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Old 22nd Jun 2019, 8:30 am   #41
mark2collection
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Default Re: Yet another Bush DAC90A

Donald,

Looking at the photos in your first post, there's an awful lot of under chassis wiring covering the chassis-hole under the dropper.

My last DAC90A restoration, post 2, you can see my 'after' photos under the chassis, showing how I route the wiring. Post is here:- https://www.vintage-radio.net/forum/...d.php?t=152212

Also in the thread above, I've attached documents so various labels can be recreated, should anyone wish to.

The board the dropper is mounted on, I believe, is paxolin(?) My set was charred, ended up scraping the worst off and used fibre washers (in addition to the original washers) to aid insulation, with a little Araldire as further reinforcement. The fibre washers were larger than the originals, and with the spots of Araldite, they give a firm hold.

Whilst the dropper was 'out of set' I ran a long-stick cotton bud through the centre, (the cotton bud having been soaked in foaming cleanser). Incredible the muck which came out. The dropper runs hot, but the chassis in general, no longer smells hot.

Mark
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Old 22nd Jun 2019, 11:18 am   #42
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Default Re: Yet another Bush DAC90A

Thanks Mark - your superb restoration/resurrection creates a bench mark for me to aim for! I'll ensure that I compare and contrast the under chassis wiring and maybe Post some pictures later.

As I've highlighted what may be overheating problems I've focused my attention for now at the dropper end of the circuit.

Here are some voltage readings: -

V5 - UY41 half wave rectifier

Anode (Pin 2) - Service Sheet 222 A.C. / Actual reading 110 A.C.
Cathode (Pin 7) - Service Sheet 205 / Actual reading 182

V4 - UL41 audio output pentode


Anode (Pin 2) - Service Sheet 190 / Actual reading 162
Screen (Pin 5) - Service Sheet 98 / Actual reading 87
Cathode (Pins 3 & 7) - Service Sheet 5 / Actual reading 3

Most of the voltages are about right apart from the Anode (Pin 2) of the UY41??
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Old 22nd Jun 2019, 11:22 am   #43
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Default Re: Yet another Bush DAC90A

Check the meter setting or try another meter.

Lawrence.

Last edited by ms660; 22nd Jun 2019 at 11:50 am. Reason: addition
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Old 22nd Jun 2019, 12:22 pm   #44
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Default Re: Yet another Bush DAC90A

Tried another meter and checked meter settings on both - they both agree on the readings in Post #42 above.

All readings are taken with respect to chassis.
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Old 22nd Jun 2019, 3:11 pm   #45
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Default Re: Yet another Bush DAC90A

Quote:
Originally Posted by DonaldStott View Post

V5 - UY41 half wave rectifier

Anode (Pin 2) - Service Sheet 222 A.C. / Actual reading 110 A.C.
Cathode (Pin 7) - Service Sheet 205 / Actual reading 182
***UPDATE & CORRECTION***

Apologies as I got my Pins 1 and 2 mixed up on the UY41 - watch out for the glass pip!

Should be: -

Anode (Pin 2) - Service Sheet 222V A.C. / Actual reading 192V A.C.

R17 drops the Voltage from 202V to 192V.

All Heater Voltages check out as well: -

UY41 - 31.0V / Actual 30.2
UL41 - 45.0V / Actual 44.4
UBC41 - 14.0V / Actual 13.3V
UF41 - 12.6V / Actual 12.4V
UCH42 - 14.0V / Actual 12.9V
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Old 22nd Jun 2019, 4:24 pm   #46
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Default Re: Yet another Bush DAC90A

That's better.

If you want to know how much power is dissipated by a resistor you can use any of the following….I&V (I*V) or I&R (Isquared*R) or V&R (Vsquared/R)

Regarding the usefulness of a mains bypass capacitor, my take is that the manufacturer fitted one for a reason, here's an interesting article as to what can happen on the mains (RCA Review, April 1937, mag page 18):

https://www.americanradiohistory.com...w-1937-Apr.pdf

Lawrence.
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Old 22nd Jun 2019, 5:43 pm   #47
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Default Re: Yet another Bush DAC90A

Thanks Lawrence

Now I know about "A NEW FORM OF INTERFERENCE - EXTERNAL CROSS MODULATION" - so I may fit a new Class X mains filter capacitor just to see if it has any effect ....?

