Nombrex 27 Signal Generator

[Justanoldgit]

I gained this sig gen in it's original box but it had been dismantled so I assumed it had problem. There was no history with it but I could see that the output attenuator pot had been replaced and most of the dust cores looked as though they had been recently tweaked.

On first powering up it seemed to work well on ranges 1-5, range 6 worked but not quite as it should. Ranges 7 & 8 were awful, the RF amplitude shot up and down throughout the range and the oscillation stopped altogether on the lower end of both ranges, apart from signal level it would stop oscillating when the main tuning capacitor was about half meshed.

I checked all resistors and apart from R1 & R3 having different values than the circuit diagram I found nothing wrong (all resistors and caps were the factory fitted ones).

I checked the spacing of the vanes of C6, tried replacing TR1 (twice) even played around with the bias resistors of TR1 but nothing changed so I cast it all aside for another day.

After a couple more wasted sessions with it I tried again but this time I hooked my scope onto the junction of R4 & C5 and suddenly things improved, the additional loading of the scope probe was actually helping it work.

I checked through the components on the emitter of TR1 and eventually found that the new pot that had been fitted was a 10K when it should be 400 Ohms so I soldered a 470 Ohm resistor across the pot and everything works as it should. Now awaiting delivery of a 470 Ohm pot before I can reset the alignment. If only I had checked the pot in the first place !

[mickm3for]

Hi welcome from me i live just up the road from you if that is Rochester kent nice to have another member in kent. Glad you fixed it

[Justanoldgit]

Thankyou Mick, yes definitely in Kent. I see from your username you are into amateur radio, I followed that path for a while, got as far as my intermediate licence but then ran out of time, interest and money. I just seem to be juggling too many balls at times.

Ray

[Justanoldgit]

To update my original post ... I fitted the new pot and things were greatly improved but still not quite right, range 7 refuses to oscillate below about 70MHz and the same on range 8 below about 120 MHz (harmonics, so much the same problem).
As it works on range 6 down to about 62 MHz the circuit is capable of working and the fault can only lie with the coils/capacitors of ranges 7 & 8.
I would be interested to hear if anyone has one of these generators that works properly on ranges 7 & 8 just to make sure I am not trying to solve a design fault.

On searching the net for information on these generators I have found various bits of conflicting information especially regarding the values of TR1 base biasing R1 & R3.

It appears there was an early version of the Nombrex 27 which had 6 coils enclosed in cans, they still managed to 8 ranges from this setup although the range coverage is slightly different. The circuit diagram is also different and the 2 resistors are 3.3K & 18K.

On later versions with open coils, these resistors became 1M & 150K although on the new diagram they are still shown as 3.3K & 18K, these values will not work in the new circuit.

I then found a circuit diagram for a Nombrex 31 which is almost the same as the 27 but with a prettier front panel and slightly different AF circuit, the RF side is identical. On the circuit diagram for this they show R1 as 1M and R3 as 150K, right values but the wrong way round which again doesn't work.

So if you are playing with one of these just remember R1 should be 1M and R3 should be 150K.

[Justanoldgit]

I wish I could find the "edit post" option !

Even I swapped the values when mentioning the model 31 circuit diagram.

This is how the incorrect diagram for the 31 looks.

and the placing of the resistors of an unaltered model 27.

[Gridiron]

I had a look at the Type 27 generator I have and, although it has open coils, R1 is 3K3 and R3 18K, factory fitted. It works on all bands up to the broadcast FM band (highest I could check easily) although the calibration accuracy is not great.
Mike.

[6SN7WGTB]

I acquired a Model 31, and have replaced the resistors as many were very wayward.

I have also adjusted the calibration and it's reasonably tidy now.