Marconiphone T19A - No Sound

[Zelandeth]

I decided that starting a new thread here was the best course of action, as the original regarding capacitors has served its purpose, and this is an entirely new problem!

Simply put, it's dead...or at least seems to be. I'm pretty convinced that both the local oscillator and amplifiers are doing anything but what they're meant to be.

Touching the wiper of the volume control with a screwdriver/meter probe only produces a very faint click from the speaker. The same is true of switching between bands.

The set exhibited similar symptoms prior to a complete recap (the only originals remaining being C24 and 32 (24uF electrolytics in a can) which reformed perfectly. Unlike the Hunts one in the corner of the chassis which was doing a very good impression of a 5K resistor. All replacements for waxies and Sprague metal bodied caps being the usual yellow 1kV rated metalised polypropelene ones from RS which have always done me good service in the past.

So...where do we stand? Pretty much as follows. The anode voltages all appear slightly high - though as nothing seems to be pulling any current, that may be to be expected - the voltage at the smoothers appears okay though, the sheets say 305V and I've got 308V. Not that far out. At least the U143 rectifier's doing its thing.

Here are the voltages I've measured (heaters are all good, so are omitted for clarity). Note these are all taken using a DMM relative to the chassis. Numbers in brackets are what they should be.

V1 - X143 Heptode/triode.

Ah: 292V (230)
At: 162V (100)
Gt: -6V
G2/G4: 86V (90)
G1: -1V
G3: -6V

V2 - W143 Pentode.

A: 288V (230)
G2: 80V (90)
K: -1V (0) - My meter having a fit?

V3 - DN143 Beam pentode(?)

A: 298V (260)
G2: 256V (230)

V4: Another W143.

A: 138V (45 - though I reckon that's a typo and should be 145V)
G: 20V (15V)

Obviously I'll be going through my work with a fine toothcomb tomorrow - but does this give anyone an inkling as to what might be up with this set?

There's still a bit of tidying up to be done besides this - for one thing the mains lead has obviously been strangely affected by the gunk from the leaky smoother, so will be getting replaced (and a suitable cord grip fitted).

Is it just that I've had quite limited exposure to these sets so far - or is this quite an oddball bunch of valves too?

[Steve_P]

Are all the valves lit. Silly question I know but we've all done it!

Check R16, R17, C31.

Valves :

X143 is a Frequency Changer.
W143 is an IF Pentode.
DN143 is an Output Valve, and a Diode.

When you switch to gram, do you have a hum when you touch the gram sockets?

The Internal external plug switch - is it set right. Switch to ext and try another speaker. The O/P transformer should be 500 Ohms primary and 0.4 Ohms Secondary.

Check resistors for High and Open Circuit. Look at R9 as well as the ones on the HT rail.

Cheers,

Steve P.

[Zelandeth]

Cheers Steve, I'll investigate those leads tomorrow. It was getting late this evening, so I didn't dig too far. The heaters are all lit (even the DN143 after the fight it put up to come out of its socket!), I didn't try the gram input though.

C31 is new, R16/17 are originals though, so they'll be my first ports of call.

The old 1W carbon (I think - the long painted ones where the paint always flakes off!) resistors all measured high (15-20%) and were replaced - I've not gone through the rest of them in any depth though - time to spend an extra five minutes with the meter methinks. I have triple-checked my soldering around that confounded tagstrip though.

The output tx has passed the touch-test in as much as the speaker made a sound when I touched the output tx terminals to check with the meter that it didn't read o/c. Checked that before I took on the set, but I'll check the actual values tomorrow. Occurs to me that the values were right in front of me now and I should have just measured it. Ah well, will remember next time. Maybe!

[stuie319]

Hello,

Originally Posted by Zelandeth

The output tx has passed the touch-test in as much as the speaker made a sound when I touched the output tx terminals to check with the meter that it didn't read o/c.

I'm not sure the o/p tx has passed. Did you 'touch test' across the secondary with the speaker connected? Try it across the primary, or, disconnect the speaker and measure continuity across the secondary, you may have just proved that the speaker isnt o/c. fingers crossed for the tranny

Regards
Stuie

[PJL]

The bias for the output valve and AGC line are derived from series resistors in the -ve supply path. -1V is probably OK. What's the grid volts on DN143? The tx is probably OK as the anode is higher than G2. V4 is the AF amp.

[Zelandeth]

Output tx is measuring 489 ohms primary and 0.4 ohms on the secondary.

R16 and 17 check okay. R9 is somewhat high at 11.7K
I decided to go through and check each resistor in turn, as there aren't all that many of them.

The only ones to raise an eyebrow were R10 which was significantly high (1.5M as opposed to 1M), and R11 which had risen from 150K to 200K (which would account for the slightly low voltage on V4's anode. If I'm following the schematic properly, R10 being high should result in the G2 voltage on V4 being low...when it measures high!

Off to do some more hunting.

[Steve_P]

Have you tried wiring another speaker on - it's own might be knackered. Also, check all your wiring as you go.

