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 AWA 528MA combined IRC tone control, on/off switch issues.
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 Return to top of page · Post #: 1 · Written at 4:16:06 PM on 25 August 2020.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1133

I am currently working on a AWA 528MA that has a 100K tone control with part of its carbon track burnt out.

One terminal of the tone control goes to C36 (.1uf), which in turn goes to the plate of the 6V6G. Centre terminal is earthed. The other side which is supposed to go to C41 (.1uf) is open. Someone has been here before a long time ago because the open circuit side terminal of the pot has been broken off & C41 is missing.

The radio sounds a bit muffled & distorted although volume is ok. Could this issue be due to the faulty tone control?

The rectifier used is metal US forces 5W4 and gets very hot. Should this be an issue, or should I replace with the specified 5Y3GT.

Finally, does anyone have a spare 100K switch pot (long shaft) surplus to their needs?


 
 Return to top of page · Post #: 2 · Written at 4:34:05 PM on 25 August 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

Control probably burnt out due to leaky paper caps off plate.
0.1 mfd seems a bit large for straight off the plate, I wonder if this is an error in the parts list.
I would try a 0.01 mfd for C36 instead and leave the other side of the pot open.
JJ


 
 Return to top of page · Post #: 3 · Written at 4:50:50 PM on 25 August 2020.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1133

C36 was definitely a .1μF which I replaced. It is C41 that is missing which is supposed to go to the faulty side of the pot.

C41 seems to go to a feedback (I think) network.

The schematic is on Kevin Chants site.


 
 Return to top of page · Post #: 4 · Written at 4:53:19 PM on 25 August 2020.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1380

Comparison of specs of 5W4 and 5Y3 reveal 5Y3 is slightly better, but it shouldn't make much difference. 5W4 takes less filament current, saving you about 3 watts.

If you're concerned with the heat level, measure the current in the valve's cathode circuit. I'm sure you would have checked the resistors and voltages around the output valve.


 
 Return to top of page · Post #: 5 · Written at 4:58:01 PM on 25 August 2020.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 941

100k (linear) switched pot is also used on the 429MA and hard to come by. Mine has a collapsed switch, the resistor track is OK.
Saw one once on ebay in the UK at a quite high price with shipping.


 
 Return to top of page · Post #: 6 · Written at 5:00:00 PM on 25 August 2020.
Johnny's avatar
 Location: Hobart, TAS
 Member since 31 July 2016
 Member #: 1959
 Postcount: 410

Sorry, I meant try 0.01 for C36.
Will edit post.
JJ


 
 Return to top of page · Post #: 7 · Written at 5:46:58 PM on 25 August 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1905

AFAIK the only manufacturer left for rotary switch pots is Alpha in Taiwan. Not widely stocked. www,tubedepot.com have them but you'll need an extension shaft as the only ones they seem to stock are for guitar amps and only 1m with a switch.

Might be worth trying Alpha directly, although there may be an MOQ and lead time.


 
 Return to top of page · Post #: 8 · Written at 6:21:09 PM on 25 August 2020.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1133

I just ratted a 100K switch pot out of a junker 429MA. Unfortunately the shaft is too short.

I've fitted it anyway just to see if the solves the problem, and it does of sorts.

I forgot to mention before that there was some hum as well. That too has disappeared with the good pot fitted.


 
 Return to top of page · Post #: 9 · Written at 9:45:51 PM on 25 August 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

I inherited a few big boxes full of pots and naturally tapped ones are rare. Sorting is fairly daunting, but there are switch pots.

I normally save bits of pot shaft. That gives me two choices as to how to deal with those too short. Sometimes I will cut them shorter & make the one piece extension shaft longer & also save a grub screw in the process.


 
 Return to top of page · Post #: 10 · Written at 10:07:38 AM on 26 August 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Hi Monochrome TV .
My email is open can you send me a picture of the pot . Is it a log pot.
I have som pots I bought on eBay years ago if I can help I will.
Regards Jim


 
 Return to top of page · Post #: 11 · Written at 11:19:44 AM on 26 August 2020.
MonochromeTV's avatar
 Location: Melbourne, VIC
 Member since 20 September 2011
 Member #: 1009
 Postcount: 1133

Hi Jim,

Apparently it is a log pot.

I'll post a picture here so all can see.

With my limited technical knowledge would someone be able to explain what this tone control is doing. It seems to be part of a feedback circuit.

https://www.kevinchant.com/uploads/7/1/0/8/7108231/527ma_528ma_[f].pdf

If I cant procure a replacement I'll use a Jaycar pot and do away with having the on/off switch.


 
 Return to top of page · Post #: 12 · Written at 1:12:50 PM on 26 August 2020.
Jimb's Gravatar
 Location: Kanahooka, NSW
 Member since 18 November 2016
 Member #: 2012
 Postcount: 659

Hi Monochrome TC.
I have had a look and I don't have what you are looking for . I do have a couple of new 250K A curve switch pots and I also have some 500k switch pots that would be log. Could you wire two 50 K resistors in series the centre of the two resistors would be the wiper position that would give you a fixed tone position or alter the combination to achieve the required tone like 75K and 25k .I know it sounds like a dopey suggestion but at the end of the day it is only tone .
Regards Jim.


 
 Return to top of page · Post #: 13 · Written at 1:23:26 PM on 26 August 2020.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4344

When you look at the instructions & specs for some of this stuff. The bull often gets a mention later.


 
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