Bush MB60 not sounding too good

[Radio_Dave]

Hi,

I've had a quick try of my MB60 but the reception isn't very good. The stations seem week and there is a lot of hiss.

If I tune just past Virgin I get a loud thumphing noise?.

If I attach a small length of wire to the aerial socket the reception is greatly improved (still with hiss). However as this is a portable radio shouldn't it work ok without an external aerial?

I admit to having no experiance with portable valve radios so maybe this is how they sound?

Any ideas

Thanks
David

[paulsherwin]

Most battery valve sets don't perform as well as mains sets but you shouldn't be hearing thumping noises! The first thing to check is the RF alignment - just move the coils along the ferrite rod until you find a peak.

If that doesn't help it's just down to standard fault finding I'm afraid. The aerial coils can become disconnected, the wavechange switch may be giving problems, or one of the valves may be low emission (battery frequency changers and IF amps are much more likely to fail than mains types).

You should expect good reception of strong stations such as BBC R5.

Good luck, Paul

[Radio_Dave]

Thanks Paul,

I've attached a photo of one of the coils which initially looks like it's moved? However if it has it's done a very good job of re-attaching itself

Thanks
David

[Biggles]

Hi Dave, the thumping sounds like some kind of instability, I've had similar before with battery sets. Look out for decoupling capacitors gone low in value or leaky, particularly the AGC decoupler. (0.1-ish paper capacitor down to chassis on the agc line), otherwise, as Paul says. I would tend to avoid moving any coils unless you have exhausted all other possibles though. e.g valve substitution is a good idea.

Biggles.

[paulsherwin]

Originally Posted by Radio_Dave

I've attached a photo of one of the coils which initially looks like it's moved?

It certainly looks as if it's moved, though this may have been done by a previous owner trying to get it to work. Just scrape off most of the wax on the ferrite rod and slide the coil about - there should be a very clear peak in the signal. Also check the connections from the coils to the rest of the set, and check the coils for continuity.

I'd leave all the other coils and IFTs alone at this stage though, especially if they look undisturbed - as Biggles says, it's better to exhaust other alternatives first.

Good luck, Paul

[Radio_Dave]

Oh well, it's not going to be easy. I've moved the coil back to where it looks like it should be and it's made no difference. The problem is confined to MW, LW works very well!

All the Hunts caps have been replaced so I guess I'll just have to look for something else

Thanks
David

[Sideband]

Originally Posted by Radio_Dave

The problem is confined to MW, LW works very well.

Try the frequency changer (DK96). This may be struggling at MW (higher frequency) if it is low emmision. Performance should be reasonable but these battery valve sets never performed as well as their mains counterparts. It's possible that the valves have been overrun at some time and it wouldn't be expensive to change them all. As has been said before though it's better to check everything else first. I would be inclined to check aerial wiring...I would have thought moving the coil along the rod would have made SOME difference. The coil should be adjusted at the LF end of MW (around 500 metres) for maximum signal.


Rich.

[mickjjo]

Hi Dave, Just checked inside my MB60, Looks like those aerial coils would need a blowlamp to shift them, stuck with lots of wax. Worth checking the LT and HT voltages in case the metal rectifier is getting tired. Are the stations appearing at the correct points on the dial?,Usually worth tweaking the aerial trimmer and possibly the oscillator one, but mark the original positions first. Another possibility would be those 110pf capacitors inside the IF cans may have drifted off value, upsetting the alignment..... .

Regards, Mick.

[paulsherwin]

Originally Posted by Radio_Dave

Oh well, it's not going to be easy. I've moved the coil back to where it looks like it should be and it's made no difference. The problem is confined to MW, LW works very well!

That is actually a very good sign! Every time I've come across these symptoms, and every time someone has described this fault, the problem has been wavechange switching, coil wiring, or open circuit coils. The other suggestions people have made aren't impossible but they're much less likely.

There should be a very obvious peak as you slide the coil along the ferrite rod. You can confirm this with the (working) LW coil. If the MW coil is open circuit you can substitute most scrap coils, even from trannie sets, so long as they're the right diameter. The tuning may not track perfectly across the whole waveband but it'll probably be 'good enough'.

Good luck, Paul

[Radio_Dave]

Originally Posted by paulsherwin

That is actually a very good sign! Every time I've come across these symptoms, and every time someone has described this fault, the problem has been wavechange switching, coil wiring, or open circuit coils. The other suggestions people have made aren't impossible but they're much less likely.

I've just checked the valves, on my newly aquired valve tester, and although not brilliant they are ok. Tomorrow I check the coils and switches.

Thanks
David

[Mike Phelan]

Hi David

My MB60 is quite sensitive - it has an extra IF stage to most D-series valved battery/mains sets.
As Paul says, there is something funny happening to the aerial coils.
I am sure that there is a market if you record the thumping!

[Aerodyne]

If the thumping is caused by LF instability, you might find a decoupler on the HT+ line is the culprit. There's C36 (Trader sheet ident) which is a 0.1uF. Try replacing it: you never know.

-Tony

[Radio_Dave]

Paul, your a genius, the MW aerial coil had 3 broken coils which I managed to solder back together.

However.... I turned the radio on and it sounded really good, all the stations were there and no "thumping" noises I did notice that the volume control didn't turn down properly, it's (was) actually quite loud with the volume off.

It played it on Amber for half an hour but when I tried tuning through the stations again Virgin had disappeared. As I fiddled with the tuning Amber vanished too, then they all went. All I'm left with is a slight hiss.

HT and LT seem to still be Ok....

Thanks David

[Aerodyne]

Sounds possibly like your local oscillator has packed up. May be the valve, may be low HT+. Check for latter, try replacement valve for former.-Tony

[Radio_Dave]

Brilliant, I changed the DK96 and it's working again. It's strange but it tested OK yesterday, but then again it was made by Pinnacle!

The only problem now is the volume, any ideas why it does'nt turn down fully?

Thanks
David

[paulsherwin]

Originally Posted by Radio_Dave

The only problem now is the volume, any ideas why it does'nt turn down fully?

Try giving the pot a good clean. Also, check that the earthy end is connected properly.

Glad you're making good progress with it.

Best regards, Paul

[Radio_Dave]

Ok I've given the volume control another squirt, It actually works very well with no crackles and with a multimeter attached to the wiper it reads nicely from 0 to 400 ohms.

The earthy end of the track is connected via a 10K resistor which checks out Ok too

Unfortunately the volume is still too loud when it should be off


Thanks
David

[mickjjo]

Hi Dave, The circuit in R+TV servicing for the MB60 shows a 1k resistor in series with the bottom end of the volume control. I think this is a deliberate design feature to still give some output when the volume control is at minimum, This may avoid running down the batteries unintentionally by users turning down the volume, The on/off switch of course is combined with the tone control on these sets. I'm not sure why your set uses a 10k resistor though . Maybe you could experiment with the value to give an acceptable minimum volume setting? .

Regards, Mick.

[Radio_Dave]

Originally Posted by mickjjo

I think this is a deliberate design feature to still give some output when the volume control is at minimum, This may avoid running down the batteries unintentionally by users turning down the volume

I think this is a very probable explanation Hopefully somebody with a MB60 can confirm this? It's certainly a better idea than Vidor used with their attache cased radios!

Originally Posted by mickjjo

I'm not sure why your set uses a 10k resistor though

Oh, er... it doesn't I typed 10k by mistake, it is indeed 1k

Many Thanks
David

[Paul Stenning]

The 1k resistor is there because the negative feedback is fed back into the bottom of the volume control. This would mean that the volume will not go down to silent, but it should go down to very quiet (somewhat lower than normal listening level).