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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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24th Apr 2020, 6:07 pm | #1 |
Heptode
Join Date: Aug 2016
Location: Dumfries, Scotland, UK.
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BUSH VHF 90 overheating resistor bother
I bought this on a whim it was cheap and I'm blooming bored being off work I removed all the brown Hunts caps and replaced them the electrolytics have all been changed in this thing there is a tag strip with a 160r 4w resistor R34 on the diagram this had been very hot at some point and had destroyed the tag strip I've made it a new strip and replaced the dead 4w resistor I still have a smoking resistor. I've tried two different rectifiers and two output valves to no avail I've replaced the 1w 220r resistor that usually caused bother ( only had a 2.5w ) any ideas I'm at a loss ??
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24th Apr 2020, 6:13 pm | #2 |
Tetrode
Join Date: Apr 2008
Location: Rhondda Cynon Taff, Wales, UK.
Posts: 87
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Re: BUSH VHF 90 overheating resistor bother
Could the output transformer be shorting to its core?
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24th Apr 2020, 6:42 pm | #3 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: BUSH VHF 90 overheating resistor bother
Check the HT load current, if that's OK fit a 10 Watt resistor, 4 Watts seems too low to me.
Lawrence. Last edited by ms660; 24th Apr 2020 at 6:50 pm. Reason: re-write |
24th Apr 2020, 7:18 pm | #4 |
Heptode
Join Date: Aug 2016
Location: Dumfries, Scotland, UK.
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Re: BUSH VHF 90 overheating resistor bother
It's a 4W on the diagram it currently has a 7w installed still red hot.
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24th Apr 2020, 7:25 pm | #5 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: BUSH VHF 90 overheating resistor bother
I don't have the full manual, just the schematic, but as a rough calc. looking at the schematic for a VHF70 I would say it would be dissipating just over 5 Watts with the receiver working normally.
Don't forget as well as dissipating the load current the resistor has to dissipate the reservoir charging current as well. Lawrence. |
24th Apr 2020, 8:10 pm | #6 |
Heptode
Join Date: Aug 2016
Location: Dumfries, Scotland, UK.
Posts: 549
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Re: BUSH VHF 90 overheating resistor bother
It still shouldn't be burning out a 7 watt though so something's up
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24th Apr 2020, 8:16 pm | #7 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: BUSH VHF 90 overheating resistor bother
Do you mean the 7 Watt resistor has burnt O/C?
Whicheverways check the quiescent load current, from a similar circuit I make it approx. 90mA. Lawrence. |
24th Apr 2020, 8:20 pm | #8 |
Hexode
Join Date: Feb 2008
Location: Wrexham, North Wales, UK.
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Re: BUSH VHF 90 overheating resistor bother
Looking at the Trader sheet for this set, the total HT current consumption is around 100mA, in which case the heat dissipated in the rectifier anode resistor would be in the region of 1.6 watts.
Clearly, something is drawing too much current. Can I suggest you start by checking the current through the output valve? Measure the DC voltage across the cathode bias resistor and divide it by the resistance (measure this, don't take it for granted!), then multiply by 1000 to get milliamps - it should be around 50mA. It would also be worth checking the HT voltage at the output transformer - it should be around 200V. If the output valve is pulling too much current (a shorting cathode capacitor could cause this) get it fixed PDQ or you may burn out the output transformer. If neither of these tests reveal anything, check the anode voltages of all the other valves. If any of them is significantly lower than the value given in the service sheet, that should locate the problem. If that gets you nowhere, I would check the AC current through the heater chain - it should be very close to 100mA. If it's too high, all the valves will be overheating and passing excessive currents. If this is the case, check that all the valves are the correct ones! |
24th Apr 2020, 8:26 pm | #9 | |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
Quote:
Lawrence. |
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24th Apr 2020, 8:44 pm | #10 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
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Re: BUSH VHF 90 overheating resistor bother
Have you checked the voltage drop across the resistor to work out the current being drawn and hence, the Wattage being dissipated? Clearly if it's much above 4 Watts, that suggests a problem. Even if it's around 4 to 5 Watts, the original seems under-specified, so as Lawrence suggests, 10 Watt resistor would be a better choice. (Modern ceramic-bodied wire-wound resistors have an upper temperature limit of 150C. 10 Watt ones are 48mm long).
Maybe stating the obvious, but as to the heater chain, presumably you've set the mains voltage to the 240 - 250V setting - not 220 - 230V? Good luck with it.
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24th Apr 2020, 8:53 pm | #11 |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
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24th Apr 2020, 9:10 pm | #12 |
Octode
Join Date: May 2003
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Re: BUSH VHF 90 overheating resistor bother
Good evening.
