Possible Valve failure in a Bush VHF72
 

I am familiar with the fact that a valve can fail if the heater element fails. The valve does not warm up and fails to function. However, can a valve also fail even if the heater is still working?

I have followed the signal path back from the last stage (V6) through V4 to V3 by just tapping the output and input pins. I get a crackle on V3 when I touch the anode (pin 7) with a probe, but not on the input from the previous IF stage on pin 2 suggesting that the valve is not functioning which might explain the abscence of signal on any band. The voltage is correct on pin 7 (150v) but slightly out on pin 8 (87v instead of 85v). All other components around V3 seem to be OK including the earth connections on pins 3, 1 & 6.

All I get at this stage is a buzz. All paper and electrolytic caps have already been replaced.

Re: Valves failures
 

When a valve becomes seriously gassy it will fail to operate although the cathode will still glow normally.
Your best check is to replace the valve with a known good one.
John.

Re: Valve failures
 

What's the set?

Re: Valve failures
 

Its a Bush type VHF.72.

Re: Valve failures
 

What are the valve-bases? I've had bad experiences with the "McMurdo" type where the contacts are forked brass stampings: over time the brass goes brittle and one leg of the forked-part that's supposed to entrap the valve-pin drops off.

And yes, valves can die in mysterious ways: the cold-welds between the electrodes and the supporting structures can fail through heat, age and vibration.

Re: Valve failures
 

I examined the base carefully after I read this but everything seems to be in order.

The valve itself is a Mullard and looks to be in good condition although I have no means of actually testing it. It looks cleaner than some of the others in the set.

I found another on eBay and have purchased it. I guess I will know more in a few days.

Re: Valve failures
 

As the DC conditions are close I would be cautious about your conclusions. Touching a pin does not always result in a response as it depends on the impedance of the circuit at that point. The grid is connected to the IF secondary which has very low resistance, you will only be able to inject noise at the resonant frequency and even that is going to be affected by the probe.

Do you get any noise when you switch between the MW and LW bands? With the waveband switch on MW, measure the voltages on V2.

PS: with this circuit you can tell the valve is conducting as the screen is fed by R7 and it is dropping the correct voltage.

Re: Valve failures
 

I don't get any noise while switching. The buzz is constant and gets louder when I switch in the FM circuit.

I took your point and measured voltages around V2 (and re-checked around V3, V4 and V6). The are fairly close with p6 at 159v and p1 at 94v which is around 6v low. I also noticed that V6 p7 has 60vAC though, which probably explains the buzz, even when the volume is turned right down.

The voltage at V7 p3/C50 is low at 228v, but C49 and C42 have close to correct voltages of 196v and 158v. The caps have been replaced and there seems to be no AC here, although I do get a momentary few volts which fades away over 3-4secs which I presume is down to the soothing effect of the caps (and possibly my digital multimeter)? C47 has also been replaced but there is 15v instead of the expected 11.8 at p3. R27 is high though at 340ohm instead of 270ohm.

This circuit has not transformer and puts the heaters in series on the AC side of V7 so I wondered whether AC can bleed through the heater to affect the valve, or could it simply be down to the higher than expected voltage at p3?

Re: Valve failures
 

Interesting fault:
1. There is a continuous buzz
2. The UL84 cathode voltage is high
3. There is a lot of AC on the UL84 anode
4. The HT voltage is about right
5. The buzz is worse when FM is selected
6. The UL84 cathode bias resistor is 340 (270)

A couple of observations:
1. Even taking into account the 340 ohms, the UL84 cathode current is slightly higher than expected.
2. The FM selection removes the cathode bypass which reduces the gain but the buzz is greater.

A couple more questions:
1. What is the UL84 grid voltage?
2. Does the buzz vary with volume?
3. Do you have a spare UL84 or UABC80 you could try?
4. Try a 0.1uF from UL84 grid to chassis, what happens to the buzz?
5. Try a 0.1uF from UABC80 grid to chassis, what happens to the buzz?

Its always a bit of a challenge when the DC conditions are about right but you still have a fault.

Re: Valve failures
 

Another way that a valve can fail is the wire can break down the Pin or dry joint on pin end, though this was more common on the 1920s B4 style valves.
John.

