Hi folks,
I’m after a circuit and advice on an Astor G2B stereogram I’m working on. Unit works ok though one channel is not as clean in sound as the other. Also when little or no input to the defective channel, there is low volume popping sound that continues until it has an input. I have re capped the unit but can’t locate what the problem could be.
Any advice appreciated.
Cheers
Scott

Output transformer internally arcing would be a possibility.
JJ

I’m after a circuit

Downloadable from here: https://www.radiomuseum.org/r/astor_concertmaster_g2b.html

can’t locate what the problem could be.

I usually go by the axiom that what you changed last probably created the new problem. However, a full recap offers a lot of scope for possible errors, such as electros installed backwards, cold joints, missed jumpers, etc. It can also make out of spec resistors play up.

Stereo gear has the benefit of allowing signal tracing comparison of one channel with the other through the various stages.

Nonetheless, I'd probably start by being a "valve jockey" -- swap each valve channel-for-channel, one at a time, to see if the problem follows the swap.

Johnny's tip about a failing O/P transformer is worth look at too, by swapping over (by jumper if necessary) the connections between channels.

Thanks guys,
Not sure about the OP transformer, swapping valves didn’t correct the problem, the signal on the defective channel is not down on volume, just not clean. The popping sound stopped but the signal is still not clean.
The caps are all ok, and I checked voltages around both the 6M5 output valves, all seems the same apart from pin 3. Circuit says a voltage of 7 volts should be present. On the good channel it’s 6 to 7 volts but on the defective channel it’s 3 times the voltage reading at 22 volts. Resistors going to pin 3 on each of the 6M5s seem ok...

Your thoughts?
Cheers
Scott

Well by measuring the voltages you have virtually identified the problem.
Pin 3 being the cathode, high volts indicate too much current or cathode resistor gone high.
Too much current means bias problem, wiring error or gone high resistors on grid, or leaky coupling cap.
OP xformer still a possability, if all else checks out OK.
JJ

What voltage do you have on G1? Should be zero.

Sounds like you might have made a mistake or (unlikely) your new capacitor to G1 is bad. GTC's axiom is spot on.

If there is a cathode resistor and it goes open, or high that will cause the grid 1 voltage and cathode voltage to move toward that of B+. It is possible (and has happened) that a faulty resistor can arc within.

Marc

With those voltages and symptoms, given you have replaced the coupling caps, measure the resistance from Pin 3 to chassis. Dry joint or resistor gone high/ open circuit is likely.

If cathode bias resistor goes open, the bypass capacitor will start to take its place by drawing current as the voltage rises above the rating of the cap.

Hi guys,

Thanks heaps for the advice.
Problem has been fixed now and it was a combination of small things:

1/ dirty signal was due to a cold solder joint on the .001μF cap, once cleaned up and re soldered, output was clean again.

2/ the voltage of 22v on the cathode (pin3 6M5) was due to the 100k resister having gone high, replacing that bought the voltage down to 7.5v which is fine.

3/ popping sound was due to one of the new caps (.047uf) I installed being faulty, replaced it, and popping sound gone.

All seems to be fine now.

Thanks again.
Cheers
Scott

Ah, some of the usual suspects in that line-up.

Regarding 6M5s, it would be worthwhile to check if there's any sign of silver migration between pins 1 and 2 and remove it. Some 6M5 glass bases were infamous for that and it can lead to a positive voltage on the grid.

Actually it gets on the glass as well & with several grotty miniature valves the chemist in me comes out & they end up getting a quick dip in some diluted Hydrochloric acid.

Yes you can wash them but make sure you have a paint marker, or similar to replace the number (assuming it was still there originally)

One of the things I often do with 6M5, is to clear pin 6 and tie it to the cathode. That way in a crunch you can use a 6CK6. It's suppressor grid is on pin 6: 6M5 is IC.

The pinout of the 6M5 was asking for trouble. Compare it to that of the 6BQ5.

The silver migration problem on the 6M5s is restricted to the early Mullard Noval base versions, You can pick these bottles by the way the glass flares out around the base. Mullard picked this up early on and stopped using silver on the pins.

Silver migration causes a very characteristic frying crackle sound. Listen to it in the background of some of Churchill's broadcasts made from the Cabinet War Rooms, it's quite different to transcription disc surface noise. I saw the console he used there and it is a candidate for silver migration. I bet they didn't know what it was in those days!

I have seen more misswires in 6AQ5 than any other valve having two pins for g1, constantly catches the slackers who on seeing a vacant pin think that they can use that as a terminal.

I have a HMV E43F that has a broken dial. I have tried 3 times to get it into Radio Waves to see if anyone has one? So I am sending it out with plain glass, in the mean time.

I do not know what clown had a go at it, but it was never going to work. The first thing I spotted when I looked at it was a EL84. Not a HMV Valve was my immediate reaction: That was quickly confirmed. Diddums had managed to realise that the pin outs were different, so I found the coupling cap & grid leak on pin 1: Brilliant; on both valves that lot should be on pin 2.

To add insult to injury, it was the wrong value coupling cap, the resistor was out of spec & the Plate resistor of the 6N8 feeding them was also a dud. So at the end of the day I sorted around 5 resistors, all electrolytics, and two wax paper caps that should never have been left in it, plus the wrong ones. Naturally I could only align the dial at the ends, but not surprisingly the IF was also well out.

Unfortunately not the first like this, some worse.

On silver migration: There are a lot of American sets cursed with what they call "Silver Mica" disease. I have seen it here up to the 1930's, but there was one type AWA made in open & sealed, of layered construction, that seem to be "bullet proof". The American ones are in IF Transformers and all over the shop & cause havoc. They have others, as we do, in the paper group that are also replace on sight. There is a brand called "Micamold"; Rarely is there any association with Mica, most are not. I have gotten to the point where anything with that brand on it, gets tossed.

Also be wary of "Oil filled" apart from PCB oil, they are often no better than Wax paper & I tossed every one in a Signal Generator recently: All leaked electrically & some can types in Philco BC-221-N Frequency meter, were leaking oil as well.

When you have fixed a lot of radio's you see a lot of strange things.