Mark
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Post by Mark on Apr 28, 2021 22:33:05 GMT 12
Hi All, Creating this thread to document the repair of this beauty. Its my first attempt so a steep learning curve - wish me luck.  Stage 1 - Clean and dismantle (done) Stage 2 - Get circuit diagram (done) Stage 3 - Replace Capacito  rs (in progress) ...to be continued    |
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Steve
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Post by Steve on Apr 29, 2021 12:23:04 GMT 12
Hi Mark, did you replace those caps or have you just cut them out? Some thoughts on this process:
1. Do one cap at a time (which isn't always practical or possible)
2. cut the body out and leave the leads in place (at least initially)
3. put an alligator clip lead on the two lead ends until you're ready to replace the cap so you can clearly see where it went - particularly useful when the doorbell rings after snipping a cap out...
4. use 630V caps for everything... they are small enough and cheap enough that picking exact voltage ratings doesn't really matter any more.
Cheers, Steve
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Mark
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Post by Mark on Apr 29, 2021 19:10:34 GMT 12
Hi Steve, I do have a system I hope doesn't let me down I have removed them - using a label maker on the remaining solder leads along with lots of photos, I can definitely see how you can end up in trouble though... - noted re the voltage. Thanks.
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Post by radioholic on May 1, 2021 17:10:20 GMT 12
Hi Mark I always draw up an under chassis diagram, on A3 paper. I find this very helpful as it avoids having to store a lot of stuff in short-term memory as one works. James  |
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Mark
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Post by Mark on May 2, 2021 11:02:16 GMT 12
Hi James - good idea - thanks for the tip.
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Mark
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Post by Mark on May 12, 2021 0:37:02 GMT 12
Hi again ....OK so I have replaced the capacitors and and (was) pretty confident there were no errors....    Unfortunately I'm still not running and with some additional issues with the tone control now "arc" - ing with visible sparks shown which I imagine shows that I have made an error with the cap values or location. Was pretty sure I got that all correct. I understand polarity is not an issue for the poly caps and followed the previous polarity for the electrolytic. It only happens with the speaker connected so wondered if an issue the speaker could creating additional load. Im seeing 470 ohms across the speaker output as shown - the speaker has 2500 ohms written on it. Thanks for any advice on next steps any suggestion on fault isolation etc. Mark |
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Post by Richard on May 12, 2021 16:13:35 GMT 12
The 430 ohms is probably the DC Resistance of the output transformer, the 2500 Ohms is the field coil resistance. The field coil energizes and becomes a magnet for the speaker to work, but this circuit also forms a negative voltage to bias the 6F6, which should have around "minus 18 Volts" on the control Grid with respect to the cathode or in this case, the chassis earth. You need to make sure that there is a negative voltage on the control grid and not positive otherwise the 6F6 will conduct "flat out" drawing lots of current and destroy the output transformer and itself. If you have a positive voltage, check the cap feeding back from the plate of the 6F6 to the tone control wiper or the coupling cap coming from plate of the previous tube/valve. otherwise you need to check the changes you have made with the schematic to make sure they are correct. I hope this helps?
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roy51
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If it ain't broke, fix it anyway
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Post by roy51 on May 12, 2021 16:17:51 GMT 12
Hi Mark
Speaker shows 2500 ohms, that’s the field resistance, the 470 ohm you have is about right for the output transformer DC resistance.
Have you checked the resistance between each terminal on the tone control and chassis, this should read infinity (or close to it) if not you have leakage between the resistant track in the pot and earth which could result in the arcing.
Hope this helps
Roy
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Post by Peter Walsham on May 13, 2021 21:48:36 GMT 12
Hi Mark
Check - or better still, replace capacitor C17 (the 250pF Silver Mica cap.) that's connected to the centre of the tone control. I'll put money on it being leaky - even if it's only leaky when it has 250 Volts on its other end - IE: breaking down under voltage. I can't, for the life of me, see any other reason the tone control is arcing at that end of the control. Having said that, there's a high DC resistance to earth (via the 500k ohm 'R5' bias resistor which, I would have thought, would be high enough to prevent arcing - even with a shorted C17. A resistance check (with the power off to the chassis) between the junction of the tone control & R5 and earth should show a resistance of greater than 500 k ohms. If lower than this, your new capacitor (C8) may be shorted (is the other end of that capacitor actually connected to the chassis earth?) or the tone control pot has a short to earth at that end of its travel. It is also possible that there could be some HF audio oscillation going on. Try removing the 6B8 1st audio valve, and trying the chassis again. If the tone control doesn't arc with the 6B8 removed, it could be safe to assume that there's some oscillating happening. Lots to think about.
