Joe,
I wonder why the 260 showed that or was it just set on the wrong ohms scale? I
have about 3 of those 260's here and absolutely swear by them. They have to be
the toughest analog VOM and most accurate I've ever used. Matter of fact, it
was the only meter I ever bought new. The one I did buy new I've had since
1987. Heh, I did burn it out once, really frying it. I had my radio-TV shop
then and had a Maco 750 11 meter amp there converting it over to 6 meters for a
friend. It was bad when he got it. Well, before I started work, I had been
servicing a TV set and had blew the fuse in the 260. So, I couldn't find a fuse
and really couldn't leave the shop to get one. Stupid me went and took some
aluminum foil out of a Marlboro pack and wrapped the old fuse as a temporary
fix. Sure enough, I set down and started to work on the amp. Needless to say, I
forgot to switch the 260 from ohms to volts and stuck the hot lead into +900
Vdc. I took it to my local electronics supply and it was st
ill under warranty. Believe it or not, Simpson put in a new main pc board at
no cost, just the shipping. Plus, did a new calibration on it. I've stuck with
that company ever since.
If you want a good little small bench meter, Simpson made a model 467 and a lot
was sold to the US government. They come in a small attache case including a
lead kit, a 5 kV high voltage probe, and an ammeter shunt. I use it to check
plate voltages all the time with a very good accuracy. You can pick these up
surplus off eBay for about $25-$45 at any time. If anyone would want a good
small digital meter, I'd sure suggest this one.
Just the other day, Simpson sent me a complete manual on a model 713 frequency
counter, schematic and parts list included. This cost me $0.00, and is why I
think Simpson is at the top of the heap on product and service.
On the amp, you should get a normal reading for a NPN transistor when you lift
the base and collector up off the pads. There's too much circuitry to read them
attached to any benefit unless it's a voltage reading.
Best,
Will
----- Original Message -----
From: "Joe Isabella" <n3ji@yahoo.com>
To: "Will Matney" <craxd@engineer.com>
Subject: Re: ALS-500 Problem (was Mobile amps)
Date: Sun, 24 Jul 2005 07:26:46 -0700 (PDT)
>
> Hi Will,
> I just found out that the VOM (a Simson 260!!) was lying to me
> about resistance. I lifted the base of the 2879 and it showed
> open, but I was still measuring a direct short on its pad. I
> lifted the 6.8 ohm resistor and it was measuring a dead short, so I
> checked a ~1k I had in the junk box and it also showed short!!
> Anyway, the base is now showing ~19 ohms to ground, and is not
> shorted to the emitter (whew!!). So, now I'm back to square one.
> I'll put the amp back in and see what's REALLY going on...
>
> Joe
>
>
> Will Matney <craxd@engineer.com> wrote:
> Joe,
>
> I'd have to look over the schematic on it to see what they've done.
> I have seen relays cause this by restricting the RF into the amp
> but you'd have a high SWR between the amp and transceiver. Without
> knowing what their bias circuit is, I couldn't say on the base to
> ground short. You have to remember that usually there are 10 ohm
> resistors to ground anyhow. They should be soldered right on the
> base lead to the foil ground. You figure one off each transistor,
> that's 5 ohms to ground. Also, if it's a double sided pc board,
> suspect a solder joint where it goes through the board to the other
> sides circuit. Really the proper way to check those transistors is
> to lift the base and collector legs from the board and test them.
> If I can find the schematic, I'll look it over to see how it ticks.
>
> Best,
>
> Will
>
>
>
>
> ----- Original Message -----
> From: "Joe Isabella"
> To: "Will Matney"
> Subject: Re: [Amps] Mobile amps
> Date: Fri, 22 Jul 2005 08:45:24 -0700 (PDT)
>
> >
> > Will,
> > eMailing you off-list... Do you know anything about the ALS-500
> > by Ameritron? I just started having a strange problem and I
> > thought I'd ask around for some advice. Since you brought up this
> > subject, I hope you don't mind me bothering you.
