Ferguson 771 repair

[cathoderay57]

Hi Jake, good work so far. You have measured 1800 Ohms across the F terminals (i.e. the speaker field coil) which proves it is OK. You will get a resistance measurement to chassis with only one of the F wires disconnected because there is a path via the other F wire to chassis probably via C25 or another which, because the set hummed when you first powered it up, has lost its capacitance and turned into a resistor. I'm curious that you got the same resistance between both F terminals and chassis, unless you disconnected the left hand wire as shown in the second pic, not the right? The one to disconnect should be the one with the link wire to terminal 1. A measurement of 460 Ohms across the middle 2 T1 terminals means that everything concerning T1 and the speaker should be electrically good. Personally I would simply replace C26 and C26 (plus C24 and C27 for that matter) but if you want to you do a basic test on the caps you can disconnect the wires from each of these (one at a time) and using the analogue meter on the kilohms range measure the resistance between each capacitor terminal and the can. There is another gotcha here though. When you use an analogue meter on the resistance range it is sometimes the case that the black negative meter probe is positive (from the meter's internal battery). You can check this by putting the analogue meter on the kilohms range and measure the voltage on its probes using your digital meter on the volts range. Once you have established which of the analogue meter probes is positive, put that probe onto the capacitor terminal and the other probe onto the can. If the capacitor is in fair condition you should see the meter needle swing up showing a low resistance initially then, as the capacitor charges up, the needle should drift back slowly to the left showing an increase in resistance. This might take a few minutes. If the resistance remains low and the needle stays over to the right then the capacitor has almost certainly had it. It is possible to restore function to some extent by re-forming the capacitor using the radio's transformer, rectifier and a high wattage resistor but this can be a bit hazardous so I recommend you don't and as said before I recommend you should simply replace them. Unless you are a stickler for originality below the chassis you can leave the existing cans in place, disconnected, and solder in replacement capacitors below the chassis where nobody can see them. The replacement modern capacitors are readily available with axial leads and are physically much smaller than the big old cans so you should be able to fit them in snugly. Some people like to open up the old cans and scrape out the innards then stuff the modern capacitors inside the old can and re-seal it. Frankly this is a very messy business and the stuff that comes out contains God knows what chemicals so you should dispose of it sensibly if you do that. I don't bother. Cheers, Jerry

[jmggurr]

Hi Jerry - I have unsoldered the right hand wire as shown below.

...but now testing again as suggested by Lawrence I am not getting a reading between either F terminal and the chassis i.e. open circuit.

I take your point about replacing C25 & C26 and I agree with your approach in that I will disconnect the three spade connectors under chassis and solder the new capacitors in place there and leave the can in situ. From the service sheet I can see these two share a negative and I can see the wire going from here to the negative side of T2. I can also see the wire from the positive of C25 goes into V5 and and also to the positive side of T2. Therefore by process of elimination the third connector must be the positive side of C26. Is my logic any good?

[Station X]

Circuit extract attached.

[cathoderay57]

Hi Jake, yes, that sounds about right. To be absolutely sure that the third capacitor tag is indeed C26 you should be able to measure a short circuit between that tag and pin 4 (grid 2) of V4. The valve pins are numbered clockwise starting one pin immediately to the left of the valve spigot, with the valve inverted. Don't forget my earlier point that replacement capacitor for C25 needs to be a high ripple type. You don't need to be too precious about the replacement capacitor values although the rectifier's characteristics determine a max permissible value for C25 of 40uF. In this case I would recommend using 2 x 15uF electrolytics, one for C25 and the other for C26, such as the following from RS Components https://uk.rs-online.com/mobile/p/al...itors/7497357/. These can take a ripple current of 450mA which should do the job nicely.

[jmggurr]

Thanks Graham for posting the circuit diagram. Thankfully I re-read the service sheet and noticed that the terminals for C25/C26 are actually colour coded which should make life easier. I will tst th short circuit suggested by Jerry anyway.

Jerry - thanks for the link for the capacitors to replace C25/C26. Before I order them is it an idea for me to also identify what I need to replace C21, C24, C27 so they all arrive together?

C25/C26 - https://uk.rs-online.com/web/p/alumi...itors/7497357/
C21 - https://uk.rs-online.com/web/p/polye...itors/2108342/
C24 - https://uk.rs-online.com/web/p/alumi...itors/7511138/
C27 - https://uk.rs-online.com/web/p/alumi...itors/7258990/

Do these look about right?

[snowman_al]

Hi Jake,
As no one else has been back to you yet, here are a couple of small changes I would make.

It will be easier to fit 'axial' caps to replace the wax paper types like C21. (The ones on your list are 'radials' and will work fine, but not fit in like the originals.) These might be a better fit: https://uk.farnell.com/vishay/mkt181...et/dp/1166871#

The other thing is C24 and C27 only have a small voltage on them, so a 50 volt or 63 volt cap will be fine. Again you might find axials better?
For C24 you might find 10uF an easier value to find too. Maybe https://uk.farnell.com/vishay/mal203...165436?st=10uf
And C27 22uF will be fine: https://uk.farnell.com/nichicon/tvx1...-20/dp/8823758

The C25/26 replacement (15uF) caps you link to are fine.

