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Old 18th Nov 2022, 12:25 am   #1
RT 1006
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Default Bush VHF90 restoration, excessive power consumption?

I'm currently restoring a Bush VHF90 picked up at a local sale room a few years ago.

I've replaced all the Hunts and wax capacitors and few out of tolerance resistors.

I've also moved the dial lamp shunt resistor and rectifier valve anode resistor from their original below deck positions and mounted them on the voltage selector tag strip to improve air flow around them.

The set gives very good results on both MW and VHF. My only concern is that power consumption seems a bit excessive. It sits at around 260mA/60w, the UL84 output valve also seems to run very hot. The service sheet states a power consumption of around 45w. Voltage readings at the various points are all within about 20v of the figures on the service sheet so I'm fairly happy that everything is fine with these. I've also replaced the original UL84 with a new one but power remains at 60w and the valve continues to run hot. In all other respects the radio works very well.

I'm just wondering if there are any other VHF90 owners out there who could confirm if a power consumption of 60w and a hot running output valve is normal or not? Any information gratefully received, thanks.

I've added a few pictures for interest.
Kev
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Old 18th Nov 2022, 12:39 am   #2
Aub
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Default Re: Bush VHF90 restoration, excessive power consumption?

Hello Kev

Is the incoming mains voltage ok?

Aub
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Old 18th Nov 2022, 12:40 am   #3
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Default Re: Bush VHF90 restoration, excessive power consumption?

Output valves do run hot....too hot to touch in most cases and the UL 84 is no exception. Measure the cathode volts across the cathode bias resistor and then use ohms law to work out the total current the valve is drawing. I think the max anode current of the UL 84 is about 48 milliamps but check the valve data to be sure.
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Old 18th Nov 2022, 2:05 pm   #4
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Default Re: Bush VHF90 restoration, excessive power consumption?

Just to note, that as a first approximation the RMS current flowing through the rectifier will be approx. twice that of the current flowing from the reservoir capacitor to the rest of the receiver.

To me the power consumption given in the manufactures service manual (Give or take a few watts) for the Bush VHF90 looks to be based on the receivers DC load current (the total DC current flowing through V1 to V6) plus the heater circuit current.

The actual RMS current flowing through the rectifier with a reservoir capacitor will always be considerably higher than the load current flowing from the reservoir capacitor to the rest of the receiver due to the relatively narrow conduction angle of the rectifier which results in a high value of peak current and hence a higher value of RMS current when compared to the current flowing from the reservoir capacitor to the rest of the receiver.

Lawrence.

Last edited by ms660; 18th Nov 2022 at 2:13 pm. Reason: extra info
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Old 18th Nov 2022, 3:47 pm   #5
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Default Re: Bush VHF90 restoration, excessive power consumption?

If the circuit voltages are correct and there's no real sign of distress then I wouldn't worry about this. As said, the output valve is supposed to run hot.

You could always increase the cathode resistor if you want it to run cooler - going down from 3W maximum output to 1.5W is only 3dB, which is barely noticeable. This will make the valve last longer too.
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Old 19th Nov 2022, 2:34 pm   #6
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Default Re: Bush VHF90 restoration, excessive power consumption?

I've done some power consumption calculations based on the method set out in the Radiotron Designers Handbook (4th Edition) of obtaining the rectifier's RMS anode current and those of A.P. Kauzmann.

Kauzmann's method gives the power dissipated in the resistance in series with the rectifier circuit, from that the rectifiers RMS anode current can be easily calculated.

Both results are based on the voltage and current information given in the manufactures service information with an assumed mains voltage of 230 VAC with the receiver switched to VHF.

Here are the results of the rectifier anode current calculations:

RDH4: 189mA (Approx.)

Kauzmann: 188mA (Approx.) Derived from the series resistance power dissipation calculation.

Which gives an approximate total power consumption (Heater and rectifier circuit) for their receiver of 66 Watts.

The figures are approximate because of the limited resolution of the graphs/curves concerned in RDH4 and Kauzmann's and also due to the fact that part of the series resistance used in the calculations is also shared with the heater circuit.

Lawrence.

Last edited by ms660; 19th Nov 2022 at 2:47 pm. Reason: addition
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Old 21st Nov 2022, 11:26 pm   #7
RT 1006
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Default Re: Bush VHF90 restoration, excessive power consumption?

Time for a progress report on this radio.

Firstly, thank you to everyone for your comments and suggestions, these have all been very helpful and informative.

Before I start I need to put my hands up to being a bit careless with my power consumption figure quoted in my initial post. I took this from the current reading on the LCD panel meter on my isolating transformer which worked out at 60w. Over the weekend I checked the current with my Fluke and this gave a reading of 205mA/49w so much more in line with the 45w quoted in the service sheet. I should have known better than to trust the cheap panel meter so lesson learnt there.

Anyway, I did a bit of experimentation with the output valve's cathode resistor (R33) as suggested above by Paul. With R33 at 220R as per service sheet, anode current is 44mA. 560R gave 27mA with a fairly noticeable drop in output though just turning up the volume control compensated for this. 320R gave 37mA. Eventually I settled for a cathode resistance of 270R which results in a reduced anode current of 40mA with no real noticeable drop in output. According to valve data for the UL84 max Ia for Va of 200v is 60mA so I'm well inside operating parameters.

