Cossor 1324 FM Alignment Generator Repair/Restoration

[OldTechFan96]

I have been interested in obtaining equipment to align FM receivers; when the opportunity arose to buy this Cossor 1324 I snapped it up. It was well reviewed by Gordon J King in a October 1961 copy of 'Service Engineer'. King's review convinced me that this is what I needed.

The Cossor is all complete, including its original demodulation probe/dummy aerial.

The weekend was spent stripping it down and giving it a thorough going over. It came up very well! I will post photographs later.

It was free from Hunts paper capacitors bar one, which was replaced. The smoothers sat on the reformer for a while but remained leaky at the 100V-200V range.

HT is correct at about 185V. A modulated tone can be picked up by an FM radio so I know I'm getting RF out. I have not checked the audio out yet. I'm not really sure how to operate the generator or what other checks I should do. I will check valve voltages but I don't know what they should be!

Any thoughts on this Cossor?

[Station X]

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=159223

[OldTechFan96]

Cossor 1324 initial testing:

The audio and RF outputs are in working order.

V4 gets hotter than the other 12AT7s. This could be normal.

HT is slightly high at 189V. Leaky smoothing capacitors likely.

Valve voltages are mostly fine.

Cathode of V3 measures 6.03V (should be 11.8V). The resistors around the cathode are all OOT, hopefully explaining the low voltage.

Both anodes of V5 are high by about 10V. HT feed resistors are spot on. Is the valve not drawing the correct amount of current? Low emission 12AT7?

The RF out lead needs replacing. Is it 75 ohm coax?

Should I replace the TCC Visconol X and Metalmite capacitors? I'll check them for leakage.

Does the 'X' output connect to the external trigger on my scope? My Hameg's T/B has a BNC connector for an external trigger.

I'm not sure about how to use this generator so I'll study the operating instructions supplied by Cobaltblue (Thank you!).

Does anybody have any experience using this FM generator?

[OldTechFan96]

I've been going through the available material on the Cossor 1324 and I am starting to understand how to use it.

The main stumbling block at the moment is trying to connect my oscilloscope to the X input of the generator. To get the generator to sweep (for IF and discriminator alignment) you need the sawtooth/timebase/X signal from an oscilloscope.

I have a Hameg 203-4 and I noticed that on the rear plastic case there are blanking plates for Z, Y and sawtooth outputs.

Does anybody know where the sawtooth signal is generated? I have looked in the scope's service manual and am non the wiser. I think it is in the TB?

If the sawtooth generator can be found I can feed it into the generator.

[Chris55000]

HiI

You can take a suitable sweep ramp from the junction of R374 and the end– terminal of S370 (Ext X Selector) on the TB. Board, however I would recommend a simple × 1 op–amp or two–transistor emitter follower buffer before you bring it outside your oscilloscope!

You might find Hameg have provided a suitable hole in the metalwork behind the rear case panel suitable for fixing a suitable 50Ω BNC connector to!

Chris Williams

[OldTechFan96]

Thanks for the info. I have found where you mean in the service manual.

Could you please tell me more about the buffer circuit you mentioned?

[Chris55000]

Hi-

An op–amp buffer (741 or TL071 type pinout) has pins 2 and 6 linked, the input from R374 is taken to pin 3 and the output from pins 2 and 6 to the BNC outlet added for the sweep output.

Connect pin 4 to a negative supply of about –18V and pin 7 to a positive supply of about +18V, which you should be able to locate in your oscilloscope no problem!

Add 1 × 10μF 25V electrolytic to pin 4 and earth and 1 × 10μ 25V electrolytic to pin 7 and earth of the IC, connecting the + side of the pin 4 capacitor and the – side of the pin 7 capacitor to the chassis–earth line of the scope in each case.

Finally, add a 100k resistor from pin 3 to earth/0V to complete the circuit.

This can be built on a small piece of vero, or the Chinese suppliers do offer ready–made "Op Amp Buffer PCBs" for a tidy looking result – look for "Dual Op Amp Preamp DC Amplification Board NE5532" or similar – if you get one of these (they're only about £1) I'll post back explaining what components to fit!

Chris Williams

[buggies]

I have no idea what the voltage spec is for the sawtooth into the X input of the wobbulator - but would it be possible to just use a function generator ( or even a 555 ramp generator) and feed that into both the 1324 and scope ext X input?

[OldTechFan96]

Chris55000: I do have two 741 op amps. I'll look into one of those premade PCBs. Why is a buffer needed between the scope and the wobbulator? Could the raw sweep be fed straight into the X input?

buggies: Good point about the X voltage into the wobbulator. I wonder if the Hameg will be able to drive the reactance valve? There is a calibration procedure for this.

Would the X-Y mode let me feed in a signal into the X input? I might be barking up the wrong tree here!

[OldTechFan96]

I have constructed Mark's PCB and it working as intended. The sweep voltage has more than doubled.

I have fed the new X output into the Cossor and I am not sure if it is sweeping. The Cossor is set to ext. FM and a carrier frequency of 10MHz.

I'm not too sure what to make of the output that I see on my 20MHz scope...

[OldTechFan96]

I have been doing more work on the Cossor:

OOT resistors have been replaced around the grid and cathode of V3. Cathode of V3 now measures 9.2V, it should be 12V.

All wax capacitors have been replaced. The old ones tested fine! The smoothers also turned out to be good.

I have been able to get Mark's op-amp PCB to output a 23V. I connected the sweep signal from the PCB into the X input of the Cossor and monitored the RF output with my scope.

I have been able to view various waveforms by messing with the controls on the Cossor and the scope. I can't make heads or tails of them! I know that the Cossor needs to be set to 'ext. FM' for it to sweep.

Next, I thought I should connect the Cossor and scope up to a valve FM radio and follow the IF alignment instructions. I set up everything by the book and nothing appeared on the scope. Increasing the output of the Cossor did not change anything. A curve like waveform did appear with the carrier frequency set to 20MHz or so. I used the crystal probe.

Am I doing anything wrong? Or is the Cossor at fault?

Can anybody please help?