Ferranti 145 - Crackle, no actual audio

[Zelandeth]

Having finally sorted the severe case of vapourising wire insulation syndrome that this set had, I've been able to replace the usual suspects - wax-paper caps which had taken up the appearance and electrical characteristics of toffees. The cardboard cased smoothing block was also evolving into a new lifeform, so was ditched (though I am holding onto it to use it as a template for the possible creation of a replica box at a later date).

Having got it to a state where I dared apply some power (via the lamp limiter). At this point I discovered that I've got two dead scale lamps...I swear, I must be keeping Maplin's supplier of these things in business...however, despite that, there were no loud bangs, no fireworks from inside the 5Z4G rectifier, and there was some sound - albeit erratic crackling rather than actual static. There's a (very, very soft) hum there as well.

I seem to have around -1.6V on G1 of the output valve, and no cracking when the wavechange switch is operated - though the volume control is suspect, and the power switch is sticking on. It also seems to be pulling quite a bit of current judging from the brightness of the lamp - though I'm not certain what sort of power this set should draw anyway yet.

Will be taking some further measurements tomorrow and seeing if I can track this down. First port of call will of course be checking, checking, double triple and quadruple checking all my work, then getting some dial lamps so that I can at least see when the set's switched on easily - though it's only being run a minute or so at a time at the moment until I'm convinced that it's not about to self-destruct. Wavechange switch will then be given a second good dose of contact cleaner.

Anyone care to point suspicious fingers at any components in particular?

Still got work to do here methinks!

[Steve_P]

Well if there's nothing, not even clicks, when the wavechange is operated, then the IF or Audio Stages are most likely at fault. As you have changed loads of caps and wire, check thouroughly to make sure it's all right. One mistake....

Servisol on the switches and controls first. Does help. Maybe there's no crackle because the wavechange switch is not wavechanging?

Put a finger on the Pick Up port on the back and see if it makes any noise. If it does, the Audio Stages are alive. If not, then the problem is on the audio side.

Check voltages against the sheet, (No. 763 I'm using) and make sure they are all OK.

Take it from there. Good luck - nice sets those!

Cheers,

Steve P.

NOTE: There does not seem to be an AF coupling cap to the Output Valve here. I reckon there is but it's been left off the diagram. If it wasn't there the set would sound awful and it would go through Output Valves and Output Transformers at a rate of knots!

[Studio263]

Originally Posted by Steve_P

NOTE: There does not seem to be an AF coupling cap to the Output Valve here. I reckon there is but it's been left off the diagram. If it wasn't there the set would sound awful and it would go through Output Valves and Output Transformers at a rate of knots!

If it's the trader sheet you have I think you are right, the capacitor had been lost when they cut the diagram in half. It is mentioned on the components list however as C22 and is shown on the under-chassis view, so it's definitely there! Note that this set has a negative bias rail (dropped across R18), this is worth checking.

Goodness me what an ugly set though!

[Richard]

Hi

If the wiring and caps were in such a bad state, then I suspect the set has been stored in a very damp place at some point, so saddly the IF coils are suspect for corrosion damage

Richard.

[Zelandeth]

Cheers folks. I'll get it on the bench after dinner and have a look around again - I did some wiring double-checking last night, and couldn't see anything amiss...will go over it again though!

Steve: I'm using Trader 763 as well.

As for the state of the caps etc, the wiring didn't actually appear all that bad until I made the mistake of trying to bend a wire! The leakage from the caps looked suspiciously recent in comparison. I'll check the resitances of the IF coils while the set's on the bench though.

Need to recheck all the valve pin connections as well, as the valveholders on this set really don't fill me with confidence.

[yestertech]

When you get a bit more life out of it, check the seemingly innocent looking flat mica caps
(in orange and blue ) I think they may have been LEM brand
I had 3 in my 145 which were leaking like sieves - they were of course the last thing I suspected - probably caused by damp storage
However, this is unlikely to render the set "dead" ..... Andy

[love kt3]

hi-istr there is a resistor inside the second if xfmr and if its open there will be no audio.
@studio 263-i beg to differ-to me its one of the nicest sets i have.

[Zelandeth]

Okay, I'm starting to get rather more confused now.

It's definitely pulling too much current - dragging the mains down to 181V on the limiter. Mains transformer isn't getting warm though, so I don't think it's to blame.

The coil around the loudspeaker base however DOES seem to be getting slightly warm (I've only run the set briefly so as not to fry any wound components like this). Especially as I've never encountered a speaker without a permenant magnet before, so don't know how fragile these things are to be honest.

