Bush DAC90A Volume Problems

[bobhowe]

Hi one of my Bush DAC90A ( Standard one ) has developed a fault with the volume turned right down it is still playing on strong signals read on this forum that it could be the UBC41 valve tried a valve out of a different set with no issues and it is still the same . Read on the Radio museum site and one member Konrad Birkner bought up a topic in 2014 & this is what he says . The initial problem was that the minimum volume was slightly off the ccw stop of the potentiometer that happened at strong signals . As previously described that is was a constructional fault . There may be a worn track on the Volume pot but before you replace it to check that the blocking capacitor is not missing. Where is this capacitor and where does it go?

Kind regards, Bob.

[Station X]

With faults like this it's always worth checking the resistance of the volume control pot from the "bottom" end of the track to the wiper with the volume control set at minimum.

[bobhowe]

Hi Graham thanks for advise can i do resistance checks with the cables in situ or will i have to lift the cable of the wiper kind regards Bob

[Station X]

It won't matter. The only DC path to the pot is via R8 (manufacturer's sheet) so check that too.

[bobhowe]

Thanks Graham will post some readings tomorrow kind regards Bob

[bobhowe]

Hi Graham hear is some readings taken on a Fluke True RMS DVM 1.259 K ohms min volume on the pot 675 M ohms max volume on the pot checked R8 & it reading 356 ohms kind regards Bob

[bobhowe]

Hi just read on another page of the RadioMuseum site but i can not access the full information that there this is a common problem with the volume not going down to zero & the remedy is to put a 330 picofarad capacitor between the Grid and Kathode on the UBC41 valve has anyone done this & what voltage rating should the capacitor be kind regards Bob

[cathoderay57]

675 M ohms max volume on the pot

Hi Bob, that's not right. If that's what you are measuring then it looks like either the carbon track on the pot has a break in it or perhaps more likely a dirty wiper is causing bad contact with the track. The max resistance should be 500k. If you meant 675k that is probably OK. With reference to putting a 330pF between grid and cathode I can't see how that would help. It is only likely to attenuate the treble response slightly. I'd be more inclined to check and replace the cathode decoupling capacitor (0.05uF) marked on the Bush service sheet as C15 (in parallel with R8 the 330 Ohm resistor mentioned earlier). Jerry

[bobhowe]

Hi Jerry took some measurements on a set with no issues 342 ohms min volume on the pot and 648K ohms max volume on the pot & R8 354 ohms should i try and clean the pot in Methylated spirits ( tried Servisol made no difference } before i condemn the pot and i will check C15 as you say kind regards Bob

[electrogram]

If it is only the spring of the on/off switch which stops the vol control reaching zero ohms it is sometimes possible to to change the position of the the switch relative to the pot by removing a bit of the metal case so that the pot can be moved slightly so that zero position of the pot is reached before the switch operates

[cathoderay57]

Hi Bob, if you've used Servisol I doubt there's much to be gained by using meths. On the troublesome pot are you sure you are measuring 675 MegOhms at max volume? If so there is definitely something wrong with it and it needs to be replaced. The reason I mention it is most meters are unlikely to measure such a high resistance. Jerry

[Graham G3ZVT]

To use a well worn saying, it's not a fault, it's a "feature".

My DAC90A certainly exhibits this inability to turn down to zero volume particularly on "loud" stations and others have reported the same behaviour. Looking at the circuit it's not surprising why this is, there is a a 330 Ohm between the bottom of the volume control and the chassis. The purpose of this resistor is to provide grid bias to the UBC41 and UL41, by making their cathodes a little positive wrt chassis. The small value of the bypass cap (.05uF) results in some overall negative feedback.

[Gabe001]

I had a dac90a that didn't go to zero on strong stations either. It's a known "feature" as Graham said I remember reading the same radiomuseum reference you quoted and leaving things as is.

[AD360 Rob]

As I'm sure you are aware by now this is quite common as the resistive track on the volume pot drifts high in value. I've had it myself to varying degrees on about 4 DAC90A sets. The only cure was to replace the pot with one that measures better at the low end. Sadly, new style plastic pots whilst working well don't have the "feel" of an original but that's something you have to live with if you want full control over the volume.

[ms660]

Per schematic (a perfect pot)....0.1 Volt in at the pot = approx. 68uV at the wiper when set to minimum (@1kHz)

Lawrence.

[bobhowe]

Hi AD360 Rob thats makes sense never give it a thought that volume pots go high in value like resistors kind regards Bob