I have been given a 56 Chevy car radio by a friend for restoration. The radio itself is in reasonable condition and should be OK after a recap etc.

However, these things had a 'power pack' mounted on the speaker frame. Of course this is missing. We don't like the chances of finding one in restorable order, as it took quite a few years to find the radio.

Looking at the circuit it needs 215v HT, which comes from the vibrator supply in the power pack.

Is it worth grabbing a low power inverter ( such as ebay item 200977034766 ) and having a go at adapting it for the HT supply??

The power pack also has a couple of 12V6 arranged in push-pull for the audio. I reckon I can come to some sort of solid state solution to this.

Or, am I wasting my time??


ben

Care should be taken with switchmode power supplies, including inverters. Despite them being packed with delicate components they can deliver a big wallop.

Also, the cheaper inverters do not deliver a sinewave but a stepped squarewave.

Think it would be a pretty formidable project to get that radio going.

Considering the missing power pack and audio output stage.

If you friend intends the radio for a restored Chevy the radio may end up as a non working display item.

On switch mode inverters, even the pure sine wave models can develop quite an amount of r.f hash.

Yeah, noted re SMPS. I was intending to mount in a steel case/box. I guess the other option is to cobble together a vibrator supply, although they are not without RF issues either.

I love a challenge, but this may be a while in the making.........

Good luck with your endeavours whatever way you go, it will be a lot of work.

Yes a vibrator supply using a genuine vibrator would be a challenge.

Many a radio mechanic in the days gone by tore his hair out trying to cut out the hash.

Oddly enough, I found putting a 80μF electro across the battery supply where it entered the case of my Ferris M94 reduced the hash by quite a marked amount.

You could make the audio into a hybrid using an integrated circuit amp. It depends on how original you wish to go? You can use 6V tubes like 6AQ5 etc. and series the heaters. A bit of resistance may be in order as a generator / alternator will get the battery to 13.8 V +/-

Switch mode power supplies are dirty albeit more efficient due to their frequency. Solid state vibrators exist. Vibrators are around 150 Hz.+

The cheap and nasty solution would be to get a low power 12v to 240V inverter (some lights have these) and try it. Watch the fingers: bites.

Marc

The cheap and nasty solution would be to get a low power 12v to 240V inverter (some lights have these) and try it.

These may not provide isolation from the DC input, if so, hopefully the positive HT inside is not the side tied to the input. Most modern inverters use a DC-DC converter (a switching power supply circuit), then switching FETs to create a "modified" sine wave output. Find and tap the HT DC ahead of these FETs. Scavenge from and use one of those 2 winding filter coils from a PC switching power supply to filter out the hash.

The big thing is that there are a variety of ways to get the B+. You may be able to find an inverter transformer. The only scrap one here in a chassis is 32V and it I believe is complete.

The heater tubes seem to behave well in the US transformerless radio's. The heaters are by their nature isolated from B+ and can be run on AC or DC.

There are monoblock IC's that will generate the pulse to drive a "ringing choke" & regulate it, but the damn things tend to be noisy. Catch 22 the higher the frequency the better the efficiency & the smaller the transformer.

It is quite easy to make an oscillator (555 NAND gates etc) and use that to drive a Mosfet. Silicon Chip has had a few projects with this sort of thing. Using a mains power transformer backwards will work but there is the current rating of the 240V winding & physical size.

The idea of the cheap / salvaged inverter is that they will have some form of step up that is actually a transformer.

B+ regulation can be achieved, if necessary using a shunted LR8.

Marc

For $20 inc post I can buy the above mentioned ebay converter, I reckon its worth a look just to see.

This is very much along the lines I was thinking. Not worth spending a lot of money experimenting.

Do remember that if you rectify 240V AC, you do not get 240VDC: You get more (P. P. vs RMS)

Marc

Given that rectified AC produces higher DC voltage, you might look at finding an inverter to produce 110 VAC and use a small step-up transformer before rectification and smoothing, to get DC as close as necessary to 215VDC. There are some multi-tap transformers that might offer a range to experiment with and keep the PSU linear. It's very hard to eliminate RFI from switchmode PSUs.

Maven