1936 Marconiphone 365 Radiogram

[Station X]

It's difficult to tell what's pulling down the HT with link 2-7 in place, but C84 has to be the prime candidate as it's connected directly to point 2.

When you start looking at other possible electrically leaky caps you find that they're all fed via high value resistors which would limit the current.

EDIT. C69 is another possibility. In theory if it were leaky it could pull 45mA via R41.

[Station X]

You could check the current drawn from point 2 by connecting a meter on say the 100mA range across points 2 and 7. Remember that some meters require the use of a different front panel socket when measuring current.

[kinnego07]

Apologies Station X I hadn't noticed that we had turned a page.

I had replaced C84, C60 and C65 but put them back to original in case I had made a mistake. There is no change in HT

All wax caps and electrolytic caps have been replaced however as mentioned in a previous post the service sheet calls for C69, C62 and C73 to be 4 ohms.

These three caps were the only obvious changes from original (radio spares parts) C69 was a wax 2 mfd 500V and I replaced it with 4.7 uF. C62 was 50 mfd 12V and I replaced it with a 47uF. C73 was a 16 mfd and I replaced it with a 15uF

I don't know if this has a bearing on the problem

I will replace C84 with a new cap again

Unfortunately I can't get a mA reading - it could be that the multimeter fuse has blown. Testing however produced first life/crackling at the speaker.

The only caps left unchanged are mica type - C74,C75,C81,C82 etc

R64 reading 1100 ohms

I'm going through the set now rechecking wiring connections and resistors

[Station X]

What's the HT voltage with only V6 inserted, 2-7 link out and 2-7 link in?

[kinnego07]

2/7 disconnected = 465V

2/7 connected = 285V

[Station X]

With the 2-7 link in the only components which should be drawing current are R41 and R42, which have previously been discussed.

What's the voltage across each of these resistors, link 2-7 in and link 2-7 out?

[kinnego07]

Link connected - across R41 = 168V R42 = 93V (with limiter)

Without link 0 Volts both sets of resistors

These are all large resistors - about 2 1/2" long and 1cm diameter

All valves removed apart from V6

[Station X]

From post 66 the measured value of R41 is 11,500 Ohms and R42 4,700 Ohms.

Current through R41 is 168V/11,500 Ohms = 15mA.
Current through R42 is 93V/4,700 Ohms = 20mA.

So 5mA is flowing INTO the junction of R41 and R42 which is something I don't understand.

R41 and R42 form a potential divider from which screen grid voltages for V1, V2 and V3 are derived. However this voltage is only applied in radio mode via S16 contacts 5 and 6.

Do the voltages change depending on whether radio or gram is selected?

[kinnego07]

The voltages remain the same in both the radio and gram switch positions.

I will double check the resistance on both sets of resistors again tomorrow. The ends of the three resistors (R41) and wiring are hidden behind the box housing Cs 84,60 & 65

Thanks again for your assistance - much appreciated

[Station X]

I think it would be very useful if you could fix your meter so that it will measure milliamps. Then we'd be able to tell exactly how much current is being drawn from Point 7.

[kinnego07]

Try as I might I can not read mA on my meter. Fuse was blown but new fuse has not helped - must be a fault in the circuitry. New multimeter required

Speaker making a LOT of noise when multimeter in circuit

Reading at R41 = 8600 ohms R42 = 5200 ohms

Reading at switch 6 = 4950 ohms

[Station X]

Your latest set of readings differ from those given in post #66!

The reading from S16 tag 6 to chassis should be the same as the reading across R42 regardless of which position S16 is in. If the readings differ you have a fault somewhere, especially if the readings are different in the gram and radio positions.

------------

To sum up this lengthy thread so far:-

1. V5 not conducting even though it's lit up and anode/screen voltages are present. The evidence for this is no voltage across the cathode resistor R51.

2. HT voltage being pulled down when link 2-7 is inserted. It's difficult to investigate this further without a means of measuring current.

[kinnego07]

I think I have a faulty multimeter. Everytime I take a reading at these resistors I get a different reading.

I rechecked the switch reading and it is the same now as R42 (maybe it's me that's faulty!)

I am ordering a new meter today so hopefully here tomorrow.

New V5 valve?

[Station X]

Originally Posted by kinnego07

I think I have a faulty multimeter. Everytime I take a reading at these resistors I get a different reading.

When taking a resistance reading you'll need to wait for the reading to settle. This is to allow any capacitors in the circuit to charge up to the voltage at the meter's terminals.

[Silicon]

I don't like using multimeters on the milli-Amp ranges. It is too easy to damage them.

I solder a 1 or 10 Ohm resistor into the circuit and measure the voltage across it.

[Station X]

Originally Posted by kinnego07

New V5 valve?

I don't think there's enough information for me to answer that. If V5 is passing anode current there'll be a voltage across R64. If there is a voltage there the question is where is the current going to? It certainly isn't going through the cathode resistor R51 as there's no voltage across it.

[kinnego07]

I will have access to my son's new multimeter this evening and hopefully get back with a mA reading.

I note silicon's post on soldering in a 10 ohm resistor.

For the first time now since testing 2/7 for mA I have faint sound at the speaker when toggling switches.

I do know from experience to wait for the meter to settle but not particularly why so thanks for the explanation.

It's all a learning curve.....

[kinnego07]

Ok, so I get 0.13 mA between 2/7 (with lamp limiter) Can that be right?

[Station X]

That's with the meter connected in place of the link?

The reading looks way too low. What range is the meter set to and are the leads in the correct sockets?

[kinnego07]

Yes, lead was in the wrong socket. My sons multimeter has a lot more features and flashing lights. My apologies

I now get a reading of around 8.6mA between 2/7