DAC90A - some basic help for the novice.

[rovereab]

I picked up my mint condition cream DAC90A today. It is a late model with the perspex discs behind the knobs and without the crack on top! Generally it is in excellent condition. It sounds slightly muffled in comparison to my walnut DAC which I guess is down to the walnut DAC having been serviced by a radio restorer before I bought it.

I have a couple of questions please:

1. What type of cable would DACs have been made with and can this type be bought now?
2. As I am basically a novice is there a useful way to start to build up knowledge of the internals so that I may attempt to establish why the sound is a bit muffled.
3. I believe I read somewhere that there is a right and wrong way to connect the DACs to the mains. Can somebody please explain further.

Thanks for any help given in response to the above.

Eamonn (with Cheshire cat grin)

[igranic]

Because I was only a few years old when this set was introduced I stand to be corrected, but the earlier DAC90 sets probably had fabric-cover twin rubber flex, whereas the later DAC90 sets were probably brown plastic flat-twin flex.

Before you start fiddlng with the internals it must be rembered that this an AC/DC set with a potentially live chassis. Touching any internal metal part with the set plugged in (but not necessarily switched on) could result in a severe electric shock or death. To help protect against this danger there are two choices: an isolation transformer (expensive), or a permanently wired RCD plug available from DIY sheds for about £6.

Being an AC/DC set, the chassis is not earthed, but is connected to one or other side of the AC mains. Since the original plug is a two-pin non-polarised type, it is possible to connect it either way round, so that the chassis is either connected to Neutral or Live. Obviously, there is less chance of electrocution if the chassis is connected to neutral, and some owners disconnect the chassis mounted plug and wire a mains lead directly to the switch so that when switched on neutral is connected to the chassis via the lamps and shunt resistor, but this still presents hazards, so an RCD or isolation transformer are IMHO essential minimum safety devices.

If you are going to retain the original plug and socket it is a good idea to mark the plug and chassis so that it is clearly evident when neutral is connected to the chassis side of the supply arrangement.

If in any doubt about safety or other hazards, consult a competent person.

HTH

[Ed_Dinning]

Hi Eamonn, plenty of detail on this site to help you cut your teeth on this set. It is a good basic set to start on but PLEASE HEED THE DANGER WARNINGS.
Muffelled sounds may be due to the AF coupling cap (unless it has been changed in the recent past just do it as it can lead to all sorts or other bother).
The tone correction cap mounted on the op transformer is another candidate and the wires to pin 4 of the UL41 should be removed and allowed to hang in space (see earlier threads).

HTH ED

[ekcobeach]

I also stand to be corrected but of the dozen or so DACs that i have or have had,all have been purchased with black flat twin wire from the earliest smooth knob model to the late one with the perspex discs.I have however noticed that the original two pin plugs can come in dark brown or black. Colin.

[Ian - G4JQT]

DAC90A - very nice, reliable, easy to work on, classic set.

Yes - black, flat twin-lead cable.

Change the audio coupling capacitor anyway if it's the original wax type - bound to be at least slightly leaky and putting a bit of HT onto the grid of the expensive UL41. (Search for "that capacitor" on this forum for more info.)

If the IF stages have not been fiddled with (if it hasn't flaked off, check the paint seals on the "screws" sticking out towards you on the IF cans), then muffled tone could be the speaker, though not very likely unless it's buzzing on sound peaks too.

If it is working correctly, but still sounds muffled, look at the tone correction cap as others have suggested. You can reduce the value a bit, say be 50%. The audio tailoring of AM transmissions has been changed over the years for various reasons, so the fixed treble cut on some sets is a bit severe now.

But this cap is also an attempt to reduce the 3rd harmonic distortion from the output valve (Dick and Smithy R&EC about 1970 I think).

There's an IF decoupling cap across the volume control (or nearby to earth) that also cuts the treble freqs. I've reduced this too for better sound.

But beware of changing component values - they were chosen for a reason and too much alteration can cause instability.

Good luck.

Ian

[rovereab]

I removed the back cover of my cream DAC90A today to start to get familiar with its internals.

