Philips radio BCA495U

Dear forum members
I am in the process of the restoration of the above set. The set is identical to BX495U and similar to BX505U. I am facing the following problems. To start with I have replaced all the black tar capacitors with the same values and also capacitor C3. C1 and C2 are O.K.

A. Low Anode voltage of valve B3 UAF42.
The incoming voltage is 225v ac and the voltage at C1 is 188V and at C2 it is 160V. As per the schematic the anode voltage should be 45V but I get only 26V. Since I am not having new UAF42 valve I interchanged B2,B3 But The voltage remains nearly same. I also tried to replace Valves from the set BX485U ( which is working correctly) the voltage increases to 30V. I tried all possible combinations but the voltage does not go more that 30V. Presuming leakage between pin no 2 and the central spigot of the valve base ( This I had faced in one set earlier) I remove the earth connection of the central spigot and directly connected pin 4 and 7 to ground but the result is same and there is no leakage also. The screen grid voltage is 160V. The anode current is 0.6ma and the g2 current is 1.2ma.

B. I.F. & R.F. Gain of the set low.
Since I have a analog signal generator I keep the multiplier at X1 position and the attenuation pot full for carrying out the alignment. This helps me in comparing various sets during restoration.I know that one must work with the lowest possible signal bet the comparison becomes difficult. In a good set I get a DET O/P voltage of - 13 to 14V. Sometimes it is -11V also. But in this set I get only -2.8V. and no stations received. When I interchange the valve B2, B3 the DET O/P voltage became -5.5 volt but again the gain is very less. I replaced both the IFTs but the result is same. Changing UAF42/UCH21 does not help. So the voltage does no go more than -5.5. Under same conditions this voltage in BX485U is -8.6V.
The anode voltage of B2 is 158V and screen grid voltage is 90V. The anode current is 3.8ma and the screen grid current is 1.2ma.

C. When the set is tuned to a local station on MW band at 620Khz the gain is less with the antenna connected to antenna socket.But if the antenna is connected to the antenna coil S13 (SW band 50-150M) there is a lot of increase in the gain where as this is not the antenna coil for the MW band. Also the end of coil is S13 is open as the band switch is in MW position.Similarly when the Band switch is moved to SW band (50-150m)no stations are received with the antenna connected to the antenna socket.But if the antenna is connected to junction of S14 and C10 ( the other end being grounded) all the stations start coming. The resistances of Coils S13,S14.S15 and S16 are O.K.
D. Even the gains in the other 4 SW bands is low but they are much better.
I shall be grateful if somebody can help me in solving the above problems.

Thanks and
Regards
P.Rajan

Reageer op dit bericht

595 keer gelezen

[image]
25 volt anode voltage on B3 should be ok.
For the lack of sensivity I think it could be a bad IF transformer,but most times these could be repaired.
To repair them you have to take them out and remove the case.
Sometimes its a bad soldering but also sometimes the coils fell out of the holder,in this case you can put them back and put i little bit wax between.
Of course you have to be shure that a IF transformer is bad before you take it out.

Henk

Reageer op dit bericht

577 keer gelezen

My guess is that the antenna tuning is not in parallel with the oscillator tuning. By skipping the antenna tuner coils you claim to have more sensitivity.

So the antemnna tuning needs some adjusting, this can best be done according to the service doc.

In general, tune in to a low frequency in a band pe MW.
Adjust to max reception (or noise) with the relevant antenne coil for the band.
Now tune to a high frequency in this band. Adjust the cap in the relevant antenne tuning circuit.

Before doing this, if possible, check if the tuned frequency coincides more or less with the scale.

A signal genearator might be helpful, but if you are aware of the stations and frequencies in your area you can use that.

Better leave the IF tuning as is in first instance.
When you have a signal generator you can check IF first.

Reageer op dit bericht

573 keer gelezen

Dear Mr. Hennks

Thanks your reply. I am attaching a sheet from the service documents of the set BX495U. This is identical to BCA495U. You can see that the anode voltage of valve B3 UAF42 has been mentioned as 45V and the screen grid voltage is 38V. So the anode voltage is higher than the voltage at g2. I think this is normal. You have attached the data from the model BX505U. Both the sets being electrically same then I do not know why this difference is there? Even in BX485U the voltages are 45V and 38V. In BX505U the anode voltage is less than the g2 voltage? In both the sets the incoming voltage given as 220V ac but the voltage at C1 is 190V and at C2 it is 160V. But in BX505U it is given as 175V and 145V. Around 15V less.
I have traced the fault of the MW band. It was my mistake. The resistance of the coil S16 is 5 ohms whereas I was looking at the continuity part. The wire from the Junction of S16 and C11 going to terminal 20 of the band switch wafer 2 was getting short. This has been rectified and the MW band is now O.K.

Regards
P.Rajan
[image]

Reageer op dit bericht

565 keer gelezen

Both the currents are the same.
so you can calculate acording to the supply and the anode and G2 resestors.

Read carfully under what conditions
the voltage has to be measured.
With or without signal f.i.

Reageer op dit bericht

560 keer gelezen

Dear Mr Maurice

Thanks your reply. If we consider the voltage chart of BX495U/BX485U then with 160V at C2 the drop across the resistance R22 + R24 is 120V at 0.6ma. and the anode voltage becomes 40V. If we consider the voltage chart of the set BX505U then with a voltage of 145V at C2 the drop is again 120V at 0.6ma and the anode voltage becomes 25V. In both the sets all the component values are same and the voltage are measured with an incoming voltage of 220V ac and band switch in MW position, gang at min capacity and so signal applied. I do not understand why this difference is there.

Regards
P.Rajan

Reageer op dit bericht

580 keer gelezen

Important for you is that current and adjustment of that tube is the same
in both the situations.

So your measurement is correct and tube should be working in that setup as normal
with that current.

With tubes the issue is alway's current in relation to the condition of the tubes
also the lack of regulation of voltage alway's look for the voltage drop in each
circuit part in relation to the resistors value.

In that way you can solve problems with resistors value are way off, capacitors that draw current ore a bad tube.

Reageer op dit bericht

572 keer gelezen

RSS Feed van deze thread

Nederlands Forum over Oude Radio's

Philips radio BCA495U (Algemeen)