If you have an RS account you can find similar cps there too.

Alan

[jmggurr]

Thanks Alan, much appreciated. I shall have a quick shop and report back.

Cheers,
Jake

[PJL]

Before you order capacitors you will need to replace all of the paper capacitors in the set. That will be the 0.1, 0.25, 0.02 and 0.01 capacitors in the trader sheet and possibly others.

[jmggurr]

Thanks PJL.

Would you be able to tell me what voltage rating I need for the paper capacitor replacements.

Also, I understand that I can normally use film capacitors as replacements for paper - is that right?

Thanks!

[julie_m]

Yes -- metallised plastic film capacitors have taken over the rôle once played by waxed paper capacitors. Use the same or higher voltage rating as the originals.

[cathoderay57]

Hello again, you might indeed need to replace all of the paper capacitors. However, if you should choose to do so then I recommend replacing one at a time and powering the set up between capacitor swaps. Providing that you have:
1. Removed or replaced with Class Y the mains filter capacitor C28
2. Replaced the 2 main electrolytics C25 & C26
3. Replaced C21
it would be tolerably safe to power up again and see what happens. Even if one of the paper caps decoupling the HT (usually on grid 2 of the valves) is dead short the worst that will happen is some oscillation noise and a resistor or 2 getting hot. The benefit of not replacing the caps wholesale is that you are less likely to make a mistake fitting the new ones and you will learn the effects and purposes of the caps as you progress through testing and replacement. For example a leaky decoupling capacitor should manifest itself by showing a lower voltage at that point compared to that shown in the Trader Sheet which gives expected voltages on all the valve anodes and grid 2s. A leaky capacitor on the AGC decoupling usually shows up as distortion on strong signals. In the end it's up to you. Cheers, Jerry

[snowman_al]

^^^ +1.
As you see there are two schools of thought. Change everything in one go or just replace the 'obvious parts' and test at each stage. Both have pros and cons...
But the one step at a time way is the most cost effective and is better to learn about how things work.
Put everything right at once is great if it does actually 'work' after.

Which ever way, your choice, always start with the most difficult capacitor to get at! Lots of notes and pictures and you still make mistakes ask any of us...

Alan

[jmggurr]

Good evening all,
I spent a while this afternoon identifying the paper capacitors to be replaced. I have a couple of questions:

1. Does anyone happen to know if the voltage rating of C5, C6, C10 & C11 is 150v - same as C12?(all are 0.1uF) in this set? The reason I ask is that these are quite hidden and while I could unsolder them to read them I don't want to get myself muddled.

2. Assuming the above are indeed 150v - are these suitable: https://uk.farnell.com/vishay/mkp183...le/dp/1413915?

3. I am struggling to find a suitable 0.02uF replacement for C16 & C17 (rated 450v). Are these suitable https://uk.rs-online.com/web/p/polye...tors/6224959/?

Thanks for your patience!

[ms660]

Some skinflint's might shoot me down:

https://www.cricklewoodelectronics.com/CFH100NH.html

https://www.cricklewoodelectronics.com/CFH22NH.html

630 volt rating covers most applications in these old receivers including under certain fault conditions.

Lawrence.

[jmggurr]

Thanks Lawrence - I shall go for them. Also realised that I don't have an obvious choice for a replacement 0.25uF - is 0.22uF close enough:

https://www.cricklewoodelectronics.com/CFH220NH.html

[ms660]

0.22uF is ok.

Lawrence.

[cathoderay57]

Hi Jake, good work and nice to see you are persevering. You would get away with a minimum voltage rating for the paper caps of 400v but as recommended it's better to go for 630v. The only variation I would add is that I don't recommend replacing the cathode bypass caps C5 and C12. It won't do any harm, but won't make any difference either. Reason is that in the case of C5 it is in parallel with R4 which is only 150 Ohms, so a few kilohms leakage in C5 is neither here nor there. Ditto C12 which is in parallel with R11 (300 Ohms). I always leave the paper cathode bypass caps such as these alone. It's different for the audio stage cathode bypass caps which, if present, are usually electrolytics (in this case C24 and C27) - I think you were planning to replace these. When you fit replacements be careful to get the polarity right - C27 with positive lead to chassis (maybe counter-intuitive) and C24 positive to top of R23. Non-replacement of the latter two could affect the audio quality and inability to reduce volume to zero. Cheers, Jerry

[jmggurr]

Hi Lawrence - Thanks for the replies. I have ordered the bits I think I will need and will report back.

Hi Jerry - I will follow your instructions and let you know how I get on!

[jmggurr]

Evening all,
Daft question for you....which end is which?

[jmggurr]

This one too:

Are they both bipolar?