Dynatron HFC2A Radiogram.

[Biffo_the_Bear]

I have an old Dynatron radiogram and the left speaker output isn't working properly so I'm after a bit of friendly advice. I'm as sure as I can be that the large cylindrical TCC Lectropack capacitor, which is a 2000uF 18v DC TCB/XH 0266, has failed.

I can't find an original replacement, and the capacitors I can find online do not come in the same values. The closest I can get is 2200uF and 25v.

Would that be a suitable replacement ?
There is one capacitor for the left speaker, one for the right speaker - the right seems to be working fine, but should I replace both so they are the same ?

[paulsherwin]

That will be a suitable replacement, but you shouldn't change it without a good reason. Swapping caps is a poor fault finding strategy. There are many possible causes of these symptoms, and a bad output coupling cap isn't even the most likely (though it's possible of course).

Why do you think it is bad?

[Biffo_the_Bear]

Thanks, I'm the first to admit that electronics aren't my strong point, but I'd like to learn. The reason for suspecting the capacitor is that I did a bit of testing with my multimeter and found that I got an output for the right speaker, none for the left (both speakers themselves work fine, it's the left output from the radiogram that does not work). So I tested the resistance over the resistor on each speaker output and got same values, so then tested the earths for the speaker sockets - all good. I then tested the positive wires from the speaker sockets to the capacitor (all OK), and positive wires from the capacitor to the next connection (not sure what it is, but call it X) - all OK. I then tested the speaker socket positive to the point X and the right speaker was fine, nothing on the left. So as the capacitor and the resistor are the only components between the socket and X, and the resistors show the same readings but the right socket is showing a reading and the left isn't, my simple brain reckoned it was probably the capacitor ? I only have a multimeter, no fancy measuring equipment. What do you think ? Is there a proper test I can do (with just a multimeter) ?

[Station X]

Do you have a model number so we can find a circuit and guide you from there?

[Biffo_the_Bear]

Yes, that would help wouldn't it - sorry ! Its difficult to read but I think its HFC2A - late 60's I think.

[Station X]

Here's an extract from the circuit.

[llama]

With that electrolytic disconnected you can do some simple tests on it just using your multimeter. As a start, discharge the cap by shorting its wires/legs together. Then set your multimeter to resistance and put the probes on the cap legs. The meter (is it analogue or digital?) should give a low resistance reading at first and then over a second or two (depending on the capacitance etc) the reading should increase until it stabilises at some high resistance reading (ideally infinite). Switching to voltage, you should see the voltage that the meter has charged the cap up to and that should slowly (very slowly for a DMM) decrease as the meter's internal resistance discharges the cap.
This is all easier with an analogue meter as changing digits can be hard to see.

If you felt like repeating this for the known-good cap you could compare readings.
This only works for capacitors in the multi-uF range, like yours.
Graham

[Boater Sam]

You could swap the capacitors from one channel to the other.

[Station X]

Perhaps, with reference to the circuit diagram, the OP could tell us where he's taking the readings which differ between channels and what the readings are including units.

[Nuvistor]

Originally Posted by Boater Sam

You could swap the capacitors from one channel to the other.

Possible but that risks two duff channels.

[Boater Sam]

Why?
One good capacitor should work in either channel. The bad one, if it is bad, won't work in either.
Or are you suggesting that a fault in the output transistors could destroy the capacitor?

[G6Tanuki]

Originally Posted by Boater Sam

You could swap the capacitors from one channel to the other.

If you're going to the bother of unsoldering a possibly-questionable capacitor, to my mind it would make more sense to replace it with a new known-good item.

[Boater Sam]

Fine if Biffo has spares to hand......................
Just trying to be helpful, hat and coat on, I'm gone..........

[Nuvistor]

Hi Sam,
I am of the school if it’s not broke don’t fix it school. It’s possible that the capacitor could break the good channel but more likely just working on an old PCB if there is no need can introduce faults. Perhaps I am over cautious but the poster admits to having little expertise and therefore may cause more problems.

If the old cap is being removed, may as well just replace it with new, better still do some fault finding.

[buggies]

If you have any capacitor in your junk box over about 100uF and 18v or more then just clip it across the suspect to see if any audio comes through.

[Biffo_the_Bear]

Originally Posted by Station X

Perhaps, with reference to the circuit diagram, the OP could tell us where he's taking the readings which differ between channels and what the readings are including units.

Thanks for all the help and suggestions, and the diagram. I'm struggling to align the circuit diagram to the "contents" of the radiogram though. Where are the left and right speaker outputs ? I assume C22 and C25 are 2000uF capacitors ? It might be easier if I take some pictures and post them up later today.

[G8HQP Dave]

Making sure that it is the right way round.

Yes, no sound may mean that the output coupling capacitor has gone open circuit so temporarily paralleling it with a good cap will check for this.

[Station X]

Originally Posted by Biffo_the_Bear

Thanks for all the help and suggestions, and the diagram. I'm struggling to align the circuit diagram to the "contents" of the radiogram though. Where are the left and right speaker outputs ? I assume C22 and C25 are 2000uF capacitors ? It might be easier if I take some pictures and post them up later today.

The speaker sockets are to the right of resistor R35.

The diagram only shows the LH channel. The RH channel will be exactly the same, but the component designations may be different.

The capacitor you're referring to is probably C22. Have you checked the 1 amp fuse?

[Biffo_the_Bear]

Thanks, the fuse does sound an obvious thing to check - I'm struggling to identify it in the equipment though. Here are some photos:
https://www.vintage-radio.net/forum/...1&d=1542477127
Are the fuses visible in any of them ? My money is on the things in the last picture, but I'm used to car fuses, not sure what I'm looking for here ?
I just connected my multimeter (digital) over the capacitor I thought was at fault (the one at the top in pic 1, out of its holder), and the resistance gradually increases to 0.93. I did the same over the other capacitor (still in its holder top right in pic 3) and it gradually increased to 0.76. That makes me think the capacitor is probably OK ?

[ms660]

Third photo, top left, red and yellow wires, the fuses should be on the other side of the chassis at that point, the ones you think might have been fuses are transistors.

Lawrence.