Radiomobile 200X and separate amp XB Polarity question

[sobell1980]

Hello all

I'm repairing the above set to go back in the car it came from will remain 12volt positive earth.

Studying the diagram, the power supply line from the battery to the vibrator in the XB amp is fitted with reversible electrolytic capacitors for use with either positive or negative earth. After full wave rectification using MR1 they revert to the use of standard or polarised electrolytic capacitors. However I'm a little confused. These caps after MR1 still use the same earth as the reversible caps which could and will be positive earth. So essentially seeing reverse polarity? The amp shares the same earth path as the head unit on pin 5. So earth is positive on line to pin 5 and anything after MR1 Electrolytic is having its negative connection to the positive earth path. Surely these should be reversible electrolytics too?


Many thanks
Dave.

[radiomobile]

The reservoir and smoothing condensers are isolated from the 12v supply by the double wound vib transformer so although the input filter condensers will "see" pos or negearth depending on the car's polarity, the 16+16 uf 350v elecs will always "see" neg earth.

[sobell1980]

Many thanks for your reply and what you say also makes sense that after the transformer it is isolated from the 12 volt supply that could be earthed either way. I will remember this for future repairs. Its just the first time I've come across reversible electrolytic capacitors. Talking of which, I can only find these available in low voltage rated examples. Is anyone aware where I can buy some high voltage rated ones? I don't really want to make it a permanent positive earth incase of it being removed and fitted to a different negative earth vehicle in the future. It also takes it away from what it's meant to be, both pos and neg earth. Unless I put a sticker on it.

[Mr 1936]

Hi

If you want to keep things standard, just replace the NP/reversible electrolytics in the 12 volt section with suitably rated new ones, e.g. 35 volts.

The HT smoothers can be replaced with high voltage standard polarised types as the HT inside the radio and amplifier has to be always positive with respect to the chassis, irrespective of vehicle polarity. Valves (and transistors) only conduct one way !

So I don't quite see why you want high voltage NP/reversible electrolytics.

[Radio Wrangler]

The works-either-polarity trick can only be done with a vibrator supply which uses diode rectification. (thermionic or solid state)

Some vibrator supplies have ancillary contacts in the vibrator to switch the transformer secondary HT windings in synchronism with the primary switching. These will reverse the HT polarity if you reverse the low voltage supply polarity.

Just something to watch out for in future.

David

[sobell1980]

Hello everyone
Thank you for your replies. I got everything ordered on Tuesday and all the parts arrived in the post yesterday morning.

In your question about ordering high voltage caps. I tend always to go with the side of caution and order the highest voltage I can find. Only because when I was early days into the hobby I unwittingly fitted an incorrect rated cap. Well it went off like a pistol and since then I go overkill which now I know is unnecessary.

After posting here last weekend I lost the HT on the set after the transformer thankfully. The rectifier MR1 gave up . I had 300 volts ac going in but nothing DC coming out. I ordered a new rectifier which arrived and I fitted. I did a brief switch on and HT was restored although I may need to fit a dropper resistor to bring the HT voltage 100% correct.

The set is drawing approx 3.30 ish amps inline with the service data

Interestingly I had to remove the ground off the new rectifier. The set was drawing too much current and the transformer was getting very warm quickly. I snipped the ground connection from the new rectifier and everything was in order. I has the exact same issue when I replaced a rectifier on a Roberts RMB. It overheated the transformer. Is this because the transformer is centre tapped?

The set now is working on both wave bands but is far from finished. There are some more caps to change and run through the valve voltages to nake sure nothing is being over or under run. I also need to service the tuning and preset mechanism. But progress nonetheless.

Very best
Dave.

[Herald1360]

Were you using a rectifier bridge perchance? That would short a centre tapped transformer secondary on alternate half cycles if you grounded the negative rectifier output. The centre tap is the negative output in a typical full wave rectifier circuit. Leaving the negative leg open will just leave the unused diode pair in series opposition across the secondary where they'll do nothing.

[Techman]

Originally Posted by Herald1360

Were you using a rectifier bridge perchance?

It sounds like it.

I think you've fitted the wrong type of rectifier.

[sobell1980]

I see, this will explain what happened with the RMB all those years ago which still has the bridge connected with the ground off it. Am I OK to leave it like this?

[Herald1360]

As long as neither of the unused diodes in the bridge are short circuit, yes.

[sobell1980]

Good afternoon

Can anyone help identify this in the photos?
It looks like a wax capacitor but has no identification markings on it or values. It was connected to pin 3 of the vibrator VIB101 and chassis ground. However it is not shown on the diagram. Any thoughts?

Many thanks

Dave

[sobell1980]

If it helps I measured it on my capacitance tester at 7.2 nF, that's if it is a capacitor of course.
If it's not shown on the circuit diagram shall I omit it? My only worry is it looks fairly original.

[Herald1360]

Quite possibly a leaky 0.05uF capacitor. I'd replace it with a 47nF 630V filn capacitor.

From its position, I'd guess it was added for some extra RF noise suppression from vibrator contact sparking. The CR across the vibrator contacts would take care of most of the problem whilst damping any spikes from suddenly open circuited transformer primary windings.

[radiomobile]

The capacitor is actually a very small 0.001uf wax tub. (1nf) It was not fitted on earlier radios and only appears on later circuit diagrams. It's purpose was ,as suggested by Herald 13/60, to remove any residual interference from spikes from the vib.

[radiomobile]

I would suggest replacing it with a 1nf disc ceramic.

[VT FUSE]

Used to be called a snubber cap. Vital for longevity of vibrator contacts.
Mike

[Herald1360]

Isn't the snubber the RC network across the vibrator contacts?

[sobell1980]

Good afternoon. I've got the week off work so first thing today I was straight back on the set.
I've replaced the cap that was questionable as not shown on the circuit diagram. As member Radiomobile suggested, I've replaced this with a 0.001 uF cap , pin 3 of the vibrator to ground. I've just about finished changing the wax caps. I will shortly be making up a better power lead and giving the set a try and running through the valve voltages. Many thanks for all the replies.
Dave.

[sobell1980]

All good news. The set is now all working. All valve voltages are textbook and as per the service data. The set is very, very loud. One question, the volume control does not entirely silence the set on minimum, is this normal on these sets? I also had a problem with a broken joint on one of the valve screen grids which gave radio silence briefly. It was a cracked joint where the wire solders through onto the tag strip. Moving the tag strip board the set burst into life so it didn't take too long to find the culprit.
The locking plate that stops the presets popping out is very worn. I have the spring tension set as much as I dare on each preset button and set also so when the button is pulled out the cams can still rotate to set the desired preset station. I've also adjusted the locking plate and tuning mech clutch but still can't quite get enough to hold the preset in. The slightest bit more spring tension holds it so I've ordered some very light additional springs to see if I can fit one to give it that tiny bit more and anchor it from the preset button locking plate to chassis. I'm awaiting for these to be delivered. It seems ashame now its all working not to put the effort into the preset mech and get it fully working. I've enjoyed the challenge so far and would consider doing another.

[sobell1980]

I'm nearly there with this now. I really don't get the time I would like or like I used to have but I still try and keep in with the hobby. It's pretty much all there and I'm just after an illumination bulb.
The one I removed was rated at 14 volts 2.8 Watts.
It's an MES type with a globe glass. It has to be a globe type otherwise the fascia panel doesn't go on and is held off by any other type. I'm also finding it difficult to locate 14volt bulbs. This is because of the charging circuit on the car and when running will see 14volts rather than 12volts. Can anyone's suggest where to locate such a bulb.
Many thanks
Dave.