fitton: 551; Ambassador

Hi Folks,

I have purchased an Abassador 551 which I am going to attempt to restore.

As I am not very experienced in radio repair, I would appreciate any help and advice

as to the correct proceedure to go about replacing parts that are suspect, and also advice on a source of radio bits and pieces in the UK

On looking at the underside of the chassis I've noticed that all the old wax

condensers/cpacitors are in bad shape and need replacing, there are also a couple

of 22k wire-wound resistors that seem to have overheated. The rest of the under

chassis is quite clean and tidy.

I have downloaded and printed out the schematic and parts list.

Any help/advice will be very much appreciated.

Ray Bancroft.

 

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Hello Ray,

As your radio is 60 years old, most if not all of its tubular capacitors will have become leaky and they should be replaced. The ones to look out for are those which are tubular in shape and have a wax coating or are brown 'Hunts mouldseals'. These will have capacitance values of between 0.001uF and 0.1uF. All the capacitors in your set are listed in the service manual and are complete with capacitances and voltage ratings. Some capacitor values for your set are no longer widely available, so you will need to obtain the closest values still available as follows ....

0.002uF  use a 0.0022uF

0.005uF  use a 0.0047uF

0.02uF    use a 0.022uF

0.05uF    use a 0.047uF

When replacing capacitors ensure that the voltage ratings of the replacements are equal or greater than the originals, so if one has a voltage rating of 1000v then replace with one rated at 1000v or if a capacitor has a voltage rating of 350v then one rated at 400v or 630v will be suitable.

Replacement capacitors are available from many UK websites but I usually buy Vishay or LCR polyester/polypropylene axial capacitors from Farnells. Justradios in Canada do however keep most of the old values and if the order is no more than £15 it shouldn't attract import duty, VAT and an £8 post office COD delivery charge !  

Silver mica capacitors which are generally flat in shape and with capacitances measured in picofarads (pf) rarely fail so leave them.

Electrolytic capacitors, which have capacitances usually greater than 2uF, may or may not be in good working order so I use an ESR meter and a bridge to test mine. The large electrolytic smoothing capacitors often 'reform' when the set is first switched on and will work OK but if they get hot (switch off immediately and stand back) or there is unacceptable hum audible then they may well be worn out and you will have to replace them. These can be difficult to match, I usually obtain NOS originals if available or good secondhand ones from specialists. If you don't have any capacitor test equipment then just replace all electrolytics if the set doesn't work well. Don't forget that electrolytics are polarised. so make a note of which are the  '+' and '-'  ends before removing them.

It's not unusual for resistors in the power supply to get hot, I suggest that you check their resistances and if more than 5% out of spec then replace them with new ones of the same wattage rating.

Good luck with your restoration.

Regards ............. Howard

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