ECH81 triode anode voltage extremely high

[MrBungle]

Everyone needs to keep a VTVM/FET voltmeter around. I've had no end of problems with RF on DMMs over the years. You need something with heavy mechanical damping as well as low pass filtering.

What is even more annoying is that some of them inject noise into the DUT from the digital side of things which leads to hours of puzzling until you disconnect the DMM and become instantly enlightened. The stupid one I had recently was I built a 10v reference with a REF102 IC and couldn't get the noise down below ~5mV even with low impedance bypassing specified in the datasheet. Disconnected the 3478A and the noise was gone. 3478A sat there reading a steady voltage though so it'd filtered all of its own noise out helpfully.

[Tractorfan]

Hi,
I've had a few scares when using a DMM (not a Fluke) that gave odd readings too.
I have since bought a (very) cheap analogue meter to give a rough cross check. They cost around five or six euros here, but can been quite handy at times, especially when I need to see a voltage trending up or down.
Cheers, Pete.

[G8HQP Dave]

A simple trick I use to measure DC in the presence of RF AC is to add a 1M resistor in series at the probe end. This is easiest to do with those probes where the spike can be removed and replaced with a croc clip. The 1M plus stray capacitance forms a low pass filter and blocks the AC well enough to keep the DMM sane. You then just have to multiply the reading by 1.1 - assuming a 10M DMM.

PS One one occasion when I was chasing an oscillator fault I tack-soldered a 1M resistor to the anode tag on the valveholder so it was there whenever I needed it. I can't remember if I remembered to remove it after fault-finding!

[indigo.girl]

Originally Posted by 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.

Great tip Lawrence - thanks. Without the valve in position I am getting a full 225V HT getting to pin positions 6 and 1. So its looking like a faulty valve. The 10K R7 is ok too. Will see if I can source an alternative ECC85 and retest

[indigo.girl]

Originally Posted by G8HQP Dave

A simple trick I use to measure DC in the presence of RF AC is to add a 1M resistor in series at the probe end. This is easiest to do with those probes where the spike can be removed and replaced with a croc clip. The 1M plus stray capacitance forms a low pass filter and blocks the AC well enough to keep the DMM sane. You then just have to multiply the reading by 1.1 - assuming a 10M DMM.

PS One one occasion when I was chasing an oscillator fault I tack-soldered a 1M resistor to the anode tag on the valveholder so it was there whenever I needed it. I can't remember if I remembered to remove it after fault-finding!

Hi Dave - just had a go at your suggestion of using a 1M resistor in series at the probe end. The DMM now reads 63V DC at the pin 8 anode of V3 so that confirms I was just picking up stray RF. Good lesson learned and I'll now be on the look out for a cheap analogue for the future.

With the 1M resistor in place my HT line reads 111V instead of the 225V so that means half of the voltage drops across the resistor. (Surely my DMM is more than 1M??). Anyhow - with voltage reduced by a half that means the true voltage at V3 anode is about 120V DC which is about right seeing as the spec says 96V but all my reading on this set are about 10-20% high.

[ms660]

1 megohm input impedance isn't unusual for some budget DMM's, a 20k analog would better than that on any range above 50 volts.

Lawrence.

[MrBungle]

If that's a DT830 DMM which I saw mentioned earlier, it does indeed have a 1M impedance. This surprised me when I had one. Was expecting 10M.

Mine has "Zin=1M ***" scrawled on the back in sharpie to remind me

[indigo.girl]

Originally Posted by 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.

Update: Tried a replacement ECC85. No change - I'm still only getting 70V at pin 6 (anode) when the valve is in (it should be 163V).

In absence of valve, I am getting the full 200V HT at both anodes and resistance across R7 is OK.

Still no FM stations.... is there any way to 'inject' an FM signal without using an oscilloscope?

[ms660]

With the valve removed check for any +ve voltage on the grid connection on the valve holder (pin 7)

Lawrence.

[indigo.girl]

Originally Posted by indigo.girl

Update: Tried a replacement ECC85. No change - I'm still only getting 70V at pin 6 (anode) when the valve is in (it should be 163V).

In absence of valve, I am getting the full 200V HT at both anodes and resistance across R7 is OK.

CORRECTION: I have remeasured using the 1M resistor at the voltmeter probe to block any RF effects. Correcting for the doubling of the voltmeters resistance I now get close to spec readings on the valve with anode pin 6 measuring 184V (slightly above spec of 163V like all other readings).

So, the valve is actually fine. So now looking for an alternative fault for the FM
I do get a crackle from the loudspeaker when I probe the anode voltages so I'm presuming this tells me that the route through to the loudspeaker is ok. Is this a reception issue?

[indigo.girl]

I have checked the aerial input connected to the pin 3 cathode of V1. R4 (180ohms) is OK and there is an intact wire from aerial to R4 so the S3 is not open circuit. I have also extended the aerial with another 2M of wire.

Set at highest volume and careful tuning I can pick up a very faint FM reception. So somethings getting in.

