Bush DAC90a

[Ed_Dinning]

Hi Folks, I've just had one of these to repair that looked as if it had only ever had the UL41 replaced and no other work done.
Main fault was a faulty second IFT, duly replaced from a scrap chassis and was found to be almost spot on alignment wise.
It then had the Peter Vaughn mods done and sounds great
Main residual problem is motorboating on LW, no amount of additional decoupling seems to cure it, but no problems when right on station (and fortunately no drift). Any ideas ?

I also carried out my latest mod as I mentioned in another thread; I used 2. 6.3V 60mA lamps, connected in parallel and with a 250R bypass resistor. Result is reasonable brightness and no sign of lamps dying if rapidly switched.
I think these lamps are readily available, I had them as HT fuses for battery sets.

Ed

[Piccy2valve]

Hi Ed,
Just starting my first radio repair attempt a dac90a curious what the Peter Vaughn mods are

Regards mark

[DangerMan]

Hi Mark,
Modifications are described here:

https://www.vintage-radio.net/forum/...ad.php?t=35784

Pete

[DonaldStott]

Thanks Pete - it's easy once you know that Peter Vaughn is Forum member kalee20.

In relation to these mods I can only agree with the comment from stevekendal in the linked Thread "Wonderful piece of research and immaculate wiring there, makes my own fumblings look rather crude"!

@Ed_Dinning (or anyone) - I'm still none the wiser as to what is meant by "motorboating" and what is sounds like. I've always thought there is a place for a Sticky with links to audio samples on what common problems sound like?

When I have the time I must go back and revisit my DAC90A - although it's working the scale lighting is very dim, there is a residual hum and the treble response is poor.

[G4CMY_Tony]

In my view, "motor boating" is a pulsing, low frequency oscillation that sounds a bit like the motor on a small boat.

[ms660]

An example of motor boating:

https://www.youtube.com/watch?v=MDIuVFiVIKo

Observant ones might be able to catch the the IF birdies.

The time periods are often governed by the time constants in the AGC circuit.

Lawrence.

[DonaldStott]

Originally Posted by ms660

An example of motor boating:

https://www.youtube.com/watch?v=MDIuVFiVIKo

Thanks Lawrence

[kalee20]

Good to know there's another modified DAC90a out there - thanks Ed!

Originally Posted by Ed_Dinning

Main residual problem is motorboating on LW, no amount of additional decoupling seems to cure it, but no problems when right on station (and fortunately no drift). Any ideas ?

Thinking in writing... motorboating is often caused by inadequate HT rail decoupling, causing feedback at AF from a later stage to an earlier stage via HT rail common impedance. But that can't be the case here as there are only
two AF stages (UBC41 and UL41) so any feedback would be negative.

Oscillation in the AVC loop? Signal level rises, so AVC voltage increases, but there is a delay in the AVC time constants so the control voltage does not get fed back: by the time it does, the signal level has overshot, the valves start throttling back, the AVC level falls, but the reduction in AVC voltage is again time-delayed. Continuous LF oscillation results. But that would surely be most likely to occur with a strong signal, bang on tune. Anyway, worth checking the AVC series resistors just in case one has gone high.

Variation in frequency in the frequency changer with AVC voltage? Sounds a bit more likely. AVC voltage rises, oscillator frequency moves off-tune, IF signal moves down the IF amplifier response curve, IF signal at the detector falls, so AVC voltage falls. But again there would need to be significant phase shift in the AVC decoupling network.

I'd try forcing an external negative bias (9V battery and potentiometer) onto the AVC line, to overcome the radio's own AVC, and see what happens. Maybe a frequency counter picking up the IF signal would help.

[Ed_Dinning]

Hi Peter, thanks for that.
Set was with its original ecap that showed OK for ESR, but I normally add an extra 0.1 or so as ESR tends to rise with frequency.
I think your mods increased AVC cap values a bit. Resistors all checked OK

Donald, hopefully motorboating has now been explained,m you will know it as soon as you hear it.
Piccy, the links to Peters mods were given above. In them he "modernises" the DAC90a circuitry to improve the sound of the set so it is more to modern tastes, rather than "mellow bellow". The reasons are also well explained in the article.

