Bush DAC90A-Dim Panel Bulbs

[Petes BxB]

Hello
My current project is the Bush DAC90A and the set appears completely original and is working fine with its all original components. But the only obvious problem are the very very dim original panel bulbs.I have the
Bush service data sheet ,so changed both the bulbs for new 3.5v and also
changed the shunt resistor -R16-to 250 ohms@7W as per the data sheet.

This worked ok bulbs much brighter...my concern is that the R16 original that I removed tested about 70/80ohms...so is it satisfactory to leave the
250 ohm in circuit? ---Or could it create another problem?
Thanks Pete

[ms660]

So far as I know, 250 ohms was the original value of the shunt resistor but it was later changed to 75 ohms to help give the bulbs a longer life.

Lawrence.

[Station X]

Lots of threads on this subject. The lamps in a DCA90A are certainly dim.

Trader sheet gives the resistor value as 75R. Manufacturers sheet says 150R.

The OP won't have been the first to have changed this resistor thinking it was the wrong value.

Which is the correct value I don't know. I left my set as it was with a 75R resistor.

[Boater Sam]

Dim as a TocH lamp with 75 ohms, and 3.5V 0.15A bulbs are almost unobtainable now.
0.2A work with 250 ohm resistor but not well. Effect on heater current is minimal.
The models with dark dial glasses as opposed to the cream ones are the most problematical to light satisfactorily.
I have resorted to LED lamps now. ( Ducks and runs away from the incoming flack ) There is a thread on here about using an illuminating panel instead of bulbs.

[DangerMan]

I believe Bush reduced the resistor value to 75 ohms in order to take account of possible lamp failure... the voltage drop will be smaller with the 75 ohms resistor than it would previously have been with 150ohms, but the bulbs will be dimmer when working.
There's also the surge to consider, if a set is switched off and quickly back on while the cathodes are still hot. It's possible there was more than acceptable lamp failure under these conditions.

I have always found that some 3.5V 0.15A lamps are brighter than others in these sets, and some surviving old ones are really quite bright enough.

[AD360 Rob]

I have found that changing the shunt resistor from 75 ohms to a value of 150 - 180 ohms gives more than adequate brightness with either the original 0.15A 3.5V lamps or the more easily obtainable 0.2A 3.5V lamps. Also that value seems to be sufficient to limit the hot switch on current surge. Some of the modern LED lamps will give a traditional glow if you pick the right colour . Personally I'm happy with the original set up as the dial lamp really only needs to be bright enough to see the scale at night or in a darkened room.

[Boater Sam]

I find that the real current rating of the modern bulbs whether 0.15 or 0.2 or 0.3 A is very variable. I often have to find matched pairs to give even illumination in sets with multiple bulbs.
There seems to be a poor standard of manufacture, not uncommon these days unfortunately.

There is a thread on here about using an illuminating panel instead of bulbs.

One of mine, here... https://www.vintage-radio.net/forum/...d.php?t=129025

[mark2collection]

The sets I've worked on in the past, required a good dose of foaming cleanser to the reflector plate & diffuser panel, as these were covered in a layer of thick dust, then grime under the dust. I have had a set where the lamps were working, but owing to the grime no light was visible Truly grubby ...

Cleaning brought the illumination up to useable levels.

Only one set (which fought me all the way) required a new coat of paint to the reflector plate, owing to bad corrosion.

Trouble is, we're getting used to LED back lighting on various gadgets, which in some cases, are sunlight readable.

That said, I like the idea

[PsychMan]

I replaced mine with a 250 ohm and made use of easier to get bulbs, results are good enough for me and no issues since making the change.

I believe originally the dial lamps were never the brightest. There's lots of threads on here around modding these sets with brighter dial lamps, each to their own.

[smeerkat]

Hi,

There was a thread on this forum a while ago, which detailed the use of a small 12V transformer (purloined from an old "wall-wart") to power two 6.3V, 0.3A bulbs in series. I gather the only problem with this method is finding a place on the chassis to fix the transformer.

Like other posters here, I used 3.5V, 0.2A bulbs with the a new 250 ohm shunt, and have had no troubles. The resultant dial light is bright enough to be usable, but not so bright that it spoils the radio's characteristic "glow" when switched on!

Regards, David

[G6Tanuki]

In the past I've successfully replaced filament-bulb dial-lights with the LEDs salvaged from dead GU10 LED 'bulbs' (appropriate series/parallel resistors being added). Since these come in different colour-temperatures you can choose between a gloomy dim yellow [to replicate those years of dust soot and nicotine buildup you found on the original failed bulb], or a nice 6500K midday-at-midsummer bright white.
The only real downside to this is that a typical LED has a rather narrow beam-angle so tends to produce a spot of light rather than a diffuse glow. The answer: more LEDs! I should post a pic of my six-LEDs-fitted-to-both-sides-of-a-strip-of-Veroboard-and-fitted-into-a-MES-bulb-base dial-light-converter.

[Ed_Dinning]

Hi Gents, I produced an auto transformer for use with these sets and it had a 6.3v tap, so it was possible to feed the bulbs directly (remove shunt resistors) and stabilise their brightness.
The main purpose of the exercise was to do away with the dropper and the danger of cracked cabinets on cream sets.

It worked well for both purposes

Ed

[Petes BxB]

Gents,
Thank you for your comments and the various suggestions for the "bright Lights".
As I have the new bulbs and R250 for now I will go with this as its at least
a reasonable result and better than the original.

My only other areas of concern are re- 2 pin male mains connector(I don't have the 'female' lead) and also there is no internal fuse,so I'll read a few of the existing threads as a way forward.

Pete

[murphy512]

I have used LED and so far it has worked well.

I have cut off the glass from the blown bulbs and soldered a diode and LED (mellow white) on the base of the old bulb.

No modification to the circuit, so normal bulbs could be used again if needed

[TrevorG3VLF]

The result looks good. I assume that the pair of LEDs are in antiparallel.
With a 100mA source and the LED running for half the time, would 50mA equivalent be a bit much. Perhaps a shunt resistor across the LEDs would ease the duty somewhat.