UK Vintage Radio Repair and Restoration Discussion Forum

UK Vintage Radio Repair and Restoration Discussion Forum (https://www.vintage-radio.net/forum/index.php)
-   Vintage Radio (domestic) (https://www.vintage-radio.net/forum/forumdisplay.php?f=20)
-   -   KB FB10 "Toaster" (https://www.vintage-radio.net/forum/showthread.php?t=70801)

westcliff 25th Nov 2011 9:30 am

Re: KB FB10 "Toaster"
 
Last night I removed the bolts holding the wavechange switch in place so that I could clean the switch properly. The first thing to happen was that the central part of the switch fell out. While this made it easier to clean (it was filthy), it took me about an hour to work out how to get it back in place.

We don't have Viakal PJL, not yet anyway, so I used contact cleaner, kitchen roll and a toothbrush. Now the switch is cleaner than it has been for decades.

Re-assembly was a bit of a nightmare, reminiscent of fitting a Bush DAC10 switching unit, except everything is much tinier in the KB. I eventually finished but it was too late to test the set, which I will do tonight.

Gus.

westcliff 30th Nov 2011 12:35 pm

Re: KB FB10 "Toaster"
 
My most recent efforts to get the set working have involved checking continuity, to ensure that all the connections I made are sound. So far, everything seems okay, but I did discover something last night which I don't understand.

The noise the set makes points to the smoothing capacitors as a potential source of the problem. Last night, I was checking their connections and I mistakenly tested continuity from the rectifier to the smoothing capacitor it is not connected to.

The DMM gave out a short "beep" and its display said 946 ohms. Is this indicative of a fault with the capacitor? Or is it just one of these things I've not yet learned about?

Thanks.

Gus.

Station X 30th Nov 2011 12:45 pm

Re: KB FB10 "Toaster"
 
If I understand you correctly, you'll be measuring R11 (Trader Sheet), but the reading is a bit low, so there may be a parallel path due to a fault. I haven't studied the diagram in detail.

westcliff 1st Dec 2011 4:07 am

Re: KB FB10 "Toaster"
 
Graham, I took R11 out and measured it - it was 944 ohms, half what it should be. I don't have any 1.8k resistors apart from some 0.25 watt ones, which I guess are not the right ones to use (the Trader sheet doesn't say).

I did find an old unused carbon comp one, rated at 1.5k ohms, which measures 1866 ohms, so I intend to use it to test the set.

Gus.

Sideband 1st Dec 2011 9:25 am

Re: KB FB10 "Toaster"
 
Not got the circuit here but I'm guessing that is the smoothing resistor? You need a 1 watt at least...I would use a 2 watt.



Rich.

westcliff 1st Dec 2011 11:35 am

Re: KB FB10 "Toaster"
 
Yes it's one of the smoothing resistors. The one I intend to use has a brown body if that's any indication of it's wattage rating. It is the same colour and size as the one I'm replacing.

Gus.

Station X 1st Dec 2011 11:49 am

Re: KB FB10 "Toaster"
 
Looking at the ERT sheet, it's described as a 1/2 Watt 1.8K resistor.

westcliff 1st Dec 2011 6:26 pm

Re: KB FB10 "Toaster"
 
Replacing R11 has made no difference. Smoothing capacitors are the next things to check. The speaker has once again been removed and I am satisfied that all smoothing capacitor connections are good. Is there a way to test electrolytics without applying power to them?

One other thing I noticed is that R2, the oscillator stabiliser resistor, measures 284 ohms instead of the 220 ohms it should be. I will change it, but it's not the cause of the problem, is it?

Gus.

Station X 1st Dec 2011 6:42 pm

Re: KB FB10 "Toaster"
 
Change the resistor, as it's more than 20% off value. I doubt it'll cure the problem. though.

Pull out the valves except for the rectifier. Connect your meter on DC volts range across one of the smoothers. Switch on. As the rectifier warms up the HT should rise and then steady somewhere above the specified HT voltage. Switch off. The reading should now fall very slowly. If it drops quickly the smoother is duff. Repeat for the other smoother.

westcliff 3rd Dec 2011 12:25 am

Re: KB FB10 "Toaster"
 
Thanks for the tip Graham.

The resistor has been changed and the electrolytics tested. One capacitor got to 330 VDC, then took around 10 minutes to drop to 75 VDC and the other reached 335 VDC and diminished slowly as well. They're don't seem to be causing any problems.

I did two other things last night. First, I re-soldered the OT wire to the electrolytic as all the wiggling had disconnected two of its three strands. The other thing was to repair a previous repair where the on-off switch chassis connection was a mostly bare wire joined to the original wire by a big blob of solder without the wires actually touching each other. The bare wire was able to touch one of the volume pot tags.

No change when the set was switched on though, still a droning hum between stations, with added crackling and distortion when a station is tuned in.

I'm running out of ideas. Any suggestions people? I'll try anything if someone tells me how to do it.

