Oscilloscope

[The Natural Philosopher]

the key is that it will show resistance slowly rising between 'live' points - if using a resistance meter connect RED lead to negative terminal

So there is one + one - and two total dummies

[tony sayer]

In article , Davey scribeth thus

I should look at the diagram of the pinouts and the one marked with the "-" sign treat that as negative. Pin 1 is the positive..

If your not certain then connect it to the existing cap and power it up but put a resistor say 10 k or thereabouts in line it, will take sometime to charge up and you should be able to read the same volts on the new cap if all is well then the volts across that resistor will go down to next to nothing as the cap charges.. if its arse about tit then the volts will remain quite high as the new cap will be pulling too much current..

[Davey]

Hmm. Maybe, if I'm not convinced by the DMM tests. Thanks anyway, useful knowledge to have.

[Davey]

That's what it looks like. I'll report when I have tested it.

[Davey]

Seems to be the case. I get a similar, but not identical, response if I connect the test leads the other way round. I'm going with the pin marked '-' as being negative, and pin '1' as positive.

[Davey]

OK, I'm getting on with this job. The two fuseholders F1 and F2 are replaced, and the board re-installed ready. Ditto a few of the new capacitors C203, C204, C164. But:

1. Looking at the new capacitors, they mostly to have a '-' mark, usually inside an oval; I am taking it that this signifies the Negative pin.
2. The big 4700uF units are in fact easy to mount: I have taken a piece of grey packing foam from the bottom of one of the RS boxes, and a strip of that wrapped around the bottom of the new cap. holds it nicely in the old clamp.
3. But on these same caps.: The old ones had FOUR wires all connected to the '+' terminal, and three to the '-'. The new caps' pins are much smaller than the old, and so I am looking at taking one wire from each of the pins, and going to a choccy-bar, or similar. Otherwise I'll never get a good joint of all those wires on the tiny pins. Any suggestions welcome.
4. I am looking at C51, which is very hard to actually see the details on. I see 8uF, and what looks like 6 V; is this possible? It seems very low, and I don't know how to work it out from the circuit diagram. The parts list just shows the 8uF. It's physically the same size as the EIRE ones mentioned earlier, and the same style.

Thanks in advance for help.

[The Natural Philosopher]

C52 is simply strange smoother for the FM discriminator and part of what may drive the lights that say 'tuned' and 'stereo'- at most it might have a couple of volts across it. probably there to smooth the AFC.

6v or 6v3 is fine

The

[Davey]

Thanks, but I was actually asking about C51....

[The Natural Philosopher]

and I was replying about C51, not sure how I typed it as C52..

[tony sayer]

Yes it does..

Angle grinder and silicone sealant are best for this especially the anti mildew sort;)..

Well you can make a loop of tinned copper wire and put the other wires in to thru that to make a binding connection as it were, but choccy block is a -way- of doing it;!...

Anything will do there 6.3 volts upwards or the lowest voltz you can find in that size;!..

[Davey]

Fat fingers. Situation normal. Thanks.

[Davey]

This means that the RS 788-7558 (10µF/20V) that I have would work, if I can fit it, I think. Or one of the other, more suitable alternatives discussed earlier, if I go that route.

[tony sayer]

In article , Davey scribeth thus

Yes, will be fine...

[Davey]

Thanks.

[Davey]

I might be powering the Module 80 up again soon. If you hear a loud bang from East Anglia, and Tacolneston goes off-air temporarily, you will know that I got something wrong.*

I have also found some more caps. that were hidden under a cover plate, maybe used as an RF screen, so I am identifying them (not easy) for possible future replacement. Their discovery has also made me confused about the ID of some of the others that I have already replaced, but at least the values are correct, so that's ok. I wish the schematic had a map with component IDs on it, and showing only one channel of the two also leaves me puzzled as to a component's ID sometimes.

• Back in my student days, three of us shared a house off-campus, and anybody finding a TV during the holidays would buy it for the house. One such set, I took back home for the summer break, and one day smoke curled out of the back, there was a loud crack and a flash, and it died. It later transpired that all sets in the street had a massive interference at the time.

[Davey]

Well, it powers up, no smoke, no bangs. Plays a signal input from the cassette player. Both channels.

Which still hum.

[tony sayer]

In article , Davey scribeth thus

Right so your saying that we're no further forward then?.

Can you tell us exactly what caps you have replaced..

[The Natural Philosopher]

OK.

There is one and only one capacitor which fits the bill, and that is the main preamp smoother C77 - or possibly one per channel.

if the hum was present but different on both channels, I would expect that the power amp input stage smoothers C90 left and right would be the culprits.

If any of these are high impedance its going to push low level ripple into the preamp and power amnp post volume control

The values of these are not critical ..C90 is 50uF and anything from

47uF upwards will work, at probably at least half the supply voltage - so 30V or better, and C77 is 300uf., so 330uF and up at I would guess at least 25V.

Can you confirm that you have, in fact, replaced all these?

If you have we will need to put on the thinking cap (sic!) again.

Because we will then be out of 'dried capacitors' and into 'something else'

Finally, is it possible to work on the set with it actually 'on the bench' ?

Because using an electrolytic to short the signal out progressively up the signal flow is a good way to isolate where its being injected.

WE know already that it's in the late preamp or power amp stages post the volume control...and that could be something as simple as a screened wire screen come off..but one step at a time.

By the way., nothing you have done so far is wasted. Leaving old caps in there that would eventually die is bad news anyway. Its good to replace the lot.

The recording you made and examining the waveform tells me that a capacitor hasn't gone down in value, either. Its gone up in internal resistance.

Which is classic when they dry out.

And finally where roughly are you located..there are various people who DO have scopes and experience...

[Davey]

I'll answer this as part of my reply to The N. P.

[Davey]

This is where I think I got confused: I had thought that there was a C77 on each power board, but now I think that there is one, in a different place, which has not been replaced yet. The 'false' C77s are, I think, C93, which confusingly is only referenced once in the schedule, although clearly being spec'd as one per channel. Grr.

C90 and C92 on both channels have been replaced. For C77, see previous paragraph.

I still have some capacitors to change: C77 (the real one), C52, C63, C55, (2) x C87. I don't have all these yet.

Yes, as long as I don't let the smoke out!

That sounds dangerous, but maybe worth a try, with guidance.

I'm game, with direction.

I appreciate that! If nothing else, it's good experience. Not that I intend to repeat the exercise.

In East Anglia, mid-way between Bury St. Edmunds and Norwich.

Of course, I could always bid for lot 1155:

and be done with it! I might just put in a low bid, just in case.......