Bush DAC90A - Adding Bluetooth

[DonaldStott]

Originally Posted by wd40addict

Let me dig my DAC90A out and see how it behaves. Have you replaced the V2 V3 cathode bypass capacitor?

Thanks - I will be really interested in what you find?

All waxy paper caps have been replaced including the V2 V3 cathode bypass capacitor.

[wd40addict]

OK, I've only replaced critical caps on mine and the set performs well. If I disconnect the wire from the IFT / C12 audio immediately ceases; even on Radio 5 which is the strongest signal round here as it's broadcast from 20 miles away.

1st Picture shows disconnected wire held by a scope probe so I can touch it on and off. If I move the scope probe to the IFT then I can still see the audio, but nothing is heard through the speaker.

2nd Picture shows a KRC-86 bodged in using the switched configuration (+470k & 0.1uf). Note this module has the 3.5mm jack PCB and 100k resistors are still fitted.

It works absolutely fine and as expected. In radio mode volume is reduced slightly by the presence of the 100k's. Plenty of volume in the Bluetooth mode, Hancock's half hour sounds good!

If you lived nearer and we weren't in lockdown I'd say come round with your set, but I can't see what more to suggest.

If anyone is still following this thread remember the DAC90A is a live chassis design and the isolating transformer at the bottom of picture 2 shows the yellow isolation transformer which is enabling me to connect scope etc.

[DonaldStott]

Thanks Paul (wd40addict) for taking the time to dig out your DAC90A and configure it accordingly - much appreciated.

Originally Posted by wd40addict

If I disconnect the wire from the IFT / C12 audio immediately ceases; even on Radio 5 which is the strongest signal round here as it's broadcast from 20 miles away.

This is how I would expect the set to behave - in mine there is no connection between the IFT and the top of the volume control but the RF/IF signal is still getting through?

Don't want to turn this into a DAC90A investigation so we'll just have to move on for the time being unless anyone has any further ideas on how to quickly resolve the issue?

Meantime both the overdriving and switching options set out in detail by Paul (wd40addict) in Post #44 above work very well and there is little to choose between them in terms of audio quality. Both are certainly streets ahead of the extremely thin, tinny and sibilant output I was getting from the LT700 setup described in my Post #48 above! The additional clarification as to the connections and wiring supplied by David G4EBT in Post #72 proved essential to ensure I had configured everything correctly!

Time to draw a line, choose one of the options and move on ... thanks to everyone for their valuable and helpful contributions! I've certainly learned a lot although it's probably run of the mill stuff for you guys! I'm always humbled by the way Forum members often go above and beyond to provide help and guidance - just wish I could reciprocate more!

And just to ensure we are taking health and safety seriously I've included this part of Paul's (wd40addict) recent Post: -

Originally Posted by wd40addict

If anyone is still following this thread remember the DAC90A is a live chassis design and the isolating transformer at the bottom of picture 2 shows the yellow isolation transformer which is enabling me to connect scope etc.

In Posts #72 and #77 David G4EBT also provides excellent advice on the best location in the DAC90A for fitting a PCB, avoiding long cable runs and optimised board layout. Provides me with a detailed step by step guide to my next stage in this Project ...!

[Gabe001]

Yes Donald a good outcome I think. I wouldn't worry about having to tune to a quiet area of the band, it's not a deal breaker and is quite a common arrangement in sets from the time.

One last piece of advice - check it again in your preferred mounting position on all bands before making the arrangements permanent, in case the set picks up some digital noise from the module

[wd40addict]

Hi Donald, sorry I wasn't able to solve this remotely. The only thing I can think of is that the AGC diode is detecting at AF and it's getting back into the amplifier somehow. Does AGC seem to work ok?

It has inspired me to permanently Bluetooth my set, so I'll have a go once another module arrives from China.

[DonaldStott]

Originally Posted by wd40addict

Hi Donald, sorry I wasn't able to solve this remotely. The only thing I can think of is that the AGC diode is detecting at AF and it's getting back into the amplifier somehow. Does AGC seem to work ok?

Hi Paul - can't thank you enough for your valuable Posts in this Thread and appreciate that it's not your job to resolve my RF/IF breakthrough problem!

Not sure which component you mean by the AGC diode?

AGC is measuring around -1.1V to -1.2V which seems a bit lowish?

[ms660]

Originally Posted by DonaldStott

Not sure which component you mean by the AGC diode?

In the schematic (Trader) V3...UBC41...d1....pins 6&7:

http://www.r-type.org/pdfs/ubc41.pdf

Lawrence.

[wd40addict]

Does the AGC voltage rise and fall as you tune through strong and weak stations?

[DonaldStott]

Originally Posted by wd40addict

Does the AGC voltage rise and fall as you tune through strong and weak stations?

Yes - forgot to mention that so AGC seem to be functioning correctly?

I'm looking at possible paths to the top of the volume control if we have disconnected the wire at IFT2? What about (Trader) C13 100pF (IF By-pass) which links to the screen of this co-axial and in turn is connected to Pin7 of V3 (UBC41)?

[wd40addict]

If you have wire disconnected does the level of breakthrough vary with the setting of the volume control or is it constant?

[DonaldStott]

Originally Posted by wd40addict

If you have wire disconnected does the level of breakthrough vary with the setting of the volume control or is it constant?

