Resistor question please

On my Ampro projector restoration I have come across a resistor that I am not sure about its value; it has a yellow body with what looks like a blue or violet, or even black end, so scruffy as to be pretty much unidentifiable as to exact colour. The schematic is not fully compatible with whats exactly in th circuitry, not unusual with Ampro of such vintage. The resistor reads 0.641 on the M scale. I have replaced it with a 1 M, but I am not happy that its right, too much? any advice please on what would suffice. .Oh and how reliable are canned capacitors, difficult to get at and even read, so would they be okay to leave in?
Many thanks as always.

I suspect that it may be 470 K
Yellow body 4
Violet end 7
If the decimal multiplier (Which would be a spot of band near the middle.) was also yellow (4) then it would not be visible as it is the same colour as the body. The resistor could have changed it's value with age.

Les.

What does the resistor do?, where does it connect to? - resistor values in valve amplifiers are rarely very critical.

Electrolytic capacitors do degrade over time, and even if not faulty would probably require reforming to avoid their destruction when first powered up.

Okay thanks, I have studied this little fellow with a magnifier and can see not even a trace of yellow. Should I go ahead with the 1 M, or would a 470 do it? Looks like I will have to reform these pesky canned electrolytics then. You need the fingers of a 3 year old to get at them! Anyway would appreciate your thoughts on that resistor. Thanks. Oh the resistor connects to the 5Z4G.

Photo and schematic? Even a not-fully-compatible schematic can give clues as to the component's function, and hence its value.

ak

That tube is a full wave rectifier. Pin 8 would likely go to high voltage capacitor. if resistor is also on pin 8 or what it connects to, odds are it is a bleeder resistor to discharge the capacitor, since it is a high value. 470k would be about right, but not critical in that situation.

Oooops sorry, the resistor is connected to the INPUT jack!!!!!! NOT the valve!!

Oooops sorry folks, my mistake the pesky resistor is connected to the input jack socket, NOT the valve!! My apologies.

paddy ryan said:

Oooops sorry folks, my mistake the pesky resistor is connected to the input jack socket, NOT the valve!! My apologies.

Click to expand...

Again you're still not very clear - is it connected from the live input down to chassis (i.e. directly in parallel with the input) - if so it's value is of little consequence, 1 meg would be perfectly fine, as would 470K, or a huge range of values.

"the resistor connects to the 5Z4G." If the tube is a rectifier diode and the resistor is shutoff cap bleeder selected to be much less than the cap leakage. Can you measure DC and AC voltage on the tube cathode pin. This will tell you if the caps are ok under load if the DC is normal and ripple < 20%... If not try to specify the model as it is not this one " Model 472" 1949

Hi Nigel, yes it is grounded to chassis. Happy to continue now, so thanks for the advice as always. I have learnt a lot from this website, as a beginner, and enjoy the learning process at 78!!!

The resistor was likely OK but the large caps likely dried out.

Looking at the schematic in post #10 of this thread part of the bottom of the schematic is missing, I remembered in an earlier thread there was another schematic. It was in post #8 of this old thread.
These two schematics are not the same. The one in the current thread uses B7G and B9A base valves. The one in the old thread uses octal base valves. What type of valves are in the actual projector ? In the schematic on the old thread there is a resistor R2 near the input which in the parts list is marked as .47M (Which is 470K)
In that part of the circuit it's value is not critical as it only has seems to be related to the microphone input gain.
On the topic of distorted sound I was wondering if the problem may be related to the optics between the sound track on the film and the photocell rather than the amplification. I also wondered if the photocel could be replaced with a photo diode or photo transistor.

Les.

Yes thanks Tony. My mistake, the resistor is attached to the jack input, not the valve! The model is the Premier 20, an older model even than the Stylist. I have traced the resistor on the schematic and am happy now. Thanks for your kind advice. No schematic with Ampro seems to match the actual projector. the list of caps and resistors and even valves is always different in real life!