Ultra Troubadour U7961 ... quiet as a mouse

[martinw]

Hi,

I've recently acquired a Roberts RT1 ... working and a UT U7961 ... not working

I'm taking the U7961 apart, to gain better access for voltage measurements etc (I'm an electronic engineer but have no experience of valves, a bit before my time ) and have removed the rear cover, four screws, and the four screws attaching the metal chassis to the rest of the radio.

I've also removed the front M-L button but the chassis still refuses to come out.

I don't want to force anything and break it, but it seems like the two front controls (left and right) are holding it on ... any suggestions please

Also I don't seem to be able to add attachments to my post, not sure if this is finger trouble or something else, as I have a few pictures of what I'm talking about that may be useful.

Cheers,

Martin

[Terry]

... I don't seem to be able to add attachments to my post ...

Welcome to the forum, Martin. I don't know if there is any restriction on adding attachments as this is your first post - I'm sure Mark will be along soon to advise but, if you use a web hosting service to store your pictures (I use Dropbox but there are many more options available) you could try adding them as images.

Is the chassis loose in the cabinet? If not, then there are still some fixings that you've missed. If it is loose, can you determine, by jiggling about, what is still holding it in?

Ah! Just spotted your comment about the controls! You must remove the knobs from both of these before you can remove the chassis! If you can't see any grub screw fixings then the knobs may well just pull off - although they might need a bit of 'persuasion' if they haven't been off for a long time!

[martinw]

Hi Terry,

Thank you for the reply, I did pry the controls with a screw driver slightly, about the same as I did with the M-L switch (which popped off quite easily) but they weren't for budging (didn't see any grub screws) and I didn't want to break anything, so I'll apply a bit more pressure now

Thanks for the advice RE: the pictures too

Cheers,

Martin

[pwdrive]

I think they might pull off.

Schematic here if you need one:

https://www.doctsf.com/documents/schemat.../U7961.PDF

Be aware that the receiver is a live chassis design....

Lawrence.

[martinw]

Thank you Lawrence, that's good to know!!!

Martin

[Mark Hennessy]

Hi Martin,

Welcome to the forum!

You should be able to add attachments now that your first few posts have passed moderation.

If you don't see the option immediately, try again in about 20 minutes.

All the best,

Mark

[martinw]

Hi Mark,

Thank you, glad to be here

Here's my recently acquired Roberts RT1

   

https://youtu.be/cvZKX4ZX_a0

The UT is not quiet anymore after I pulled off the control knobs on the front and had a bit of an investigation.

   

Although it's not very melodic

I couldn't get any continuity across the Live input on the on/off switch, the Neutral was fine so I jumpered (soldered) a wire across (just to get it going).

It still didn't spark into life though  but after a bit of voltage probing around the big green resistor I thought I'd move the input voltage selector, it too pulls off and then you simply twist it around to the desired voltage input range and press it back in again.

   

Hey presto, lights, buzzing and stuff

Then the big yellow capacitor started to melt !! Power off !!!!

   

So to cut a long story short the UT powers on now, not a bad start, it doesn't tune into any radio stations, although it does make that noise that radios makes as it is about to start tuning into a channel, woo, woo, for want of a better descrition every now and again and I need to find a replacement capacitor.

Any ideas?

   

Cheers,

Martin

[pwdrive]

That capacitor needs to be replaced with an X Class type, a modern value will be 47nF (0.047uF) X Class 2 type is what's normally used.

EDIT: Also be aware that if you've bypassed the live switch contacts then the chassis will be live with the receiver switched off but still plugged in to the mains if the mains lead has been wired correctly to the mains plug.

Lawrence.

[ppppenguin]

Assuming that's C22 on the circuit diagram you can just cut it out completely. It should be replaced by a 47nF class X modern components. You were lucky it just got hot. They often go bang.

All similar wax capacitors are, almost by definition, now faulty. Notably C18 and C19 must be replaced. Leakage in C18 will put a +ve voltage on the grid of the output valve. This will damage the valve and the output transformer. Modern polyester (or other plastic dielectric) capacitors rated at aleast 400V are suitable.

[martinw]

That capacitor needs to be replaced with an X Class type, a modern value will be 47nF (0.047uF) X Class 2 type is what's normally used.  

Lawrence.

Thank you Lawrence, I'll have a look on t'internet for one

Martin