I'll be doing some proper resistance checks again tomorrow - think my meter's battery is dying because I seemed to be getting somewhat erratic readings this evening. I did discover that R4 (HT feed to screen grid on V1/2) has crept up to 16K from 10K, and that R9 (V3 triode grid bias/AVC delay) is open circuit. I somehow doubt I need to be worrying about AVC at this stage - but what other effect that may have on the circuit I'm not sure.

A new battery and some further hunting tomorrow evening will hopefully yield some more useful data, including voltages which will hopefully give me some more idea of what's going on (or not!).

[yestertech]

Beware fault conditions affecting the LS field coil, they burn out pretty easily as they're thousands of turns of very fine wire - I always reckon on around 100v across the field coil (with some exceptions ) Much more and it could indicate a short in the HT or a valve underbiased etc.
As you say, when you can do some accurate voltage readings we'll know more ANDY

[Steve_P]

Take the valves out and measure the mains on a variac as you power it up. If it's still drawing too much current then it's probably the transformer. Has it fully dried out?

If not put the rectifier in and try again. Smoothing is done by the caps and the speaker field coil. Check the speaker coil is OK as well as the smoothers.

Cheers,

Steve P.

[Zelandeth]

Okay, that ties in with how I thought it was constructed - and how fragile I thought it was. A variac is unfortunatlely the one useful bit of kit that I don't own, and thus far haven't been able to track down at a price I can afford. Varying the lamp used in the limiter from 10-100W is the best I can do on that front I'm afraid.

Smoothing caps are all okay (new caps, wiring to them quadruple checked now - unless I've got dud new ones of course!).

First things I'll be checking will be indicated reistances of the field coils/hum reduction/voice coils in the speaker, and the voltages present there. As I said, discovered that my meter was throwing a fit this evening so I don't trust any of the resistance readings I've just taken.

Valves all out, mains current draw is non-existant. I'll try with the rectifier back in if appropriate once I've checked the stare of the speaker coils etc tomorrow.

[PJL]

The lamp limiter will do fine. Follow Steve's suggestions and power up without valves which should barely light the bulb. Then try with just the rectifier and again it should barely light the bulb but don't leave on for long. If the mains transformer stays cool I doubt that is the problem.

A DMM won't give a meaningfull reading as it won't loook much like a sine wave.

[Zelandeth]

Speaker field coil: 1.04K (trader value: 1K).
Hum neutralising coil: 0.02R - spot on.
Speech coil (admittedly being measured in series with the hum neutralising coil and output tx secondary) seems to be okay, with those adding up to 2.9R.

It seems happy to sit indefinitely without the rectifier in without drawing any current. With the rectifier in and the rest of the valves out, it seems to after a short warmup period to pull quite a bit of current again. The DMM is claiming 1.9A current draw (with all the valves in) on the limiter once the set's warmed up.

I'm reading a steady 91V across the speaker field coil.

[PJL]

You won't get an accurate reading across the field coil as it will have alot of ripple.

Measure the resistance across the HT (C28).

If the lamp is glowing with only the rectifer you have a problem so don't leave it on for long. What HT (across C27 and C28) do you get with just the rectifier? If it doesn't rise above the trader sheet value then switch off immediately.

[Steve_P]

Change them. The only ways (apart from man made ones) that this voltage can get to earth with other valves out is C6.

Cheers,

Steve P.

[Zelandeth]

I'm out at my parents place this evening, but I'll check the voltage over those caps and check the wiring to/replace C6 when I'm back in the flat tomorrow.

Is that one a waxy or an electrolytic originally? I don't have the sheets with me, and if it's an electrolytic I might well need to order in replacements as I don't have a huge range of them in stock. Got a huge bunch of polypropelene ones though.

Is there any chance of the rectifier itself being at fault here?

[Steve_P]

It's not an Electrolyic. Must be a waxy.

Cheers,

Steve P.

[Zelandeth]

Okay...this set is starting to irritate me now. Think I'm going to have to resort next to the point-to-point following of the schematic, examining every single component connection until I find the mistake.

We seem to have 0.78K across C6, and only 70 odd volts there. C6 itself however is fine, and has been tested by substitution.

HT looks to be about 170V.

There's plainly something quite wrong - I'm just at rather a loss as to find it.

[Steve_P]

This is an AC set. So, you can take one valve out and the rest will still light up. Leave the Rectifier in place and pull the other four.

If the HT is still as low as you say at C6, then it's either a wiring fault, the smoothing caps, the rectifier valve or possibly the speaker.

Cheers,

Steve P.

[Zelandeth]

Yep...that is still with all the others out.

Smoothing caps are brand new, so my gaze is currently fixed firmly on my wiring, as that's where a lot of work has been done.

Will do a detailed check on it again over the next couple of days and see if I can figure out what's going on!