LX437AB

Hi.Radiophiles, It came to me for repair/recover one portable radio LX 437 AB.I don't have its schematic,but supose  may be similar to model LI 437/AB.My problem is that I don't have the correct filament tensions when switched to the mains.As a matter of fact,the wires that should conect to the batteries have been cut so I can not test it with the batteries once they are not there.And some help would be very welcome if somebody could explain me the wiring of the tubes filaments,once I find very dificult to understand the complexity of the switch wiring together with the dificult access to components and lack of data of voltages.

May I wish to all members and their families a Merry Christmas and a Happy new Year with greetings from our Sunny Algarve!

In general

If the pin 1 & pin 7 is in parallel for all except output (Here DL94), and it has 1 & 7 tied together then it's 1.4V filament and the two 1.5V batteries are in parallel.

The alternative is usually all filaments in sieres, but that's unlikely as the DL94 is 100mA 1.4V parallel and 50mA 2.7V Series.

Examining the LI437AB (which is alleged to be the same), suggests the 2 x 1.5V cells are in series. This does NOT mean 3V of mains. This is because a pair of 1.5V batteries are nominally 3.4V to 3.2V (new, no load to near dead).

Recommended voltage is actually 1.35V per filament for good life. Between 1.28 and 1.39V is fine. So on mains your filament volts should be 2.56 to 2.78V if the battery connections simply switched to mains PSU. But they are not.

To start with till Mains PSU is tested I always use batteries. In this case 2 x D cells in series for LT and 7 fresh alkaline PP3 9V cells or 8 slightly used PP3 or 45 to 48 off  AA cells (or AAA cells) in 8 way holders (you can bypass 3 positions in last holder).

On Battery  (Nominal 2.8V 125mA)

DK92 (V2 50mA) in series with ( DAF96 and DM71 each 25mA) 

DF96 (V1 RF ) in series with DF96 (V3 IF)  25mA

DL94 in series mode, 2 x 50mA

On Mains

On this set the LT on mains is via 1.6K and adjustable 600 Ohms from the HT rail. The switch is shown in mains position, so V1 pin is no longer the Battery +. All current is via V5, DL94, so 50mA Series.  DK92 (50mA) is to 0V, so is end of "chain". V4 DAF96 and V7 DM71 are still in parallel (25mA + 25mA), but the 2 x DF96 are switched from series 25mA 2.8V to parallel 50mA.

So we have at 1.35V per filament (Series mode),

Logically (BUT NOT How it's done!)

DL94 = 2.7
2 x DF96 parallel = 1.35
DAF96 + DM71 = 1.35
DK92 = 1.35
Total = 6.75V

This is not a very happy design on mains! The total from slider of 600 Ohms (pin7 DL94) to Chassis (Pin 1 DK92) would be 6.75 nominal. The reasonable range is 6.3V at lowest mains voltage to 6.9V at highest mains voltage. However it looks like the two DF96 remain in series on Mains!

How it really works

LX444 seems to be similar (Battery)

On Mains the LX444 is given

So in this case R37 has about 23mA and 2.7V. On the LI437 I think 120 Ohms, which is 22.5mA at 2.7V on DF96 filaments in series. R21 is to balance the 6mA or so HT current.

Conclusion

On battery the nominal supply is 2.8V 125mA.

On Mains the nominal supply about 48mA nominal 8V (6 filaments in series) or about 7,7V to 8.3V at extreme limits of AC mains voltage variation. As if the design originally had DF91 x2 , DAF91 and no DM71 instead of DF96 x 2, DAF96 and DM71,

To test and align such Mains PSUs I use dummy valves (tubes) with resistors for simulation of filaments AND resistors form g2 and Anode to EACH filament pin to simulate total HT current  which adds to the filament currents. This is why there are additional resistors in the filament circuit. They are to balance the HT currents which vary between nearly 7mA per tube to under 500uA (DAF96, DM71),

Hi Antonio,

I uploaded the schema and all original Philips infos. There is a separate schema of Hf-wiring for all 3 wave-rages and a page with power-supply wiring and a voltage chart.