Hey guy's,

I need to replace the Selenium Rectifier in my Healing 400h, and I need a little help calculating the resistor value to drop the voltage out from the silicon diodes that I'll replace it with.

I figured there's is about a 10V drop on the selenium compared to a silicon. I'm just abit stuck on what the current draw would be on the 1R5 1T4 1S5 3V4 tubes the unit uses.
Need to know to use ohm's law

Cheers

The second sheet in AORSM's has the voltages and currents drawn.

Do note that on mains that is a dangerous animal. Isolation transformer recommended.

On mains it draws 17 Watts at 225VAC mains, rectifier output 185VDC 16mA

Voltages measured with a 1000 ohm per volt meter.

I believe the schematic shows a 7.5 volt drop across the type MU58 selenium rectifier.

Take heed of Marcc's warning as that set has a "live chassis". Electrocution can lurk in such sets for the unwary.

Although Marcc has quoted the requested figures from the AORSM, this short video may be of general use on the subject:

http://www.youtube.com/watch?v=ZLgUFG3zmfc.

It's not as straightforward as the current flow isn't continuous. The current consists of large peaks (at the top of the AC waveform) that charge up the filter cap, and when the AC waveform drops below the voltage in the capacitor, no current flows. This peak current is something like ten to 20 times the average DC current consumed off the B+. So a resistor around 50 ohms should get you in the ballpark.

Marcc : Thanks for those figures . Gives me a safe starting region.

With a few assumptions

10v drop ~133ohm

7.5v drop ~ 99 ohm

Wa2ise : Thanks You'll probably be right, I'm just being paranoid.

GTC : Fantastic link, thank you. Very practical, compared to my accademical original solution

I couldn't agree more with both of you on an isolation transformer (and the warning).

I was actually abit horrified when I opened it up to find it didn't have a transformer, safety doesn't seem to be a major factor with the few portables I've owned .Currently have an astor portable with a horrible exposed mains terminal thats on my rewire list.

I'm currently looking for a small enough transformer to fit within the case itself. May end up using two 12v transformers (connect the 2 12v winding together to get a ~240 output)

I was actually abit horrified when I opened it up to find it didn't have a transformer, safety doesn't seem to be a major factor with the few portables I've owned.

It was widespread practice that children were completely barred from operating household appliances and in the case of the humble radio it was often left to the man of the house to decide when it was on and even what station and programmes were heard. With this (and at the risk of sounding biased) there was a small margin for error compared to how life is today where the children have a bigger influence on the use of electrical appliances and because of this there are Australian Standards, state authority approvals, Commonwealth A-Tick and C-Tick certification and even though they aren't formally recognised in Australia the plethora of overseas standards such as BSA (the British equivalent of AS). CE, CSA and UL ensure that manufacturers today do take notice that their products can be used by those without an awareness of the danger of electricity and take this into consideration when designing new products.

It is often said that in most cases you can lay your hands on any top-of-chassis component and not get a shock but this has been proven wrong on too many occasions. In some radios there are valves with hot top caps. If there is a fault in the radio's tuned circuit it can be possible to get a shock from the exposed tuning condenser. If the paper insulation is failing on the field coil of the loud speaker you can get a shock from that and in some radios there is 700 volts flowing through that. In my early days as a collector I got a belt from a field coil of a HMV table model and it wasn't a pleasant experience. Peter Lankshear once described the restoration of a nameless console he called Henry in Electronics Australia. I remember it containing a smoothing choke which had exposed terminals - again with around 700 volts across them.

There's also the issue, which some members including me, sometimes touch on - the problem we have with people who lack the competence to carry out repairs unwillingly setting booby traps for repairers and collectors to fall victim to. They don't do this intentionally though whether it is deliberate or accidental, wrongful repairs and modifications are dangerous.

The one thing I've never been able to work out is that with many manufacturers there was a pattern throughout the valve era where just about everything was exposed, followed by putting things in cabinets with the rear exposed and then followed by a completely safe cabinet that was sealed on all sides, top and bottom. Some manufacturers digressed from this by releasing a fully enclosed model and then later releasing one with an exposed rear and then chopping and changing between these methods for decades.

