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Old 20th Nov 2023, 7:54 pm   #1
Spencervs
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Default Bush VHF80C valve heaters

Hello all! Looking for some advice on my radio that I've been using for years. When I got it, one of the bubs was dead so when replacing it, I recapped all the old electrolytics, hunts and waxies. Worked wonderful as usual but I noticed MW stations that were strong kept fading in and out of volume so I decided to have another check today. I noticed that the valve heaters were very high.... like around 150vac high. That can't be right, surely? As you all probably know, I'm not an expert but I checked the valves on The Valve Museum where heater voltages seem to be at a maximum of 30v. I have had a search for others who have serviced this radio and they mentioned the lamps starting bright then dimming down, but mine light up a dark room and stay bright! So something must be up.

Unfortunately the service manual doesn't specify any voltage values for the valves used. I did notice that R34 (2.7k)measures at 2.2k and R35 (680 ohms) measures higher at 850 ohms, so I suspect this may be part of the reason and that one of the thermistors is open circuit (TH1).

So my questions are:
  • What voltages should I be expecting?
  • Are these minor differences in ohms the culprit?
  • And where can I get a new thermistor?

I really hope someone can help me because I love this radio! Thanks in advance

Spencer
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Old 20th Nov 2023, 8:00 pm   #2
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Default Re: Bush VHF80C valve heaters

Usually the thermistors are wired in series with the valve-chain so if one has gone open-circuit the valve filaments wouldn't light.

How are you measuring the filament-voltage? You need to measure the voltage directly across each valve's individual filament-pins, not between the filament-pins and the radio chassis.
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Old 20th Nov 2023, 8:57 pm   #3
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Default Re: Bush VHF80C valve heaters

Ok... time to wear my dunce hat that is exactly how I was reading the voltages. so my heater voltages now are as such:
  • V2 (UF89) - 9.6v
  • V3 (UCH81) - 15.4v
  • V4 (UF89) - 10v
  • V5 (UABC80) - 30v
  • V6 (UL84) - 34.8v
  • V7 (UY85) - 30.8v

Do these seem usual? I have to admit due to the tight spaces I missed a pin and there was a small spark and the ME5 bulbs blew! With this the sound did lessen quite a bit but seemed to return to normal.

I suppose the bright side to this is that without the bulbs, the set seems to work as normal...a hint that the thermistors are ok?
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Old 20th Nov 2023, 9:09 pm   #4
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Default Re: Bush VHF80C valve heaters

You can check specified heater voltages on this website:-

http://www.r-type.org/exhib/aaa0234.htm
http://www.r-type.org/exhib/aaa0430.htm
http://www.r-type.org/exhib/aaa0171.htm
http://www.r-type.org/exhib/aai0038.htm

and so on.
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Old 20th Nov 2023, 9:36 pm   #5
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Default Re: Bush VHF80C valve heaters

I saw this before but thought because it said "maximum" that it doesn't necessarily mean that those are perfect for the specific circuit. So to confirm those are often the values I should expect to see in a circuit?

If so that's great news because I was so confused and it takes the guess work out of some of my other projects!
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Old 21st Nov 2023, 11:14 am   #6
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Default Re: Bush VHF80C valve heaters

Your readings look a bit low to me. Check the HT voltage and see what that looks like.

Check that the set is configured for your mains voltage.
Check all components in the heater chain.
Check the capacitors connected from the heaters to chassis.
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Old 23rd Nov 2023, 1:21 am   #7
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Default Re: Bush VHF80C valve heaters

Hello! Ok I have measured the HT voltage on the valves and now have a load of numbers and a few of them are... unexpected and some even go well beyond the specified maximum such as the 216v on UL84. Please do give me suggestions, I do think some of the resistors are out of tolerance and I noticed after my MES bulbs blew that the speed of the valves heating up dropped and though I only took a few readings before that, the heaters seemed a higher value. Though that could be wishful thinking, so perhaps replacing those will help as well as replacing the 680 ohm resistor that reads as 900 ohm and the 2.7k that reads as 2.2k might help matters.

In order of the service sheet:

V1 (UCC85)

Heater - 15.9v
Anode - 162
Grid - -0.17v

V2 (UF89)

Heater - 10.2v
Anode - 117.5v
Screen - 111v

V3 (UCH81)

Heater - 11.4v
Anode - 194v
Screen - 98v

V4 (UF89)

Heater - 13v
Anode - 183v
Screen - 107v

V5 (UABC80)

Heater - 25.3v
Anode - 84.2v
Grid - -0.6

V6 (UL84)

Heater - 32v
Anode - 216v
Screen- 130v

V7 (UY85)

Heater - 31.4v
Anode - 236v

Most of my grid readings were a bit high as you can see from the ones I included though I am unsure if this is relevant at this stage. A lot of the resistors look pretty faded and sad, so my next step is to check them and see how they are doing. Any thoughts in the meantime?