Re: Valve failures
 

Most times they just go low emission.

Re: Possible Valve failure in a Bush VHF72
 

As this thread is now about a fault in a particular radio, it really belongs here.

Re: Possible Valve failure in a Bush VHF72
 

+Bill, thanks. The conversation has indeed gravitated to a specific radio fault so I guess this is correct. I see you changed the title of the thread to reflect that as well.

PJL, thanks for your responses. Regarding your questions:

1. What is the UL84 grid voltage?
Vg(p2)=2.4vdc/9.2vac; Vsg(p9)=160vdc/0.08vac

2. Does the buzz vary with volume?
No, it does not. It stays constant regardless of volume.
This is why I'm trying to figure ot the source of the AC voltage on the cathode and is comming from.

3. Do you have a spare UL84 or UABC80 you could try?
Unfortunately not. I do have a EL84 and an EABC80 in another radio but I believe these operate on different heater voltages.

4. Try a 0.1uF from UL84 grid to chassis, what happens to the buzz?
This kills the buzzing noise.

5. Try a 0.1uF from UABC80 grid to chassis, what happens to the buzz?
This has no real effect.

I also tried decoupling C43, but that made the buzz considerably louder.
So I guess this implicates the UL84?

Re: Possible Valve failure in a Bush VHF72
 

It does look like a faulty UL84 with that DC voltage. You can confirm it is not from the UABC80 by disconnecting the coupling capacitor to the UL84 grid (I am using the trader and numbering is different).

It is most likely be leakage to the grid, the DC could be due to rectification. Firstly make sure the valve base is really clean and check for any obvious signs of leakage such as charring.

With the valve out of the socket try measuring the resistance from grid of the valve to each of the other pins. If it shows any sign of cold resistance a trick you can try if you have a gas hob with spark ignition is to wire the valve up to put the spark between the grid and all other pins together. This tends to blast any impurities away from the inside of the glass. Think H&S and cover the valve with a Pyrex dish and don't touch the wires. Stop once sparking only occurs inside the anode cavity.

Re: Possible Valve failure in a Bush VHF72
 

Already followed your suggestion regarding disconnecting the coupling capacitor between the UABC80 and the UL84. This is the one designated C43 on the diagram I have here and mentioned in my last post. Disconnecting it makes the buzz louder and, as you say, rules out the UABC80. This seems to confirm that the buzz problem is down to the UL84.

I also tried wiring a 1.5k resistor across that UL84 cathode bias resistor (i.e the one with value of 349ohms but which should be 270ohms). This gave me around 284ohms across that point (should have really used a 1.2k but didn't have one) but this changed nothing.

I have cleaned the pins on the UL84, but there is quite a bit of brown deposit on the inside of the glass. The valve looks quite 'dirty' in comparison to the other valves in the set. There is indeed resistance between the pins. Since I wasn't sure what to expect, I compared the UABC80 and found no resistance whatsoever between any of the pins, even with the highest resistance setting (2000M) on my multimeter. However, on the UL84 I do get cold resistances although these are pretty high:

- about 105M between pin 2 and 3
- 78M between pin 2 and 7
- 18M between pin 2 and 9
- 17M between pin 2 and either of the heater pins (4&5)

Unfortunately I do not have a hob with spark ignition as I would have tried your suggestion. Instead I'm going to try and get a valve from eBay. The cheapest one I have seen so far is £11 though!

Re: Possible Valve failure in a Bush VHF72
 

Hello,

I have a used Mullard UL84 you can have for the cost of postage. PM me if interested.

Michael

Re: Possible Valve failure in a Bush VHF72
 

That's a very kind offer. I have PM'ed you.

Re: Possible Valve failure in a Bush VHF72
 

I received the UL84 from Michael today. I replaced the 270ohm resistor wich read as 349ohm with a new one and tried the replacement UL84. I am happy to report this did the trick! The output stage is now completely silent. A quick tap on the inputs of the UL84 and the UABC80 shows that there is an output. The old UL84 was evidently the problem.

I am now turning my attention to problem 2: no radio signal. I've started a new thread for this problem.

https://www.vintage-radio.net/forum/...063#post627063

Just wanted to also say thanks to Michael for suppplying the replacement valve.