Generally 20% either way with voltage is the considered norm.

Where have we set the balance control?

Bass is achieved normally by adding more capacity to the OP plate circuit albeit some put the tone in the grid circuit.

There is also negative feedback in some amps, taken from the secondary of the OP transformer. Mess that up by reversing the primary and it is liable to oscillate. Change the loading on the secondary,you change the feedback and one knows not what the ramifications are of that, but it is unlikely to be good.

I rescued a Philips123L from a recycler, with that sort of arangement. Incompetent (not recycler) had disconnected the feed back as it oscillated and he did not have the brains to realise what he had done. In changing the OP transformer & drilling a hole in the speaker. Output was low, speaker was stuffed but not the cause of low volume.

Would be handy look at a circuit.

As far as I can tell this stereo is virtually all original and untouched except for the replacement caps I added.

The schematic of it is here:

Image Link

When I measured it I had balance in normal position.

If the voltages are normal etc. This goes back to the changing of the caps. As before, if it was working and then you changed the caps that is where the problem lies.

The wires coming off of c21c & r36c as well as having those components in the wrong order, or on the wire on the wrong side may have unpredictable results.

This actually takes feedback from the OP secondary, so a phase fault is critical.

Phasing also effects the speakers, that is why they have a negative & positive. We do not need them working in opposites. When correct (using DC) the cone jumps out. Make sure permanent mag speakers are all correct.

Speakers in series are like batteries in series.

Thanks for that I'll go back and examine those specific caps.

Perhaps R36 was damaged by a bit of heat?

When you say above changing the loading on the secondary can mess up the feedback can me attaching a subwoofer to the speaker outputs affect this loading enough to cause an issue?

What exactly does the feedback do / achieve? Which wires on the diagram are the feedback wires? Certainly is a lot to these old radios it's certainly interesting!!

Depending on how they are presented to the output depends on how they load it, however placing another speaker in the circuit must change the impedance and increase the load.

I would like more info on the sub woofer and how it is attached (electrically) R32's are the feedback wires and do note that the extension speakers are 15 Ohm. Negative feed back was introduced a long time ago to increase the sound quality, especially at the bass end.

There should be more detailed info on the web. I think it was the "Williamson" Amplifiers that first put it to good use.

I would actually disconnect the sub and see what happens? You may have to rethink it, or how it is wired.

With the subwoofer I have wired the output on the rear of the unit to a high to line level converter so I can then plug into the rca input of the sub (it has it's own transformer).

Examining the schematic it seems this places the converter in series with the speakers. What effect do you think this could have? I guess it could add resistance to the circuit and load up the output transformer if there is a high resistance to the converter in which case maybe it needs to be wired parallel?

The converter I used is this

Image Link

Ok I measured the impedence of the converter and its huge approx 500 ohms. So obviously designed to work in parallel.

Looking at the schematic again I do indeed have it connected parallel anyway (I misread the schematic before but the bridges are still there and the negative of the converter is connected to the negative of the connection block).

So it would seem this arrangement shouldn't have much effect...? Can hardly be drawing much current?

Hmmmm......

I would still be inclined to fault find with it out of the equation. I do not know without seeing its circuit, how it inputs.

If it presents as a resistive or inductive load it still may have undesirable effects. The converter as I see it would plug into the 15R speaker plugs.

What may have to happen is that the plugs may need to be DC blocked or wired via a high impedance transistor type audio transformer?

I (and others) have had issues with some regulated power supplies having their outputs mains grounded and this could be an issue as the secondary is actually grounded to the chassis, which the circuit indicates, is mains grounded.

I had a tape recorder that caused issues with an Astor JJ if one dared use it when plugged into the mains.

Marc

Hi,
I am attempting to get an 11-77 up and running for a friend of mine. I have some electronic repair skill but I'm trying to get experience on some of this older stuff as a basis to really learn to understand it. I have replaced all the Electrolytics and paper caps with top quality items and everything seems to be working apart from the 12ax7 preamp tube. The heater doesn't seem to be working, all the other tubes are heating up fine and the pilot lamp is on. When I pull the 12ax7 out and re seat it I can hear a pop through the speakers so it seems that the output stage is functioning. I'm getting 5.4V across heaters everywhere including the dud preamp. I have tried multiple different 12ax7's and I have checked the socket too. I am a noob, and any help however basic would be greatly appreciated. I only wish my grandfather was still alive as he repaired these things for a living all his life....
I can attach photos if that helps.
Cheers,
France