Needed circuit diagram for Codar PR-30 Preselector

[daro]

I brought an Codar PR-30 preselector from a local Ham-fest & it was missing the EF186 (6EH7) so I replaced the valve with one I had in my stock of valves & I tidied up the wiring & replaced the 2 Belling & Lee connectors with BNC connectors.

I am after an circuit diagram for this unit because the RF gain control pot has gone O/C & the pot has been replaced before & the replacement is an 5OOK "C Law" pot with an 2 pole switch for turning on & off the power to the unit.

The model I have is the one with the built in AC power supply (PR-30X), Other then that the unit works well but I can't control the gain of the amp as it now only works flat out when the pot is turned up on full.

Can anyone out there help me out with this problem, I can only find on the NET a 2 page instruction sheet from BAMA site & it doesn't have a circuit diagram.

Thanks in advance
Dave Carr

[Oldtimer74]

Sorry cannot help with the circuit but if the RF gain control is a pot in the cathode of the valve (most likely and easy to find out by inspection ), it should probably be about 5K. Preferably use a wire-wound one as it has to carry some current.
Pat G3IKR

[Top Cap]

Hi Dave, I tried to draw the circuit many moons ago but was a bit stuck with the front end, the coil taps are a bit of a mess to try to reverse engineer.
However I came up with the attached which may help.
Les

[Top Cap]

Hi Dave, I managed to find the circuit of the PR40 which I believe has an identical front end. So I have changed my original effort to reflect this and at the same time appended the original circuit. You should be able to follow the circuit better. You will see that L1, L2 and L3 are switched coupling coils with a common that either goes to the other antenna terminal for dipole connection, or it is grounded to earth by the antenna switch (S3) when using a long wire. The output coils L4, L5 and L6 are all wired in series but switched as required by the other half of the range switch (S2) which grounds the coils as required. The feed to the tuning capacitor and the grid of the EF183 is taken from L6.
Best Regards
Les

[daro]

Thanks for the diagram Top Cap, I noticed that the pot controls screen grid voltage on the valve.

This is very similar to controlling one of the gates on the duel gate MOSFET and the other gate was used to apply the signal.

[Oldtimer74]

Ah a different method from what i expected. I am surprised though that you say the set doesn`t work unless the pot is turned to maximum. this suggests to me that either the pot is faulty or possibly the bypass capacitor is leaky thus excess current is being taken. So that a slight change in the pot reduces the voltage very considerably and hence gives the symptons you quote.
Pat G3IKR

[daro]

Pot is O/C, I checked it with an DMM we only get voltage when the wiper is touching the end which has HT on it.

I might try an old trick from the old days, that being dismantle the pot & apply some graphite from a lead pencil on the faulty end & see what happens.

[Top Cap]

Check out the 10nF decoupler on the wiper of the pot just in case it caused the demise of the pot. I am assuming that the track may be damaged but it could also be a poor rivet connection to the track. A magnifying glass may be useful in determining the cause.
Les

[daro]

Track was damaged so I checked the cap off the wiper & it is good.

I repaired the damaged track with conductive silver paint & unit is now working fine again.

[JohnBG8JMB]

There appear to be 2 versions of pr30 coils- I have a photo showing 3 separate coils, but ccts here show 1 multi-winding coil. Is this the same as the CR70A rx rf section?. The PR40 used a FET and a transistor - I have the cct
John

[Top Cap]

Hi John, the input does indeed consist of three separate input circuits. The aerial on range 1 is coupled to L1, on range 2 to L2 and on range 3 to L3. When the slide switch is set to Dipole position, the dipole aerial is connected directly across these input circuit windings by one half of the switch. Only one pole needed as L1, L2 and L3 are common wired to the other half of the dipole. When long wire is selected then this common end of L1, L2 and L3 will be grounded by the slide switch. The secondary coils L4, L5 and L6 are wired in series but shorted out by the other half of the switch. In position 1 all windings are connected to the variable capacitor. In position 2, L4 is shorted so only L5 and L6 are tuned. In position 3, L5 is shorted so only L6 gets tuned. Hope this clarifies the situation.
I firmly believe the FET version is made the same with regards to the input circuitry.

[JohnBG8JMB]

What I ment was, are L123456 all wound on the same former [like cr70a] or on 3 separate formers? my photo shows 3 separate formers but the ccts here show only one.
John'

[Top Cap]

Hi John, the former on my PR-30 is just one former, it is one of the celebrated air cored inductors that is a series of coils supported by four plastic rods (for want of a better word). The wire is melted into the plastic, I think the tank coil in the AT5 transmitter was similarly constructed.
Les

[JohnBG8JMB]

Les Thanks - I think there must have been several versions
john