Each time I changed R33 I measured the temperature of the UL84 output valve with my IR thermometer as best I could. At 220R the valve ran at about 165 degrees, at 560R at ran at about 140 degrees, not a huge difference in temp as the valve still ran hot even with the lower current through it so clearly hot running is just a characteristic of these valves.

The existing valve has clearly had a hard time as it's quite discoloured inside however it seems to be working ok so I'm just going to leave it in the set and keep the new one until it's really needed. For interest I've put up a picture of the existing valve alongside the new one. Incidentally, what are the signs of a failing valve? I'm assuming low audio output and/or distortion?
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Old 22nd Nov 2022, 10:30 am   #8
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Default Re: Bush VHF90 restoration, excessive power consumption?

That's certainly a well used UL84, but they are tough as old boots and last well. Mullard / Philips learned from the relatively poor reliability of the UL41 and made lots of changes. Late UL41s are also much tougher than early ones.

Output valves usually fail in 2 ways - either they develop internal leakage causing excess current or humming (UL41s are notorious for this) or their emission drops leading to low volume and distortion. If the valve is leaky you will normally find +ve volts on the control grid, even though the coupling cap is good. The valve emission can fall quite a long way before producing obvious audible effects, and they can soldier on for years like that.

I'm surprised you didn't see a bigger variation in valve dissipation when changing the cathode resistor.
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Old 22nd Nov 2022, 11:07 am   #9
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Default Re: Bush VHF90 restoration, excessive power consumption?

Don't forget that about 25% of the total power dissipated in a hard run UL84 is from the heater so a reduction in anode dissipation will give a proportionately lower reduction in total dissipation and hence temperature.
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Old 25th Nov 2022, 8:08 pm   #10
RT 1006
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Default Re: Bush VHF90 restoration, excessive power consumption?

Another update on this radio.

When I first started work on the set the dial lamp shunt resistor was missing so I fitted the correct 33R resistor back in circuit. The dial lamps were painfully dim. Investigation revealed the original 3.5v 150mA bulbs must have been replaced at some point with 6v 300mA bulbs, this probably explains why the shunt had been removed. Fitting 3.5v 200mA bulbs has increased brilliance to a reasonable level but they're still a bit dim so I have ordered some 2.5v 200mA and will do a bit of experimentation with them once they arrive.

The rectifier valve's cathode resistor is running a bit hot. It's a 5 watt resistor as per the service sheet but I've decided to up rate it to a 9W wire wound which should handle the heat a bit better.

At the moment I'm wondering if I should carry out the wave change switch mod as detailed on this forum. The switch seems to be in quite good condition and works well. Intentions are to use this radio as a daily set though that'll almost entirely consist of FM listening so the switch is going to get very little physical use. Should I just leave it as it is or should I modify it to prevent potential problems in the future?
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Old 26th Nov 2022, 5:25 pm   #11
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Default Re: Bush VHF90 restoration, excessive power consumption?

I just did a heat measurement on a UL41 / it was over 100C / and that was in two radios in my collection (Radio DAC90a) Cheers Ian G
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Old 11th Dec 2022, 5:43 pm   #12
RT 1006
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Default Re: Bush VHF90 restoration, excessive power consumption?

Another update on this radio.

I decided to carry out the wave change switch mod. A bit fiddly and time consuming but in the interests of long term reliability probably the best thing to do. I followed Paul Stenning's excellent article on this mod making just a few minor changes to cable and component placement.

Just been cleaning up the chassis ready to fit back into the case.

As the tuning dial is moved for one end of the scale to the other its operation is a bit jerky due to the cord riding up either side of the concave part of the drive spindle. I'm sure I remember seeing a post on here by someone who experienced a similar problem and it was suggested to use chalk as potential lubricant. I've tried running a stick of chalk up and down the tuning cord but it doesn't seem to have made much difference. Perhaps I'm getting the chalk mixed up with something else? Can anyone suggest another substance that would help as I can't seem to find that original post on the subject.

Thanks
Kev.
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Old 11th Dec 2022, 9:35 pm   #13
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Default Re: Bush VHF90 restoration, excessive power consumption?

Quote:
Originally Posted by RT 1006 View Post
Another update on this radio.

As the tuning dial is moved for one end of the scale to the other its operation is a bit jerky due to the cord riding up either side of the concave part of the drive spindle. I'm sure I remember seeing a post on here by someone who experienced a similar problem and it was suggested to use chalk as potential lubricant. I've tried running a stick of chalk up and down the tuning cord but it doesn't seem to have made much difference. Perhaps I'm getting the chalk mixed up with something else? Can anyone suggest another substance that would help as I can't seem to find that original post on the subject.

Thanks
Kev.
French chalk? It's often sold is sticks for marking out, but powdered French chalk in also used as an industrial lubricant.

Basically, unscented talcum powder:

https://waltersandwalters.co.uk/french-chalk-powder

I'm not suggesting you buy half a kilo, but you might like to try normal scented talcum power if you have any to hand.

Hope that might help.
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