Bush AC31

[Radio Tech]

Hi folks,

First seasons greetings, now I have got hold of a Bush AC31, the cabinet in a sorry state what with scratches and minor chips. On starting work on this the glass scale was dropping out and the loudspeaker loose, I took the LS out and after clearing the dust, I noticed the cone was rather dry, I managed (I hope) by gently rubbing some nivia hand cream on the cone I managed to salvage the LS. Of course all the usual waxies got replaced and the electrolytic caps replaced, the one thing that bothered me was that for the main smoothy which was supposed to be 32+32 I only had a 50+50. One of the pins in the output valve socket broke so a fiddly job replacing that pin.

Will put some pictures later

Ken

[Silicon]

I have noticed that some speakers in Bush radios have brittle cones.

I assumed it was due to some kind of 'Bakelizing' process.

Surely there must be something better than Nivia?

[cathoderay57]

Originally Posted by Radio Tech

the one thing that bothered me was that for the main smoothy which was supposed to be 32+32 I only had a 50+50.

Hi Ken, the max reservoir capacitance for an EZ41 rectifier valve is 50uF, given a minimum series resistance of 325 Ohms. I don't know the series resistance of the transformer HT winding, but you are probably OK, if a little close to the max that the rectifier can handle at switch-on. Probably not a good idea to turn the radio off then on without waiting a while for the rectifier heater to cool down. Jerry

[cathoderay57]

Hi again. Further to my Post #3, the Trader Sheet #1066 and the Manufacturer's Data Sheet show a single HT secondary with no centre-tap with both anodes of the EZ41 strapped together for half-wave rectification. The winding resistance is given as 140 Ohms. The Valve Museum data sheet gives, for EZ41, a required anode series resistance of 325 Ohms for a 32uF reservoir capacitor. Since you are using 50uF, to safeguard the rectifier, I'd recommend inserting a 150 Ohm 5W resistor between the rectifier cathode and the first smoother, or you could insert it on the AC side between the transformer HT winding and the anodes. I don't think the position would make that much difference. Cheers, Jerry

[ms660]

The Bush AC31 manual I'm looking at is confusing, the rectifier in one of the pictures in the manual is shown as an EZ40, the specifications quote an EZ41, the voltage tables quote an EZ40, was there a specification change?

Lawrence.

[cathoderay57]

Hi Lawrence, interesting, never noticed that. The total HT current is shown on the Bush data sheet as 53mA so I guess an EZ41 would have been working fairly hard. Jerry

[Radio Tech]

Hi Jerry

Thanks for the advice on the rectifier, I will search out the 150 Ohm 5W resistor and insert as advised. I must admit some of the information given in the trader sheets are a little confusing.

I have powered the set up but trouble is now there is howling as you shift the tuning, at first I thought it was the EBF80 which had gone microphony but I am having second thoughts about this. I haven't got a sig gen to ensure the IF's are aligned correctly so its going to be hit and miss. Is there a way of checking this without a sig gen? I note Silicons comments on the hand cream I used for the LS cone, I have in the past always found this works for cones that have been exposed to excessive sunlight and heat.

More to follow.

Regards

Ken

[cathoderay57]

Hi Ken, usually the cause is bad grid 2 decoupling capacitors on the frequency changer and IF amp valves, but you say you've replaced the waxies so it sounds like they have been done. make sure, if there are any brown or black plastic cased Hunts capacitors, that they have also been replaced because they are also paper types and invariably leak just as badly as, if not worse than., the waxies. Also the paper decoupling caoacitors on the AGC line to the grid 1 circuits of the frequency changer and IF valves can cause trouble if leaky. The other thing to check is that the frequency changer and IF valves have clean pins and sockets, and that the wavechange switch contacts are clean. Also check you have't got a dry solder joint particularly decoupling capacitor leads going to chassis. Cheers, Jerry

[Radio Tech]

Thanks Jerry

I have shelved the project at moment due to busy times in my caring role, when I am not quite so busy I will go back to it and also do all the scratches on the cabinet., So watch this space.

Ken

[Heatercathodeshort]

Torn or very frail speaker cones respond well to a 50/50 mix of water and WP woodworking adhesive applied with a fine hair brush. John.

[Radio Tech]

Hi Folks

Happy New Year to all.

Now here are some pictures of the set in question, you may notice that I have replaced a number of those waxies, I also mentioned the set howling when being tuned, I think I may have found the culprit, a 0.001uF from the anode of V1 (hexode section) of which I missed as it was hidden so well, problem is now getting the replacement in place, te soldering iron bit is a little short to reach comfortably so got to use a bit of my technique to get it in.

Regards

Ken

[Radio Tech]

Hi folks

I have replaced the main smoothing cap with a new 33+33uF much shorter and fatter than the old can type. I have also replaced the coupling cap to the grid of V1 a 1nF (1000pF) but the set still tends to squeal when tuning from one station to another, I am wondering if the EBF80 has some particular fault with it causing this, it's got me stumped at moment which is unusual.

Best wishes

Ken

[Radio Tech]

Hi Folks

Had another look at this troublesome set, I have as a matter of course gone through this resistors and found five of them extremely high in the value they should be. I checked the HT voltages around the valve bases and they seem around 10V higher than they are stated on the circuit diagram, that said the cathode voltage on the triode section of the frequency changer should read around 3V according to the diagram but this reads zero, the associated resistor is ok, all this was measured with no signal input, so any help on this very much appreciated.

Ken

[ms660]

3.3v when switched to gram otherwise 0v.

Lawrence

[Radio Tech]

Hi Lawrence

I didnt realise the set had to be switched to gram,the voltage at that point is ok, the strange thing I cant work out though is if the freq changer is removed there is still a burble sound as if something is trying to oscilate, I have changed all the caps that would matter and I havent got a spare EBC80 or EL41 to see if either of these valves are causing it. The HT votages are around 10V higher than they should be.

Your thoughts

Ken

[ms660]

Check all your work, de-couplers, ground connections etc.

Lawrence.

[John10b]

Ken for clarification, are you able to tune in and listen to a station? I note you said that you have instability when tuning between stations. Thanks.
John

[Radio Tech]

Hi John

It was the fact that the coupling caps on the triode / hexode were messed, and most of the resistors had gone very high in the stated values, I had to change all the resistors and caps in the end and also ensure the pins on the valves were clean. The set is back in its cabinet so all that needs to be done is touching up the scratch marks . I must admit this has been one of the worst sets I have ever restored and got working.

Regards

Ken

[John10b]

Hi Ken. Glad to know you got it working ok.
John