Ferguson 771 repair

[snowman_al]

Does not matter which way you put those in (yes call them bipolar).
It is the electrolytic caps that have to be installed correctly. They are marked with a negative band or arrow normally. Alan

[jmggurr]

Great, thanks for that. Did a quick check of what has been posted to me and noticed that Farnell were out of stock for the two 15uF replacements for C25 & C26. They are due in next week so I shall report back then.

[jmggurr]

Afternoon all. So the last bits arrived from Farnell so I set about putting the two 15uF capacitors in place of C25 & C26. I remove C28 completely and also changed C20 & C21. The humming is still present when I power up however - it is subtle and if I adjust the volume there is plenty of crackle. Am i likely to have made a mistake already? Picture attached. Forgive the solder work.

[cathoderay57]

Hi Jake, a few questions if I may:
1. What DC voltages are you measuring across C25 and across C26?
2. Are both filaments of the 5Y3 lit up?
3. Have you changed C24 and C27? C27 in particular can cause hum.

If point 3 (above) doesn't remove the hum, then turn up the volume a bit so you can hear the hum then try shorting out R20 to chassis. Hum should stop. If it does, the hum is probably coming from V3 for which there could be several causes. At that stage it will be worth measuring the anode and screen (grid2) voltages and comparing them to those shown in the Trader Sheet table. Be careful as you will be measuring lethal voltages 150-350v at up to 80mA which can kill you (sorry if Granny is teaching you to suck eggs) Cheers Jerry

[jmggurr]

Hi Jerry! C25 is 315v and C26 215v. V5 is the 5Y3 and it is glowing at the tip of the filament. See picture:

I haven't yet changed C24 & C27. I wanted to check in at each stage so I don't get in a muddle. I have removed C28, changed C25, C26 and C20 & C21 at the same time because one was blocking access to t'other.

I think in general I am obviously very naive and don't have any idea what to expect the radio to do or not do at each stage.

Sorry about all the questions!

[cathoderay57]

Hello again. C25 voltage is healthy and I can see both filaments of the 5Y3 rectifier valve are lit so we can assume that valve is useable. The C26 voltage is a bit low, should be 240v DC not 215v but that's not surprising as you haven't replaced some of the other capacitors yet which might be leaking DC and adversely affecting valve bias voltages. I recommend that you change C24 and C27 next then see what happens to the hum. The scratchy volume control that you mentioned earlier is a good sign that something is working. A squirt of switch cleaner through where the control connections come out and possible replacement of C16 and C17 will sort that out. Cheers, Jerry

[jmggurr]

Evening all,

Well I have now replaced C24 & C27 and C16 & C17. The hum is still there but very quiet - much quieter than before - and does not increase with the increased volume.

I know you mentioned testing the anode and screen voltage of the valves - but I don't know which pins I should test on each valve for this. I there an obvious way or are there other things I need to test next?

Many thanks for the pointers so far!

Jake

[snowman_al]

First do not expect silence, there will be a slight hum, but not enough to spoil normal listening.
If you know which is the Gram switch on the push buttons, and select it and turn the volume control to half way, if you touch the top socket (PU on the trader sheet) you should get a buzz from the speaker. BE CAREFUL to choose the PU socket. The other is the loudspeaker output (Ext LS on the trader sheet) and has HT voltage on it!!! Just another gotcha...

Voltage checks for Anode and Screen Grids are as simple as it comes on this set. Negative meter lead on the chassis then:-
V1 6A8G anode is pin 3. Screen grid is pin 4.
V2 6U7G anode is pin 3. Screen grid is pin 4.
V3 6Q7G anode is pin 3. No screen.
V4 6V6G anode is pin 3. Screen grid is pin 4.
Record what you get.

Count the pins clockwise from the key as in the picture. This is from underneath the chassis of course! Just be careful, as you have been advised before.
Alan

[cathoderay57]

Nice work Jake. Alan's advice is a sound next step. Once you get the valve voltages recorded, it will give a clue as to whether the valves are functioning more or less within correct limits. It is likely that the push button switch contacts are very dirty so if you get chance give them a squirt of switch cleaner and work them a bit. You should then get a crack or click from the speaker when you press them providing the volume is up a bit. Have you plugged any form of aerial wire into the aerial socket yet? If you try that you should hear some noise if the front end of the set is working at all. I look forward to your next update. Cheers, Jerry

[jmggurr]

Afternoon all,
Thanks for the safety tips by the way - I am being very careful with how I handle the set.

I should have mentioned that I do get crackles and pops at high volume when I turn the tuning knob back and forth and I did the screwdriver trick Alan suggested on C17 and got a good buzz!

Voltage readings are as follows (anode, screen):

V1 208, 72
V2 216, 73
V3 129, -
V4 196, 216

That said, I still haven't changed the remaining papers capacitors. Is that my next job?

Jerry -I will brave the cold and bring in a bit of galvanised wire I have in the shed and mock up an aerial and report back!

Thanks all!!

[jmggurr]

It works!!

[jmggurr]

Well, it worked for a few minutes listening to classic rock on MW but now nothing, no hum, nothing!

[snowman_al]

Don't worry,

Does anything smell hot or look different (scorched / blistered) to before?

Then check the HT voltage on C25 and C 26. Let us know.

PS. just check your previous repairs for a loose connection or cold solder joint too.

[jmggurr]

Hi Alan,
Thanks for your reply. Nothing smells or looks different. I have had a poke at the joints and nothing seems loose. The voltage readings are 446v for C25 and zero for C26. Getting zero's on all the valve anode and screen readings now.
Thanks!
Jake

[snowman_al]

Poo,
That looks like L16 (the speaker field coil) has gone open circuit...

Be careful now as C25 may stay charged up a long while. Disconnect from the mains, and measure the voltage on C25 - make sure it is less than 20 volts before you touch anything...
Then back to checking the speaker again as in post no. 16 - 18. Check the F to F resistance (was 1.8k) and look for a wire off somewhere.
Alan

[jmggurr]

Oh dear, my mistake. When I mentioned I had checked my joints I hadn't thought to check the one on the speaker! All good now listening to classic rock again.

Thanks so much for the help - I was genuinely thrilled when I heard the first voices coming out of the speaker! But I guess I am not finished - do I need to go ahead and replace the remaining paper capacitors? And am I fairly safe to operate the radio now it is "working"?

[snowman_al]

Phew,
good news then.

No not finished yet, the voltages you have on the anodes and screen grids are all lower than they should be.
Something is drawing more current than it should still. Change C5 and C10 next and see if that brings them up a bit.

Look at the table under the section 'Valve Analysis' on the trader sheet. That gives an idea of where you want to be.

Also could you measure the voltage on pin
8 (cathode) of V4 the 6V6 sometime?

Do you get stations on other bands too?

Alan

[cathoderay57]

Well done chaps, sounds like it's well on the way to a good result. I'll butt out now and leave you both to it. Cheers, Jerry

[jmggurr]

Thanks very much for your detailed instructions Jerry - I couldn't have got this far without you!

[jmggurr]

Originally Posted by snowman_al

Do you get stations on other bands too?Alan

Sorry Alan, missed this. I can get a very clear BBC R4 on LW and some other bits but with poor reception but nothing on SW. The buttons are still very sticky and my contact cleaner ran out so I need to get another can to give those switches a good clean.

I will have a go at the other things you mention tomorrow and report back!

Thanks all!!