DAC90A Audio Problems - Help!

[John G8MWF]

I picked up a DAC90A in a VERY sorry condition and it is continuing to fight me!
The set 'motor boats' only when tuned to a station and the audio is un-intelligible.

The capacitors, resistors etc. have been changed and supplies have all been checked without any obvious faults.

I also have a working DAC90A and the only difference I can find is the output transformer primary resistance on the 'worker' is 1200 ohm and 800 ohm on the non worker - even though the manual shows a resistance of 500 ohms!!

When I first got it the set had been used with the 210 Volt tap set and the UY41 heaters were O/C. My fear is that the transformer has now got shorted windings as a result of this high HT and that is what is causing this problem.

ANY help would be much appreciated.

[Silicon]

It would not be unusual for a previous 'repairer' to have introduced some wiring errors.

When you say you have replaced the capacitors, can you be more specific.
Do you have a service sheet and schematic to work from?

[kalee20]

Doubt if the output transformer is faulty, if it works at all it's probably (though not certainly) OK.

Is the loudspeaker OK? For some reason, the coil suspension seems to come unglued. Swapping with your working DAC90a - or anything else - would prove that.

You can also feed a decent audio source in at the volume control, such as from a portable CD player, and see if that sounds clear.

[Boater Sam]

Motor boating is usually caused by failed smoothing or decoupling capacitors.
Which caps have you changed?
Make sure there are no connections to pin 4 of the UL41, bush used it as a tag strip.

[Herald1360]

Originally Posted by kalee20

You can also feed a decent audio source in at the volume control, such as from a portable CD player, and see if that sounds clear.

Maybe suck eggs time, but unless you're running the DAC90A from an isolated supply, the audio source must be floating. Standalone battery operated is easiest way to guarantee this.

[John G8MWF]

Originally Posted by Boater Sam

Motor boating is usually caused by failed smoothing or decoupling capacitors.
Which caps have you changed?

Spot on! I placed a 0.1uF across the smoothing caps and the problem was cured!
Only the wax paper caps have been replaced so I am going to replace the smoothing caps as a matter of course as it seems they may be drying out.
I would not have gone for this fix as there was no 'hum' on the audio!

My next issue is to check the I.F. alignment but I am a bit wary of breaking the cores on the transformers and I do not recognise the trimmer. Is a special tool or method needed to adjust these?

Everybody's help is VERY much appreciated - thank you.

[Nuvistor]

Check the IF alignment with a sig gen, 'rock' the gen frequency around the IF , if it's sharp and on freq then no need to twiddle the cores.
Frank

[Boater Sam]

GOOD!
Its great to be right now and again, thanks.
Restuff the can? Looks better than hemorrhoids underneath.

If the IF and RF cores have not been got at LeAvE ThEm AlOnE.
The alignment will not be far out and the risk of cocking it up is high.
If the stations are in the right place on the dial, then the RF is OK too.

There is some merit in adjusting the aerial trimmer (preset capacitor) only to match with the aerial if you use an exterior one but it should match the inbuilt frame aerial as it is.

Resist the temptation to be a twiddler unless you have good reason to realign and you have the necessary equipment to do it properly.

[David G4EBT]

Originally Posted by John G8MWF

I am going to replace the smoothing caps as a matter of course as it seems they may be drying out. I would not have gone for this fix as there was no 'hum' on the audio!

My next issue is to check the I.F. alignment but I am a bit wary of breaking the cores on the transformers and I do not recognise the trimmer. Is a special tool or method needed to adjust these?

Whatever shortcomings the DAC90A has, the twin smoothing/reservoir capacitors isn't one of them.

I've never had to replace them, and I think when people do, it's from the mistaken view that they should be replaced as a matter of course along with all the waxies just because the set is over 65 years old. Unless you have pronounced hum, they're best left alone. As often as not, if there is hum, it's much more likely to be from the UL41 output valve and I dare say that many smoothing/reservoir caps have been changed only to find that it made no difference to the hum level as they were looking in the wrong place for the cause.

As to the IF alignment, if you check the alignment with a signal generator, I'd be surprised if it isn't spot on. Luckily, because it is difficult to fiddle with the IFT cores, they generally defeat the best endeavours of the meddling hands of the phantom twiddlers through whose hands they passed over the years.

Seems to me that you've done what's needed to the set! 'High fives' - maybe best to quit while you're winning!

Ooops - crossed with Sam's post - same sentiments.

[PsychMan]

Id highly recommend the reformer circuit listed on this site.

When I did my Dac90a recently, I used it to reform the smoothing caps. They started off with quite high leakage, but eventually came down to well within tolerance. They work fine now and run cool. Id never consider replacing one now without giving it a proper blast on the reformer first

[HamishBoxer]

Well I am going to differ,the last DAC90A I did in 2016 the smoothing cap would just not reform.One from the BVWS a top quality replacement cured the hum.

