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-   -   KB FB10 "Toaster" (https://www.vintage-radio.net/forum/showthread.php?t=70801)

westcliff 7th Dec 2011 2:06 am

Re: KB FB10 "Toaster"
 
Thanks Pete. As far as I know, that capacitor is correctly connected (V4 pin 5 to V3 pin 7), but I've put the radio away for the night so I'll check tomorrow. It's been six months so far; one more day won't hurt. :thumbsup:

Gus.

Sideband 7th Dec 2011 11:39 am

Re: KB FB10 "Toaster"
 
Can you post some pictures around the smoothing capacitor and rectifier? I know it will probably mean removing the speaker but that only takes a minute or so.

Rich.

westcliff 8th Dec 2011 1:55 am

Re: KB FB10 "Toaster"
 
Pete, that capacitor was attached to the correct pins. I tried attaching it to chassis and the hum was still present. I have replaced it as a precaution. The output valve was also replaced but there was no change.

Rich, I will try to get some pictures tomorrow evening when I get back from my work's xmas drinking session. There's not much to see as the leads from the smoothing capacitors are all covered in sleeving and heatshrink.

Gus.

Sideband 8th Dec 2011 8:54 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486470)
Pete, that capacitor was attached to the correct pins. I tried attaching it to chassis and the hum was still present.

If that is the audio coupling capacitor and attaching it (temporarily) to chassis didn't stop the hum then it confirms that the fault is in the output stage or smoothing circuit...it can't be anywhere else!

This is sounding more and more like a wrong connection somewhere. There are several different circuits for the HT decoupling and smoothing on these sets (no surprises there)! Do you know what version you have? One of the versions uses a tap on the output transformer to provide hum cancellation.


Rich.

*edit*. Looking at the pictures you provided, it looks like you have the version with a large 15K resistor providing HT decoupling (the green wirewound replacement in yours). Can you confirm this?

westcliff 8th Dec 2011 3:15 pm

Re: KB FB10 "Toaster"
 
Rich, that large green resistor near the wavechange switch is one of a pair which are supposed to make up 5.9k ohms. Its value is 4.5k ohms and its partner is 1.8k ohms (on the Trader sheet, it is described as 2 x 8.2k ohms in parallel plus 1.8k ohms in series).

There is a connection from a smoothing cap to the OT; is this the hum cancellation? The lead goes to the central of three connections on the bottom of the OT.

Gus.

PJL 8th Dec 2011 8:50 pm

Re: KB FB10 "Toaster"
 
This set seems to have been one problem after another! These are the facts as I have read them:
1. The hum remains if you disconnect the audio coupling cap and connect it to ground
2. Changing the output valve makes no difference
3. You have replaced the 32+32uF reservoir and smoother
4. Someone has been there before and had disconnected the rectifier and added a 25uF capacitor

I think it might be good to have the voltages on V4 anode grid screen and cathode.

At the moment suspects might be:
1. The wiring around the HT supply
2. A faulty rectifier
3. Leakage in the output valve socket
4. Poor earthing
5. Faulty output transformer

I would favour a wiring error but the voltages might help in diagnosis.

There should be two 32uF capacitors, one on the centre tap of the OT (reservoir) and one to the screen of V4 (smoothing).

Station X 8th Dec 2011 9:03 pm

Re: KB FB10 "Toaster"
 
1 Attachment(s)
Here's part of the circuit.

Sideband 8th Dec 2011 9:28 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486562)
Rich, that large green resistor near the wavechange switch is one of a pair which are supposed to make up 5.9k ohms. Its value is 4.5k ohms and its partner is 1.8k ohms (on the Trader sheet, it is described as 2 x 8.2k ohms in parallel plus 1.8k ohms in series).

There is a connection from a smoothing cap to the OT; is this the hum cancellation? The lead goes to the central of three connections on the bottom of the OT.

Gus.