Thanks to everyone for their responses although everything does seem to be relatively o.k. - I may also look at fitting a new piece of Paxolin to fabricate a new dropper panel if I can find a suitable source.
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Old 22nd Jun 2019, 6:06 pm   #48
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Default Re: Yet another Bush DAC90A

Keeping a mains bypass capacitor as part of the original circuit might or might not improve the quality of reception at your location as it stands, but one things for sure, the nature of mains interference rarely stands still.

Lawrence.
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Old 23rd Jun 2019, 10:10 am   #49
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Default Re: Yet another Bush DAC90A

Just a few final questions then I think we can more or less wrap things up for this DAC90A.

1. I have a Class X capacitor that I can fit as a mains filter/by-pass - putting historical accuracy to one side (apologies) I'm going to mount that underneath the chassis. Is there anything wrong with just connecting this across the live and neutral terminals inside the chassis?

2. The diffuser was attached with some sort of glue that I would like to remove - recommendations please for an appropriate solvent that won't attack the diffuser itself e.g. IPA, Acetone, Lighter Fluid?

3. There have been several comments that the "bubbling" at the bottom end of R13, the dog and bone resistor, is quite normal. Instinctively I'm thinking - that can't be right, surely that's caused by overheating although the readings are well within tolerance - intrigued!

4. Apart from the Pin 4 modifications on the UL41 I've not tried any of the other changes mentioned in various Threads over the years - are there any that I should really consider before putting the chassis back in the case?

Thanks
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Old 23rd Jun 2019, 11:01 am   #50
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Default Re: Yet another Bush DAC90A

Just a note on measuring AC voltages on non-sinusoidal waveforms.

Meters that are not marked as 'True RMS' usually use an average of the rectified waveform then assume it is a true sinewave and adjust the displayed value to show the equivalent RMS. Many AC voltages in a mains radio will be a long way off a sinewave and the reading can be a long way out. This is true of the DAC90A heater chain as part of the dropper is shared by the rectifier although in this case the error should be slight.
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Old 23rd Jun 2019, 11:15 am   #51
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Default Re: Yet another Bush DAC90A

If you replace the mains bypass capacitor it should be in the same circuit position as shown in the schematic, ie: across the switched mains.

The "bubbled" HT filter resistor fitted in your receiver, would under normal conditions, be dissipating approx. 1.3 Watts (Isquared*R)...the Current info (I) is given in the Trader sheet, layout wise it's in quite a hot position, on top of that if g1 of the UL41 had +ve grid at anytime then the screen current (as well as the anode current) through the valve would have increased which would cause more power to be dissipated by the resistor, the valve and the rectifier.

Lawrence.
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Old 29th Jun 2019, 12:04 pm   #52
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Default Re: Yet another Bush DAC90A

Carefully worked my way through replacing ALL the paper capacitors some of which were in a terrible physical shape - split and oozing!

I've also done the UL41 Pin 4 modification.

I'm aware of the issues surrounding the proper measurement of capacitors at their rated voltage - of course that man David G4EBT had already covered this in an earlier Thread which he directed me to: -

https://www.vintage-radio.net/forum/...d.php?t=157011

This set is still working very well with a strong Radio 4 on LW and several other stations on MW. There is, however, loud background noise and interference between stations - don't know if this is what Forum members refer to as "motor boating" but I'll record a sample and stick it on YouTube for clarification.

I've yet to tackle the resistors under the chassis some of which are measuring way over tolerance and will be replaced.

In terms of resolving any remaining noise and interference I'm seeking pointers as to where I should be looking e.g. particular resistors, valve pin voltages etc.

And I'd like to "bump" a couple of the questions I asked previously in my Post #49 above: -


2. The diffuser was attached with some sort of glue that I would like to remove - recommendations please for an appropriate solvent that won't attack the diffuser itself e.g. IPA, Acetone, Lighter Fluid?

4. Apart from the Pin 4 modifications on the UL41 I've not tried any of the other changes mentioned in various Threads over the years - are there any that I should really consider before putting the chassis back in the case?



Thanks again to everyone for their ongoing support and help.
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Old 29th Jun 2019, 12:12 pm   #53
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Default Re: Yet another Bush DAC90A

In the trade, motor boating was a term given to predominantly LF noise that was caused by instability within the receiver, usually varied from a phut phut phut (old style motor boat noise on a Butlins boating lake) to a well tuned fart who's frequency would often vary with the setting of a control (typically the volume control) or by the placing of ones hand near the circuit, nothing to do with external noise sources per se but sometimes a particular received signal could trigger the fault within the receiver.