> >
> > The amp was working one day until I stopped using it, then I
> > attempted to run it later and could get no replies. When I looked
> > at the current meter on the front of it, it was no longer drawing
> > much current. It is showing a little current draw, though (less
> > than 5A or so, which only shows up with RF drive). It has the
> > 2879s, and there is no collector short to ground, but I'm seeing
> > what appears to be a direct short on the base. I haven't had a
> > lot of time to troubleshoot it, but does that mean there's a
> > base-emitter short?? Is that a common failure mode? I had planned
> > to isolate the transistors and re-check them, but I just haven't
> > gotten to it yet.
> >
> > Any thoughts?
> >
> > Joe, N3JI
> >
> > Will Matney wrote:
> > Dan,
> >
> > Yes, they are really the same wide-banded amps that have been
> > around for years and the same as in the Motorola app notes. Most
> > use the 2SC2879 transistors. The only thing is for them to be
> > linear, and clean, they have to be converted to class AB1 or AB,
> > an output filter should be used, and a bandswitching network
> > added to the input to level the SWR across the bandspread.
> > Generally these are made by the one doing the conversion as
> > there's too many differences in amps to make a one-size-fits-all
> > type of circuit. They generally make them using switchable pi net
> > circuts switched in with relays from a rotary switch on the front
> > of the amp. Henry did this on their mobile amps and is the upper
> > board over the main one. These 11 meter amps dont have this. What
> > I'm making is a good RF keyer, and the bias circuit to set the
> > transistors to class AB, or AB1. The keying circuit they've been
> > using on a bunch of different models just tie the parts together
> > in mid air, and look like a rats ne
> > st. Terrible construction, but the rest of the amp is usable.
> > Another thing that will have to be used is an attenuated input as
> > 5-20 watts will open up a set of 2SC2879's to their rated output.
> > That is if the transceiver can't be cut back. However an
> > attenuator is easy to build. You can buy these amps second hand
> > off eBay for a good bit less than new. I may even build some new
> > ones but that's a good bit in the future. For homebrewers, I can
> > get the cabinets, and heatsink along with other parts they may
> > need. There is also pre-punched PC boards but not etched. The
> > punched holes are for the transistors. These holes can be made
> > though by drilling the board first, and then cleaning up the rest
> > using a spiral bit in a mini-router, or a Dremel tool with a
> > router base added. The best drill to use is one of those cone
> > bits. The way they've been building them is by dead bug
> > construction. I prefer to use an etched board myself, but do use
> > surface mount with through hole components by
> > bending the leads. Those amps are too hard to repair without the
> > parts mounted on the top, and unsoldered from the top. Those
> > traces can be routed too if you have a steady enough hand. Hope
> > this helps.
> >
> > Best,
> >
> > Will
> >
> >
> > ----- Original Message -----
> > From: "Dan Sawyer"
> > To: "Will Matney"
> > Subject: Re: [Amps] Mobile amps
> > Date: Fri, 22 Jul 2005 06:59:46 -0700
> >
> > >
> > > Will,
> > >
> > > Is it practical to adapt these to work on 80 through 10 meters?
> > >
> > > Thanks,
> > > Dan
> > >
> > > Will Matney wrote:
> > >
> > > > All,
> > > >
> > > > Some have been buying those 11 meter mobile amps and
> > converting > > them over for amateur use. The ones which use
> > un-etched PC > > boards, etc. The RF keying in these are
> > generally rats nests and > > pretty weak circuit wise. Most dont
> > last very long and are for > > use only with about 5 watts max
> > input and have no SSB > > capabilities. I am now offering
> > retrofit kits for RF keying, and > > bias control. Most are made
> > class C only and the ones with any > > bias are not adjustable
> > only using a diode clamp. I can do > these > on tie strips or on
> > small PC boards made to mount upright > on > edge. The tie strips
> > take up the less room. If any are > interested > in these, give
> > me an e-mail. I'll be glad to help in > converting > these in any
> > way possible.
> > > >
> > > > Best,
> > > >
> > > > Will
> > > >
> > > > Amateur Transformer & Supply
> > > >
> > > >
> > > >
> >
> >
> > --
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