Ahh now that's interesting. One of mine is the same and it has a tap on the output transformer. Now carefully check the wiring around the transformer. The cathode of the rectifier (pin 8) is connected directly to the tap (centre of the three). This means one of the other three connections goes to the anode of the output valve and the last one goes to the smoothing resistor 1.8K. It is important that the transformer primary is connected correctly. The idea of the tap is that it helps to reduce hum to a low level by cancelling out ripple from the rectifier. If the transformer is connected wrongly it can add to the hum rather than reduce it. If the transformer appears to have been undisturbed however, we can assume it's connected correctly. If it looks as if the wiring to it may have been disturbed, it will do no harm to reverse the two outer connections (leaving the tap where it is) and see if the hum increases or reduces. There is the remote possibility that the transformer is faulty. Hopefully your doner set has the same transformer.

If everything seems OK in this area, there is not much left other than the smoothing caps themselves. They should both be 32uF.

What is still puzzling is the fact that the radio has worked OK until the cabinet was fitted......:-/

I've played both of mine recently and there is very little hum.




Rich.

westcliff 9th Dec 2011 1:09 am

Re: KB FB10 "Toaster"
 
1 Attachment(s)
PJL, you've got the facts right but to your list we can add that the aerial to chassis connection was broken, and both the 200-220v and 230-250v power connections were in use at the same time. Hence the smoky power transformer.

Of your list of suspects, I favour poor earthing, then leakage in the output valve socket.

I re-soldered the aerial to the nearest chassis point, which is shared with the V4 grid bias resistor (R12), the V4 heater filament and the V4 control grid resistor (R9).

The V4 socket has been scrubbed with Servisol and my little boy's toothbrush several times on both sides.

Rich, the rectifier cathode is connected directly to the centre of the three terminals on the OT, although it is connected to one of the 30uF capacitors along the way. The other two OT terminals are connected to the 1.8k ohms smoothing resistor R11 (the connection is on a tag on the wavechange switch), and to the V4 anode.

You asked me to post pictures of the smoothing capacitors and the rectifier area. The rectifier is shown in one of the pictures I posted the other day and the capacitors are shown below.

The left hand capacitor is connected to the rectifier, the OT and C18, the tone correction capacitor. The rectifier lead is the red one which passes the power transformer screw and the OT lead is the black one more or less parallel to the red one. The tone cap lead is the orange one coming out from underneath the power transformer.

Gus.

bhamham 9th Dec 2011 10:51 am

Re: KB FB10 "Toaster"
 
1 Attachment(s)
Just an observation... I've restored two of these sets and mounted new smoothing & reservoir caps in the same direction as the original i.e., 90-degrees from how you have yours.

On a set (not an FB10) I did a few years back I mounted a smoother not in the original manner and it played havoc with tuning the set with a lot of hum.

PJL 9th Dec 2011 3:27 pm

Re: KB FB10 "Toaster"
 
Voltages V4 grid, anode, screen and cathode. If it's a poor earth it should show up with a few measurements. Also measure the voltage between grid and cathode.

Sideband 9th Dec 2011 3:56 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486697)
The V4 socket has been scrubbed with Servisol and my little boy's toothbrush several times on both sides.


Yuk!! I hope he doesn't use it afterwards!!;D

Well there doesn't look too much wrong there but of course it's difficult to see minor errors on a photo.

There really isn't much left to cause hum of that level. I think we've established that the hum is still present with the coupling capacitor temporarily connected to earth. That leaves the output stage and power supply which in this case includes the choke winding on the output transformer. With everything connected correctly, presumably the hum is still present. Identify the control grid of the output valve and short it to earth. If the hum is still present it would be worth changing the output transformer. You will need to check that the one in your doner set is the same type with three connections on the primary.


Rich.

westcliff 11th Dec 2011 2:34 am

Re: KB FB10 "Toaster"
 
I spent the whole of Saturday tidying up the chassis connections mentioned in post 89, above. There was a huge blob of solder and old wires to get rid of; my puny soldering irons and I found it hard work. The result was no change, although LW now hums louder than MW. To be honest, I can't remember if this was the case previously.