Gus.

frsimen 3rd Dec 2011 12:50 pm

Re: KB FB10 "Toaster"
 
Gus,
Reading through your posts, there are a few potential problems that come to mind. The first is the position of wiring within the set. Make sure you keep the mains lead well away from V3 and its wiring, as it would be very easy to introduce unwanted hum by that path. The same is true of your heater transformer wiring. Make sure you route it as it was originally, away from the components.

The other problem could be poor connections to chassis. You have mentioned that you have found some already. In particular, check C9 (the AGC decoupler) and C13, the HT decoupler. It would be a good idea to check all the wiring and component changes you made. Also,check around the wavechange switch in case you have things touching that shouldn't, since you did the work in that area.

You can't be too far off success, as it has been working on and off.

Paula

westcliff 3rd Dec 2011 8:17 pm

Re: KB FB10 "Toaster"
 
Hi Paula, it's good to hear from you again.

The transformer leads are both routed along the edge of the chassis. The power lead's route goes between V3 and V2; it does go close to the 1.8k ohm part of R10.

I have already tested all of the work I've done and at the continuity level at least, everything is okay.

I have turned my attention to the wavechange switch again. The two terminals on S1 show continuity when S1 is closed and also when it is open. I have disconnected the 70pF capacitor which is mounted across both of these terminals, to eliminate it as a suspect. It is innocent so it looks like I will have to remove the switch again.

Gus.

Station X 3rd Dec 2011 8:26 pm

Re: KB FB10 "Toaster"
 
C2, 70pF is not wired across S1.

The reading across S1 will be low when it's open on account of L3.

frsimen 3rd Dec 2011 8:34 pm

Re: KB FB10 "Toaster"
 
Gus,

When S1 is open you will still measure some continuity, around 110 ohms, because L3 is wired across the contacts. If you're measuring a much lower resistance than that with the switch contacts open, there is something amiss.

I wouldn't worry too much about the mains lead running near R10, that's unlikely to be a problem. If it's close to R4-R7, C12 or C14 that could result in hum. Does the noise go away when the volume is turned down? If not, does it change as you tune the radio with the volume turned down?

Paula

Station X 3rd Dec 2011 8:39 pm

Re: KB FB10 "Toaster"
 
I think L3 is 11.0 ohms, not 110 ohms.

westcliff 3rd Dec 2011 9:03 pm

Re: KB FB10 "Toaster"
 
Graham, I know what you're saying is correct, but on my FB10 there is a 70pF flat wax capacitor wired across the two S1 terminals. Again, on my FB10, there is a 0.001 uF tubular capacitor from S2 to chassis. Neither of these feature on the circuit diagram on Trader sheet 969, but the set was working well with them in place.

Paula, the volume control has no effect on the hum, which is fairly loud whatever volume setting is used.

When S1 is open there is continuity at 16 ohms.

The mains lead runs near C12, but not that close.

Gus.

frsimen 4th Dec 2011 12:04 am

Re: KB FB10 "Toaster"
 
Gus,

16 ohms is perhaps a tad higher than it ought to be, but not too far out. Station X is right, it should be 11 ohms not 110 ohms.

The decoupling arrangement on this set is a little unusual in that V1, V2 and V3 all share one decoupling capacitor, C13. Its value (0.1uF) is sufficient to decouple well at RF, but won't be so good at 50Hz. When everything is working properly, that shouldn't matter, but I wonder if you have an AGC line fault, with the control grid of V1 floating and causing 50Hz to appear across C13. The AGC is fed via the switch and your latest trouble started after you worked on that. Try checking the resistance from V1 control grid to chassis. It should be around 2.7M ohms if all is well (assuming the component values on the Trader sheet are correct). If it's much higher, trace the wiring back through the switch S1, L2, R4 and the volume control.

You could also try removing V1 and V2 to see if the hum goes with them unplugged.

Paula

Sideband 4th Dec 2011 12:05 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 485347)
Paula, the volume control has no effect on the hum, which is fairly loud whatever volume setting is used.

Then the fault MUST be in the circuits after the volume control, i.e output stage or power supply. There is very little to check here assuming connections are correct and agree (hopefully) with the circuit.

As the set has worked OK and the hum originally seemed to be associated with fitting the case it still seems likely that there is a short somewhere. Photo's of the underside could be a great help here as I feel that something very fundamental is being missed. These are relatively simple sets...by that I mean there are no odd circuits involved and there are only a few places where hum could be introduced.


Rich.

westcliff 7th Dec 2011 12:25 am

Re: KB FB10 "Toaster"
 
3 Attachment(s)
Thank you again for your helpful comments.

I have taken some pictures of the underside as Rich suggested.

Gus.

DangerMan 7th Dec 2011 1:07 am

Re: KB FB10 "Toaster"
 
Hi Gus,

I can't really see where the coupling capacitor to V4 from V3 is connected, so it's worth checking it's on the right pin of v3 etc..
If all else fails, try disconnecting it from v3 anode, and put it to chassis. All the hum should go, leaving just a small residual. If it doesn't, either the smoothing isn't working properly (wiring or dodgy capacitor) or you may have a heater-cathode short in V4.
HTH

Pete


All times are GMT +1. The time now is 7:33 am.

Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.