Issues with Philips B2F70U.

[Montythegreat10]

Oh, I believe they may actually be motor start capacitors... Anyway, I tried tapping and wiggling the valve with the set turned on, and nothing came of it.

[Montythegreat10]

I am not sure whether mine are start or run capacitors, they are Cambridge capacitor type As. Anyway, I tried wiggling and tapping the valve and nothing happened.

[Sideband]

Run capacitors are for continuous use, start capacitors are for intermittent use. They may be OK in this application since they are not being used as start capacitors and supplying a high current to start a motor. They will probably have an AC rating of about 400V which gives them a DC peak of 566V so they should be well within their rating.

[Montythegreat10]

Ok, so they wouldn't affect the voltages and function of the radio, I guess?

[Sideband]

As long as they are good and the value is correct, they should be fine.

[Montythegreat10]

Ok, thanks for clarification. It seems likely that it is the valve then, and unless there is any alternate thing I can try, I will probably take you up on your very kind offer of that EL84. The voltages were high on the UBC81 as well, do you think this is due to the EL84 being bad as well?

[Sideband]

The UL84 is the output valve and draws the most current (somewhere around 45mA). The power supply is simple unregulated half-wave and the output voltage is directly dependent on the mains input voltage and the current drawn from it. The less current drawn, the more output the power supply will give (up to the peak of the rectified voltage which assuming 240V will be near 340V....240 x 1.414 = 339V). Therefore if the output valve is not drawing any current for whatever reason, the output of the power supply will be much higher depending on the small current drawn by the remainder of the valves (which is probably no more than about 12mA). Therefore all the voltages measured will be proportionally higher on the remaining valves.

Don't forget also, now you are using a solid-state rectifier, all the voltages will be high at switch-on until the valves start drawing current. Once they are warmed up and working, the voltages should be somewhere near the published level.

If you PM me with your details, I'll get the UL84 in the post.

[Montythegreat10]

The UL84 very generously gifted by Sideband arrived, and I tried it in the radio, and there is still high anode voltage and no cathode voltage! I have double checked all of the connections, and I have no clue how this could possibly be the case on a known good valve! Some help would be appreciated here, as this seems like an impossibility.

[Gabe001]

My theory is that you lost the connection of R10/smoothing/reservoir capacitors to the metal chassis when you replaced the latter. With the old metal capacitor, the outside metal case is not usually isolated from the negative terminal so the negative terminal can be used as a chassis connection point. Not so with the new capacitor. Maybe all you need is to run a wire between your negative terminal of the replacement capacitors and the metal chassis.

[Sideband]

There must be something very fundamental that is wrong. Go over everything that you have done. The only way you would get 0v on the cathode is a short to chassis from the cathode connection (unlikely in your case as there is no cathode capacitor across the cathode bias resistor) or if the valve wasn't lighting up (so drawing no current) or if the cathode pin was physically open circuit either within the valve (now eliminated as a possibility) or within the valveholder. If the cathode resistor was open circuit, the cathode would tend to rise towards HT so you would be measuring a high voltage on the cathode.

Make absolutely certain that there is continuity between the cathode resistor and the contact in the valve holder.

A good point is made by Gabe001 above. You could have an open circuit earth somewhere so check around the smoothing caps and make sure that you have earth continuity everywhere you should have!!

[Montythegreat10]

Thanks for the tips. I have already checked continuity straight from the chassis to the holder, and it reads 150 ohms, as it should. I take your point about the Capacitors, that may be something to try, but I think I remembered that the negative of the Capacitors was already connected to the chassis through the circuit.

One thing I thought is that I am currently running the set through a 40W light bulb, and I corrected my mistake with the filament resistor, doubling the value of it, since the radio last worked. Since there was loud cracking from the speaker which seemed to increase current consumption and anode voltage when it happened with the old valve, but silence and a steady voltage with the new one, it leads me to belive that the old valve was internally shorting, but this new one is fine, however there is simply not enough emmisions from the heater to get it to conduct at this lower voltage, and increasing the size of the light bulb will get it to conduct and turn on.

[Gabe001]

I never had a set not work because of the lamp limiter. The volume will be lower.

[Montythegreat10]

The reason I was thinking that also was because the filament seemed very dim, barely visible even with the bench lights turned off

[Sideband]

Originally Posted by Montythegreat10

the filament seemed very dim, barely visible even with the bench lights turned off

Then there is something wrong! The heater of the UL84 is quite visible....why not check the AC voltage across pins 4 & 5? I think it should be 45V AC but check the valve data to make sure.

You have to check thoroughly these very basic things or you'll be going round in circles forever!

[Montythegreat10]

I am thinking that I should put it onto the 60W bulb and see what happens, because it does seem that there will be a cutoff where if the heater voltage is low enough, the valve will not conduct and will draw no current, meaning that the anode voltage will stay high and there will be no cathode voltage. On a somewhat unrelated note, is it a terrible idea to run the radio chassis while it is on its back, because this allows me easy access to the bottom of the PCB to do measurements.

Thanks very much for the valve by the way, it arrived in perfect shape, and the radio does not crackle when it is in, unlike the other one, making me think that the other one was shorting internally

[Montythegreat10]

Just tried it on the 60W bulb, and the filament is at 65VAC, so that doesn't seem to be the problem. The negative of the capacitors is already connected through to the chassis. It is a real mystery why there is no cathode voltage then, as it appears that the valve is getting all it needs to draw current, and yet it isn't doing anything, which would point to the valve being bad, but I know the valve is good? It seems like this radio is bending physics in its quest to not work! The output transformer is fine by the way, when I put the meter across the primary on diode check, the speaker makes noise.

[Sideband]

Are you measuring ACROSS the filament pins 4 and 5? 65 volts is TOO HIGH for the heater voltage! it should be 45V so you are 20 volts over....You'll burn it out!

[Montythegreat10]

Yeah, I believe so... it doesn't look really bright though, which is strange for how much voltage is being put through the filament. All of the other valves are working, and both the dropper resistor and the resistor replacing the rectifier filament are correct, so I don't see how the voltage could be that high. As I said previously, the valve is being given plenty of voltage in all the right places, and is good itself, and yet it still is not pulling any current.

[Sideband]

Check your meter on a known AC source in case it's giving wrong readings. We always assume that the meter is correct....I've been caught out before. Check the meter on a known AC voltage....even the mains but make sure you select the correct voltage range unless it's an auto-ranging type. Also check it on known DC voltages as well. Do this before doing anything else.

I bought a new meter about 18 months ago after my 30 year old meter started playing up and giving odd voltages where none should exist.....

[Station X]

Extract from circuit attached.

Is there any screen grid voltage present? Apologies if this has been asked before, but this is quite a long thread to wade through.

I note that post #22 describes the output valve as an EL84, I hope a UL84 was fitted originally.