20-10-2015, 04:02 PM
(19-10-2015, 06:19 PM)Terry Wrote:(19-10-2015, 04:04 PM)Nowhere-Man Wrote: ... After a few moments, I finally realised the radio was silent but definitely alive with juice. The valves were clearly glowing and I got a 400 VDC reading from the H.T. Smoothing Can. I also noticed on powering off the can drops kind of slow and gradual. So, definitely not wise to touch anything with power off as the caps charge quite well ...
P.S. I ought to test the output transformer too.
The exceptionally high HT voltage plus the slow discharge points to there being no load.
It would be a good idea to check the primary of the OP Transformer, as you say, but I suspect that this not the fault.
Having no idea of the age of the set, the connection between the smoothing and the reservoir sections of the smoothing can may be either a resistor or a choke, and I would guess that this is where the problem lies.
Older HMV sets used what is often incorrectly called a 'mains energised speaker'. This is not connected to the mains at all but what makes it different is that it doesn't have a permanent magnet like most speakers do. Instead it has an electromagnet which also doubles as smoothing choke.
If your set has one of these, you should check it for continuity.
Do you have a circuit diagram for this set?
The model number will also help others to help you - just HMV on its own is of very little use.
This is the HMV 1124. Should you wish to take a look at the schematic or any schematic, just google up Radio Workshop and it's easy to access any valve radio.
O.K. so now I've found out what is wrong. You are basically right that the voltage was way too high. It now turns out I have no H.T. output across the H.T. smoothing caps. This has never happened to me before and is a new experience to get to grips with. I did test the inline smoothing resistor to be sure this wasn't a factor. That resistor is a few Ohms out of spec but not crucial. The bottom line is when I power up the radio, the rectifier sends a high voltage to the first H.T. Smoothing cap but there is no output. There should be a voltage through the wiring from the first to the second H.T. cap whereas there is nothing.
The schematic gives 3 smoothing capacitors. The two big cans are 16 MicroFarads at 400 volts DC. The first can is showing a high voltage on only one terminal but the outgoing terminals show no voltage and it's the red wire from that point that powers the set. What could have happened?
My job now is to make a new smoothing circuit and take it step by step.
I've sort of tucked this thread away from the main Valve Radio Section with a view to making the odd observation or maybe even digressing to some theory discussion on old radio design. Not sure if it's of much interest but this is where I'm at the moment - a basic H.T. issue.
