Bush AC41

[Sinewave]

So I relflowed what looks like to be a dry joint with C10 and the mod tone can be heard, but very quietly, it's still much stronger further along the scale. If I leave it at 500m and adjust the generator its stronger at about 470k which is the intermediate frequency. So something's still a little out somewhere.

[Sinewave]

Despite that, there are a handful of stations I am picking up reasonably well, despite using some speaker wire as an antenna which isn't quite properly placed. I'm also adjusting the MW cores whilst tuning to a station, without the generator and comparing it to another working radio, since on MW the mod tone isn't quite strong where it should be.

I don't have a proper dummy antenna, so for now I'm just using a 0.1uf cap.

[cathoderay57]

I've attached a sketch of the dummy aerial used for medium frequencies on the Avo All-Wave Oscillator. Jerry

[ms660]

A standard dummy antenna should be fed from a 10 Ohm impedance, that means a matching network should be fitted if the output impedance of the signal generator is more than that.

Details for connecting to a signal generator that has an output impedance of 75 Ohms are given in the link.

Lawrence.

[Sinewave]

Thanks. I'll have to see when I'll be able to get any inductors.

In the meanwhile, I do have an Avo All wave, I can't remember how 'ready' it is, but maybe I could use the dummy antenna with my Marconi TF144H/4S set.

[Sinewave]

Ok, my Avo all wave dummy antenna is working fine.

I checked C10, will need to make some other checks and work out why I can't get the tone where I should on the L9 and L4 adjustments.

[John10b]

Glad you are making good progress, if you can’t “peak up” the IF, it is probably one of the capacitors across the coil, as mentioned earlier.
You can gently remove the IF Can, and take a look at the capacitors, it could be a bad joint on the rivet of the capacitor or, as I have found in the past, a fractured lead from the capacitor itself.
John

[Sinewave]

Thanks John.

The IF peaks, but it's the MW RF which doesn't. Do you think that there could still a hidden issue in IF linked to the RF side not peaking on MW? I can peak on LW fine.

[John10b]

Didn’t read the thread properly, I would suggest the IF,s are ok, from what you say, that’s good. So the problem could be switch contact or one of the capacitors across the coil, I think this has been already suggested, so more fault finding is required, so keep going you have made excellent progress so far, now time for Rugby.
John

[John10b]

I should have said you should try adjusting C10 , as suggested by Jerry first, you may not have a fault. I have the Bush Service Manual not the Trader sheet so the components are numbered differently.
John

[Sinewave]

I think there's a few different drawings about. From what I see on mine, C10 is a fixed value, 515pF?

[cathoderay57]

Do you have the means to test the value of C10, or substitute a replacement capacitor (a 470pF in parallel with 47pF is close enough, preferably silver mica). Jerry

[Sinewave]

I've got a capacitance range on my meter, but not a proper capacitor tester.

I do have the values you've suggested, but in film/foil.

[Sinewave]

Tried those caps in parallel, didn't make any difference. Unsoldered the tab and cleaned everything up to make sure the grounding is fine and clean, still no difference....

[cathoderay57]

I confess that I'm not sure that I properly understand the fault symptom. It would appear that while frequency calibration and signal strength are OK on SW & LW, the sensitivity drops off at the low frequency (long wavelength) end of MW. Am I right? If so it would be helpful to align the oscillator such that you can receive the tone at 500m when injecting an AM signal from the Sig gen at 500m (600kHz) albeit weakly. Then tune the receiver and the Sig gen up to the HF end of MW, stopping at a couple of points along the way, and advise how clear the tone signal is at each waypoint, and whether the wavelength on the receiver scale at each waypoint is within calibration. If everything sounds good apart from weak signals at 500m this could simply be that the coil(s) has/have lost Q owing to damp ingress. Hope that helps to clarify things ( for me, at least). Jerry

[Sinewave]

I've been trying to find some time to get back onto this. The sensitivity drops off at the higher frquency and of the MW band.

I've just been checking the AGC decoupling resisitor and cap, cap which has been replaced.

I'll hopefully over the weekend make the other checks suggested and report back. If the coils have lost Q, then there's not an awful lot we can do about that.