img]https://***********************/65535/49672267441_8815877236_b.jpg[/img]


and the burnt parts taken out from the top of the pot.
[img]https://***********************/65535/49669817187_bddbc33e5f_b.jpg[/img]

[Radio Wrangler]

That looks like a resistor.

Next step is to identify which one on the circuit diagram.

I haven't figured out yet how the BFO is implemented in this radio, still less why tuned with a pot, not a small variable capacitor.

David

[Station X]

And let's not forget that components burn out for a reason. There'll be a short-circuit or leaking capcator somewhere through which current has flowed burning out the resistor.

[ORAWA01]

I cannot locate the BFO pot, and the resistor in the schematic. It looks like S4 but S2 as well, but there is no resistors directly connected to either S2 or S4.
What is directly connected to both S2 and S4 look like caps to me in the schematic.

The set having a non isolated mains transformer is not helping and could have possibly caused the burns. It seems very volatile with sparks from the parts when plugged into the mains even with the power switch off. When even a wire antenna was being attached to the antenna input screw, and ground wire to the ground input screw, it was sparking badly with bright red flames.

[Herald1360]

That BFO circuit is a nice bit of bean counting!

R8 is the BFO pot and varies the resistance between the IF valve suppressor grid and ground. The switch on the pot looks like it disconnects the IF valve control grid from the AGC line and takes it to ground via a 5k resistor when BFO on is selected. With some added resistance in the suppressor grid circuit, the IF stage duly takes off to provide a crude BFO. There's some confusion with the Standby switch contacts in the same area of the circuit diagram which doesn't help with clarity!

(Reminiscent of an old bodge I used to copy SSB on a domestic set which consisted of a variable wirewound resistor in the cathode circuit of the IF valve and some judicious anode - grid feedback via a "trick" capacitor)

How the resistor cooked is a bit of a mystery- maybe running the BFO too hard causing excessive grid current, a breakdown in the IF transformer or flashover in the IF valve perhaps?

Edit: Just seen crossed post...

The set sounds lethal from your description of connecting an earth! The only isolation between mains and the earth connection is C5 in parallel with R16(?). I would replace C5 with a suitable Y-class safety rated capacitor and make doubly sure that mains neutral not live ends up connected to the HT- line.

To be honest, what you can get away with on a non isolated set on 120V mains can be rather too risky on 240V. I would seriously recommend running this one from an isolating transformer or at the very least via its own dedicated RCD.

[ORAWA01]

Originally Posted by Herald1360

That BFO circuit is a nice bit of bean counting!

R8 is the BFO pot and varies the resistance between the IF valve suppressor grid and ground. The switch on the pot looks like it disconnects the IF valve control grid from the AGC line and takes it to ground via a 5k resistor when BFO on is selected. With some added resistance in the suppressor grid circuit, the IF stage duly takes off to provide a crude BFO.

(Reminiscent of an old bodge I used to copy SSB on a domestic set which consisted of a variable wirewound resistor in the cathode circuit of the IF valve and some judicious anode - grid feedback via a "trick" capacitor)

How the resistor cooked is a bit of a mystery- maybe running the BFO too hard causing excessive grid current, a breakdown in the IF transformer or flashover in the IF valve perhaps?

Ahhhh .. I can see the BFO variable resistor of 1K? So would it be R7, 300R which is the burnt resistor?

When I opened the radio, there was also an electrolytic cap burnt as well, which was duly replaced.

[Station X]

Originally Posted by ORAWA01

IThe set having a non isolated mains transformer is not helping and could have possibly caused the burns. It seems very volatile with sparks from the parts when plugged into the mains even with the power switch off. When even a wire antenna was being attached to the antenna input screw, and ground wire to the ground input screw, it was sparking badly with bright red flames.

There is no mains transformer, isolated or otherwise. This is an AC/DC universal set.

Given that you have sparks, smoke and flames I'd suggest that a more structured approach to fault finding is needed. I'd suggest starting with the power supply. This uses a semiconductor rectifier, so it will produce HT with no valves plugged in. I'd make sure that there are no shorts from HT to ground and if not check that HT is present.

[Radio Wrangler]

That sounds like integrating a Miller Transitron oscillator into the IF amp stage.

Transitrons will probably be familiar to most people as timebase sweep oscillators in oscilloscopes from a certain era, before triggered sweep took over.

I think one RSGB book showed a transitron oscillator as a general coverage sig gen.

David