Though out of all I have had through the workshop I think it was the first that needed changing.

Re alignment I have never needed to touch the if,s.Only thing that can occur is the dust core can come detached.

[ms660]

Whatever the outcome with the HT electrolytics, if the motor boating still only stops with a 0.1uf connected across the HT, then you need to look elsewhere.

Lawrence.

[John G8MWF]

Originally Posted by Boater Sam

Motor boating is usually caused by failed smoothing or decoupling capacitors.
Which caps have you changed?
Make sure there are no connections to pin 4 of the UL41, bush used it as a tag strip.

Originally Posted by PsychMan

Id highly recommend the reformer circuit listed on this site.

When I did my Dac90a recently, I used it to reform the smoothing caps. They started off with quite high leakage, but eventually came down to well within tolerance. They work fine now and run cool. Id never consider replacing one now without giving it a proper blast on the reformer first

Thank you all for the advice regarding the I.F. - I will be leaving it alone!

I think that the smoothing caps, dated December 1957, do need replacing. The speaker appears to have a sub audio hum on it i.e. it is visually vibrating even with the audio turned down. When a new 15uF cap is placed across the original 16uF cap it disappears.
If you measure the HT rails using a DVM on the AC range, with just the original caps in place, you get about 3.5 volts - presumably ripple - When the new 15uF caps are tagged across them, not my favourite method, this is virtually 0V.

I have never tried a reformer circuit before and generally just use a series lamp and gently let events take their course. I always assumes that once a cap had 'dried' out then that was it!

[Ed_Dinning]

Hi Gents, it is not unknown for the internal resistance of the e-caps to rise with ageing, especially the HF resistance. There may be little hum as the LF resistance is OK.

Given the normally good history of these particular caps I would tend to leave tgem in place and just use the parallel 0.1. I often fit one of these across the HT line in a set restoration where I do not replace the e caps as it will reduce motor boating an whistles on tuning.

Ed

[Boater Sam]

I agree with Ed, the HT line on the DAC90A is not particularly well decoupled, some of the accepted mods on this chassis include separate screen feed resistors and extra decoupling for the IF and 1st audio stage instead of the commoned parts.
But if there is evidence of speaker cone movement at a low frequency than I would also suspect the reservoir and smoothing caps as being low in value even if they don't heat up. Am I correct in thinking that motorboating is always due to instability in the audio stage/s rather than the IF?

[kalee20]

Not always! Instability in IF amplifier may make it oscillate. This generates a massive input into the detector. The AVC voltage shoots up, reducing IF gain, below the threshold of oscillation, the AVC voltage decays till it starts again. Result: Phut-phut-phut in loudspeaker.

[Boater Sam]

Obvious really, now I know. Thank you.
Funny how some sets like to motorboat, others never. But the decoupling on the Bush sets of this ilk leaves a lot to be desired. They seem to have economised on components as near as they dared, but I suppose that was the business then, they never wanted then to last 70 years!

[John G8MWF]

The set is now in very nice shape and I would still like to fix any faults that may not have been present when the set was made.
I still have mains hum at low volume and from previous discussions that I read on this site I understand that this is quite 'normal' at low volume.

My issue is that the other DAC90A that I own has very little hum at low volume!
With this in mind I changed the HT smoothing caps but this didn't seem to make much difference.
V4 was also changed with a known good UL41 with identical results.

The only thing that I can think of is that the output transformer has a primary resistance of approx. 600 ohms as opposed to the 500 ohm stated in the manual.
Is it possible . . . that the transformer has been fried over the years and has not only increased in resistance but has reduced in inductance - reducing its choke effect on lower frequencies and so the increase in hum ?

Also, this model 'appears' to be an early model, circa 1950, with the original 250 ohm lamp shunt resistor so that the chassis has a higher restive path back to the neutral side.
Unlike the other set touching the chassis with your hand, I have checked the mains polarity!, causes the volume to reduce - presumably because it is floating a bit high than the later models? Does this also cause more hum, a feature, on the earlier models?

Your thoughts as always would be most welcome.

[Voxophone]

Have you isolated pin 4 on the UL41 yet? This pin is not actually used by the valve for external connections but Bush used it as a convenient tag for some other wiring. If you haven’t done so already, desolder all connections to pin 4 and transfer the two resistor ends to the nearest tag on the main board (to which they are currently connected by a wire jumper). Some people have reported that this reduces hum.

Aside from this, make sure that the metal body of the mains switch/volume pot is connected to chassis. If it is floating, this too can lead to hum.

Good luck!

Liam

[Boater Sam]

Heater wiring routed near the audio valve grids can cause pickup, if it is not altered by the volume control it must be in the output stage.
The pin 4 trick sometimes works but many UL41 valves do have heater cathode leaks. There is a fearsome method of reducing this if you are brave enough, flashing between the joined heater pins and the cathode pin either with a charged capacitor or a spark generator of a gas oven, only for the brave or well off!