Ideas for making a DAC90A sound less muffled

[Nickthedentist]

Hello everyone,

While waiting for valves for my Baird 232 sets, I decided to resurrect a Bush DAC90A which had been languishing in the garage for years.

After replacing all the wax caps and doing some other minor repairs, it works really well... except that it sounds rather muffled to my ears.

It's not terrible, it just is slightly less intelligible than other AM sets which I have around the house. I haven't got another DAC90A around to compare it with.

I'm certainly not proposing any radical modifications, but wonder whether changing the value of one or two components slightly could improve the high-frequency response a little.

All ideas welcome

Nick.

[mickjjo]

Hi Nick, Try removing the .003uF cap connected from V3 anode to chassis, I've done that on all my DAC90As and they sound much nicer, ( I believe Gerry Wells also does that mod! ). You could try halving the value of the 0.01uF cap across the output transformer. .

Regards, Mick.

[Nickthedentist]

Thanks for the prompt reply, Mick

I will try removing the 0.003uF cap first as suggested, if only because I'd completely overlooked it during my re-capping: you can see it hiding under the tagboard in my attached pic. (Don't worry, I didn't forget about the mains filter cap or the one across the OP transformer primary though!).

Nick

[mickjjo]

Very neat work there Nick! , If I remember correctly, the .003uF cap is hidden underneath the tagboard. The original design was fine for the much higher fidelity 1950s transmissions, but for todays A.M. stations, less treble cut is needed. .

Regards, Mick.

[Nickthedentist]

Originally Posted by mickjjo

Very neat work there Nick!

Thanks, Mick. These are such lovely little sets and are so easy to work on, that it would seem rude not to do one's very best

Originally Posted by mickjjo

If I remember correctly, the .003uF cap is hidden underneath the tagboard.

Yes, on the bottom right of my pic above, running from the lower rightmost tag to chassis at a 45 degree angle.

I can't thank you enough, Mick. The mod took 2 minutes to do (didn't even have to warm up my iron, just got out my finest cutters), has saved future problems because of my earlier carelessnes, and best of all, the radio now sounds fantastic. It's just like a pillow has been removed from in front of the speaker aperture.

Nick.

[paulsherwin]

The fact is that a generation of radio listeners really liked that muddy, top free sound. The DAC70 has a similar sound, as do many other radios of the period. My reasonably upmarket FM only Ekco A277 only sounds right with the tone control towards the maximum treble position. In the 70s elderly people with hifi amps would use them with the treble turned down all the way. This was known as a 'nice tone'.

My personal suspicion is that this attitude dates back the the era of horn speakers when any top cut produced a more pleasant sound.

Of course, accurate hifi reproduction was a tiny minority interest until the 70s and the general listening public had no idea what electronic sound reproduction was supposed to sound like

Paul

[Nickthedentist]

That's true, Paul, but for me, too much treble attenuation makes me feel like I've got a heavy cold, and makes listening "uncomfortable".

I've also just found this thread: https://www.vintage-radio.net/forum/showthread.php?t=679

Leon suggests that removal of this cap isn't necessarily all good news. What does everyone else think?

Cheers,
Nick.

PS Sorry for the mixed fonts, but I can't get it to look right even after formatting everything to Verdana/2

[paulsherwin]

You just do what works. Some manufacturers did deliberately degrade the performance of some models to protect their more expensive sets, and as discussed above tastes change about sound quality. I think any easily reversible mod is perfectly acceptable.

Originally Posted by Nickthedentist

PS Sorry for the mixed fonts, but I can't get it to look right even after formatting everything to Verdana/2

Don't try to specify fonts explicitly, as it can confuse some browsers and you don't know how the forum software will interpret your intentions. Even changing the font size can cause some browsers to switch fonts, depending on the default fonts and what alternatives are available.

Paul

[Nickthedentist]

Originally Posted by paulsherwin

Don't try to specify fonts explicitly...

I didn't; merely pasting in the link to the other thread seemed to mess everything up

[geofy]

That's a nice row of tag strip work Nick, you would get a job at Mr Marconi's factory with that type of work. Though the original resistors should be facing the same way, not like the two red banded ones, what appear to be 220 or 22 Ohm?

Apart from the cap modes mentioned, is there any cap across the volume control that can be reduced in capacitance, as long as you don't over do it if the cap is part of the detector rf demod circuit.

I recently repaired a Roberts R200 and found that disconnecting the small cap across the volume control improved the sound.

Paul is right, at one time a radio was judged on it 'good tone' rather than its accurate reproduction of treble (medium wave not allowing much treble anyway). Some of the BBC medium wave transmitters use 'Optimod' to make speech more intelligeble (even though the subject might not be!) though it gives a hard edge to the sound.

[Sideband]

For what it's worth Nick, you may find that the old capacitor had changed value significantly.... I've found many values like this measuring nearer .1uF! You could experimentaly try a new .0033uF in this position...don't forget it's an RF filter as well as a bit of top-cut.


Rich.

[Panrock]

Originally Posted by Nickthedentist

After replacing all the wax caps and doing some other minor repairs, it works really well... except that it sounds rather muffled to my ears.

Nice job you've done there Nick! I'd go along with the solutions proposed.

Originally Posted by paulsherwin

The fact is that a generation of radio listeners really liked that muddy, top free sound....In the 70s elderly people with hifi amps would use them with the treble turned down all the way. This was known as a 'nice tone'.

This attitude is still widespread today. I generally supply restored vintage radios to the general public (ie. not collectors or knowledgeable enthusiasts) and again and again I hear comments like 'what a nice tone' , 'what a shame they don't sound like this today', and these are not old people. But this is really for another thread.

Steve

[Nickthedentist]

Thanks for all your kind and useful comments, everyone

Originally Posted by Richard_Newman

For what it's worth Nick, you may find that the old capacitor had changed value significantly.... I've found many values like this measuring nearer .1uF!

Just tested the cap on my Peak Atlas LCR, which said 3.956nF, so only 32% high. But on my 500V Megger (thanks Jeffrey) it reads just 1.5MOhms .

Originally Posted by Richard_Newman

You could experimentaly try a new .0033uF in this position...don't forget it's an RF filter as well as a bit of top-cut.

I will, when it's next out of its cabinet (which may or may not be soon).

N.