Restoration novice part 2

[LyntonP]

I was recently presented with a Marconiphone T89DA.
I found the trader sheet 1486 which was common to this and other radios, and also read a post on the forum from a member who had restored one.
On examination resistor R33 next to V6 (trader sheet numbers) had come unsoldered from the circuit board, and that the dial screen was very distorted on one side possibly due to heat damage.
I resoldered the resistor and powered up the set via an isolation transformer and a lamp limiter. The dial lights lit up the valves glow, and there was no indication of any short circuits from the limiter. Prior examination of the dial lamps gave no clue to their voltage and since the trader sheet only showed one lamp (mine had two in series), I tried to check their working voltage only to have one fail immediately. The set is completely dead on all wavebands.
I turned my attention to the distorted dial screen which I covered with some card and applied a warm iron to try and reform it. It immediately stuck fast to the card! So after two mistakes I have left it in the cold shed and returned to the warmth of the house!
In the post I read from another member in 2011 where he described returning his identical set to working condition I have noted the components he changed which I will have to order.
Any further suggestions would be welcomed to progress my first ‘live chassis’ set.
Lynton

[WaveyDipole]

I sounds like you will already know which capacitors to replace from the other thread. Any plastic moulded paper capacitors (e.g. Hunts) are best replaced on sight. Once you have completed this step, then troubleshooting can begin in earnest.

Since this set is built on a PCB, one thing you could do while waiting for the capacitors to arrive is to check the tracks, particularly near the bases of the valves (which get hot), to confirm that there are no cracks. You could also check the IF cans for open coils, or the tuning capacitor for a short. Bear in mind that resistors can "go high" or even open circuit. Their values may not be critical, but any that are significantly out of tolerance are probably best replaced.

Other than that, I would wait until the paper caps have arrived and have been replaced. Then you will be in a position to check voltages against the service data and troubleshoot further.

[Nuvistor]

As well as looking for cracks in the PCB one area that can be susceptible to tracking is the PCB area around the UL84. The PCB material can allow a circuit between pins 2 and 3, grid and cathode putting a positive voltage on G1.

Do you have an isolation transformers? Worth thinking about getting one if not. Other than that confirm the chassis is connected to neutral. Check the on/off switch works correctly.

[LyntonP]

Thanks Gents. I will have a look tomorrow. I wish I had just a little of your experience.
By the way Frank both of your radios are working very well, in particular the ECKO which has an astonishing amount of volume. Did you sneak a small linear in the back of it?

[WaveyDipole]

Sorry, to clarify the last line of my post #2, I meant to say wait until the replacements caps have arrived and the paper caps have been replaced. Don't use paper caps as replacements, especially NOS ones.

[LyntonP]

Yes I understood your message but thanks for the clarification
Lynton

[LyntonP]

Did a little more work on the Marconiphone today and ordered the necessary caps plus a few more bits and pieces and the complete service sheet cd so I should have the correct wiring diagram soon.
Effected a temporary repair to the dial lamp diffuser and had another look at the lamps in question. It seems that they are 28 volt 0.1 amp (two lamps connected in series). Checked the supply voltage on the circuit board and this is correct. Searched all over the internet but could not find them other than a site in USA. Any suggestions where I might find them in the UK?
Tomorrow I plan to make a support frame so I can turn the chassis over and back without disturbing things and as soon as the new components arrive start checking some voltages in earnest.
Great fun this|
Lynton

[Nuvistor]

According to the circuit they 12V at 0.1A.

[LyntonP]

Blimey! I could have ordered a spare today! One of the lamps I could not read any specification. The other showed (not very clear) that they were 28 volt 0.1 amp. I will test the surviving lamp tomorrow.
Lynton

[LyntonP]

I should have the correct wiring diagram very soon. The trader sheet was not specific to my model.
Lynton

[WaveyDipole]

Originally Posted by LyntonP

It seems that they are 28 volt 0.1 amp (two lamps connected in series). Checked the supply voltage on the circuit board and this is correct.

If you mean that you measured the AC voltage feed from the circuit board to the dial lamps (at points 27 and 28) then this will not be a reliable indication for a number of reasons:

The two lamps form a parallel resistance with the varistor X1. With the lamp missing and the circuit open at that point, the overall resistance formed by this section of the circuit will be higher than when both lamps are in circuit, i.e. when the load is applied. Consequently the voltage reading across X1 will be on the high side.

That section, in turn, forms part of a series resistor chain, including varistor X2 and its parallel resistor, a couple of other series resistors and the valve heaters. Voltages across various parts of the chain will tend to vary after power up as the two varistors and valve heaters warm up. They eventually settle when the radio is warmed up but measuring cold will not give the same voltage reading as measuring when the radio has warmed up.

Finally, if you are using a lamp limiter, this also affects the input current and likely the voltage at the mains input, and in turn the voltages across individual parts of the chain.

Best to make sure you have the correct service info and then use whatever has been specified.

