Bush AC34, no sound.

[Pepelephew]

I think this sketch is clearer
bill

[Steve_P]

Why are the two IF valves so different? The cathode voltages both low as well, though one is lower than the other. Also, why are the two Anode voltages different.

There should be a hiss noise in the speaker - is there?

As for the oscillator section. the lack of any voltage on the grid of the triode at DC may or may not mean anything. But I'd check R6, C14, C10, R3. R2, C9. These parts are common to every waveband and are most likely to fail.

Cheers,

Steve P.

P.S. There shouldn't be a direct link between the UCH42 and the UF41. Are you sure about that?

[Tim]

Screen grid volts on the IF valves are different too.
While your at it have a check of the resistors feeding them from the HT and any associated decoupling capacitors. Swap the two UF41's over and see if the voltages remain the same(something fauly in the circuit) or if the voltages swap over(circuit OK, valves faulty).
It's clear the audio stages work( as you had a loud buzz when you touched the probe on the UBC41 grid) so as others have said this seems to be an RF fault.

[Station X]

From your sketch and assuming AC34 Trader Sheet component designations.

Pin 1 (Heater) wired to heater chain.
Pin 2 (Anode of hexode) wired to L14.
Pin 3 (Anode of triode) wired to wavechanger.
Pin 4 (Control grid of triode) wired to R3 (47K yellow, violet, orange) and C10.
Pin 5 (Screen grids of hexode) wired to screen grid of UF which is also at 46V and C14.
Pin 6 (Control grid of Hexode) wired to wavechanger and C5 50pF
Pin 7 (Cathode) wired to R3 (47K yellow, violet, orange), R2 and C9.
Pin 8 (heater) wired to heater chain.

[Station X]

Originally Posted by Steve P

P.S. There shouldn't be a direct link between the UCH42 and the UF41. Are you sure about that?

It's a link between screen grids, so maybe they share a dropper resistor? They're at the same voltage ie about 46V

[PJL]

The 2nd UF41 is going to be an RF amplifier and if Bill has selected MW, it is probably out of circuit as it will only be used on SW.

The voltages on the UCH42 are a problem. Graham, the anode voltage should be low if the valve is not oscillating as, when oscillating, the AC signal on the triode grid is rectified to make the grid go negative which reduces the anode current.

This spec http://www.mif.pg.gda.pl/homepages/f...30/u/UCH42.pdf says the triode should be drawing over 6mA which I strongly suspect it is not. There are two possibilities:

a) The UCH42 is dead
b) The DMM probe capacitance is killing the oscillator

I suggest the following actions:
1) Measure the resistance from UCH42 pin 7 (cathode) to chassis so we can calculate the current
2) Try the other UCH42 you have

[Pepelephew]

graham,
sorry , the blue wire goes to the base of the unit in the pic, not sure of its correct description

[Station X]

Originally Posted by PJL

Graham, the anode voltage should be low if the valve is not oscillating as, when oscillating, the AC signal on the triode grid is rectified to make the grid go negative which reduces the anode current.

Agreed, but at 104.4V it's a lot higher than the 60V quoted in the service sheet.

Originally Posted by Peplephew

graham,
sorry , the blue wire goes to the base of the unit in the pic, not sure of its correct description

That'll be L14 and L15. I'll update post #144

[PJL]

Originally Posted by Station X

Agreed, but at 104.4V it's a lot higher than the 60V quoted in the service sheet.

Agreed, the high anode voltage means the UCH42 is drawing far too little current and therefore is low emission. If we try the other UCH42 that might help. Not oscillating the anode voltage should be down to 30-40V or so.

[Station X]

Well I have to do some metal bashing now, so by the time I get back from the workshop I hope to hear that this set is working on MW.

I assume it has an aerial?

