Bush DAC90A hum - AGAIN!
 

I had restored yet another Bush DAC90A which worked very well for several months with no real audio problems but has gradually turned out to be a bit of a hummer! I'm not talking about the usual background hum that we all know to expect, but a hum that can still be heard when a station is tuned in? The hum also increases as the volume control is turned up?

In order to avoid lots of helpful Posts starting 'Have you tried ...' the answer will probably be YES i.e. changed paper caps, reservoir/smoothing caps, over tolerance resistors, swapping valves from known working sets, Pin 4 mods, replaced volume pot-on/off switch, replaced lots of crumbling wiring etc. etc. Thanks to David G4EBT for help and assistance in testing and working through these changes.

So I'm a bit stuck... ! ​

Earlier this year I restored an Ecko U122 which had a similar loud hum issue but this was resolved through the addition of a CRC (Pi) Filter between the reservoir capacitor and the load. This comprised a 220Ω resistor and a 33uF electrolytic capacitor - thanks to Lawrence (ms660) for help with this. Now I'm wondering if some similar RF filtering would help with this DAC90A although I do appreciate that the Ekco uses grid bias and the Bush sets use cathode bias?

I'm also considering the option to replace the UY41 half wave rectifier valve by disconnecting the anode and cathode connections and substituting a 1N4007 and 200Ω resistor, checking of course that the UY41 heater is still dropping the correct voltage. See here - Replacement of valve rectifier with silicon diode And as part of my ongoing education, would it be possible to use 1N4007s in a full wave rectifier configuration.

This DAC90A set has been used as my experimental test bed e.g. installation of a Bluetooth module, so I'm not concerned about circuit modification and realise fully that any further changes may be at the expense of originality.

I have several sets of the same vintage in the room where I work and none of these exhibit the level of hum heard from this particular set?

Re: Bush DAC90A hum - AGAIN!
 

Are you on a pantry transmitter?

Re: Bush DAC90A hum - AGAIN!
 

No.

Re: Bush DAC90A hum - AGAIN!
 

You changed the smoothing and reservoir caps, did you re-stuff the original Can ?

John.

Re: Bush DAC90A hum - AGAIN!
 

Could that be modulation hum? Have you removed C22, the mains filter capacitor which is intended to reduce modulation hum issues? If so, it will need to be replaced.

Annoyingly modulation hum can also be caused by other power supplies/radios nearby.

One thing is for sure, a 1N4007 will very likely make the problem worse. Use a UF4007 in preference.

Paula

Re: Bush DAC90A hum - AGAIN!
 

Fixed a DAC90A yesterday which had the same problem. Checked for ripple on the HT lines. The oscilloscope showed 15volts P - P ripple across the reservoir capacitor which is OK and the lower potential HT line had hardly any ripple at all.
A quick under chassis examination revealed the UL41 output valve grid stopper resistor was anchored to a spare tag on the valve holder. Relocating the resistor to an adjacent tag on the component group board effected a cure.
No hum at all from the loudspeaker with the volume control turned fully down.

DFWD.

Re: Bush DAC90A hum - AGAIN!
 

Attachment shows the repositioned UL41 grid stopper resistor.

DFWB.

Re: Bush DAC90A hum - AGAIN!
 

Thanks to everyone for their helpful responses on a topic that has been 'round the houses' too many times to mention on this Forum.

For the avoidance of doubt I used one of the new 16uF/32uF dual cans from the BVWS. And just for the sake of completeness, putting back the original dual can made no difference either.

C22 has been replaced with an X2 mains filter capacitor but I've never found this to make a difference.

Thanks for the tip about the 1N4007 so I'll be sure to use a UF4007 if I decide to replace the UY41.

I do the kalee20 UL41 Pin 4 mods as standard on any DAC90A restorations - mentioned in my OP.

Re: Bush DAC90A hum - AGAIN!
 

If the onset of the hum was gradual, it suggests more of a wear and tear type fault. You've already eliminated valves by swapping them out for different ones. Everything else seems ok - resistors within spec, new caps etc. An RC filter will mask the fault temporarily not deal with the underlying cause.

What is the state of the shielding of the signal wire. Is it intact? Are your ift cans properly connected to chassis?Finally, could there be a fault with the volume control track itself, as Jerry recently described in the Schneider Melodie thread, or even a valve base going leaky?

Re: Bush DAC90A hum - AGAIN!
 

Have you checked the earth tags?

Re: Bush DAC90A hum - AGAIN!
 

If the hum increases as volume is turned up, it says that the hum starts at, or before, the volume control.

So, disconnect the lead from the IFT to the volume control, so that there's no input. It'll kill the audio of course, but whether the hum remains, or is also gone, is the important bit.

