Hi there.
I'm hoping one of the smart and clever people here can solve a bit of a quandary for me.

I'm currently working on restoring a CALSTAN 529 5 Valve radio. Its my first restoration and I'm struggling a little in following the circuit.

My current issue is the centre tap of the power transformer. The circuit says it is connected to earth via a 300 ohm resistor and that there is another path up to the grid of the 6V6 power tube and through a couple of capacitors to one side of the choke.

The question I'm confused about is why does this centre tap go to the plug on the back of the radio that goes to the speaker and just loops through this plug without connecting to anything in the speaker. This is the first thing that happens to the centre tap on leaving the transformer. On the other side it appears to go where the circuit says it should. This means that if the speaker plug is removed the centre tap is removed from circuit.

Does this serve a purpose of which I am unaware?

I've sent Brad a copy of this part of the circuit together with a picture of the speaker plug.

Yes I know I got the masking around the plug a bit dodgy. Next week.

Thanks in anticipation.

Peter

That arrangement was to remove B+ from the circuit in the event that the chassis was powered up while there was no speaker load attached. A safety measure to ensure that the output valve didn't run red in the face trying to work into a load of infinite impedance.

Photos uploaded.

GTC: Many thanks for that. That makes sense.

While I have your attention. I'm also struggling to understand the values of the resistors on the circuit.

Are they fractions of a Meg? .1 .12 .5 etc.....

Thanks

Peter

Normally the early circuits were in Megohms on the 6V6 grid .5 (0.5) is 500K and check them, normally the grid resistors on 6V6 and the detector 1st audio plate are cactus.

Check all resistors as best as, when you rid the set of paper caps. If the 300 ohm back bias resistor (CT to chassis) is out of spec to not replace it with a higher wattage one. Fuse is a decoration and only for the primary side. The back bias resistor is more effective where there is a HT overload.

That's a "shielded" transformer shown. That's why a grounded cable is best.

Marc

Possibly. Read the colour codes and go from there.

Marcc: When you say the fuse is for decoration. Do you mean the one on the back panel. I've put that there on the active line as it enters the chassis. The 300 Ohm resistor is spot on in value. It is made up of two 150 ohm wire wound resistors in series.

GTC: A number of the resistors do not seem to have any colours on them so I assume they have faded. They are just a brown colour.

I've replaced all the capacitors but I am yet to attach the broken speaker transformer lead.

Cheers and thanks

Peter

A number of the resistors do not seem to have any colours on them so I assume they have faded.

They most likely won't be too far off spec. Old carbon resistors tend to go high as they age, so you'll likely find them above what they should be, but usually not all of them will be too much above spec.

The problem with a fuse in my experience when its on the primary side of the old transformer radios is, that tends to only protect that side it has to be a certain size, or in some cases slow blow to protect itself from inrush current and rarely if some thing goes awry on the secondary is it any more use than putting an ash tray on a Hogg.

The number of things I have seen do a melt down secondary side without affecting fuses & circuit breakers is amazing & that incudes the last act of the UPS, which has now been modified.

I am concerned in respect of two WW back bias resistors WW are normally high wattage. What I prefer, and what most sets had, where there was back bias; were resistors pretty much "sailing with the wind". AWA often used (not only for this purpose) similar resistors in the plate circuit of some rectifiers.

The object here is, that when there is a short circuit, or overload it acts as a "fusistor" and fries, rather than burn out the transformer. 300 Ohm at 13V means around 0.6 watts which means that exceeding 1Watt is excessive. Are they original?

Marc

QUOTE: Are they fractions of a Meg? .1 .12 .5 etc.....

Marcc: I've asked Brad to post a photo showing the resistors. However I believe that they are original. Not sure I understand what you mean by resistors "sailing in the wind".

Thanks

Peter

With resistors there is an important thing called, I (current) squared by R (resistance) = Watts. That tells you the wattage the resistor has to dissipate: Not the wattage of the resistor. The ramifications of this, is normally measured in smoke density. Viz if "W" exceeds the rating of the resistor it fry's.

So "sailing with the wind" means that one uses a resistor on the back bias, or rectifier plates, that is as close to the limits of its rating as you can get, so that if the current becomes excessive the resistor acts as a "Fusistor" and burns out, instead of the transformer.

Actual "fusistor's" were made expressly to fry on overload as a circuit protector.

If the resistor is actually Megohms it will be labelled Meg. So 1.2 Meg will be written 1.2 Meg.

Marc

Photo uploaded to Post 11.

Methinks that there has been a problem in that area that thing in a clamp looks like an accident. That black WW resistor type, I have seen in Japanese signal generators around the 60's but might date earlier. I suspect that they are far from original. Astor would be a brown one as in the pic.

That 50K in the pic & 500K do fail high regularly, especially if associated with an out put valve. The choke is 14 Henry 60mA. So if we said it was drawing the full 60mA (which it won't) through the 300ohm resistor it would have to dissipate 1.08 Watts.

I would leave it but! There may be a wattage value on them, or the parts list. 528 which has a filter variation used one Watt. That is a resistor in a set that you do not ever make 'heavier' in wattage. Modifications have ramifications.

Marc

A question .... the 2 x 4μF electrolytic (??) caps in parallel on the Back Bias line in the circuit diagram ... are they orientated correctly??