ECH81 triode anode voltage extremely high

[indigo.girl]

Hi everyone,
Well, this is the weirdest problem I've encountered to date. I'm working on a Philip F5G01A radiogram. The output valves are working ok and when I inject and audio signal from my MP3 player at the pick-up input I get amplified audio from both speakers.

I wasn't getting any radio reception so started checking valve voltages. V4 was OK and so was the heptode readings from V3 (ECH81) with anode (pin6) at 230V and grid (pin1) at 69V. However, the triode anode (pin 8) was off scale (>600V DC)!! (note: its grid was on spec at -9V). I switched off and checked the nearby components and found that R51 (18K @5W) had gone open circuit so I ordered another expecting this to be the issue.



I fitted a new R51 today but I am still having the same issue with a high triode anode voltage of >600V. As you can see from the circuit this feeds into a LC network and switch. When I flick the switch and node 6 makes contact with node 5 and I get a bright flash/bang from the side of the switch!! Despite the dramatic effects its possible to switch on power again and everything is still functioning (despite ECH81 pin 8 being at >600V).

(When I take V3 out of its holder the pin 8 position reads 230V DC as expected as it is fed by the HT line. This is a bit high with the spec stating 208V)

Whats going on here!!

[Al (astral highway)]

Hello IG,

Can I invite you to post the circuit? ;-)

[Nuvistor]

If you are using a DMM try an analog one.

Frank

[indigo.girl]

I don't have an analog. I'm using a basic DMM XL830L

[Nuvistor]

Is the radio working? I was just wondering if the DMM was being fooled by the anode having the sine wave oscillator voltage on it.

Frank

[indigo.girl]

Looking at the circuit I am getting >600V DC (or whatever it actually is) on either side of C26. It also goes off scale when reading in AC mode so its clearly not a straight forward DC voltage here....

[Al (astral highway)]

Originally Posted by indigo.girl

Looking at the circuit I am getting >600V DC (or whatever it actually is) on either side of C26. I

Hey, can you include the power supply circuit, please?

[Nuvistor]

You could disable the oscillator and see if the voltage changes, is the MW wave working ok?

[indigo.girl]

Originally Posted by Nuvistor

Is the radio working? I was just wondering if the DMM was being fooled by the anode having the sine wave oscillator voltage on it.

When I switch to MW I get good reception and sound. If I switch to VHF or LW mode I get the flash.

[Nuvistor]

Originally Posted by astral highway

Hey, can you include the power supply circuit, please?

It a full wave rectifier EZ81 with 208VDC on its cathode, that could be higher oc course if the mains voltage is on the high side.
Frank

[indigo.girl]

Hold on folks the flash is (an embarrassing) red herring. I was grounding my DMM at a position that accidentally connects with the HT line (hence the flash when I switch and it connects the circuit). So forget the flashing problem. But I am still getting the high anode readings at ECH81.

I can tune into MW and LW stations (when I test the pin8 high voltage I lose all audio btw). However, when I switch to VHF I get no audio (not even a noisy background) and no voltage readings from V3.

[indigo.girl]

Originally Posted by astral highway

Hey, can you include the power supply circuit, please?

Is this any more help?

[Cobaltblue]

Hi IG

The HT gets switched off the V3 triode on VHF

Have you subbed the ECC85 yet.

If its spent all its life on AM chances are the ECC85 will not be working now.

Cheers

Mike T

[indigo.girl]

Ok - so perhaps the V3 readings are corrupted by the DMM but must be functioning OK since I MW and LW stations. I'll stop worrying

Moving onto V1 to look at VHF it seems that V1 is working albeit under par at the pin 6 anode. I'm reading 208V at pin 1 anode (178V spec) but only 70V at pin 6 anode (should be more 163V)


PS: I am also getting >600V readings on at position Q where R7 connects (other side of R7 is 230V HT as expected). (False DMM reading again I presume)

[turretslug]

When the triode section of the triode-hexode is working, there will be quite a high RF voltage at its anode- though (hopefully) nothing like 600V!, more like low tens- it sounds as if this RF is getting into the meter circuitry and giving it headache and confusion. An inexpenisve DMM was probably only designed with AC voltages of typical power line frequency in mind, maybe going into the AF range if your lucky. RF frequencies are very good at getting into unintended places and causing strange effects- something like a Fluke would be better shielded and internally designed to be less susceptible but those who work with things like transmitters still need to beware of the possibility of strange and misleading readings. Back in the day, the usual analogue multimeter would have been much less prone to this sort of thing- though probably still liable to RF effects in extremis.

The radio goes quiet when trying to measure the oscillator anode voltage because the extra stray capacitance of the meter connection off-tunes it- you might be able to get a (weaker) signal back by off-tuning in the direction of higher frequency. Or it might be disturbing it sufficiently to make it oscillate in a different way at a totally different frequency (less likely). Oscillators can be touchy things....

[indigo.girl]

Thanks for your explanation TurretSlug - that all makes good sense - so nothing to really worry about (although this is the first time I had strange readings with this DMM).

So, not much of a problem after all...except still no VHF. Is the low anode voltage at pin 6 likely to be this issue. Or is it more likely to be a tuning/reception problem?

[ms660]

The low voltage on pin 6 is looking desperate, remove the ECC85 and check the voltage on pin 6 of the valve holder, it should be approx the same as the HT voltage, if it is then measure the resistance across R7+S8, if that's ok (10k +- the usual) then a replacement ECC85 would be the best bet.

Lawrence.

[M0FYA Andy]

This supports a view I've always held - it doesn't matter how many digital meters you have, it's also vital to have a trusty old Avo 8 or similar!
Andy

[Silicon]

If I had a digital voltmeter that was giving unrealistic readings on a circuit containing both DC and RF voltages I would start by trying to filter the RF out.

A series RF choke in the test leads and maybe a parallel capacitor at the DMM terminals should make a difference.

[Skywave]

Originally Posted by turretslug

. . . RF frequencies are very good at getting into unintended places and causing strange effects - something like a Fluke would be better shielded and internally designed to be less susceptible but those who work with things like transmitters still need to beware of the possibility of strange and misleading readings.

Which is my experience exactly. Several years ago I was employed by a firm who designed and built VHF transmitters for Band 2 (88 - 108 MHz). We used Fluke DMMs extensively, (although there were a couple of AVO 8 meters available), but it was found essential to wind the test leads of those DMMs around a ferrite toroid - one for each test lead - when working in the vicinity of a TX with several hundred watts sloshing about.

Al.