Hi,

while trying to find the issue with a motor not running in a jukebox wurlitzer 2000, I noticed that the still present RS 1 selenium rectifier on the amp was disconnected. Underneath the plate of the amp a replacement was put into place prior to buying it, a KBPC 610.

It seems to still be working, however I was told that it might be to light or weak and was adviced to use a KBPC3510.

I was hoping for some advice on this one.
I attached a part of the schematic, since it seems the f-2 is an 8 amp fuse very closeby.


The motor is to be revised professionally some time next week, as a likely culprit.

 

The KBPC610 should be fine it's rated 6 amps and a 2 amp fuse F-3 is in the circuit.
The F-2 is not part of the rectifier circuit.

 

Hi Nick,
A point to note when replacing Selenium rectifiers with Silicon, is the high forward voltage drop of Selenium compared to Silicon.
This will result in a much higher DC supply voltage to the circuits.
E
https://www.antiqueradios.com/forums/viewtopic.php?t=305477

 

A point to note when replacing Selenium rectifiers with Silicon, is the high forward voltage drop of Selenium compared to Silicon.
This will result in a much higher DC supply voltage to the circuits.

Yes.
Before I understood the difference (still in college) I replaced a selenium rectifier that had failed, with a silicon diode in an old B/W TV.
The TV worked with the replacement, but a vertical white line appeared on the edge of the screen, apparently due to the increased voltage from the silicon diode. I realize now, I should have added a resistor in series with the diode to compensate.

Note that the schematic shows the top of RS-1 output as minus, but the bridge diodes show the top as plus.
My guess is that the minus indication is correct and the diodes were drawn backwards.

 

That circuit is almost as old as me (78).

 

That circuit is almost as old as me (78).

So which one is in better shape?

 

Hi Nick,
A point to note when replacing Selenium rectifiers with Silicon, is the high forward voltage drop of Selenium compared to Silicon.
This will result in a much higher DC supply voltage to the circuits.
E
https://www.antiqueradios.com/forums/viewtopic.php?t=305477

For the benefit of us youngsters, what is the voltage drop of a selenium rectifier? I remember my father having a battery charger that had one, and the smell it made when it failed, but I was too young to understand the concept of forward voltage drop.
I imagine that the IV curve doesn't have a very steep slope (i.e. a lot of ohmic losses)

 

Click on the link in post #3, for a starting point regarding forward volt drop/plate

 

So which one is in better shape?

I am doing pretty well. At 78 I feel like 48.
At the Covid vaccination clinics I am about the only person my age who was not triple my weight and in a wheelchair.

 

Hi Guys,
A Moderation note from a VERY old Moderator, requesting that you youngsters please keep on Topic.

Mod.

 

Click on the link in post #3, for a starting point regarding forward volt drop/plate

Thanks. They were worse than I thought!
I learned transistors first, I learned valves later,I never needed a selenium rectifier.

 

The last place I saw selenium rectifiers used was in older high current battery chargers for things like fork-lifts. Horrible smell when a stack started to fail.

https://www.steampoweredradio.com/pdf/international rectifier/international rectifier 1961.pdf

REPLACE All SELENIUM TV RECTIFIERS WITH THE
SILICON " UNISTAC" TV 500 AND THE SILICON TV-502
(DOUBLER & COLOR TV TYPE)
Here are two silicon quality rectifiers designed for direct mechanical replacement of selenium stacks.
The "UNI ST AC TV-500" is rated conservatively at 750 MA peak capacitive load and will replace practically any single selenium rectifier in TV sets. The "UNISTAC
TV-502" is a dual unit for voltage doubler circttit application and for color TV replacement, and is rated at 750 MA at 800 PRV.

 

Last saturday my dad lent a helping hand, since we suspect a defect in the amplifier.

Indeed it seems that there is something wrong here.
The motor that drives the carroussel no longer works, and the crank on the selector drum is no longer moving.
The record player however still spins.

We decided to follow the power, with all plugs disconnected to the jukebox.
115V comes in, which we can follow up to the chassis plug.
However, the rectifier not only does not provide 24 v DC, we also saw it simply doesn't receive 24V AC.
Would it be right to assume that the transformer (circa 1958) has failed, and that it's the reason the motor turns slowly, until it stopped ?

Also we measured the mentioned resistor of 1650 ohms, which still looks to be ok.

 

A transformer can indeed fail,and that can be a show-stopping problem. So the very first thing will be to disconnect the selenium rectifier bridge and measure the resistance of the 24 volt section of the transformer. If it is open, that is the problem, or fuse F3 has failed. if it is not, It may be the rectifier bridge failed. and you can use a silicon diode bridge replacement.

 

In addition, I can add that the amplifier makes a loud "toong" noise when getting power, although it's not always. It seems to be happening when it's been off for a while and then switched back on. It certainly wasnt there earlier.

Youtube wurlitzer electrical fail

 

There is a fuse in the transformer connection to that rectifier, and that is the very first thing to check, BUT not to replace, if it has failed. Mostly, fuses fail because of excess current above their rating. So if the fuse has failed then discovering the cause of the failure is the next step.
That loud sound at switch-on sounds like a brief overload while a capacitor is charging. So the next step is discovering what item is drawing excess current so that the capacitor charging is now an overload.

 

In a old charger I've had the internal transformer 'fuse' fail, because the selenium rectifier failed, because the filter capacitor failed. These things seem to cascade from the root cause.

 

I do not recall any of the simple chargers that I have repaired including any filter capacitor at all. Not a required component. At least, none of them that used a transformer-rectifier scheme. Those with a switcher would be different, and include a filter capacitor. And certainly most of the regulated chargers would contain a capacitor. And those also serving as power supplies.

 

I do not recall any of the simple chargers that I have repaired including any filter capacitor at all. Not a required component. At least, none of them that used a transformer-rectifier scheme. Those with a switcher would be different, and include a filter capacitor. And certainly most of the regulated chargers would contain a capacitor. And those also serving as power supplies.

These were old dual units. Power supply and charger with a LC filter. Old boat-anchor stuff.

 

Last saturday my dad lent a helping hand, since we suspect a defect in the amplifier.

Indeed it seems that there is something wrong here.
The motor that drives the carroussel no longer works, and the crank on the selector drum is no longer moving.
The record player however still spins.

We decided to follow the power, with all plugs disconnected to the jukebox.
115V comes in, which we can follow up to the chassis plug.
However, the rectifier not only does not provide 24 v DC, we also saw it simply doesn't receive 24V AC.
Would it be right to assume that the transformer (circa 1958) has failed, and that it's the reason the motor turns slowly, until it stopped ?

Also we measured the mentioned resistor of 1650 ohms, which still looks to be ok.

Looking at the circuit diagram, the bridge rectifier is for the relay supply to pull in the other mains primary winding.
Has the relay burned out as well?