It could be that hum is originating in the grid circuit, in which case progressively shorting the grid to chassis (from an AC point of view) will reduce it. But it could also be arising in the diodes.

(Thanks for doing my modifications, by the way!)

Re: Bush DAC90A hum - AGAIN!
 

Thanks Gabe001 for your suggestions and ongoing support.

When I did the Bluetooth module installation I took the opportunity to replace the somewhat tatty signal wire with a new length of co-axial cable. Before anyone asks, the Bluetooth module was completely disconnected and all associated wiring removed to ensure that this wasn't the source of the hum.

The existing on/off switch-volume control was replaced with a modern 470k log pot.

Not sure how to check for a valve base going leaky?

I'll certainly recheck the IFT cans/chassis connections along with all other earth tags as suggested by PJL above.

Thanks kalee20 for these suggestions and I can certainly disconnect the lead from the IFT to the volume control and listen out for that hum!

Not sure what you mean by 'progressively shorting the grid to chassis' and 'it could also be arising in the diodes' so I'd appreciate a bit more detailed guidance on those - thanks.

I now do a number of your mods as standard with any DAC90A (and DAC10s) including the UL41 Pin 4 mod, the dial lamps resistor mod and the UBC41 anode to chassis cap mod.

Re: Bush DAC90A hum - AGAIN!
 

Ah. Yes! I agree, putting myself in the reader's position, my text isn't clear...

So. When the volume control is progressively reduced to zero, the resistance between wiper and chassis tag is progressively reduced to zero.

You say that hum reduces as the volume control is turned back - well, I'm thinking that maybe there is a leakage current in the grid circuit which, when the grid impedance is high, allows a hum voltage to be generated but when the impedance is low, the voltage is miniscule.

Of course, in the DAC90a, the grid isn't connected directly to the wiper: it goes through a capacitor. But the effect may be demonstrable, all the same.

On the other hand, if the hum is arising by leakage from something to the detector diode anode, then it will disappear with the signal when you disconnect the wiring to the volume control 'hot' tag.

Re: Bush DAC90A hum - AGAIN!
 

[QUOTE=DonaldStott;1425072]

For the avoidance of doubt I used one of the new 16uF/32uF dual cans from the BVWS. And just for the sake of completeness, putting back the original dual can made no difference either.

[QUOTE=



[/QUOTE]

My reason for asking if you had re-stuffed the original can is that if the Can is not washed clean of all the old capacitor residue it is highly corrosive. It may only take a few weeks for the new capacitor wire to corrode through. But you have avoided the problem by using a new dual cap.

John.

Re: Bush DAC90A hum - AGAIN!
 

Re valve bases just check the heater pins and adjacent pins for continuity. You will need a multimeter than can measure resistance up to 10Mohms at least. A DMM that measures only up to 2Mohms isn't suitable. Disconnecting the signal pathway at various tags (starting with the volume control) and seeing if the hum persists or not may help "bisect" the area at fault.

Re: Bush DAC90A hum - AGAIN!
 

I meant "resistance" not "continuity" apologies for the confusion

Re: Bush DAC90A hum - AGAIN!
 

Pickup from switch mains live to volume track/wiper?

Re: Bush DAC90A hum - AGAIN!
 

Just out of interest I connected up my 'scope to the speaker terminals of the DAC90A to determine the frequency of the hum. The DAC90A was of course plugged into an isolation transformer. The resultant waveform was slightly 'dirty' but calculating the reciprocal of the Period I'm getting, guess what, 50Hz.

Rechecked all my valve voltages, including heaters, and there were no surprises.

Cleaned up all the chassis tags and connections I could find and replaced a number of components around V4 and V3. The hum has reduced very slightly but I'm still getting around 48dB?

I disconnected the lead from the IFT to the volume control and as expected, this killed the audio but the hum was still present - so this helps narrow things down to one end of the set?

When I short Pin 6 (Control Grid) of V4 (UL41) to the chassis I'm getting a loud buzz but I read in previous Threads that others were reporting a reduced hum by grounding the UL41 grid??

Control Grid to chassis is 0.00V
Cathode to chassis is 4.32V
Cathode to Control Grid is -4.17V

V4 had been replaced with a CV1977 while V5 is a NOS UY41.

Re: Bush DAC90A hum - AGAIN!
 

As buggies said, probably pickup from the mains wiring to the volume/on off control - or even within said control.

I had a similar problem with a DAC10 that hummed for England until I rerouted the mains wiring away from the 'hot' end of the pot.

Re: Bush DAC90A hum - AGAIN!
 

I’ve got some sticky backed aluminium tape that I can use to try and shield the mains wiring?

What about fixing any pickup from within the volume-on/off control?

The connection from the IFT to the volume control is a co-axial cable so I’m assuming that this will have sufficient shielding?