Hi all, I posted else where about the output transformer, which has been replaced and the set works great. One thing I don't understand though is that there is a 516ohm resister (as measured) between v5 and c22, its not on the schematic I have. I have replaced it (because the original had over-heated at some point and lost its shell) with a 560ohm 5w resistor, it naturally gets quite warm. Anyone know about that resistor or why its there and not on the schematic? A later production change not added to the schematics?

thanks
Darrin

If the filter choke is missing someone has replaced it with a resistor.

Have you measured the anode and screen voltage of the 6M5.

The resistor you mention is obviously in series with the main HT line, as it comes directly off the rectifier.

May have been put there due to too much voltage out. Perhaps the mains transformer is not original?

If the filter choke is still in circuit, I suggest reduce the value of that resistor keeping an eye on the output voltage.
JJ

Thanks JJ, the choke is in place. The transformer looks original as far asi can see. The voltage at the resister is 269v and 243v at the Junction with c22.

I have a feeling that while the transformer is original, they used a different one during production at some point and fixed it by whacking in this resister. I think I will attach it to the chassis to help with heat dissipation.

Have you measured the 6.3 volts AC filament voltage?

If spot on I would get rid of that resistor, and measure HT at the 6M5 again.

All should be OK, from what my info is.

Resistors getting hot is either bad design or a fault, or monkeyed.

Notice that you are posting same questions on different mediums. Hard to help if that is the case.
JJ

Thanks JJ, will checks those items. I do post on different things to gather a wide range of input.

My calculations on the figures given indicate the radio is drawing approximately 50mA,

Which is about right. Through a 560 ohm resistor this works out at 1.3 watts.

Hot, but not excessive.

The radio really needs further investigation, especially the biasing components around the 6M5 and the valve itself.

Then I'm sure you could get rid of that resistor, or at least reduce the value to half of 560 ohms.
JJ

Thanks JJ, the filament voltage is 6.59v, the 6m5 anode is 204v. I'd like to remove that resister, just seems a failure point. The original one had lost its shell, probably from excessive heat. If I replace the resister with a link, what should the the 6m5 anode voltage be?

I should also point out that the replacement output transformer has a primary impedance of 5k, which may be less than the original.

Check the valve data on the 6M5, but anything up to 250 volts is not excessive.

Another thought, maybe someone changed the filter choke, for a lower impedance one and have added the series resistance to compensate.

Won't hurt to remove the resistor and do some checks.

Resistors getting hot in anything is bad design and waste of power.

JJ

Or 220 volt radio. Going by filament voltage.
Is there a mains tap?
JJ

I took a picture of the power transformer and looking at the zoomed in picture, I can see now different values have been written over the original printing on the secondary side. The original value printed on the secondary lugs was 250V, has been written over with 265V, so I guess that explains the resister.

Transformer pic: https://drive.google.com/file/d/1SFDhK_YXElALXNCNhmKi_-ljYtGLPt3w/view?usp=sharing

Wonder if I can use the 250/260v tap instead of the current one (220-240v) and lose the resister ?

Yes, and that would bring down the filament voltage to 6.3 volts(fairly important).
JJ

Thanks JJ, will try that later and see how it goes.

cheers
Darrin

As that is showing over voltage, I would shift to the 250V / 260V tap. Then assess the voltages.

If the choke has failed the WW resistor/s to replace it should be the resistance of that original choke. The voltage drop quoted between 6X5 and the Plate voltage will give a voltage drop figure.

The voltage across R7 & R8 "back bias", is the bias for the 6M5. By ohms law the proper current draw of the set is derived from them, as all cathode current passes through them.

There is a voltage chart on AORSM data..