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 AWA 573MA sound with volume at minimum
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 Return to top of page · Post #: 1 · Written at 6:23:00 PM on 15 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

I have a problem with an AWA 573MA where the radio can still be heard quite clearly with the volume at minimum. All wax paper caps, electros and out of spec resistors have been replaced. All valves except the 6BA6 were replaced because of either having shorts or being very weak. The volume control has a resistance of 30 ohms across it at min volume but putting a jumper across it makes no difference.

I thought it might be a feedback problem but removing C22 makes no difference. Bypassing R8 drops the volume right down but it's still faintly audible. I noticed on a 565 this resistor is 50 ohms but lowering R8 as far as 30 ohms makes no difference. AGC is working well with nearly -6 volts on a strong station. All voltages are a bit high but well within 10% of spec with the exception of the 6AQ5 screen grid, spec is 165 volts and it is at 190 volts. I've swapped the 6AQ5 a couple of times with no difference. The only clue I've got is that it seems to have much more volume than I'd expect, putting the volume control to halfway gives it full volume. The volume control is the original 500k and measures good. I'm going to go back through it checking all the resistors but does anyone have any ideas? I havent worked on one of these before but I'm assuming this isn't inherent in this model.


 
 Return to top of page · Post #: 2 · Written at 7:28:43 PM on 15 June 2014.
Gandhn's Gravatar
 Location: Cameron Park, NSW
 Member since 5 November 2010
 Member #: 770
 Postcount: 385

As the volume drops when you bypass R8 makes me think there is audio getting on to the AVC line. Can you put an oscilloscope on C5 to see if there is anything there other than the AVC voltage.
Another thing worth checking is the phasing of the feedback via C22, possibly the feedback is positive rather than negative if either transformer winding has been reversed. Does this wiring look original?
Harold


 
 Return to top of page · Post #: 3 · Written at 8:28:55 PM on 15 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

Thanks for that Harold, I've been thinking it's something involving AVC. I'll get the scope out in the morning. OPT wiring is original but disconnecting C22 didn't make any difference except to bass response. I'm thinking of trying another 6AQ5 as well, the only slightly wrong voltage was here.

Warren


 
 Return to top of page · Post #: 4 · Written at 8:06:00 AM on 16 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

There is no AF on C5. It's a later model with the revision that includes C26, C27, C28 and R18. There is a small amount of AF on the R18 side of R3 but nothing by the time it gets to C5.

Warren


 
 Return to top of page · Post #: 5 · Written at 9:07:44 AM on 16 June 2014.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

I haven't checked that circuit, but in some circuits the AVC feedback is connected to a center tap on the volume control, so the bias kicks in at a certain level. I had a set with volume issues related to wear and loose connections around that center tap, as well as the resistors and caps on that part of the circuit.

Maven


 
 Return to top of page · Post #: 6 · Written at 10:06:17 AM on 16 June 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

Is there a circuit for that? AWA clone to another (common with them) If its a pot tapped at 40K that is not uncommon.

Has this got conventional, or delayed AGC. Often delayed AGC is taken from the plate of the last IF. and leaking Mica's whilst not that common in Australian sets can let go.

My normal process with those, if it's leakage and not a short, is to use the insulation tester to test them (one end of cap, out of circuit).

Marc


 
 Return to top of page · Post #: 7 · Written at 10:56:20 AM on 16 June 2014.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1179

Kevin Chant has the circuit for the AWA 573MA here for anyone needing to offer advice.

http://www.kevinchant.com/uploads/7/1/0/8/7108231/573-ma.pdf.


 
 Return to top of page · Post #: 8 · Written at 2:23:04 PM on 16 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

There is no tap on the volume pot. It seems to me that AGC is taken from the diode so isn't delayed.

With respect to my previous post regarding there being no AF on the AGC at C5, my scope technique is a bit rusty. Using AC coupling instead of DC shows about 20mV of AF on C5. That sounds like a lot to me and might explain the amazing audio gain this set seems to have. I'm out of my depth here but is C5 supposed to decouple any AF or is a small amount of AF intentionally fed back through to increase overall gain? The circuit for the modification including C5 is shown but no values are specified. It had a 0.047 originally so that's what I replaced it with. I think I will replace C5 again in case the new one was faulty but I'm only fishing here.

Thank you everyone for your assistance so far, it is greatly appreciated.

Warren


 
 Return to top of page · Post #: 9 · Written at 7:44:23 PM on 16 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

Okay, this is getting really weird! You might need to look at the circuit to follow this. During all this I had a jumper on the volume control to eliminate any possibility of this being the problem. I removed R3 which disconnects the AVC, the volume increased which shows that a) the AVC works okay and b) AF on the AVC isn't my problem.

I then removed C22 which disables the feedback circuit. The problem is still there.

Next I removed C19 which isolates the tone control circuit, no change.

In desperation I removed the AF coupling capacitor C16 from the volume control thinking that would fix its wagon. Nothing has been reconnected at this stage but the radio STILL has good volume, albeit slightly distorted. There is no connection between the IF and AF except the diode, B+ and ground!

Can AF get onto B+ and cause this? I've checked and double checked all components and wiring and replaced the 6AV6 and 6AQ5 multiple times. I'm out of ideas...

Why did AWA put so many useless components in this radio? I've eliminated about half of the component list and it still works fine.


