Bush DAC 90A Conversion

[Chicoull]

I have just finished restoring my Bush DAC 90A, but want to take it a little further by adding Bluetooth. I have a GEC set which has a GRAM input, so adding BT to that was easy.

The bush not having GRAM is a bit different. My setup is a BT r/x which is powered by a 240Vac/5Vdc power supply. The radio input to the Volume Control is disconnected and the output of the BT fed in at this point.

The BT connects fine and volume control is available at both my iPad and the radio. However, with the radio input disconnected, there is a radio channel still transmitting over the BT. With the BT switched off the radio transmission is clear, but at a fixed volume - the radio Vol knob having no effect on this signal.

Any ideas where the fixed volume signal is coming from with the input disconnected at the volume control?

It is my intention to connect the radio and BT inputs to the volume control via a DPST switch - giving me the best of both worlds.

[Cobaltblue]

On the DAC90 the volume control is on the grid of the UBC41 which also contains the detector diode and AGC.

My guess the unloaded detector is inducing the signal perhaps via the cathode onto the triode.

Cheers

Mike T

It does sound like the cathode decoupling cap of the UBC41 is a bit leaky. Does the radio station change with tuning? If that is the case merrily tune to a dead part of the band. BTW I do like the idea of using Bluetooth to isolate the radio from the input device. You could probably steal enough power from the dial lamp voltage to run the BT unit.

[Chicoull]

Hi, I have replaced all the usual caps but not the micas. There is one mica strapped across the volume control - would a leak here cause the problem?

The radio signal does change with tuning, but I would prefer to resolve the problem rather than tune out of it...

I will check the new caps tomorrow to see if there are any leaks.

Not sure about the unloaded detector idea - by swapping over from the normal input to the BT input, am I not doing the same as I would be if I connect to the GRAM input on my GEC - switching to GRAM seems to be the same as disconnecting the volume switch input?

I intended to power the 5V DC unit from the AC input, but I like the idea of using an in house supply rather than adding the 5V unit - I will look at tapping off the lamp circuit.

Hope to resolve this because the BT sound on my GEC is great and I would like to emulate that on my Bush. I know I could use a pantry transmitter, but the BT is a neater solution.

[julie_m]

Probably teaching granny to suck eggs, but remember the whole Bluetooth module must be treated as live once it is connected to the DAC90A (or any other transformerless set).

Try a resistor of about 10 kΩ across the Bluetooth output, which should make the input it is connected to less sensitive to picking up any stray signals. This will have to be switched out when you wish to listen to the radio. If the radio still breaks through, try replacing decoupling capacitors, and consider adding additional decoupling physically near the UBC41 and UL41.

Don't touch any capacitors in tuned circuits, unless you are prepared for a full re-alignment.

[Chicoull]

Before breaking into my restored Bush, I opened up an unrestored 90A this morning and disconnected the input to the volume switch - without connecting the Bluetooth.

The second radio has the same issue - a tuneable radio signal with fixed volume at the speaker. So, two radios - one fully restored with replacement valves, new caps, resistors and wiring - versus an untouched original.

The same issue in both suggests that the stray signal is in the design so I will revisit the circuit diagram - try and trace the route and attempt a block - possibly additional decoupling as suggested.

As to the Bluetooth output - it is stereo, so I have a couple of 1k resistors tied together to give a mono output and a 10k resistor across the output for a load.

[Cobaltblue]

Quick test put a 500 K fixed resistor between the lead you have lifted off the volume control and the common end of the volume control (cathode of V2 and V3) this will return a load onto the detector

See if that improves it.

Cheers

Mike T

[Chicoull]

Hi Mike,

I thought you had the answer there....it made sense, however no change - the signal may even have been a little stronger!

thanks, Charles

[Cobaltblue]

Do you get zero volume when the set is configured normally?

(when the control is fully anticlockwise that is?)

Cheers

Mike T

[Chicoull]

Yes, with the radio wired as normal - the volume control functions correctly.

Regards, Charles

[ms660]

Try bridging the cathode bypass capacitor (0.05uF) with a much larger value such as an electrolytic and see if that makes any difference.

Lawrence.

[Cobaltblue]

Is the BT unit connected directly across the volume control?

Cheers

Mike T

[Silicon]

Have you modified the wiring of the 'ground' end of the volume potentiometer?

In the original circuit the ground end of the resistive track in the volume control is connected to the cathodes of V2 and V3.

These are about 1V positive with respect to the chassis.

The output signal ground of the Bluetooth device may not be isolated from its negative power supply connection.

If you connect the power supply negative to chassis you will probably change the biasing of V2 and V3 and alter the AGC behaviour.

There will be a small amount of IF frequency at the cathode of V2. It may find its way into the audio amplifier, get rectified and turn the Bluetooth device into a crystal set.

[Chicoull]

In answer to your questions:

1. I bridged the cathode bypass cap with 2.2uF - no effect.

2. The BT unit is connected between the volume input and chassis, but the stray signal is there even with the BT disconnected from circuit.

3. I have not modified the ground end of the pot. I take the point that there could be an issue with the power supply negative, but I refer back to item 2 - the signal exists without the BT in circuit - and to my previous post regarding checking an unrestored 90A - it has the same signal issue with the volume input disconnected.

regards

Charles

[Silicon]

If I understand the previous posts, you have disconnected the 'top' of the volume control from IFT2.

However IF voltages are still present at both diodes in V3.

A small signal may be capacitively coupled from these diode anodes to the cathode of V3.

V3 may be operating in grounded grid mode and passing a small fixed volume signal to the audio output stage.

If you temporarily connect a 0.1uF capacitor from chassis to each diode anode (pin 5 and 6) it may reduce the volume and confirm the source of the signal.

[Chicoull]

Hi Silicon,

I will act on your suggestions tomorrow, tonight, vodka and irn bru has reached the point of disabling my abilities - will let you know how it goes.

regards,

Charles

[Chicoull]

Hi, I acted on Silicon's suggestion this morning.

A 0.1uF cap from V3, Pin 6 to chassis resulted in no stray signal on LW but MW signal still there. This cap was then removed.

A 0.1uF cap from V3, Pin 5 to chassis resulted in success! No stray signal on MW or LW.

Jim Reeves has been playing on Bluetooth for the past hour with no problems.

Am awaiting delivery of a DPST Rocker Switch to enable switching between the radio and BT.

Many thanks for all your inputs to resolve this issue.

Best regards, Charles