Hi everyone,

I have a radiogram that I have repaired (though it's still got a couple of strange issues). I have been unable to find any indication on it as to who manufactured it. The chassis is also quite unusual. I have sent a couple of pics to the administrator for uploading - hopefully someone can throw some light on it. The valve lineup is: 6BE6, 6BH5, 6BD7, 6M5 and a rectifier valve that I can identify.

We have replaced all the caps, along with the audio OP transformer, a new 6M5 and all seems to be fine for a couple of hours operation then it starts to crackle and pop from the speaker, lowering the volume has no effect, so it sounds like its around the 6M5 somewhere.

Anyways, any advice appreciated.

Scott

Seeing as you have mentioned 6M5, it should be noted that the 6M5 is somewhat renowned for this problem:

"A common problem with 6M5 valves concerns migration of metal between the screen and grid (pins 2 and 1) which causes them to appear to be gassy. Scrape between these two pins with a metal scriber and in many cases the valve will again work well, saving it from the rubbish bin." (Rodney Champness in Silicon Chip, 2001).

Not saying that is necessarily an issue in your particular case, but something to be aware of nonetheless.

Some valves had silver on the pins & that is notorious for migrating.

Do check that it is not a bad solder joint or debris. The fact that it waits till it is hot could be causing an arc on a bad joint bad socket connections are also implicated. Did you check the resistors?

Crackling & hissing is also the realm of silver mica caps. Normally these are not changed.

Marc

The valve lineup is: 6BE6, 6BH5, 6BD7, 6M5 and a rectifier valve that I can['t] identify.

That lineup, with a 6X4 rectifier, was used by brands Astor, Breville, Peter Pan, Precedent and Pye around 1954/55.

I cannot find a model that says radiogram, but yours may be a house brand made to order for a retailer and that used the chassis of one of those brands mentioned.

Or it could be something else entirely.

Thanks Gents, I'll take a look at it today and report back. The resistors seemed ok, but as you say, it could be one of them.
Cheers

I replaced the 6M5 valve today (and also scraped the pins) and the unit started crawling and carrying on as soon as it was switched on. Having a look at the circuit, it does have 2 silva mica caps which are original. I'll take it back to the workshop tomorrow and replace the 2 caps and check it from there. As the audio OP transformer was second hand, it could quite possibly be the problem.

Check for a brown mica capacitor from the anode of the 6BD7 to ground. They aren't so reliable in that position.

Also have you tried replacing the 6BD7? They are not that reliable either.

Any of the valves with silver-plated pins should have the pins cleaned with steel wool. The pins should not be black. The 6M5, 6BD7, 6BH5 may be that type.

Hi guys, got it back in the workshop now. I think it may be the mica cap from anode to gnd on the 6bd7. When u pull the tube, the static becomes both loud and continuos. Putting the 6bd7 back in the noise stops as the tube heats up then comes and goes. I don't know the value of the mica cap though, there is nothing written in it other than the brand 'simplex' MS.... Any ideas on what to replace it with?

Replace with 100pf at a high voltage.

However, to test if it really is that capacitor, you can simply remove it.

Cheers. Now to find one!

If I just remove it, it won't damage anything?

Ok, removed and all is ok. What's the purpose of it?

It filters the remains of the IF frequency out of the recovered audio.

Without it you may notice increased distortion or strange instability.

Replace it if you can.

Thanks, Any suggestions where I can get one locally ( Melbourne) as I need to get the unit back to a client by Monday

I don't think that it is possible to buy low value, high voltage caps at Jaycar any more, although you could check their catalogue.

What you could do is get a 400V (at least), 0.01μF cap and put it in series with a normal 100pf cap, which are both available. Sounds messy, but these days there's not much choice if you're in a hurry.