I had a look at the output transformer and noticed that of the two wires that connect to the speaker, one doesn't move when you pull it and the other can be wiggled up and down. It is still firmly attached inside the transformer. Can this be a problem?

I have not taken any voltage measurements yet, but intend to do that tomorrow. Nor have I shorted the 6V6 control grid to earth; again, I'll have a go tomorrow.

Bruce, thank you for that observation about the orientation of the smoothing capacitors. I will probably try rotating them 90 degrees before I try replacing the output transformer. I have some lovely brand new 32+32uF cans but they are unfortunately too big to fit behind the speaker.

Is it an easy job to change my floppy frame aerial with its broken frame for a ferrite one? Does anything else have to be changed? I'm asking because the donor set has a ferrite aerial attached to the output transformer and if I have to change the transformer, I might as well change the aerial at the same time.

Gus.

Herald1360 11th Dec 2011 11:09 am

Re: KB FB10 "Toaster"
 
To fit a ferrite rod aerial instead of the frame, it will have to have the same inductance for its tuned windings if the tuning is to track properly and the coupling arrangements will need to be the same. You may be able to achieve the inductance by sliding the windings along the rod, but if the frame has an isolated coupling winding and the rod a tapped coil you may need to make some extra windings to suit.

It sounds like an interesting challenge- make sure everything else is working as well as possible first or you could end up in an awful tangle!

PJL 11th Dec 2011 12:47 pm

Re: KB FB10 "Toaster"
 
Frame aerials work extremely well and are fairly tolerant of interference.

westcliff 11th Dec 2011 5:08 pm

Re: KB FB10 "Toaster"
 
PJL, I have measured the voltages you suggested. All are DC volts.

Control grid: varies between 7 and 13 volts.
Anode: 236 volts
Screen: 206 volts
Cathode: 9.26 volts
Control grid/cathode: -8.92 volts

The voltages at the anode, screen and cathode look fine. I don't know what the control grid voltage should be, nor the control grid/cathode voltage.

The hum lessened as I took some of the measurements, but was still present.

Gus.

Top Cap 11th Dec 2011 6:18 pm

Re: KB FB10 "Toaster"
 
Don't give up on it Gus, if you get too dis-enchanted with it I am always happy to take a second look for you. You only need to send the chassis and you would need to wrap the valves well in bubble wrap then bubble wrap again if sending by post. Not sure where in Essex you are, I have no transport of my own but might be able to persuade someone to come over and pick the chassis up. I do have a soft spot for FB10's as they bring back a lot of happy memories working in a village radio shop in my young teens. I am located not far from the M25 in Watford. Les

Sideband 11th Dec 2011 6:51 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 487296)
Control grid: varies between 7 and 13 volts.

:o If those voltages are correct, then there is something seriously wrong around the output valve! Firstly and most importantly the control grid shouldn't have any DC voltage on it at all. The control grid should measure zero or at least very little (maybe a few millivolts) with respect to chassis. Please tell us how you measured this voltage.

Quote:

Originally Posted by westcliff (Post 487296)
The hum lessened as I took some of the measurements, but was still present.

That may indicate an earthing problem somewhere


Rich.

westcliff 11th Dec 2011 7:13 pm

Re: KB FB10 "Toaster"
 
Rich, I crocodile-clipped the black lead of my DMM to the chassis and applied the other lead to pin 5 when the set had warmed up.

Gus.

Station X 11th Dec 2011 7:54 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff
Control grid: varies between 7 and 13 volts.

Disconnect the end of C15 (Trader Sheet) which is connected to pin 5. This is "THAT CAPACITOR" in the FB10. Check the voltage on pin 5 wrt chassis again. With any luck it will now be 0V. Check the voltage on the disconnected end of C15. If you see a voltage here, then C15 is definitely faulty.


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