I noticed that on the right hand side as you face the back of the radio a component has "exploded" onto the side of the radio. On removing the debris from the case it looks like an end cap of a wax component. I remember hearing about wax capacitors.

I have attached a couple of pictures for you to look at. To me I reckon the radio has never been restored judging by the dust I vacuumed out of it!

Please allow for me knowing nothing about radio restoration (I'm a dab hand with refitting classic cars though lol). Are these wax capacitors simple to obtain and install?

Would the main UK Vintage Radio site maintenance CDs be a good introduction for me? Basically "HELP"!!!

I live near Chelmsford, is anybody local to me and up for drink in a pub one evening to discuss how to get going?

Regards.

Eamonn

p.s. I won't be switching the radio back on having discovered the exploded component!

http://i101.photobucket.com/albums/m...b/P4240021.jpg

http://i101.photobucket.com/albums/m...b/P4240023.jpg

[Nickthedentist]

Hi Eamonn!

The most likely culprit to have exploded is the mains filter cap. The good news is that the radio will work OK without it being replaced (but you should remove its wires to prevent them short circuiting!).

You should replace the audio coupling cap though, to prevent damage to the audio output valve.

I would also remove the chassis from the case to have a proper look at everything.

See Paul's other site for general restoration info (esp. the Capacitors section): http://www.vintage-radio.com/repair-...ion/index.html

And you can get hold of the circuit diagram as a PDF for £1.99 by clicking on the white and brown box in the top right corner of the screen.

There's a nice article about restoring a DAC90A here, with EXCELLENT pics too: http://steamradio.tripod.com/dac90a/

Cheers,
Nick.

[Steve_P]

The CD's are circuit diagrams. Or you can buy the ones you want as and when.

http://www.service-data.com/

As far as the techniques of restoration go, http://www.vintage-radio.com/ and select the section on Repair and Restoration Information. All you need to know.

Books...There are plenty. Look under Book and Product Reviews.

And of course us mottley crowd....

Cheers,

Steve P

[paulsherwin]

The exploded cap is almost certainly the modulation hum cap which is connected across the mains. You should replace this with a modern X2 rated component, though the set will work without it.

You should change all the wax caps. This is very easy to do in the case of this set. Once this is done there's a good chance it will 'just work'

Paul

[BassoonBloke]

Hi Eamonn,

I have to admit that i've never worked on a DAC90, by judging by where that component (a capacitor) is, i would say it is a mains filter cap, and can probably be safely removed, with no effect on the performance of the set (i'm sure someone will set me right on this if i'm wrong).
You can replace this with an 'X' rated cap later on if you want to. (it will cause the mains fuse to rupture violently if it has gone 'dead short').

Nice set, good luck,

Alan.

[Paul Stenning]

The only other potential problem is the UL41 output valve. These often become leaky internally, which means you have voltage on the grid and therefore excessive anode current even with the relevant caps replaced. Fingers crossed yours is good. It's best not to run it again until at least the output valve grid coupling capacitor has been changed, then check the voltages.

[rovereab]

Thanks folks, I am very green with all this so please bare with me as I ask what may be very stupid questions!!!

Regards.

Eamonn

[G8DLG]

Eamon.
I would replace that exploded capacitor straight away,this is a very important component,although the set may appear to work without it.
You should use a 1000v rated polypropylene or similar.
All the DAC90s used 2 core red and black conductors,the 90 had black outer and the 90a used brown as all the later Bush sets.
If the connector is wired correctly the lead will exit toward the aerial,so you will know the preferred way to connect the plug.

[Paul_RK]

"Very important" seems to overstate it somewhat: at least, I'm at a loss to think of anything the mains filter capacitor does apart from reduce mains-borne interference and occasionally explode

Paul

[paulsherwin]

Originally Posted by Paul_RK

"Very important" seems to overstate it somewhat: at least, I'm at a loss to think of anything the mains filter capacitor does apart from reduce mains-borne interference and occasionally explode

It's there to prevent modulation hum. That said, I agree many radios seem to be acceptably humless with it removed

Paul