McMichael 365

[delaitt]

Hello,

I've restored this radiogram and i'm doing a number of electrical checks to make sure it is ok. i've been using a variac during the testing phase.

However, when connecting the set directly to the mains, it appears the DC voltage from the rectifier is slightly high as follows:

1- Mains 235V - directly plugged-in
Voltage between the 2 anodes of the rectifier: 750V
DC voltage: 403V
Filament voltage: nearly 4V (for both rectifier and other valves)
DC current: 90mA

2- Mains 220V - via variac
Voltage between the 2 anodes of the rectifier: 700V
DC voltage: 373V
Filament voltage: nearly ~3.6V (for both rectifier and other valves)

3- Mains 200V - via variac
Voltage between the 2 anodes of the rectifier: 625V
DC voltage: 330V
Filament voltage: nearly 3.2V (for both rectifier and other valves)
DC current: 70mA

The transformer has 3 positions for the mains and is set to 240V.

When the set is directly connected to the mains, the filament voltage is almost 4V which is good and the transformer remains cold. However, it seems strange that the AC voltage for the rectifier is slightly high i.e. 750V by default. V5 is a DW4/350.

There is a valve reading table in the McMichael service manual which says that the DC voltage for 200V AC should be 330V. however, it isn't clear what position has been using on the transformer (i.e. 200 or 240).

Should i consider adding a couple of resistors in the two anodes of the rectifier to reduce the AC voltage to the valve rectifier? I find it a bit strange to have to do this.

Thanks

Thierry

[TrevorG3VLF]

The circuit on the CD shows no connection to the HT winding.
The output anode should be 245V.
The output screen should be 255V.
Presumably the valves have 4V filaments, I have not looked this up.

The rectifier is supplied with 350-0-350V. You would expect a valve rectifier to give approximtely the same DC out as AC in.
I would suggest that setting the transformer to give 4V filaments is the way to go.

You could drop excessive voltage with a single resistor connected to the rectifier filament centre tap. The resistor will drop much more voltage than simple ohms law would suggest as the current comes in pulses. The dissipation would also be higher.

Have you replaced the output valve grid coupling capacitor?

[snowman_al]

As Trevor says, go by the heater voltage being correct.

The other thing to remember is that the readings in the service data are for a moving coil meter (AVO etc.) and these were typically 500 ohms per volt or lower. So you would expect to see higher readings with a digital meter as there is practically no current draw by the meter.

I do not have the circuit diagram, but 400 volts at the rectifier cathode assuming it is connected to a small value cap would be a reasonable figure from 350-0-350 ac.

How and where are you measuring the current by the way?

Alan

[Sideband]

Everything looks good to me....I agree with Alan (snowman). Don't bother with the resistors, don't worry about the apparent anomalies. The valves won't be bothered by a 10 or 20% discrepancy. The most critical is the heater voltage. If that is correct, just sit back and enjoy the radiogram!

The voltages given were taken on a particular day, at a particular mains voltage with a (probably) 500 ohm per volt testmeter. Your meter either digital or analogue will be far more sensitive than that so as stated by Alan, will give higher readings. A 10 or even 20% variation is not unusual.