Last edited by Spencervs; 23rd Nov 2023 at 1:32 am.
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Old 23rd Nov 2023, 1:29 am   #8
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Default Re: Bush VHF80C valve heaters

Quote:
Originally Posted by Spencervs View Post
Ok... time to wear my dunce hat that is exactly how I was reading the voltages. so my heater voltages now are as such:
  • V2 (UF89) - 9.6v
  • V3 (UCH81) - 15.4v
  • V4 (UF89) - 10v
  • V5 (UABC80) - 30v
  • V6 (UL84) - 34.8v
  • V7 (UY85) - 30.8v

Do these seem usual? I have to admit due to the tight spaces I missed a pin and there was a small spark and the ME5 bulbs blew! With this the sound did lessen quite a bit but seemed to return to normal.

I suppose the bright side to this is that without the bulbs, the set seems to work as normal...a hint that the thermistors are ok?
From the top of my mind that looks about right, but I haven't looked up the nominal voltages. In a series circuit, heater voltages are only a rough indication anyway since they were optimised to give correct emission at 100mA (in case of U valves) and voltage could be off a bit depending on production tolerances. If they're way off for one valve, that valve might be defective. If you want to know if the heater circuit as a whole is working optimally, you measure the current. Should be 100mA, preferrably +-5% but +-10% is allowed.

Last edited by Maarten; 23rd Nov 2023 at 1:34 am.
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Old 23rd Nov 2023, 4:25 am   #9
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Default Re: Bush VHF80C valve heaters

Your heaters are being under–run by at least 10%, the correct voltage drops across them, when the correct current of 100mA is being supplied, are :–

V1 (UF89) 12.6 V,
V2 (UCC85) 26 V,
V3 (UCH81) 19V,
V4 (UF89) 12.6 V,
V5 (UABC80) 28 V,
V6 (UL84) 45 V,
V7 (UY85) 38 V

This is a total of (12.6+26+19+12.6+28+45+38) which totals 190.2 V.

Allowing 10 V for two 6.5 V 0.15A scale lamps, that leaves 40 V to be dropped across a heater dropping resistor, so if you used a 390 ohm 7 W wirewound ceramic resistor, with a thermistor in series, this would be correct for a UK 240 V supply.

Check the value of the series dropper resistor fitted is as specified by the maker, if the heater current is correct, the volts dropped across the resistor will be 1/10 of its marked value in ohms, its as simple as that!

In the case of the VHF80C,, the heater dropper is a 150 ohm component that is both a rectifier surge–limiter and heater–dropper in one, assuming a 240V supply, there would be approximately (150 × 0.18) dropped across it when the valves have fully warmed up, so if everything is correct there should be 215 V.A.C.at pin 9 of the UY85, about another 10 V for the two scale–lamps, plus 15 V for the NTC heater thermistor, leaving 190–195 V for the seven valves – check the thermistor (I think its a VA1005 or VA1015 for 0.1A radio use) hasn't cracked in half or the shunt resistor R36 (680 ohms) across it hasn't gone open circuit!

Chris Williams
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Old 23rd Nov 2023, 1:19 pm   #10
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Default Re: Bush VHF80C valve heaters

. . .From the readings posted by the OP in post No. 7, either the two voltage selection flyleads have been fitted in the 210–220 V position or the valves aren't drawing enough h.t. current due to their heaters being under–run!

The heater voltages quoted in post 7 add up to 138 V, so assuming that the heater current reduces in proportion, that equates to 70 mA flowing in the chain, a 30% reduction from normal!

If the heater chain thermistor or the smaller one across the two scale lamps both went o/c along with the bulbs themselves, that would reduce the heater current by about 30% according to my calculations!

It's possible there could be a partial heater–cathode short in one or more of the valves, but that would result in those valves above the shorted one being much brighter than normal and those at the chassis–end of the short being dim or out – if all 7 look equally dim, you can rule out a heater–cathode short!

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Old 23rd Nov 2023, 2:17 pm   #11
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Default Re: Bush VHF80C valve heaters

. . .Regarding replacement thermistors, your best bet is to ask on the "Service Information, Parts and Sets Wanted" section – there's very likely to be Members who will have something suitable!

Chris Williams
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Old 23rd Nov 2023, 6:53 pm   #12
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Default Re: Bush VHF80C valve heaters

. . .I forgot to mention that some of the AC/DC "U" prefixed Murphy Radio circuits show the approximate heater voltages along the chain – most Murphy Radio circuits had their voltage measurements taken using 245 V.A.C. input, which is what U.K. supplies tend to be – the OP's supply could be a bit lower!