I actually think I have that issue of electronics australia (bought a good 10+ years worth at auction) and the name "Henry" rings a bell.

Do you think the permeation of this methodology of construction was a carry over of the original "hobby" (for lack of a better word) construction of fixed receivers (both battery and mains)? ie single core wiring screwed onto exposed terminals etc? (see any 30's radio book for examples). Having said that I prefer single core wiring in my amps.

P.S. I should point out that I was lucky enough have received 2 years of paranoid instruction / training by a very professional former serviceman.

I guess that I was so "shocked" because I have to be so paranoid when building a valve amp (even just for myself), and here's a mass market product. I'm probably just a victim of having a modern perspective while dealing with 50+ year old technology.

Here is another BIG trap for the unwary restorer.

Certain Astor radiogram models from the early to mid 50s have the output transformer body isolated from the chassis and connected to the HT voltage.

I'm sure Astor had a valid reason for this set-up, but I don't think the safety aspect was one of them.

As Astor built PYE they did that in them as well, instead of using ISO types.

The general theory I believe was that by making the body live there was less potential difference between the body and core & that was supposed to reduce stresses & corrosion.

I actually think I have that issue of Electronics Australia (bought a good 10+ years worth at auction) and the name "Henry" rings a bell.

There were two articles on Henry. The first was to get the set going again and a year or two later ol' Henry got a bit of a restoration to improve performance.

Do you think the permeation of this methodology of construction was a carry over of the original "hobby" (for lack of a better word) construction of fixed receivers (both battery and mains)?

Hmm, maybe to a slight extent. Many and perhaps most 1920s home brewed receivers had a timber baseplate to mount the fibre breadboard on and the controls were mounted on a second piece of fibre acting as the front panel. That was it, though with a HT of between 45 and 90 volts these sets were pretty much anything but dangerous.

Factory built sets of the same type were generally housed in full coffin-style timber cabinets with timber, Formica or marviplate front panels.

Heading into the 1930s we had the tombstone era and these radios rarely had backs on them, regardless of the type of chassis or circuit installed. The Airzone Radiostar of 1937 had no back but was available as a mains AC/DC model. No transformer, just a barretter to limit current flow and some passive components to drop the voltage for the valve filaments. Big problem if the electrician wired the powerpoint wrongly or the radio was fitted with a bayonet plug and the operator touched the chassis - there was no isolation at all. Also, the control knobs on the Radiostar were quite small. If you changed station under the same fault conditions with wet fingers the close proximity of the grub screws to your fingers would send a reminder of how the set was powered.

I should point out that I was lucky enough have received 2 years of paranoid instruction / training by a very professional former serviceman.

Best way to enter the game. I wish I'd come in the same way. Don't get me wrong, I've had a good run but I just wish I knew a little more of the technical stuff. I've had to learn things along the way rather than have an old hat actually demonstrate a restoration right in front of me. It is good experience that can't be replaced with anything else I reckon, providing that person knows his stuff.

an old hat

Old hand?

Or that.

Glad I found this thread as my next fix up is this model.
I've had this radio sitting around for years and it was never working.
I have had a couple of goes trying to get this one up and running and it never occurred to me the rectifier had failed.(All new learning for me)

Did anyone get the right resistor values when replacing the metal rectifier with a diode?
The only thing I have noticed is the large wirewound resistors are running very warm. (R2,R3,R4,R5,R9)
Don't worry,I know it's a live chassis,so it's unplugged when I feel the warmth of the resistors.

For what its worth (perhaps little) I recently replaced a metal rectifier (shorted) with 1N4007's. The original was a full wave, but the transformer that was used to replace the fried one was for a double diode valve rectifier. In this case, similar to AWA in some sets with 6X5 I chose to put 100 Ohms in series with the rectifiers.

This would have an effect on surge current, however, the load voltage is actually bang on spec.

Current squared by resistance gives you wattage. One of the things I & others have done, is to buy the feed through resistors that can be panel mounted (RS have them) These in conjunction with heatsink compound, can be mounted to a side panel or similar & that will allow the dissipation of considerable heat.

Marc.

that will allow the disputation of considerable heat.

Not to mention the dissipation, too