Philips Radiogram chassis. Philips Radioplayer 174.

[joebog1]

A question for the boffins please.

Years ago I was given a chassis from a Philips radiogram.
Its a very simple circuit ( or so it seems) but after replacing the PSU electros and all waxies ( most of which had been devoured by cockroaches) It works.
BUT its deaf as a post!! By that I mean sensitivity is woeful. It receives the local stations on broadcast band, but does nothing on both shortwave bands.
I have the circuit ( attached ) and the parts list.
It would appear that the back bias voltage derived from the centre tap of the transformer is too negative. Its about -10 volts and that makes the AVC and sensitivity very poor. Voltage is supposed to be about -6.4 or so.

My question comes in two parts:
1: The screen volts on both the 6AN7, and the IF amp, 6BH5 is at about 115 volts, NOT as the circuit voltage table suggests, 57 volts.
2: The 6M5 ( EL80 I believe) output bottles are very worn looking, with those runny black stains on the envelope. I dont have a new pair, and I dont want to pay the $50 for a "perfectly matched audio vintage hi-fidelity pair" that "that" site gave me as an option for replacement.
NOW as I am assuming the ouput pair are low, would they cause the bias voltage to rise that high?
OR is the Philips circuit diagram wrong?

I have several circuits from different Philips models radiogram' and they all show the same 57 volts on the screens.
Will anybody have a go at deciphering my questions please

Thanks in advance
Joe

[Nuvistor]

The high screen volts are perhaps due to the AGC voltage biasing the IF and FC valves off. You could try removing AGC, and test the sets sensitivy, the grid circuits of those two valves will still need a path to chassis.
Have you checked the value of the resistors in the HT supply, particularly R18.
I would not be buying new EL80's until other tests on the set show them as being faulty.
Frank

[ukcol]

Another vote here for R18 (100ohms 1W).

A related but off topic post > http://www.vintage-radio.net/forum/s...d.php?t=137640

[joebog1]

R18 was out of spec very slightly at 107 ohms, so I replaced it in desperation.
I have since fitted an 82 ohm resistor in an attempt to reduce the bias.
Its interesting to note, that another Philips broadcast band only model, but with almost the same valve lineup, uses a 47 ohm resistor for R18. The other difference is it only has a single ended audio amp, utilising one 6M5.
A careful viewer will notice I have replaced most of the resistors in the HT chain, as they were all on borderline 20% or slightly greater tolerance in a futile attempt to get it right.

I am still somewhat confused with the screen supply voltages, although I don't have a 1000 ohm per volt meter, I do notice that my AVO 8 pulls the bias line down on the front end to about -3 volts and massively increases sensitivity.
Perhaps the reason I am still confused is that in my experience Philips circuit diagrams were usually spot on. So how they get 57 volts and I get close to 120 volts has lost me

Joe

[Nuvistor]

The service notes state the radio must not be receiving a station, that would increase the AVC and lower the current through those valves and increase the screen voltage. You have much too much AVC voltage for some reason and that will increase the screen volts.
Frank

[joebog1]

I have followed the instructions to the letter!

Hence the post in the first place.
On another note I have recovered the dial glass.
I did two scans one with black background
( thanks to "All things must pass" LP cover)
and a white background scan.
Its interesting to note that Philips always printed the dials "backwards" to convention in australia, which was to put low frequency on the left and high frequencies to the right, while Philips did the opposite. High frequency on the left and low frequency on the right.

Joe

[ms660]

Interesting push pull stage.

Cathode coupled?

Class A ?

Lawrence.