Bush DAC10

[Mike. Watterson]

HT plus regular peak audio plus amplitude of any spike caused by inductance of transformer and noise causing the output valve to turn off suddenly as the voltage on an inductor is related to the RATE of change of current and worst case energy is the current getting turned off at a current peak.

So on 250V approx HT I fit a ceramic 1KV part usually. You can arrange to have output valve arc with out the cap. Though usually called a "Tone corrector" (it is as the transformer may boost higher frequencies if not generously large), it's actually a snubber and protects valve from arcing or insulation break down on primary winding.

This is also why it's a bad idea to leave speaker disconnected on a transformer, especially valve types. The voltage from a crack or pop, even from wavechange or tone switch, (dis)connecting aerial, earth or gram could make the 600V or 800V paper capacitor fail. Some sets have them from anode to 0V, which is nasty for the transformer if the capacitor goes short.

[Mike. Watterson]

Also if you don't have a bench PSU or a variac or a lamp limiter, you can turn on for about a minute, off for 3 or 4, repeat with longer on times and the Electrolytics may reform. "Good ones" reform in about 10 minutes. Leave the DVM connected on HT line. If they have reformed they run cold. Check with mains disconnected. Some might take an hour.
If the bung is perished or there is a crusty deposit the electrolyte may have dried out and thus you get hum, even though there is no leakage current and the can is cold.

[Boater Sam]

Its across an inductor with a high DC voltage and an alternating voltage (the audio) which generates high voltage spikes as the current reverses.

[pip5678]

Thanks for the explanations of requirement for high voltage rating capacitors.

[defender]

Originally Posted by rambo1152

I tend to re-stuff smoothing blocks in old radios even if they seem OK.

Actually, I find it a quite satisfying chore.

Here, for the OP, are some images of how I do it, and note with modern components there is always ample room in the can, even when an extra one is used in parallel to make up the required value with what I had to hand.

What voltage did you use and how close to the original do you have to keep the capacitance?

[Graham G3ZVT]

There is usually room for 400v capacitors so that's what I use. Just use the nearest modern values, so that 16/32 was replaced with 15/30 (2*15).
You can use a higher value than the original and sometimes improve the hum situation, but beware of doing that with the cap connected to the rectifier cathode without reference to the valve specs.

[defender]

Thanks for that I mesured the old caps, the 16uf part still measured 16uf the 32uf part came up as an inductor and the 47uf that had been bodged on to replace the 32uf part measured 57uf9 (PEAK LCR45)

[Boater Sam]

The bodged one will be leaky, that's why it reads high. Obviously the 32uf original is toast.

[Tim]

Hi

The DAC10 is actually quite an easy radio to restore, at least from the electronics perspective. As others have said it's best to get rid of all the wax capacitors. The last one I did ISTR I used some little brown polyester ones from Rapid electronics. All common values at 400v DC. Most of the capacitors are on the tag board, and although the above mentioned replacement caps are designed for PCB mounting the leads are long enough for this job. Most importantly they are nice and cheap!
I also seem to remember that there are two or three wax capacitors under the tuning capacitor. It's quite easy to remove the tuning cap(only 3 wires I think), or possibly just undo the mounting screws and swing it out of the way , so it's not that scary.

Originally Posted by pip5678

Out of interest please could I ask why this particular capacitor needs to be so highly rated? Thanks

Due to the inductance of the output transformer(back EMF's etc), the anode voltage on the output valve can peak to much higher than the HT when in use. A capacitor in parallel with an inductor forms a tank circuit( albeit a de-tuned one) so all sorts of funny things can happen.

[defender]

Thanks for that, now I need to know what gauge wire I need to re wire it and best place to get it and colour so as not to look out of place.