Ed

[Helder Crespo]

Originally Posted by Ed_Dinning

Hi Folks, I've just had one of these to repair that looked as if it had only ever had the UL41 replaced and no other work done.
Main fault was a faulty second IFT, duly replaced from a scrap chassis and was found to be almost spot on alignment wise.
It then had the Peter Vaughn mods done and sounds great
Main residual problem is motorboating on LW, no amount of additional decoupling seems to cure it, but no problems when right on station (and fortunately no drift). Any ideas ?

I also carried out my latest mod as I mentioned in another thread; I used 2. 6.3V 60mA lamps, connected in parallel and with a 250R bypass resistor. Result is reasonable brightness and no sign of lamps dying if rapidly switched.
I think these lamps are readily available, I had them as HT fuses for battery sets.

Ed

Hi
Quick question: I'd like to try the output transformer mod this weekend, replacing the 0.01uF with a high-voltage 6.8nF+4.7k. Would a 1/2W resistor be enough, or should this require something larger?
Thanks!

[kalee20]

Larger. But for trial purposes 1/2W will be OK.

It's only going to dissipate significant power if you listen to material with a lot of treble content, and you have the volume well up. The maximum it would dissipate is what the UL41 can throw out, i.e. 2.5W in the DAC90a, so if you like it and buy a resistor for permanent, go for a 3W type.

[Helder Crespo]

Thanks! I definitely would like to go permanent in case I'm happy with the perceived treble response. I will test it with a modern source - the output of a bluetooth module - which is part of a totally reversible mod in my second DAC90A. I would not expect much change from an AM source, although I could be wrong in the light of your description in the thread referenced above.

[Ian - G4JQT]

Originally Posted by kalee20

.

Oscillation in the AVC loop? Signal level rises, so AVC voltage increases, but there is a delay in the AVC time constants so the control voltage does not get fed back: by the time it does, the signal level has overshot, the valves start throttling back, the AVC level falls, but the reduction in AVC voltage is again time-delayed. Continuous LF oscillation results. But that would surely be most likely to occur with a strong signal, bang on tune. Anyway, worth checking the AVC series resistors just in case one has gone high.

Variation in frequency in the frequency changer with AVC voltage? Sounds a bit more likely. AVC voltage rises, oscillator frequency moves off-tune, IF signal moves down the IF amplifier response curve, IF signal at the detector falls, so AVC voltage falls. But again there would need to be significant phase shift in the AVC decoupling network.

Sounds like the 'fault' on my completely rebuilt Ekco A22 when using my pantry tx. Only motorboats on that strong station at high volume. Lots of extra HT decoupling fixes it, but that shouldn't be necessary.

[Ed_Dinning]

Hi Helder, the other mods round the output valve are also worth doing.

Ed

[Piccy2valve]

Dangerman, Thank you for the link, i will look at putting peters mods into my first radio project , the dac90a that i am starting now

[TrevorG3VLF]

I wonder whether there is a phase problem. The IF transformer was changed and the phase may have been changed with overall stray feedback changed from negative to positive.
Sometime ago I repaired a set by replacing a chewed up IF transformer secondary with a single tuned transistor transformer top coupled to the primary which probably changed the phase. The set was partially umstable which was cured by cutting the AGC line and adding more low pass filtering. The set had a ferrite bead in the AGC line near the first valve indicating that the design was dubious.

[Helder Crespo]

Originally Posted by Ed_Dinning

Hi Helder, the other mods round the output valve are also worth doing.

Ed

Thanks Ed.

[Helder Crespo]

Originally Posted by Ed_Dinning

Hi Helder, the other mods round the output valve are also worth doing.

Ed

Hi Ed (and Peter)
These seem easy to do (the hard work was already done by Peter). I have one very practical question: where can I get components around London without having to buy 50 of each or something like that (as with RS and etc.).
Also, could you help me with your recommended power/voltage ratings for each of the new components? Thanks!
Helder

[John10b]

Cricklwood Electronics, North London,is a good source of supplies.
Cheers
John

[Helder Crespo]

Originally Posted by John10b

Cricklwood Electronics, North London,is a good source of supplies.
Cheers
John

Thanks John!