By the sounds of it, think simple first. Do not run the set again like this as has been stated irreplaceable parts could be destroyed. if the resistor is getting that hot and burning out, I would suggest that you carefully visually check all of the work that you have done I.e capacitors soldered to the correct tags. check that the electrolytic capacitors are the correct way round. If this looks all ok then I would suggest seeing what the resistance between the HT line and chassis is. If it is very low then it is a case of elimination by disconnecting connections from the HT line until it disappears. It could be something as simple as a resistor leg touching a screen or a screw shorting the HT line. Never ignore the fact that new components could be faulty straight out of the box. I had a brand new HT electrolytic go S/C on me soon after fitting in a pre war Murphy set. The rectifier valve certainly did not like it!! Christopher Capener
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24th Apr 2020, 9:24 pm | #13 |
Nonode
Join Date: Oct 2017
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Re: BUSH VHF 90 overheating resistor bother
I'd recommend a systematic disconnection of all the HT load paths from the rectifier cathode then reconnect them one by one to work out where the excessive load path is. Start with just reconnecting C54; just because it has been replaced doesn't mean it is either good or of an adequate voltage or ripple rating. Then reconnect the primary of the AF o/p transformer leaving off tag 4 denying HT to the rest of the set. By breaking it down like that, you should soon figure out which part of the circuit is out of whack. Cheers, Jerry
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24th Apr 2020, 10:05 pm | #14 |
Nonode
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
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Re: BUSH VHF 90 overheating resistor bother
Hi!
The set hasn't been mis–wired somehow so that the heater–chain current as well as the h.t. supply current is flowing through R24 is it? The easy way to test this is to make an ohm–meter test between mains live and pin 9 of the rectifier valve and another test between pin 9 and chassis, the resistance between pin 9 and chassis should be the resistance of the heater chain, dial lamps, heater dropper, etc., plus the 150 ohms of R24! Chris Williams
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24th Apr 2020, 10:19 pm | #15 |
Octode
Join Date: Sep 2017
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Re: BUSH VHF 90 overheating resistor bother
I agree with Jerry and Christopher that the urgent necessity is not to repower the circuit until current path has been determined. In particular, C51 and C53 have to be checked for s/c. That current in the resistor is coming from somewhere, and could be doing/have already done a lot of damage to components that are less easily replaced than capacitors - the O/P valve and O/P xformer. OP says that he has replaced the cathode resistor, so presumably that isn't a suspect.
Mike |
24th Apr 2020, 11:00 pm | #16 |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
Did you change or reform the main smoothing cap? What's the anode current like on the output valve?
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Kevin |
24th Apr 2020, 11:04 pm | #17 |
Heptode
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Location: Dumfries, Scotland, UK.
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Re: BUSH VHF 90 overheating resistor bother
Right let's start again why has a UY41 sheet been tagged the set has a UY85 , as mentioned the set as it arrived in its untouched form had done the same thing so the problem predates anything ever having been replaced.
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24th Apr 2020, 11:41 pm | #18 |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
Surely the UY41 reference was simply to provide the information that the anode current limiting resistor in this sort of application will dissipate something like three times the apparent figure from the dc current load on the rectifier. The figure of 3x will be similar whatever the rectifier type number may be.
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25th Apr 2020, 7:51 am | #19 |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
The theory about the wattage of the surge limiting resistor is the same for the UY41 as the UY85, don’t worry about the valve type in this instance, the information is correct for any half wave rectifier valve. I presume that Lawrence found the information about the UY41 before any information about a UY85, the statement about the wattage size is still valid.
As noted in earlier posts, standard fault finding techniques should prove if the set has other faults, either mistakes or faulty components.
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Frank |
25th Apr 2020, 10:41 am | #20 | |
Dekatron
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Re: BUSH VHF 90 overheating resistor bother
Quote:
This can also be verified by reading towards the end of section 13 on page 10 of this Philips valve book: http://www.nj7p.info/Manuals/PDFs/Tu...art-4-1972.pdf Doing a power dissipation calc for a surge limiter when the load is resistive is relatively straight forward, doing the same when a reservoir capacitor comes into play is not so straight forward, hence the guide in the info I posted. My rule of thumb for replacing/fitting a surge limiter would be to double the value of the normal DC load current and use that value to calculate the power dissipated then double that for the resistors power rating, others might have a different take on it. Remember that the rectifier only conducts for a short period of time relative to a full cycle and during that time all the energy supplied to the load from the reservoir capacitor during the non conduction period has to be replaced during the conduction period and at the same time the load will still be demanding to be fed, the current peak will be considerable. Lawrence. |
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