Always check the simple and easy stuff first!

Cheers,

Steve P.

[Zelandeth]

Right progress!

The original speaker I've not tested by substitution, but it is definitely capable of producing sound of some description - may try another shortly.

I have proved now that it is actually alive - it's sick, but alive. Feeding a proper signal into the gram input with the volume at maximum gives sound, albeit very quiet. Likewise with the radio - using a screwdriver on the antenna terminal as an aerial (I'll stick my usual drum of speaker wire on there later) definitely gives the sound of reception on MW - though with some really of squeaks and indescribable noises in between stations! Fingers crossed that behaviour will rationalise itself a bit once there's an antenna on there.

Audio from the gram input didn't sound distorted or anything, just very quiet, definitely affected by the volume control though.

One thing I have noted as well is that R2 and R5 are getting quite warm - despite the 1W originals being replaced with 2W rated parts - I don't know how warm they'd be expected to get at the best of times though so that might be a false lead.

[PJL]

Sounds promising as it is taking some current - did you measure the DN143 grid? This will tell you if the DN143 is biased correctly and also the total current of the set.

[Steve_P]

Leave them for now. Wrong end of the set.

Try swapping the W143's over. See what happens.

The O/P valve is in fact EBL21. Rather funny system this. AM decoded in EBL21, Pre-amp is one of the two W143's, (EF22), and final pentode is the EBL21. Try inserting a signal at Pin 3 of the DN143 (EBL21).

I think one or more of these valves may be low emission.

Cheers,

Steve P.

[Zelandeth]

Tried switching the W143s around, doesn't seem to have made a huge amount of difference. The set is still very quiet, and even with an antenna, the reception on MW seems to be very "thin" for want of a better word.

A meter probe on pin 3 of the DN143 doesn't seem to have any real effect.

[Steve_P]

Check that the non-valve side of R9 goes through VR2. The other end should go to the mains transformer centre tap - NOT the chassis.

Check the wiring again against the diagram. Check C28 and C30 if you haven't already and try lifting the wiper of RV2. Are RV2 and RV3 OK.

This is starting to look to me like a Low Emission EBL21.

Cheers,

Steve P.

[Zelandeth]

Non-valve end of R9 to tone control (R21 on my sheets) - check. Control working (if noisy) - check. Other end disappearing to one of the mains tx tappings - check.

C28 measures 0.32nF (350pF on the sheet).
C30's new and connected correctly and measures spot on.

Volume control pot is okay as well.

I'll double-check all my wiring tomorrow, as I know full well, even when working one lead at a time that mistakes can happen.

EBL21/DN143's rare/expensive valves to get hold of?

It's worth noting at this point I guess that it *looks* like the valve has been driven very hard - aside from the gettering, there's a lot of deposits inside which look to me like sputtered electrode material - very bright silver rather than the glossy dark silver of the getter - and only in line with the openings in the anode cylinder.

[Steve_P]

Well EBL31's are, but I don't think EBL21's are.

http://www.vintage-radio.com/contacts/suppliers.html

Ask for some quotes! Not Malcolm Bennett - he's not too well. Or put an advert in the Parts Wanted section below and see if anyone can help.

Cheers,

Steve P

[Zelandeth]

Cheers, I'll stick a post in parts wanted and drop the folks on that page a line. Even if I don't need it now, can't hurt to have spares!

[Zelandeth]

So now we wait a bit. New DN143 has been ordered from Valve & Tube Supplies. Until then, I guess we wait. Reckon the valve should be here Thursday/Friday next week.

That's definitely the fastest response I've ever had to an email! The response to my query as to whether they had one in stock was responded to in four minutes from the second I hit send! Now just to cross my fingers that the set plays ball once it arrives.

I'll get the rest of those out of spec resistors replaced in the interrim as well and get a replacement for the duff dial lamps (which I've borrowed out of one of my own sets for testing...just got to remember which one now!).

[PJL]

Did you measure the grid volts on the DN143? It should be around minus 5-6V. If it's less then the velve is low emissions otherwise it's probably OK.

Most of the HT current is taken by the output valve. This design biases itself by using the HT current. The correct operating condition for a DN143 is as above. If it's low emissions it will not be passing current and the bias will remain low. This type of design does have a self-regulating effect.

[Zelandeth]

Interesting, just measured that voltage again (couldn't figure out which scribble on the scratch pad was that one!) and it's sitting stable at -1.63V.

[PJL]

I think the sum of the resistors is about 80ohms so the whole set is taking 20mA. The output valve should be taking 80mA with that low bias figure so the emissions are very low as Steve has suggested...Peter

[Zelandeth]

It lives! The new DN143 seems to have done it - coupled with me jumping halfway across the room when switching on and it warmed up because I'd forgotten to turn the volume down.

I don't seem to be able to find a great deal in the way of stations - plenty of buzz at various points on the MW band, along with a couple of stations...will try again tomorrow with a better antenna and see what happens.

Glad to have finally made some sort of recognisable progress now though!