Philco tropic A3629 info

[Paul Mathew]

Any info/schematic on the Philco Tropic would be appreciated. Valve line-up is 1AC6 converter; 1T4 RF/IF; 3V4 Power output;1U5 diode pentode AF. It has a curious mains converter unit attached to the back panel, with what appear to be 2 selenium rectifiers. Would it be safe to use this arrangement, or come up with something else for mains HT?

[Herald1360]

It looks like a reasonable unit but I'd be inclined to reverse engineer it to see what makes it tick and test it offline on resistive loads to simulate the radio filament and HT loading before going "live".

The selenium rectifiers may be past their best in which case they can be replaced with suitable silicon diodes or bridges with extra series resistance to simulate that of the original selenium devices.

[Paul Mathew]

It has since occurred to me that I powered a battery set previously with a string of 9V batteries for the HT, but only had 7 at the time, giving around 63V, which still did the trick. For the filaments I used a D-cell 1.5V battery. Not sure how long this arrangement would last, if played for any length of time? I may try and forage for a suitable transformer as well, and do some ss rectification. I'm not keen to bang the mains straight in!

[PJL]

Battery version of the A3626 I have here. Other than the Bakelite case, there are lots of differences though.

I would not trust a simple dropper battery eliminator like this as there is no regulation and it is easy to wipe out all the valve filaments. You might also want to check for exposed metalwork as the chassis may be live.

[Paul Mathew]

I decided to use the power pack attached to the back of the unit, but replaced the selenium rectifiers with a suitable diode, as per the guidelines here: http://w3hwj.com/index_files/RBSelenium2.pdf
All the voltages seem spot on. All electrolytics and paper/wax caps replaced and the radio worked well until after about 15min of normal playing, the volume faded to a very low but audible level with a hum (motorboating). Lots of info online about possible causes, from faulty caps, resistors and possibly internal leakage in the valves. After much time checking resistor values and poking around, I'm at a stalemate with the volume issue............

[Paul Mathew]

Success at last! Re-checking voltages I found 0 volts on pin 3 of 1U5, and 88V on pin 3 of the output valve, 3V4. The two grids are connected via a 4,7M resistor and grounded at the 1U5 by a cap which measured 63nF on my meter. I'm not sure of the colour coding for this one. Just orange and black. A schematic would have been great. Anyway, I replaced the resistor with a close enough value as it had gone high, and replaced the cap as I assumed this must be pulling the voltage down to chassis. I used 100nF as this seems to be the value of this cap on other AC/DC schematics.I still think the pin 3 (g3) of 1U5 is a bit low at 21V, but the radio works well, so I think, let sleeping dogs lie.

[Nuvistor]

I think the capacitor is a 0.01uf, a type known for failure.
See if anyone else as a better thought.

[crackle]

That cap looks like brown black orange so 10.000pF or 10nF or 0.01uf
And yes it will likely be leaky.

Mike

[Paul Mathew]

Thanks for the info. I'll try a 10nF cap. Not sure if it's connected, but have now picked up intermittent microphonics!

[usradcoll1]

Originally Posted by Paul Mathew

Success at last! Re-checking voltages I found 0 volts on pin 3 of 1U5, and 88V on pin 3 of the output valve, 3V4. The two grids are connected via a 4,7M resistor and grounded at the 1U5 by a cap which measured 63nF on my meter. I'm not sure of the colour coding for this one. Just orange and black. A schematic would have been great. Anyway, I replaced the resistor with a close enough value as it had gone high, and replaced the cap as I assumed this must be pulling the voltage down to chassis. I used 100nF as this seems to be the value of this cap on other AC/DC schematics.I still think the pin 3 (g3) of 1U5 is a bit low at 21V, but the radio works well, so I think, let sleeping dogs lie.

Pin 3 on the 1U5 valve is the screen grid and is common to have a much lower voltage than the anode voltage, so 21 volts is fine.
Dave, Midwest USA.

[usradcoll1]

Originally Posted by Paul Mathew

Thanks for the info. I'll try a 10nF cap. Not sure if it's connected, but have now picked up intermittent microphonics!

Gently tap on the 1U5 valve. They were great for microphonics, especially at higher volume levels. Try replacing it or wrap a rubber band around it.
US firms did that a lot.
Dave, Midwest USA.

[Paul Mathew]

Thanks for the info Dave. Tapping on the 1U5 causes ringing like crazy! Rubber bands on and testing. One for the diode, and one for the pentode! Less oscillations which seem to fade pretty quickly. Who would have thought! Going to look for another 1U5 to see if there's any further improvement.

[usradcoll1]

The US firms had special wide rubber bands made for the purpose.
It looks like a high quality speaker used with isolating grommets to minimize microphonics. You might want to check them to make sure they aren't dried out and brittle. That would help as well.
Dave, Midwest USA.