Cossor 480K radio RF/IF Tuning

[Lanman]

Hi newbie here.
I am restoring a Cossor 480/470/835 vintage valve set. From no output I have managed to get a very weak output. By changing V4 an OM4 valve and several defective components.

Trying to align the tuned circuits with a signal generator that goes from 0Hz to 12 Mhz.
Successfully aligned the IF stages, then came the RF and Oscillator stages which requires an 18Mhz signal input.
Here,s where I have a problem as the sig geny only goes to 12MHz and it is quite expensive to buy one capable of 18Mhz.

Does anyone know of an alternative method of aligning these stages without an 18Mhz signal.
Any thoughts greatly appreciated.

Thanks

Llewellyn

[ppppenguin]

Hello LLewelyn and welcome to GVR.

These sets from 1947 are covered on Trader sheet 835.

Unless somebody has been twiddling, it's relatively unlikely that the alignment will be far out. Your problems with weak output could easily be due to reasons other than alignment.

Fairly obviously you can align MW and LW without a source of 18MHz. If you really need SW then you cold try aligning by ear, for maximum output on received stations. your signal generator may also produce harmonics, so you could try it at (9MHz or 6MHz to see if you get any results.

I assume you've changed all the wax capacitors. Especially C22, 0.01uF (Trader 835, other sources of data may vary). If this capacitor leaks it will likely damage both the output valve and transformer.

[Lanman]

ppppenguin, 
Thanks for the info, Yes the first thing I changed was C22 . Tuned the Sw band by ear, but it was pretty well on track. 
No harmonics were detected at 9 and 6Mhz, even on square or sawtooth outputs.
I will continue with analysing the gain overall to see if I can improve it.

TA

llewellyn

[ppppenguin]

That's a fairly unusual signal generator if it can produce square waves at 6 and 9MHz.

[coopzone]

Hi,

I would suggest you chop the problem up a bit. When you say low output it could be in the RF stage but it's just as likely in the audio or detector stage, so try and test each stage as you go. for example:

1, start with voltages compared against the trader sheet, Any low? fix this first.

2, audio. Is the speaker ok - you can connect another one or connect it to another radio maybe? I mention this because in the past i had a radio where the speaker had gone rusty! and the cone had very restricted movement - end result very quiet.

3, If it's a AC only set and has a GRAM/Record deck input connect an audio source to that - do you get plenty of volume? If it's an AC/DC set and ONLY if you signal source  is battery operated (without earth, i use an old mp3 player) you can connect it via a 0.01 to the volume control. You should also take all the other precautions needed to stay safe if doing this.

If it's low audio at this stage you know you need to start on the AF side of things, in particular check cathode resistors etc.

4, Time to start the RF side if this has not pointed you in a direction yet. Feed in your IF frequency, preferable with modulation. Check for signal at the volume control using ether a scope, bench amplifier (i use a pcb lm386+9v+speaker cost about £2.00 from ebay), or an old crystal earphone - be careful it can be very loud.

5, Last check RF amp/mixer stage. Mostly looking for voltages to be correct, you probably need a scope to see more. But it's worth checking connections, switches, coils (for o/c) etc - again i had a DAC90 where it worked but low signal and it turned out one connection to the frame areal was loose. Quick clean and replace all was good.

Hope it helps, remember always try to break a problem down into smaller chunks.

Derek

[Terry]

Hi, Derek, welcome to the forum where I'm sure you will be very happy.

I'm a little confused by this, though:

4, Time to start the RF side if this has not pointed you in a direction yet. Feed in your IF frequency, preferable with modulation. Check for signal at the volume control using ether a scope, bench amplifier (i use a pcb lm386+9v+speaker cost about £2.00 from ebay), or an old crystal earphone - be careful it can be very loud.

Surely, if the AF stages are in perfect working order at this stage, you've already got an audio monitor so, no further facility is needed?

When checking the alignment, it is vital not to let the AGC mask the true peak. It is possible to override the AGC but is not usually necessary on a radio if you follow the correct procedure: keep the volume control setting high (you might want to turn it down briefly when operating the wave-change switch) and the RF level as low as possible. If the volume increases whilst adjusting the alignment (assuming it is necessary), reduce the RF level, not the audio level!

This also applies to checking the alignment without a signal generator: find the weakest stations you can and, if the volume increases appreciably, find an even weaker one! (Sets fitted with frame or ferrite rod aerials can use the directional properties of these aerials as an RF level control by rotating the set to get the desired, low, level signal)

[JohnBS]

Is your sig gen a DDS jobby which will actually produce a half-decent square-wave at 6MHz? Do you have a 'scope to check the output?
At 6MHz square, you should have a decent signal at 18MHz, (9MHz no good as there's no second harmonic in theory).
I'm glad you managed to check the IF without some disaster occuring, as many of these old sets have big issues with the tuning slugs (falling off brass screws or breaking etc etc) and it takes a long time to repair!
John