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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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23rd Oct 2018, 12:38 pm | #1 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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AR88 S-meter and an odd capacitor
Hi again guys
Have been busy replacing capacitors in my AR88D which was previously set up as a R1556 version. Now I have 2 questions for you: - It has the original RCA S-meter which goes from -6 to 100 DB, can anyone give translation from this scale to the more modern S0 - S9+60dB? - While replacing capacitors I found some differences from the schematic diagram: a/ R20 was connected to earth from V5 pin 3 instead of pin 5 b/ a Hunts 4000 nF capacitor connected between the grounded end of R20 and ground at V9 pin 1: What is the purpose of this capacitor? As far as I can see it is not original. Can it be a RAF mod? Appreciate your comments on these items. Jan |
23rd Oct 2018, 1:01 pm | #2 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: AR88 S-meter and an odd capacitor
Hi again
Just adding a picture of the capacitor mentioned in my previous msg. the capacitor can be seen between the 2 oil filled capacitors just under the angled metal plate. Jan |
23rd Oct 2018, 1:22 pm | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
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Re: AR88 S-meter and an odd capacitor
The original AR88 S-meter is very rare.
It's marked "decibels above one microvolt" which makes perfect sense. It's the S1 to S9 business on amateur radio equipment which makes no sense. There is no agreed standard for what signal level S9 means. There isn't even complete agreement on what the difference in level between S points should be. If you measure the signal levels for different S point indications on a range of radio equipment, you'll see massive differences and scales compressed wildly at one end or the other. Also there are large differences in the sensitivity and directionality of different people's antennae. So feel free to give any S-reports you feel like When my AR88 was my main receiver, I used to consider the pointer straight up to be S9 and imagined a linear scale of 1 to 9 in the left half of the meter. It was no more misleading than anyone else's. There are various small deviations from the schematic in lots of these receivers. At one time in the 1950s they rebuilt a lot of them. Look for a label on the right-hand side gusset which braces the front panel to the chassis. If you have the label, it's on the side normally hidden against the cabinet, so you have to pull the cabinet off to see it. Some rebuilds got a PVC wiring harness. David
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23rd Oct 2018, 1:26 pm | #4 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
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Re: AR88 S-meter and an odd capacitor
Just seen your photo.
Nasty leaking 'bathtub' capacitors. THey need changing. Wear oil proof gloves. Do not get the stuff on your skin. It's believed that the oil is notPCB Polychlorinated BiPhenyl type, but no-one is sure. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
23rd Oct 2018, 2:12 pm | #5 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: AR88 S-meter and an odd capacitor
The pin 3/pin 5 discrepancy sounds like a straightforward production wiring convenience, as pin 3 and pin 5 of the 6SG7 valve are both cathode connections- this keeps cathode connection inductance to a minimum and both are connected to ground in the 1st and 2nd RF stages that deal with up to 32MHz, but not in the 455kHz IF stages where lead inductance is far less significant. There has been the occasional report of the pin 3/pin 5 connection failing internally in the valve, resulting in a valve that doesn't work in the IF strip but does in the RF box- I can't see any harm in linking pins 3 and 5 in the IF strip valve sockets- if you can be bothered!
If you check closely, I reckon you'll find that the cathode resistor mentioned actually goes to an isolated stand-off tag there, not a chassis tag, the wires go to the signal strength meter and the capacitor simply decouples the non-grounded wire at IF, Colin |
23rd Oct 2018, 9:17 pm | #6 |
Tetrode
Join Date: Jan 2018
Location: Stockholm, Sweden
Posts: 78
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Re: AR88 S-meter and an odd capacitor
Thanks David and Collinear for your comments which cleared a a few things for me, as to the "bathtub" capacitors - they have been rebuild already with new capacitors but I kept the original look.
And Collin you are right abt the the standoff tag, I thought it was a ground point as I got a 0 Ohms reading when measuring from that point to the chassi, but now when I know why the capacitor was there I will replace it with a new one. Jan |