WANTED - Speaker for AWA 429MA
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Got back to this again.
Replaced high resistors. Put in two extra tag boards, one to support the much shorter than original filter caps, and the other to support one of the caps for the volume control which otherwise would be supported by long flimsy leads. This tag board also supports the three 1/2 watt resistors used to replace a 2.5Mohm AGC cap. I know the warnings against using these tiny resistors, but couldn't get 2.5Mohm resistor and I hoped that three in series would have a high enough voltage rating, and this seems to be the case as radio performance doesn't seem to be affected.
Measured valve base voltages indicated by the table in the documentaion. At around 240V line voltage filament volts are low, 6.3s reading 5.9v and 5.0 reading 4.6v. This seems low but if line voltage was 260V (upper rating of transformer tap in use), then these filament voltages would come in on spec. So OK I guess.
Screen grid to chassis volts were pretty close using my digital meter, with line voltage at around 240V. I guess they would go lower with a 1000ohm/volt analogue meter. Anode to chassis are all low using the same meter. 5Y3 rectifier spec anode 190ACV reads 185ACV, 6BE6 spec 200VDC reads 159VDC, 6AR7GT 200 reads 160, 6AQ5 190 reads 177. I am guessing that these low voltages are due to the low filament voltages as the digital meter should drag them down less than a 1000ohm/volt analogue meter used to get the spec voltage. Can't find anything else to account for them. The valves all test OK.
The voltage across the output valve back bias resistor R8 in the high tension centre tap lead is specified to be -3.0V in the circuit for N78/K61 versions. The modified circuit diagram (posted above) for 6AQ5 versions doesn't specify it (though it does say socket voltages are all the same). The N78/K61 version uses a 75ohm R8 resistor and the 6AQ5 version uses 175ohm. I get -7.5V across this 175ohm resistor, which checks spot-on at 175ohm.
Despite these apparent anomalies, the radio works quite well, sounds good, and is sensitive enough to just pick up Sydney commercials from about 60km away using three feet of aerial under an iron roof, and it hasn't been aligned yet. If it really needs alignment it can only get better.
Having trouble finding a 3 core mains cable with small enough diameter to pass under the radio after it comes out of its hole underneath between the two halves of the cabinet. The original brown two core cable of course does this easily. Some light fitting 3 core is quite small diameter - will see if I can get that in brown.
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Location: Belrose, NSW
Member since 31 December 2015
Member #: 1844
Postcount: 2601
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Note to newbies when replacing old (non-preferred value) resistors and caps.
If you need a 2.5 megohm resistor, for example, don't make it up with several preferred value types. Just use the closest preferred value, either 2.2 megohm or 2.7 megohm will work just fine.
The original resistors mostly had a 20% tolerance in those days. These days even 5% resistors (the loosest tolerance you can readily get) will usually be within 1%.
As a rule when picking a replacement I usually go higher for low-value resistors and lower for high value. So a 250 ohm 1 watt resistor for example would be replaced with a 270 ohm 1 watt.
Oh, and if your 500k resistor (for example) reads 520k when you actually measure it, leave it alone, it will be OK.
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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"Just use the closest preferred value, either 2.2 megohm or 2.7 megohm will work just fine."
Trouble was Jaycar only had 1Mohm max in 1/2 watt, an 2 and 3Mohm in 1watt, and I needed 2.5Mohm. Could have done 2 + 1/2Mohm in 1 watt but already needed the other 1/2 watt resistors elsewhere so went for packs of them.
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Location: Wangaratta, VIC
Member since 21 February 2009
Member #: 438
Postcount: 5555
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I would be very surprised if any paper & many oil filled types were good now. If you HV tested them (out of circuit) at their rated voltage, over 99.9% would leak like sieves.
Something is not right. All of the voltages are low? Something is dragging the voltage down. Re check your methodology.
What is the mains voltage e.g. Mine normally sits on 245 on load. that means that the transformer primary is tapped at 240V. If you put it on 260VAC you will get low voltage. Check & fix that.
The reason there is too much voltage across the back bias is that it is drawing too much current, that is probably what is pulling the volts down. Normally with a rectifier like 5Y3 if the set was not drawing enough current the B voltage would go high.
What filter cap voltages are you using & did you put them in the right way? I never use less than 500V with 5Y3. I have had 500V caps fail under that & some 450V ones are rubbish.
With a digital meter I would expect the screens (where fed by a resistor) & plate voltages on the first Audio to read higher than quoted. As said voltage tolerance +/- 20%.
Several sets used a 1.75Meg resistor in a divider on the AGC: 1.8Meg can be found in the 1/8 to 1/4 watt ones. AGC is mostly voltage not current.
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Thanks for these thoughts Marcc. I will compose a point by point answer. But will say now that there are no paper caps left in it.
Last night checked the power transformer resistances and have found both primary and secondary to be about 10% lower than spec. I want to double check that today first.
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Location: Wangaratta, VIC
Member since 21 February 2009
Member #: 438
Postcount: 5555
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Basically as I am obliged to primary side, especially if I change the mains cable. I normally attack the transformer both sides with an insulation tester. These can also be used to leakage test the HV NP caps, as I have pointed out many times.
You can run the set without the rectifier which eliminates HV DC and allows by removal of the B+ the testing of the 5V winding to ensure that it in not going to ground. Preferably with an insulation tester. I have seen that winding short. Do not run 5Y3 sideways.
I still think the back bias resistor is saying overload if there is 7.5V across it and it is supposed to be 3V 43mA that voltage is normally the grid bias of the OP valve. Grid bias 250V for N78 (6BJ5) is quoted at -5 at that voltage the N78 is dragging 40.5mA.
