Roberts R55/Amplion "Convette" LC4

[Nick]

Two questions really:

• Anyone have a schematic or details on the Amplion "Convette" LC4
  
• Can you re-cover an old Roberts radio?
  

The LC4 was in the radio when I bought it - schematically (I haven't drawn it out yet) it seems quite crude. I don't want to power it up as it contains old electrolytics etc. I have a variac, but I'm wondering first if anyone has either rebuilt one of these or has a suggested, preferably modern-ish (no selenium stacks), replacement. The inside is very crowded... I believe that there should be a B126 for HT and an AD35 for LT - the LC4 replaces both of those.

The covering of the radio is best described as "tatty"... mind you, it's not unreasonably so, considering that the radio is 70 years old (1955). I'll re-cap and replace resistors etc. where necessary. The inside is very clean and the speaker looks good. All 4 valves test good on my Hickock 539C... The plastic grille on the front was broken but thankfully all the pieces were inside, so very carefully under a microscope and with tweezers and superglue, the grille was re-assembled and it's now almost impossible to see the repairs.

Has anyone re-covered a Roberts or radio similar to this... Any hints?

Nice little set. I want to do it justice and fully restore it to use with my pantry xmtr channels.

[ppppenguin]

There's a thread here about the Amplion: https://www.vintage-radio.net/forum/show...?p=1251314 I would want something with better regulated LT in view of the fragility of the heaters of battery valves.

[Nick]

There's a thread here about the Amplion: https://www.vintage-radio.net/forum/show...?p=1251314 I would want something with better regulated LT in view of the fragility of the heaters of battery valves.

Yep - saw that, but it's not the same model - I've posted "over there" too... RM hasn't got anything on the LC4 either, though it does have other Amplion "Convette" models... It's a nasty thing really - the inside of the metal case is rammed and has no earth etc. Bits of card are used to keep HT & LT mostly apart, electrolytics bulging...

I will almost certainly strip the innards of this one, keep the mains xfrmr, earth the case and replace the electronics with a modern, current-limited, linear version without PCB-filled caps and selenium rectifiers...

I can see that the LT on this model is centre-tapped full-wave rectified with CLC smoothing and the HT is half-wave CLC. I expect the electrolytics are completely shot and I dislike selenium plate rectifiers, so really not a lot left to keep!

[ppppenguin]

A lot of battery eliminators were pretty evil, at least by modern standards. I'm not surprised, they had to be built down to a price in an age when rectifiers and capacitors were expensive. LT hum had to be kept pretty low with directly heated valves. At least the earlier battery valves were a bit more robust than Dx9x types.

I'd be wary of the transformer too, but at least that can be tested with a megger.

Building a modern PSU is entirely valid, but using a rechargeable LT cell (voltage a bit low but probably OK) and PP3s or coin cell stacks (as researched by Mike Watterson) has a lot to commend it.

[Nick]

A lot of battery eliminators were pretty evil, at least by modern standards. I'm not surprised, they had to be built down to a price in an age when rectifiers and capacitors were expensive. LT hum had to be kept pretty low with directly heated valves. At least the earlier battery valves were a bit more robust than Dx9x types.

I'd be wary of the transformer too, but at least that can be tested with a megger.

Building a modern PSU is entirely valid, but using a rechargeable LT  cell (voltage a bit low but probably OK) and PP3s or coin cell stacks (as researched by Mike Watterson) has a lot to commend it.

I have a Megger (actually a Robin) so will test the xfrmr. I could possibly find a modern sub-miniature xfrmr, but we'll see. There's also no fusing, temp overload or actually any safety features at all... it's amazing anyone survived the '50s...

[Mike Watterson]

The Convettes, IMO, are display only. The voltage will rise too much on LT if the O/P valve is unplugged or goes O/C. They are rubbish.

I've used a shaver transformer with full wave via 2 x 1N4007, two caps, series resistor and bleed resistor to limit HT when no LT. If you want closer to ideal 80 to 85V then add a shunt regulator.

A DC PSU with series resistor and 2 x 1N5407 as a series pair as shunt regulator for the 1.35V. Use an LM315 at 6.8V for a 7.5V battery set.

The NiMH are a bit higher than the NiCd. The filaments are ideally run at 1.4V max, and 1.35V was recommended for a PSU. Using 1.3V is fine. The end point of the Zinc cell was 0.9V to 1V, but the DK9x would typically stop at 1.1V. So NiCd are marginal and NiMH are fine.

Similarly a 90V HT pack is nearly 100V no load and new, but end point of about 56V! So 75V to 85V HT is fine.

[Mike Watterson]

A 45V or 90V rechargeable pack will have short live as cells are progressively destroyed by reverse charging if one goes flat first. Even 12V packs have shorter life than 6V ones. The multicell 16V or higher laptop packs monitor each cell.

[Mike Watterson]

I've also experimented with SMPSU designs running from 1.3V and 6.8V with a reed switch operated by filament current. Getting it low noise is the issue!