Hi everyone,

For some months now I have been working on a 1956 Astor CPP radiogram.
It has numerous issues and has now all capacitors replaced also virtually every resistor too. The 3 or 4 remaining resistors are within 10% of their rated value.

The remaining issues relate to the radio not working well (poor volume and a distorted sound) the gram side works fairly well though I think is a little lacking in overall volume.

Attached is a link to the schematic.

https://www.kevinchant.com/uploads/7/1/0/8/7108231/cpp.pdf

I suspect the 6AD8 valve maybe a problem, but after many hours working on it, I’m
At a standstill and needing advice.

Looking forward to hearing from someone.

Thanks in advance
Cheers
Scott

Quite a complicated radio.

You can replace the 6AD8 with a 6N8, they have the same connections, and the 6N8 has a bit more gain.

Does the volume/distortion problem occur with the record player?

Do the voltages in the schematic correspond to what you have?

The record player uses the 6AD8, the 6AV6, and all the rest, it's complete overkill, there should be huge amounts of volume.

The radio uses the 6AD8 as the IF amp instead, the audio starting at the 6AV6.

Also check out the band-switch, it's probably the most troublesome thing in a locally-made radio.

Hi Robert,
Thanks for the reply.
To answer your questions, can I interchange both the 6N8 and 6AD8 (swap them over)? Will the 6AD8 work where the 6N8 currently is?
The voltages are fairly close to the schematic voltages with a few volts either way. The selector switch has been checked and seems to be working properly.
The gram volume is clean but seems to distort if turned up - a friend has one of these and his is certainly a lot beefier than this one which is why I know it’s not working correctly.
The radio signal is very broken up at any volume except max when it actually clears but is not loud.
Hope that helps
Cheers
Scott

Yes, you can swap the 6N8 and 6AD8 for testing, I take it you don't have spares.

I don’t have any spares here no. By swapping them that will at least see how the gram side performs but it prob won’t help the radio side??
I do know the 6N8 valve is ok as I swapped it out with another set last week to test it. The other set has now been sold.

Remember, the valve line-up in that set is overkill, most radios didn't have the RF AMP stage (the 6N8), so it still (if it was working) would be more than enough. You should notice some kind of change though.

Do voltage measurements of the AGC line in various parts of the set. Due to the bandswitch that line goes all over the place so you will need to make sure it is correctly reaching the 6N8, the 6AN7 and the 6AD8 grids.

If you are happy with the voltages throughout, I'd start working back from the output stage seeing where the distortion is being introduced. Is the gram/radio switch contacts ok? There is some trickery going on here changing the IF stage to a pentode input for the gram. There is some similar switching used for the proceeding 6AV6 stage.
Good luck. Looks to be a nice unit.

Hi Scotty et al,

A couple of 'thoughts' ...

* A typical reason for low volume, particularly after swapping a lot of parts, is Alignment, of IF's and Local Oscillator. Be aware that adjusting IF's is fraught with danger ... breaking or cracking a tuning slug could be very difficult to fix.

* Your comment about breaking up unless at max Volume suggests a bad contact, Pot, switch ....

* The switching is complex .... usually, the Gram switches in after the Detector and AVC in this type of circuit.

* Why does the circuit use a Double Diode Pentode 6N8 in the RF amp??? And in the IF (6AD8)??? The diodes don't appear to be connected.

* I have a spare 6N8 and a 6AD8 which you can borrow to use as substitutes. And / or, I could test your 6N8 and 6AD8. I live in Eastern Suburbs Melbourne ... contact me on my Email if interested/required.

Thanks Ian
Yes the whole thing is a bit of a mystery to me.
I’ll check the selector again. Interestingly, when I switch between radio and gram, there is a slight delay before the audio switches.
I find the circuit very difficult to follow and as I said in my initial post, having spent enormous time going through, I’m still none the wiser.
Bring based in regional vic sourcing valves means on line or a trip to Melbourne. I’ve restored numerous radiograms but this thing has really got me stumped.
I find it almost impossible to try and follow the signal path given the complex way this thing works…
Cheers
Scott

Hi Scotty,

The advice by Robbbert and DangerousDave is good. The circuit diagram is somewhat complicated by all the Band coils, caps and switches etc ... so my approach would be to follow the KISS principle ..... ignore the Gram, ignore the various Band circuitry, ignore the various switch positions ... and follow the signal with the switches in their current positions. That is, Antenna to RF Amp (6N8) Grid, then out of Pin 6 to Mixer (6AN7) Grid, etc ... that pathway is conventional.

Do you have, or have access to, a Scope?? Signal Generator?? ... as my next step would be to work my way backwards, as per DangerousDave, injecting various signals at appropriate points.

Thanks Ian
No crow or signal generator here unfortunately
I have a crow in Melbourne that I’ll bring back with me next trip.
Cheers
Scott

Although it looks complex, the radio side is indeed quite conventional for a multi-band set, as Tinkera123 has said.

Therefore I'll describe as best I can the signal path for the GRAM side. The diagram shows the Gram/Radio switch in the radio position, so you need to imagine it in the Gram position.

The diagram has convenient numbers in circles, so I'll refer to them as (nn).

So, we start at the gramophone (137), through (139), up to the coil (116). Since the coil is tuned to IF, the audio passes through it unmolested to the grid of the 6AD8. Output is taken both from the anode and G2 (I think this makes the stage more triode-like), the anode side passes through (117), again unchanged where it meets the G2 side. Then through capacitor (15) to the Gram/Radio switch where it becomes common to the radio audio.

So, through the Volume control, go up through capacitor (10) to the grid of the 6AV6. The output of this stage goes to the tone controls which there's no need to describe, but the most direct route is through (12), down through (71), into the bass control, across to (73), up and across into the grid of the first 6AU6. The output of this goes to (13) where it splits into two.

Now, this set has a class B output stage, where each 6AQ5 amplifies half of the waveform in turns. For this to work we need an inverter for one half. The uninverted half goes down and across to (78) the lower 6AQ5. The other path takes us through the 2nd 6AU6 for inversion, then to (77) the upper 6AQ5. The anodes of both output valves now go to the centre-tapped speaker transformer where the signals are rejoined.

The speaker output has some of its signal used as negative feedback to reduce distortion and tame the amplifier a bit. This is done via the cathode of the first 6AU6.

Hopefully it makes some sense.

Hi all,

Robbbert ... yep, to all that ... tortuous.

Scotty .... thankfully, I have always had a Scope and several appropriate Generators. To do the Broadcast Band you wouldn't need anything fancy. I'd wait until you get your Scope. In the meantime, I would re-clean and exercise all switches ... and run it, or 'burn it in', as others on this Site say.

Where there is distortion or perceived distortion. An Oscilloscope and Signal Generator with a good audio wave form, is the swiftest way to find it. I use an Oscilloscope to calibrate the IF; Meanwhile it will show me distorted signal using that tone and I can then use the oscilloscope to find the source.

Being Push Pull it only takes one of the PP tubes to be down & different to cause distortion.

Marc

Hi Robbert,
Thanks for the great travel path description, makes it easier to follow. So it does seem
The 6AD8 could be an issue given it’s the starting point for the gram amplification. In terms of testing it through each stage, is there a way to do this safely given the voltages around the areas?
Thanks to the other guys too, I’m determined to get this thing to work properly.
Cheers
Scott