Rectifier No.205

[BakeliteBear]

Does anyone know what the modern equivalent for the old GPO rectifier No.205 is? Does anyone still make them?

[BakeliteBear]

Since know one seemed able to help I contacted Bob Freshwater who kindly supplied the following information:

"A rect 205 can be made from two 1n4001 diodes wired across each other but inversely. i.e. in parallel but with one side + and – (the other side the same)."

[russell_w_b]

I would've thought that any silicon diodes back-to-back would do. It's only to clip off excessive audio when the amplitude reaches 600mV or so (transients caused by dial or cradle-switch clicking).

[Nickthedentist]

They can be soldered to the print side of the PCB between T1 and T2 if an "original" look is important.

I fitted them once and have to say that the effects didn't seem as good as the proper device... but I didn't have time to test the diodes, replace them etc. so my findings may be irrelevant.

Let us know how you get on.

Nick.

[Pete_kaye]

I did this on 2 of my 332 types and the effect was good using IN4001s.There was still some clicking on lifting/replacing handset but not unpleasant.I had to solder extra leadson the ends of the joined diodes then cover in tape.

[BakeliteBear]

Firstly - thanks for all your help.

I have had a play and have created my version of the rectifier as shown in the image. I don't have any originals to compare with, but it seems to be OK.

I would like some help with the electronics here though. My diode theory is v.poor and I am not really clear how they operate in the circuit. Can anyone help?

Also various people I have spoken to, have quite contradictory views as to whether fitting them in 200 and 300 series telephones is necessary or desirable. Indeed, one dealer I know takes then out of the 700 series telephones.

I have been told the following.

1. They are quite unnecessary in the 200 and 300s and that if the dial contacts are clean and the telephone wired correctly there should be no extraneous noise.

2. Fitting the diodes can have a detrimental effect on speech volume.

3. They should be fitted to 300s, but not 200s – although one of my friends tells me that they always fit them in 200s without bellsets.

Can anyone offer some clarity to this fog??

[Nickthedentist]

Nice work, top marks!

I certainly find they make a worthwhile difference in 700-series instruments.

Were they ever retro-fitted to earlier 'phones by the GPO? It would be easy to do some simple experiments to see if their effect is worthwhile.

Keep us informed, please.

Nick.

[Darren-UK]

Simple answer. If your telephone has clean dial contacts and doesn't 'click click click' to the point of being irritating - then don't bother with the diodes. If, on the other hand, the contacts are clean but the clicking is an irritation - then fit the diodes.

Which telephone are we actually talking about here, by the way?

[Nickthedentist]

Even when the dial contacts are perfect, I find these rectifiers prevent a horribly loud "crack" when operating the gravity switch to clear the line before making a follow-on call.

Nick.

[BakeliteBear]

Thanks for the encouragement Nick!

In this instance it was a 332.

I agree that the ‘trial and error’ approach is probably the best practical solution, but I would like to understand the theory behind it if anyone can help. Surely all models of a paricular telephone should behave in the same way?

Incidentally a missing strap from the dial terminals on the chassis between 2 and 3 will cause horrendous noise - so its one to look out for!

[Station X]

In theory these diodes are not needed, as the receiver is shorted out by the auxiluary dial springs during dialling. However there is always some slight pulsing noise due to induced currents through the receiver. Also if the auxiluary dial springs are dirty or maladjusted the shorting is ineffective.

Diodes don't conduct until the voltage across them reaches a certain value. This value is low enough to permit speech currents through the receiver, but high voltage pulses are short circuited. Two diodes connected back to back are needed as speech currents and high voltage pulses can be of either polarity.

Personally I like the sound of dial pulses. It's all part of the charm of an old phone.

[BakeliteBear]

Thanks Graham - that very helpful.