Philips B6X34A/54 Balance Control
 

I am not familiar with vintage Philips Radios and therefore not sure what to expect from the balance control on a B6X34A/54 that I am working on. The control has almost not effect fully turned right or left in either the stereo or mono modes. I have ohm'd out the control and traced the wiring along with checking the capacitors associated with the control and all seems fine.

Looking at the schematic it appears the control only works in the stereo mode where the wiper of the control gets grounded by the stereo/mono switch. I have verified that the switch does apply the ground to the wiper. Either end of the balance control goes to between a resistor divider supplying plate voltage to the tube. The control is then coupled by a capacitor which blocks the plate voltage from the control. The audio from the tube is taken off the plate of the tube through a different coupling capacitor (before the divider network) and on to the next stage. This means that the balance control only affects the signal present in between the resistor voltage divider that feeds the plate of the tube.

Anyone familiar with how these early 60's Philips balance controls worked? Is it normal to have almost no impact on the actual balance, or am I missing something.....? Otherwise radio plays and sounds good in FM, MW, and SW modes.

A piece of the schematic is attached for reference.

Thanks for your feedback!

Mike

The values of the plate load resistors are too blurred to read, but the magnitude of the balance adjustment will depend on the ratio of those to the 100k balance control.

AC-wise, at one end of the control setting, R60 is shorted so total AC plate load is R58, and at the other end, 100k is in parallel with R60, so total plate load is R58 plus (R60 in parallel with 100k). The opposite effect occurs in the bottom section.

When balance is centered, total AC plate load is R58 + (R60 in parallel with 50k).

You should be able to hear the volume decreasing/increasing if you listen closely to one side as you turn the control.

A second look at the blurry stuff in series with the balance control has me wondering what exactly is there. Are those symbols for shielded cable?
Also, I think I read the lower capacitor value instead of the control value, which appears to be printed in its center and is unreadable.

old_tv-nut, Your are correct the schematic is very blurry. R60 which is connected to B+ is 60K and R58 which is connected to the plate is 10K. The balance control is a 1 meg pot and coupled between the R60 and R58 resistors by a 100K cap. Since the balance control is DC blocked it only affects the signal and not the plate bias. This along with the fact that C77 (a 10K cap) couples the signal directly from the plate on to the next stage is what is perplexing.

Mike

old_tv-nut, Also, you are correct the balance control is connected by shielded cables which are seen around the balance control on the schematic.

Mike

OK, so the effect of the balance control is to parallel a resistance between 1 meg and zero across 60k, so the parallel value goes between zero and about 56 k, and the total plate load goes from 10k to 66k. This will change the gain of the amplifier stage, but calculating how much it will change also requires knowing the triode plate resistance and the impedance of the following network.

Can you measure the resistance from one end of the balance control to ground when in stereo mode to verify that the wiper contact is not open?

Balance Control tested OK. When I checked the balance control it measured somewhere between 990K to around 40 ohms between the wiper and one end of the pot, with a similar measurement between the wiper and the other end when turning the pot the other direction. I have also measured at the wiper to make sure it gets grounded when the stereo switch is activated.

Mike

Problem solved. In this radio the audio tubes are on the chassis while all the resistor/capacitor parts for each output tube are mounted on left and right circuit boards. The tube sockets are then individually wired to the circuit boards. It looks like someone melted two wires together where they connect to the right circuit board at some tine in the radio's history. This effectively shorted the right side of the balance control to the top of the right side bass control which caused the signal path from the output of the preamp stage to bypass a 150K resistor which is in series with the input of the bass control. This skewed the balance control's action. With the short removed the control functions as I would have expected, mostly quieting one side or the other as the control is turned.

Why they designed the balance control to only work in the stereo mode is still a mystery to me.

Mike

You don't need Ballance in mono mode...