I was becoming so frustrated with this I had decided that tomorrow (Monday 22/02/2021) I would reinstate my DAC90A and put all back the way it was!

Fortunately it's still on the bench (table) and lashed up to the DIY 5V DC supply, the KRC-86B board and all the wiring/components associated with Paul's (wd40addict) switching option.

I quickly disconnected the wire from IFT2 that runs to the top of the volume control. The breakthrough varies with the setting of the volume control and gets louder as the control is turned to max? At this setting, the radio station is clearly audible and not just in the background?

[wd40addict]

Have you checked R8 & R9?

Do you have another UBC41 you could try?

[DonaldStott]

Originally Posted by wd40addict

Have you checked R8 & R9?

Do you have another UBC41 you could try?

R8 & R9 have both been replaced.

All five valves have been swapped over at various times with valves from a known working DAC10 and with NOS valves - that includes the UBC41.

I've tried all the usual suspects and although this issue of breakthrough eludes me, I don't want to be taking up more of your valuable time?

[wd40addict]

I think I'm out of ideas assuming the new components have been wired correctly. It feels like leakage from the diodes to the triode of the UBC41, but very difficult to diagnose at a distance.

[DonaldStott]

Originally Posted by wd40addict

I think I'm out of ideas assuming the new components have been wired correctly. It feels like leakage from the diodes to the triode of the UBC41, but very difficult to diagnose at a distance.

Agreed, thank again Paul - you've gone the extra mile in providing help and support so now it's time to move on to the next stage.

Using the circuit layout provided by David G4EBT in Post #72 above I'm in the process of transposing that into a PCB design using Sprint Layout software. I'm currently looking at something that's around 120mm x 30mm which should just fit on '... the rear chassis apron adjacent to IFT2 and V2 & V3 valve bases' - see Post #74 by Gabe001. The component layout on the PCB still needs to be optimised and the standard pad and track spacing applied so this should take at least 10mm off the length, hopefully? Careful cable routing will be required along with ensuring that the transformer end of the PCB is nearest to the mains on/off switch.

The eventual outcome will be a PCB design that can be exported as Gerber files and thereafter sent to JLCPCB for fabrication.

NOTE: Another thank you to Forum member Terry_VK5TM (based in Tintinara, South Australia) who persuaded me last year to switch from using Eagle software to Sprint Layout and for his patience, online tuition, advice and guidance.

[David G4EBT]

Originally Posted by DonaldStott

Using the circuit layout provided by in Post #72 above I'm in the process of transposing that into a PCB design using Sprint Layout software. I'm currently looking at something that's around 120mm x 30mm which should just fit on the rear chassis apron adjacent to IFT2 and V2 & V3 valve bases

The depth of the chassis in 50mm so 30mm for the PCB is fine, but to ft it on the rear apron, I think you'll find it needs to be no more than 100mm long, which should be achievable by nudging the W01 bridge and the capacitors closer to the transformer. The PCB will need to be mounted on 10 mm stand-offs for safety so the transformer pins and AC connector pins are well clear of the chassis and as you've indicated, the AC input needs to be at the right hand end of the chassis for ease of wiring to the mains switch on the pot.

EG: M2.5 stand-off pillars:

Metal:

https://uk.farnell.com/c/fasteners-m...ze-metric=m2.5

But preferably nylon:

https://www.ebay.co.uk/itm/M2-M2-5-M...-/163450068892

The PCB will need to be towards the top edge of the chassis so it will clear the valve-holder pins etc. To give an example of how it would fit, I've made a mock-up with the transformer mounted on a strip of card 100mm long x 35mm tall in place on the rear apron of a scrap chassis kindly loaned to me by Howard, 'Half a Mullard'.

I would reiterate that I don't think that to mount the PCB above the chassis on a metal strip between the tuning scale supports is a good idea. It would be close to the heat generating Voltage Selector board with the high wattage resistors, and there would also be heat rising from the valves almost directly below the PCB, and running the wiring down to below the chassis would be rather long and inconvenient to wire up.

Just my own musings - it's not my project of course, so feel free to disregard my suggestions if you wish.

Hope that might help a bit.

[DonaldStott]

Originally Posted by David G4EBT

... Hope that might help a bit.

David you are the Master of Understatement!

Not only have you designed the layout for the PCB, worked out the optimal location for it in the DAC90A but now you have fabricated a cardboard prototype to test your theory of how it would fit!

Really not much for me or others to do apart from my Painting-by-Numbers task of transposing the PCB design using Sprint Layout software.

You may have to re-think this statement (my emboldening) "Just my own musings - it's not my project of course, so feel free to disregard my suggestions if you wish."

I'll look again at the design in Sprint Layout in an attempt to knock 20mm off the length of the PCB, starting with ' ... nudging the W01 bridge and the capacitors closer to the transformer'.

[DonaldStott]

Making very good progress with optimisation of component placing and track/pad layout with the help of David G4EBT. The planned PCB wil comprise a 5V DC supply, the KRC-86B board and all audio IN/OUT connections. Current size is around 125mm x 32mm.

Fabricated several cardboard mock-ups with PCB layouts using Sprint Layout software which allows you to print out your design actual size - a great feature.

The picture below shows one of the latest iterations on card with just a few of the larger components in place.

[Gabe001]

That's really tidy. Will you have to drill the chassis?

Will you have to drill the chassis?

Modern outdoor sticky pads are very good.