 
 Return to top of page · Post #: 10 · Written at 9:23:49 PM on 16 June 2014.
Maven's Gravatar
 Location: Canberra, ACT
 Member since 23 August 2012
 Member #: 1208
 Postcount: 584

Can AF get onto B+ and cause this?
Here's my guess. AF is an AC signal and Ground is common to AC and DC circuits. If there is a significant capacitance on the B+ circuit (and we know there is) then maybe the Ground phase of the AF/AC can cause some modulation on top of B+ even when the positive phase is absent. I base this on the way data signals work in digital circuits, but will be happy if somebody knows better.

Another path might be through the output transformer. Just speculating..

Maven


 
 Return to top of page · Post #: 11 · Written at 10:14:26 PM on 16 June 2014.
TV Collector's Gravatar
 Location: Ballarat, VIC
 Member since 4 January 2011
 Member #: 803
 Postcount: 456

Removing C16 proves the fault is nothing to do with the volume control and associated circuit which is useful. I'm thinking there is a problem in the area of the 6AV6. Are there any components or wires that connect to pin 1 (grid) and pin 5 (detector diode) close together? I suspect that the signal is being coupled somehow between the two circuits. Make sure the valve socket is clean and in good condition. It's unlikely but there could be some leakage across the socket. Also check that all grounds are WELL connected.

Make sure R9 (10 Mohm) is OK as well, these high value resistors are known to give trouble.


 
 Return to top of page · Post #: 12 · Written at 11:06:00 PM on 16 June 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

6AV6 can be a cranky micro-phonic little horror, and the grid resistor is indeed a very common fail. Take careful note of R5 on the screens. Originally they used 2 x 22K 1W parallel in that series and someone in accounts has ordered tens..... So what? The sum total of the screen grids draw, especially if the volts are up, exceeds 10mA and that is over a watt through 10K, so they fail.

Has that set been modified? It is obvious that there was a problem with the IFT and the resistor added in the AGC will change the attack. Add the mod if it is not done.

Do check the value of the replaced parts eg interstage coupling. The grid resistors on OP Pentodes seem to fail with regular monotony.

Note back bias voltage R17. If it is not within a bulls roar the set is not working properly (chassis positive CT negative) Often that is shorted out, or the first filter cap is wired wrong and grounded & that will cause issues.

One issue that I have noted and there have been several instances noted on US site:---
Do not calibrate Sig gens with a frequency counter with the modulation (tone) on.
The IFT's will not calibrate properly, if the set is overloaded with signal as the AGC cuts in. HMV normally does not use 455kHz IF's.

If the speaker secondary phasing is reversed (inverted) the sets tend to oscillate, but it may not be at a noticeable frequency.

Distortion is a worry check the plate resistor on the 6AV6 Distortion is one area where a CRO shines as a spotter. I use one when setting IF's. Too much signal is normally associated with the earth side of things albeit a faulty (open) plate bypass cap on the OP valve can cause distortion.


 
 Return to top of page · Post #: 13 · Written at 12:04:40 PM on 17 June 2014.
Blountone's Gravatar
 Location: Como, NSW
 Member since 27 April 2014
 Member #: 1561
 Postcount: 13

Hi, Just a comment.
I have several sets with the same issue. Volume will not go quite with volume pot at zero. Really frustrating issue. Check that the volume pot really does start at zero and goes up to max and does not start at some offset even a low value will cause this problem. Also place two 100pF caps around to 50Kohm resistor that is between the If and the volume pot as Rodney Champness has recommended in Silicon chip to filter out the IF from the audio stage. That last step may not fix your fault but is a good mod!!!! Please give feedback when you sort it out.


 
 Return to top of page · Post #: 14 · Written at 8:51:04 PM on 17 June 2014.
Scraps's Gravatar
 Location: Blue Mountains, NSW
 Member since 10 March 2013
 Member #: 1312
 Postcount: 401

I didn't get a chance to do much today.

The only thing still connected to pin 1 of the 6AV6 is R9 which was replaced. I'm starting in to wonder if the problem is in the socket. I flooded it with WD40 and blasted it with a hair dryer for 5 minutes which didn't change anything.

I've swapped the 6AV6 a few times, including a NOS. R5 on the screens is one of the few resistors that was in spec and didn't get replaced, indeed all the resistors apart from R5, R6, R16 and R17 were out of spec and replaced. This set must be a later build as it already had the modification. Bias voltage with a good 6X4 is 7.8 volts.

I've reconnected R3 because without the AVC it really is quite loud and the problem doesn't seem to be associated with the AVC. C16, C19 and C22 are still out of circuit. With the scope I can obviously see AF on the plate of the 6AV6 and just for fun disconnected everything from pin 7. That finally shut it up! I also took the opportunity to check C20 which was okay for insulation and capacitance. The only thing I haven't either checked or replaced in this end of the set is C15 but my guess is that it bypasses the IF.

One strange thing I noticed with the scope was that I could pick up 15-20mV of modulatred IF when the probe was anywhere with a few centimetres of the 6BA6. I don't know if this is normally or not? I seem to remember this valve had a shield on it.

I think as a last resort I'll try replacing the 6AV6 socket. If that doesn't work I'll put it aside until the solution presents itself!


 
 Return to top of page · Post #: 15 · Written at 9:55:55 PM on 17 June 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 5239

I would expect RF to be floating around 6BA6 as it is at the end of the day an RF amp. What can destabilise it. is having wire away from the chassis. most of the RF tubes should be internally shielded 6AV6 is not and if there was a shield, it was probably the one, as stray RF in that area is common.

I am not surprised that pin 7 shut it up that is the Plate of the 1st AF Triode so there would be no signal to the Pentode: Not a solution. C15 is RF decoupling the diode if the signal is loud & its shorted signal would go down.

If the set still works with c16 & c19 disconnected I would have a serious look at the miswiring in that area.


 
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