[Pepelephew]

hi lads,
I have another uch42, fitted it and here are the voltage readings
pin 1 brown 22.8 ac
pin 2 blue 137.1 dc
pin 3 green 108.6 dc
pin 4 0.0
pin 5 purple 50.0 dc
pin 6 green 0.002 ac
pin 7 yellow 1.28dc
pin 8 brown 36.2 ac

Bill

[Pepelephew]

Originally Posted by Station X

The voltage on pin 3 (anode of triode section) of the UCH42 is high indicating that it isn't oscillating. It should read about 60 VDC, but you show 104.4 VDC. Measure the value of R6 which should be 15K. (Brown, Green, Yellow).

resistor on tag board R6 brown,green,yellow reading 187k ohms
Bill

[Pepelephew]

Originally Posted by PJL

The 2nd UF41 is going to be an RF amplifier and if Bill has selected MW, it is probably out of circuit as it will only be used on SW.

The voltages on the UCH42 are a problem. Graham, the anode voltage should be low if the valve is not oscillating as, when oscillating, the AC signal on the triode grid is rectified to make the grid go negative which reduces the anode current.

This spec http://www.mif.pg.gda.pl/homepages/f...30/u/UCH42.pdf says the triode should be drawing over 6mA which I strongly suspect it is not. There are two possibilities:

a) The UCH42 is dead
b) The DMM probe capacitance is killing the oscillator

I suggest the following actions:
1) Measure the resistance from UCH42 pin 7 (cathode) to chassis so we can calculate the current
2) Try the other UCH42 you have

resistance from pin 7 to chassis 227.4 ohms
Bill

[ThePillenwerfer]

Brown, Green, Yellow is 150K. In most locations 10 to 20% out is OK so that seems a smidge high to me.

- Joe

[PJL]

Brown Green Yellow is 150K so near enough.
Cathode current is 1.28/227.4 = 5.6mA which seems a bit low to me
New UCH42 shows similar voltage readings, I guess it's possible both have been damaged by the faulty UL41

Do you get any noises from the speaker when you change wavebands?

Give the waveband switch another clean with Servisol (remember the wafers have 2 sides so you might need to use an earbud to get at the back) whilst we have a think!

[PJL]

OK, I am stuck, the DC conditions say it should work although a couple of them look a bit out.

Try it on the LW position and measure the UCH42 voltages again.

[Pepelephew]

Hi lads,
Things are looking up....changed over the 2 uf41. To find I can receive a
Couple of stations on mw and one on sw, nothing on lw.Anything that's coming in is on the same end of the tuning mechanism. The intertwining fins are in the up position. When I tune them downwards just before the half way mark everything's disappears.
The one station I could identify was at 1215 kHz / 247 m. There is nothing else at the other end of the band. I had to replace the tuning cord when I started this and I don't have it calibrated properly yet so I am not sure where I am on the bands.

[Station X]

Originally Posted by Pepelephew

resistance from pin 7 to chassis 227.4 ohms Bill

That'll be R2 220 ohms red, red, brown, so it's in spec.

Originally Posted by Pepelephew

resistor on tag board R6 brown,green,yellow reading 187k ohms

R6 is shown in the Trader Service Sheet as 15K. This would have a colour code of brown, green, orange. (Sorry I got the value correct, but the colour code wrong earlier.

Is it possible that the resistor is in fact coded brown, green, orange, or has been replaced with an incorrect value? In either case it's way off spec.

This assumes of course that the set is wired like an AC34. So far apart from the way the screen grids are wired together we haven't seen any wiring differences in this area.

[Station X]

Originally Posted by Pepelephew

The intertwining fins are in the up position. When I tune them downwards just before the half way mark everything's disappears.

It's possible the vanes of the tuning capacitor are shorting out because they're bent or have conductive debris trapped between them.

[Pepelephew]

Hi Graham,

I gave you a reading from R14; brown, green, yellow.

There is a resistor brown, green ,orange (I'm not sure where R6 is positioned), but the reading from that one is 18.08 Kohms.

I did blow out between the fins but I will look again and check for any bends etc.

Bill.