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Old 23rd Nov 2023, 7:59 pm   #13
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Default Re: Bush VHF80C valve heaters

Quote:
Originally Posted by Spencervs View Post
I have had a search for others who have serviced this radio and they mentioned the lamps starting bright then dimming down, but mine light up a dark room and stay bright! So something must be up.
I can confirm that the lamps start bright, dim down (a lot) and then come up to probably not quite as bright as when first switched on. They are not overly bright and shouldn't be bright enough to cast a shadow in a dark room. I suspect the wrong bulbs were fitted. I'm using 12V 100mA in mine. You need to check carefully around the thermistor and any parallel resistors. You need to get this right and of course, as already stated, make sure the voltage tap is correct. Underrunning valves is probably somewhat less harmful than overrunning but still not desirable.
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Old 24th Nov 2023, 2:18 am   #14
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Default Re: Bush VHF80C valve heaters

Wow thank you all so much for the helpful replies! So I am still waiting on the bulbs. The manual specifies 6.3v 100ma soni have a set of those coming and some 12v too as someone suggested. The bulbs never dimmed on warning up, they were fairly bright. I have replaced R36 as the previous was measuring over 1k and the readings are the same. I am wondering if someone could see if my dropper resistor is the correct way? I found the service manual confusing regarding this.

Regarding the thermistors, one is cracked on the metal cap and actually came off but when I jammed it back on, it still measured around 10k out of circuit. So I assume this is ok?
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Old 24th Nov 2023, 9:01 am   #15
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Default Re: Bush VHF80C valve heaters

One side of resistor R35 will be wired to R34 and TH1, That's the 'B' side. The other side will be the 'A' side.

For 200->229V 'A' is connected to the UY85's anode. B is connected to S4 and C61.
For 230->250V 'B' is connected to the UY85's anode. A is connected to S4 and C61.

Component designations refer to the manufacturer's service sheet.

If in doubt reverse the connections briefly and check whether the HT goes up or down.
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Old 26th Nov 2023, 10:39 pm   #16
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Default Re: Bush VHF80C valve heaters

Hello all, so I have had a play with it and replaced my MES bulbs which did seem to increase the heater current to 90ma. I then switched the connections on R35 and we are now at 95ma on the heaters and I think reflect what I expect based on their information. However the HT has dropped for many of these valves with the exception of V6 and 7 where it has increased.

V1 (UCC85)

Heater - 28.1v
Anode - 125v (pin 6) / 143v (pin 1)

V2 (UF89)

Heater - 13.5v
Anode - 83v

V3 (UCH81)

Heater - 21v
Anode - 170v

V4 (UF89)

Heater - 13.6v
Anode - 165v

V5 (UABC80)

Heater - 34.1v
Anode - 72v (pin 9?)

V6 (UL84)

Heater - 47.6
Anode - 226v

V7 (UY85)

Heater - 41.7v
Anode - 248v

Does this look better or worse? and can I assume that R35 is now set correctly for my 240v input? I tried to understand the rotation on the service
data but its not clear which connection is meant to be A and B.

My concern for V5 was that the anode was on pin 9 rather than the pin it was meant to be on the service sheet! I really don't understand that.

Thank you all again for your help! I do love this radio and I want to treat it well
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Old 27th Nov 2023, 8:57 pm   #17
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Default Re: Bush VHF80C valve heaters

UABC80 pin 9 is anode. Where does the service sheet say it should be Don't forget there are three diodes in this valve as well. Pins 1,2 and 6 are the diode anodes with pin 9 as the triode anode.
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Old 1st Dec 2023, 12:37 am   #18
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Default Re: Bush VHF80C valve heaters

Hi all, so I've been tinkering with the radio and I have replaced all the resistors and capacitors that were out of tolerance. I found there was one resistor (R10) which is connected on series from another 680k resistor at pin 2 of V2 (UF89) to pin 2 or V3 (UCH81) which actually reduces the volume of MW/LW to the point of unlistenable. If I remove it then the volume is good... Does this mean there is a bad valve or two? Or did I connect this resistor wrong?
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Old 1st Dec 2023, 4:56 pm   #19
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Default Re: Bush VHF80C valve heaters

. . .Almost certainly that indicates the a.g.c. decoupling capacitor C37 (0.04u) is leaky or open circuit, or the a.g.c.feed resistor R20 (2.2M) has gone high in value – replace R20 and C37 and try your set again!

. . .R6 and R10 should be connected together at their lower ends – this is the A.M. a.g.c. line!

. . .Check your d.c. resistance readings from pin 2 of V2, V3 and V4 to chassis, V2 and V3 should read about 3.5M, and V4 about 2.7M – if the readings are substantially higher than this, check R6, R10, R20, R21, R22 and R37, whilst if the reading you get is much lower, C37 has gone leaky!

. . .To identify the A and B fly leads, remove V7 and measure ohms from V7 pin 9 , the one directly connected to V7 pin 9 is "A", whilst "B" connects to the on/off switch then back to "Live" of the mains input lead with the switch in the on position – for 230–250V supplies the mains input fly lead "B" goes to the top tag on the dropper and fly lead "A" connected to V7 pin 9 goes to the tag lower down!

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Old 1st Dec 2023, 6:55 pm   #20
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Default Re: Bush VHF80C valve heaters

The AGC rail is a very high impedance circuit (hence the resistor values of 2.2M). This means that the slightest leakage in any AGC decoupler will have a serious effect on the AGC rail. If you don't have a proper cap tester (not one on a multimeter....they are no good for checking leakage), it's best to just replace them...if they are original waxies, they will be leaky enough to affect AGC.
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