All of the cathode current goes via the back bias resistor so at the moment we are looking at 100mA total, that just has to pull the voltage down so what is drawing 60mA.
It would be wise to check the heater valves for a heater cathode short. the plate of 6BE6 should be 200V +/- 20%. The fact they measured that with a 1000 opv meter is of no significance. both types should be the same, not withstanding that some digital's have issues with RF riding on DC.
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Thanks Marcc. Haven't got back to this -taken advantage of the first calm morning for weeks to spray my house for spiders - cobwebs everywhere.
Unfortunately I haven't got an insulation tester, but will measure 240vac and total HT current to get an idea of what might be happening.
I have dug out some spares for 6BE6, 6AQ5 and 5Y3 as quick checks in case there is a crook valve. Unfortunately don't have a spare 6AR7GT.
Re 5Y3 my spec sheet says it can be run sideways if pins 2 & 4 are in a vertical plane. They very nearly are on this radio and on the strength of this I have been working on the radio standing on the transformer end, with no apparent ill effects. At least no blue sparks!
Grid voltage for 6AQ5 is specified at -8.5V for 180 plate voltage, -12.5v for 240v plate. I calculate 43mA through the 175ohm as well. Will double check this resistor.
The cathode is grounded through the speaker transformer secondary on this radio built to the modified circuit. So don't understand "all of the cathode current goes via the back bias resistor."
Re cathode shorts should have checked these when testing but didn't know there was a problem. Swapping the spares might help show up such a problem too if plate voltages jump up.
Correction made above: cathode grounded through secondary, not primary winding.
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Note I made a correction to post 37.
"all of the cathode current goes via the back bias resistor."
Wasn't looking at the total picture. Of course all HT runs to the mains secondary centre tap, through R5, and then to 5Y3 plates and output grid.
The specified total HT current is 43mA for the original circuit. 3V across R5 of 75ohm gives 40mA, not far off.
This is not far from 7.5V across R5 of 175 ohm gives 43mA in the modified circuit.
So the way I see it HT draw is OK, and the loss must be AC somewhere. I don't understand where your 100mA comes from.
Note correction made - should have been mains secondary centre tap, NOT mains primary centre tap.
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Location: Latham, ACT
Member since 21 February 2015
Member #: 1705
Postcount: 2212
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STC830 I have the 3 core cotton covered power cable if you need any. It's 4 bucks a metre plus postage which would be about 7 bucks. Email is unhidden.
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Thanks for reminding me of that. I need something of small enough diameter to run under the radio where the two core went. What diameter is it? Does it have a moulded plug fitted? You might know the problem as I think that you have repaired a 429MA.
I have tracked down 6mm diameter three core at a lighting supplier but at $6/m in dull gold which doesn't look out of place given the gold highlights on the radio. But can't for the moment find a plug suitable for 6mm diameter cable. I noticed a light in the display area fitted with a similar gold cord with a moulded plug, but one meter only, and they don't supply the cord separately.
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Location: Latham, ACT
Member since 21 February 2015
Member #: 1705
Postcount: 2212
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I have cords with the molded plug as well. As far as measurements are concerned I would have to get back to you tonight. The cord is about 1 and a half metres and I'm sure it would fit.
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Location: Latham, ACT
Member since 21 February 2015
Member #: 1705
Postcount: 2212
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Send me a email and I will send pictures
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Location: Wangaratta, VIC
Member since 21 February 2009
Member #: 438
Postcount: 5555
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Nomenclature looks odd. The transformer primary is the one on the mains. The rest are secondaries.
If we look at the original 429MA in AORSM's it has N78 (6BJ5) listed as the OP with R8 75 Ohms as Back bias. However, on looking at that again the parts sheet quotes R4 as the back bias & at 2.5 Meg that's got to be a whole barrow load of crap as its AGC. I should have spotted that.
So what should be across the back bias is the grid voltage of the N78 (-5 250V). 6AQ5 is a much hungrier valve and contains a trap. The pin outs are not the same. If the socket is wired for N78 then pin 7 is the screen of it. If a 6AQ5 is put in there you put B+ onto the control grid 1. That's going to make it seriously unhappy.
6AQ5 grid 1 is both pins 1 & 7. Check the socket to see what it is actually wired for? I've already had a stuffup like that recently in a HMV.
Marc
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Location: NSW
Member since 10 June 2010
Member #: 681
Postcount: 1351
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Transformer primary error: have fixed that in post 38 - sorry.
I am working from modified circuit from the Vol 10 of AORSM which is given in post 18. This has 6AQ5 output, whereas the circuit in the previous edition has N78/KT61.
The original circuit that Kevin Chant has (AWA service data) doesn't appear to call R4 the back bias resistor. So maybe an error in AORSM.
The original circuit has 75ohm back bias for N78/KT61 and 175ohm for 6AQ5. There are also changes to the value of resistors feeding the grid of 6AQ5, to the volume control circuit, and the additionof feedback on the cathode of 6AQ5 by grounding it through the speaker transformer secondary.
I don't think that I have an original circuit radio as mine has the 175ohm back bias, and the volume control and cathode grounding changes shown in the ammended circuit form Vol 10 AORSM.
I haven't noticed any mistakes in base wiring of 6AQ5, but will have a close look at that in case it was mistakenly wired up as for the N78/KT61. It could put the screen grid connection onto the cathode of 6AQ5. Indeed seriously unhappy - would it even work?
Yesterday I checked the total radio power consumption (made easy if you have a incandescent bulb current limiting setup - just take the bulb out and measure AC current between the socket pins). Got 157mA mains current which gives 37.8watt at 240 volts, full volume on a strong local station. Specification is 35watt, so actual current is 8% high. Don't know what to make of that without doing more checks.
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