[LyntonP]

I have been checking a few details and specifications of the valves. Since the valve heaters are in a series chain their total voltage adds up to 168.6 volts (information from valve museum). If the dial lamps were both 28 volts then this brings the total to 224.6 which sounds reasonable. If the lamps were only 12 volt (as shown on the Marconiphone diagram) then this would bring the total of the series chain to 192.6 or is there a bit of trickery I am missing? I can see two “Varites” (thermistors)? Also a 250 ohm resistor in the circuit but I am not sure what effect these have on the voltage supplied to the chain?
The trader sheet isn’t much help as the circuit shows a 250 volt pigmy lamp as used on the similar Ferguson radio.
Any ideas?

I have received some new components for the set but I’m sure I will need more since a closer inspection has revealed further damaged components (split 3 watt resistors etc).
I’m sure this is a has been a high usage set and the generated heat has damaged even more components.
Finally a question regarding discharging the bigger high voltage capacitors. What value and wattage resistor is recommended to safely discharge the capacitors without damaging them?
Cheers
Lynton

[Nuvistor]

The valve heaters add up to 168.6v as you stated, the voltage drop across the two 12 v bulbs is 24v. X1 will also have an effect but much less than the bulbs so for simplicity I will discount it, it’s main purpose is to keep the radio working if a bulb fails.

The 250 ohm resistor will drop 25 v and X2 is rated at between 200 and 260ohm at 100ma, say 230ohm, this will drop a further 23v. The 2.2k resistor will also have an effect but ignored for now.

So 168.6+24+25+23=240.6v which is close enough.

This of course presumes my T89DA Circuit is correct and my sums are correct.

Any doubt just measure the current in the heater chain, should be 100ma +- 5%, I think that the tolerance.

[LyntonP]

The 250 ohm resistor will drop 25 v .

Hi Frank Thanks for the excellent and informative reply.
What formula are you using to calculate the 25 volt drop, or does that come from experience?
I think we are both looking at the same diagram from Marconiphone, a two page document?
As noted I have a lot more work to do on this set but every step, and all the answers from the forum are a great learning experience. Just wish it was a bit warmer and I could spend more time in the shed!
Thanks for all your help.
Lynton

[julie_m]

What formula are you using to calculate the 25 volt drop, or does that come from experience?

Ohm's law: V = I * R.
The voltage across a conductor is equal to the current flowing through it in Amperes, times its resistance in Ohms. Since the heater current = 0.1A, and things in series have the same current flowing through them, this means that a resistance of 250Ω will develop 250 * 0.1 = 25V across it.

[LyntonP]

Hi Julie yes I am aware of ohms law, but can it be applied to ac?
I wasn’t aware of the value of the current in the heater chain. Is this typical for a six valve set?
Lynton

[Nuvistor]

Ohms law can be applied to AC in this case, I and V are in phase, only resistance no or virtually no impeadance.
The valve type indicates the heater parameters.
Exxxx series eg EL84 has a 6.3v heater used mostly in parallel heater circuits.
Uxxxx series eg UL84 has 100ma heater used in series heater circuits
Pxxxx series eg PL84 has a 300ma heater used in series heater circuits.

Need to check the specifics an EF80 valve has a 6.3v 300ma heater so could be used in series string with Pxxx valves or parallel with Exxx valves.

Many valve types and specifications at
http://www.r-type.org

[julie_m]

Yes, Ohm's Law works with AC -- at least if you stick to sine waves and simple resistors, and measure all voltages and currents in the same way (all peak, all peak-to-peak or -- preferrably -- all RMS). Once you start introducing inductors and capacitors, you start needing to use complex numbers (but the maths still works). And some devices (thermistors, filaments and pretty much any resistor if you are really bothered) have a resistance which changes with temperature; but again, as long as you are talking about equilibrium conditions, the maths will work fine.

Valves with a "U" prefix have a heater designed for series connection, with an operating current of 100mA. Generally a rectifier or output valve will need a more powerful heater than a small-signal valve, so these valves will drop a higher voltage.

Hope that helps!

EDIT: Crossed with Frank.

[Nuvistor]

This page gives lots of information on what the valve number means, ie. pentode, triode etc.
https://www.vintage-radio.com/repair...numbering.html

[Al (astral highway)]

Originally Posted by LyntonP

I wasn’t aware of the value of the current in the heater chain. Is this typical for a six valve set?

Hi Lynton,

The current flowing through the heater of various valves used in vintage radio and TV varies widely.

Taking some completely random examples, in ECL86 triode/pentode, it's 660mA. In EABC80, triple-diode triode, it's 450mA. In ECC88, VHF double triode, it's 365mA.

Battery sets are another thing altogether. And so are big transmitting tubes, (slightly OT) which take big Amps of current, 5A or even more is common.

Beefy line output valve PL81 (TV) only needs 0.3A on its heater, but it also drops over 20V so the power taken is considerable.

If the valves are in a chain in series with a dropper resistor (often replaced with a dropper capacitor in restoration) they have all been selected such that they run on identical heater currents. As Julie points out, this is no bother as there are many valves with the prefix U- , all with 100mA heaters, and which are designed partly to simplify this process.

If they are run from a winding on the mains transformer, and need the typical 6.3V for the heater, it makes no odds. as they are then wired in parallel. Each will take the current it needs.