[ms660]

See if you can identify the station and if it lines up where it's supposed to on the tuning scale.

Sometimes FM reception can be weak and very patchy indoors if just using a short wire antenna, metal objects can sometimes affect reception.

Lawrence.

[Skywave]

Originally Posted by indigo.girl

Still no FM stations.... is there any way to 'inject' an FM signal without using an oscilloscope?

Above is from post #28.

To inject a signal to your radio - the whole set as it stands, or any part of it - you need a signal generator, not an oscilloscope. An oscilloscope is used to examine waveforms at various points in your item under investigation ('radio' in this case). In brief, a signal generator is used to produce a signal; an oscilloscope is used to examine a waveform - which might be one initially produced by that signal generator.

Yes, for a lot of radio repairs, you can get by without having either, but when there are unusual faults to deal with, having either - or better still, both - usually makes the whole job a lot easier. And, of course, experience and having appropriate technical info. really helps too!

Personally, for most electronics repairs, along with a DMM and analogue meter, I regard having the availability of both items (S.G. and 'scope) as essential.

Al.

[indigo.girl]

Originally Posted by ms660

See if you can identify the station and if it lines up where it's supposed to on the tuning scale.

Sometimes FM reception can be weak and very patchy indoors if just using a short wire antenna, metal objects can sometimes affect reception.

I can't identify the station - its too weak to hear enough plus I have sent off my glass tuner plate to be reprinted so I can't match it up.

I have moved the set to another position in the room and no change. I can normally get good FM reception here.

I've been looking again at the circuit and I'm not sure how the FM signal accesses the circuitry. I have marked the FM output in red and the HT in blue. The VHF switch sends HT to V1 but also block the FM signal into V3. What am I misunderstanding?

[ms660]

I've been looking again at the circuit and I'm not sure how the FM signal accesses the circuitry. I have marked the FM output in red and the HT in blue. The VHF switch sends HT to V1 but also block the FM signal into V3. What am I misunderstanding?

The IF output from the VHF tuner to the control grid of the mixer when in FM mode is as shown on the schematic...VHF tuner IF out>>contact 17 on LW switch>>contact 16 on LW switch>>contact 3 on MW switch>>contact 2 on MW switch>>C21>>mixer grid pin 2.

You have to remember that the LW and MW switches only operate when one of their buttons is pressed, on FM non of the MW and LW buttons are pressed so they are effectively in the receiver off position as shown on the schematic, pressing the FM button switches in the HT to the VHF tuner as you've already observed.

Pressing the FM button will return any engaged MW/LW switch to the off position, the schematic shows all switches in their off positions.

The AM mixer functions as an FM IF amplifier when the band change switch is in the FM position, in that position HT to the AM oscillator is disabled (FM switch contact 10) and is diverted to the VHF tuner via FM switch contacts 8 and 9.

Lawrence.

[indigo.girl]

Well that's a confusing schematic! So in fact only the move from positions 10-9 to 9-8 actually occurs when the VHF button is depressed.

[ms660]

Not quite, there's contacts 15,16 and 17 on the FM switch regarding the schematic in the snippet.

The primary of the FM IF transformer is shorted out by two of those switch contacts (16 & 17) when AM is used, the short is removed when FM is selected (16 & 15, 15 isn't actually shown on the switch block diagram) there's also 5,6 & 7.

Theories for R51 next.... anyone? I've got my own.

Lawrence.

[indigo.girl]

Not quite, there's contacts 15,16 and 17 on the FM switch regarding the schematic in the snippet.

The primary of the FM IF transformer is shorted out by two of those switch contacts (16 & 17) when AM is used, the short is removed when FM is selected (16 & 15, 15 isn't actually shown on the switch block diagram)

Ah, I think my confusion has been that Long wave contacts numbers 15, 16, 17 are NOT the same as the FM contacts numbers 15, 16, 17. I have been thinking there is only one set of contacts for each number. But I've now spotted a set of FM 15, 16, 17 close to C27. So, pressing VHF only makes a connection between 15-16 on the FM switch (i.e. near C27) but not the contacts 15-16 near the AM aerial. I get it now

[ms660]

Yep, that's it, in the snippet the FM switch contacts in the schematic are "f" 5,6&7...."f" 8,9&10...."f" 15,16&17.

Lawrence.

[indigo.girl]

Ok - so seeing as AM stations are working well this means that the FM can only be in sections of the V3 circuit that are exclusively used for FM. The V3 triode part is not used by the FM pathway so can be ruled out as the fault. But the heptode of V3 is used by both AM and FM. But S14 is only switched into this part of the circuit when FM is selected. So in theory the S14 inducer could be faulty so less signal (only that induced in S17 rather than S17 + S15) gets to the pin 2 grid of V4. Hence very quiet reception on FM.

There no open circuit found about each end of S14 so looks intact....so doubt thats the problem.

I'll move onto checking the rest of the circuitry for V4 and V5 that's exclusive to FM