Lawrence.

Last edited by ms660; 29th Jun 2019 at 12:21 pm. Reason: addition
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Old 29th Jun 2019, 4:28 pm   #54
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Default Re: Yet another Bush DAC90A

Voltage Readings for V4 and V5 are in Post #45 above (eventually!) so here are the others: -


V1 : UCH42

Pin 2 - Service Sheet 98V : Actual 82.6V (Hexode Anode)
Pin 3 - Service Sheet 47V : Actual 43.2V (Oscillator Anode)
Pin 5 - Service Sheet 47V : Actual 55.2V (Screens)
Pin 7 - Service Sheet 0.8V : Actual 0.67V (Cathode)

V2 : UF41

Pin 2 - Service Sheet 74V : Actual 82V (Anode)
Pin 5 - Service Sheet 47V : Actual 55.1V (Screen)
Pins 3, 4 and 7 - Service Sheet 1.0V : Actual 0.55V/0.55V/0.54V (Cathode)

V3 : UBC41

Pin 2 - Service Sheet 74V : Actual 54V (Triode Anode)
Pin 7 - Service Sheet 1.3V : Actual 0.55V (Cathode)
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Old 29th Jun 2019, 10:25 pm   #55
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Default Re: Yet another Bush DAC90A

UBC41
Anode voltage low would indicate excess current.
Cathode current voltage low would indicate low current.

I would look at the cathode resistor/capacitor.
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Old 30th Jun 2019, 10:54 am   #56
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Default Re: Yet another Bush DAC90A

When working normally the DC anode voltage of the triode will depend on the triode's bias, the triode's bias is predominantly controlled by the current flowing through the IF amplifier valve which in turn is controlled by the AGC voltage, which depends on signal strength.

As I understand it the voltages given in the Trader sheet were measured with no signal input (and with an Avo 7 meter)

Lawrence.
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Old 30th Jun 2019, 1:36 pm   #57
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Default Re: Yet another Bush DAC90A

Hello Donald,

Original dropper panel in good condition if wanted, FOC.

Ian
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Old 30th Jun 2019, 1:39 pm   #58
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Default Re: Yet another Bush DAC90A

Thanks Ian - PM sent.
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Old 1st Jul 2019, 10:14 am   #59
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Default Re: Yet another Bush DAC90A

Quote:
Originally Posted by ms660 View Post
In the trade, motor boating was a term given to predominantly LF noise that was caused by instability within the receiver, usually varied from a phut phut phut (old style motor boat noise on a Butlins boating lake) to a well tuned fart who's frequency would often vary with the setting of a control (typically the volume control) or by the placing of ones hand near the circuit, nothing to do with external noise sources per se but sometimes a particular received signal could trigger the fault within the receiver.
Thanks Lawrence - when we were children (!) my Parents could never afford a family holiday at a Butlins holiday camp but I get what you mean! Your other reference to "... a well tuned f*rt ..." certainly raised a smile.

I've often wondered if there was scope to include audio samples as Stickies which would cover the basic types of "noise" that people encounter e.g. modulation hum, motor boarding etc.

Quote:
Originally Posted by TrevorG3VLF View Post
UBC41
Anode voltage low would indicate excess current.
Cathode current voltage low would indicate low current.

I would look at the cathode resistor/capacitor.
Thanks Trevor - this is on my list of resistors to be replaced (the capacitor has been done) as it measures 25% over so lets see (hear) if that makes a difference?

Quote:
Originally Posted by ms660 View Post
When working normally the DC anode voltage of the triode will depend on the triode's bias, the triode's bias is predominantly controlled by the current flowing through the IF amplifier valve which in turn is controlled by the AGC voltage, which depends on signal strength.

As I understand it the voltages given in the Trader sheet were measured with no signal input (and with an Avo 7 meter)
Thanks again Lawrence - I'm working my way through these stages one at a time and am beginning to understand the way in which they are interconnected.

Of course I need to ask another "daft boy" question - how do you measure with no signal input?
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Old 1st Jul 2019, 10:42 am   #60
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Default Re: Yet another Bush DAC90A

D.C. Voltages (and sometimes currents?) are often measured with no signal i/p, since the presence of a signal will cause the voltage or current concerned to rise or fall depending upon what is being measured.
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