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Mark
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Post by Mark on May 13, 2021 23:14:30 GMT 12
Thanks for your input Guys, the voltage on the control grid of 6F6 seems to vary between -9 to -12v and there doesn't seem to be any leakage on that pot. The "arc-ing" seems to have calmed down a bit but still hear it occasionally and 6F6 has a very subtle high pitch harmonic vibration coming from it- not good I'd say. I'll spend some more time on it and check as much as I can against the diagram, regards, Mark
Just saw your comment Peter - I'll digest and look into those possibilities!
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Steve
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Post by Steve on May 14, 2021 15:17:58 GMT 12
If its arcing you hear, not arcing you see... then it could be silver mica disease. This manifests as a crashing sound over everything. So it could be that one of the silver mica caps that is dealing with high voltage is on its way out... something else to think about and check. And if its tone related then it could be the one between the plate of the output valve and the tone control...
Cheers, Steve
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Mark
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Post by Mark on May 16, 2021 21:56:35 GMT 12
OK so heres an update - unfortunately I have to order in some more capacitors to prove some of your diagnosis but here we go with recent observations... (from Peter) "A resistance check (with the power off to the chassis) between the junction of the tone control & R5 and earth should show a resistance of greater than 500 k ohms." - resistance reads 900K (yes the other side of C8 is connected to earth) (from Peter) "Try removing the 6B8 1st audio valve, and trying the chassis again. If the tone control doesn't arc with the 6B8 removed" - No arcing with 6B8 removed. I have uploaded a video (with audio) here: drive.google.com/file/d/19o5fP1lxypaIxZHdc-FEsfVNMdrcrihk/view?usp=sharingThe video shows: Arching starts when band is switched Arching stops when tone control moved to right. Arching stops when Gram/Radio switch is switched - disconnecting the radio cables* *Assumption being that the switch (that sits up next to the turntable) connects two cables coming from radio output - the turntable cables have been removed (current and previous state shown). The arching happens when the two cables that come from the chassis are switched to connect.    On another topic - just confirming a massive voltage between the speaker chassis and the radio chassis. I have had a couple of jolts when touching both (I'm very careful now) - see below the sparking that happens when the cable leading to the switch above touches the speaker chassis. Just checking this is "by design" and not part of my issue. Thanks as aways for any comments, Mark  |
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Post by Peter Walsham on May 16, 2021 22:42:53 GMT 12
MMMMmmmm
Thanks for your detailed information. The plot thickens.
I think, before you proceed any further, that you try and locate the cause of the speaker frame becoming live. The best way to go about doing that, is to disconnect the 4 wires between the speaker and the chassis, and using your ohm meter, see whether the output transformer primary is shorting (or leaking) to the speaker frame, or whether the speaker field coil is shorting (or leaking) to the speaker frame. If you're lucky, it may be one of the speaker connecting wires that has perished, and is making contact with the cable clamp on the speaker itself (assuming it has one). It's also very possible that the output valve anode speaker lead is the one that's shorting/leaking to the speaker frame. If this proves to be the case, then that may well be the cause of the tone control arcing, as the speaker frame will be radiating audio (and lots of it, due to it's mass) and this could be being picked up by the grid circuit of the 6B8 audio valve, and possibly causing HF (but inaudible) audio oscillation. I did write some more, but have deleted it, as what I wrote could have been very confusing. See what you come up with regarding the speaker problem before we move on. If you feel that this is getting out of your depth, then you would be very welcome to drop the chassis & speaker in to me (in Pukekohe), and I would be more than happy to help you out.
Peter
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Mark
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Post by Mark on May 17, 2021 13:49:27 GMT 12
Thanks Peter, (To be honest I've been out my depth for for a while with this -but enjoying the swim  ) I'll take one more look tonight and if i can see where this mains short is originating (couldn't see anything obvious like a speaker cable short) , Thanks for your offer to take look at it, if there's no revelation tonight I'll get in touch via email. Regards, Mark |
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Post by Peter Walsham on May 17, 2021 14:15:11 GMT 12
Hi Mark
No problem at all. It's not very easy to make suggestions or diagnoses by way of a chat forum - as a matter of fact, it's downright difficult with an 'odd ball' type of a problem, such as what appears to be happening with your radio. The fact that the speaker frame appears to be 'live' is a real worry in its own right, as it shouldn't be. I think I would have to suggest that sorting out the reason for that would be a priority. I understand and would have to agree with your comment about 'enjoying the swim'. I still get a 'kick' out of hearing music (or, unfortunately, talk back now) sounds from a radio that has been silent for a very long time.
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) I'll take one more look tonight and if i can see where this mains short is originating (couldn't see anything obvious like a speaker cable short) ,