Mowin' Down The Enemy: A Diversion

It is high summer here on the prairie and a young man's thoughts turn to yard work and how hateful a thing it is...not to mention it is mid July and that means mid October is only four months away.

A couple of things happened this week that were of interest and may prove useful in a technical sort of way.

Coming back from the grocery store last week I spotted a rather nice looking Toro walk behind mower parked at the curb. After we unloaded the groceries and put them away I got in the pickup and headed back and the mower was still there.

I knocked on the door and asked if the mower was unattached and the nice couple said "Take it, please." So I swung the tailgate down and off we went with a 2009 vintage self propelled walk behind mower. It appeared as if the recent heavy rains we had had briefly flooded the mower

I got it home and it did have compression which was good. After taking off the air filter so that I could dry it out I waited until after dark to see whether there was spark, and to my surprise there was.

Pull the spark plug out, then connect the plug to the wire and turn the engine over a few times by hand. If there's spark you'll see it. If not, you're going to have to get a magneto coil, but they're cheap enough on fleabay for the imported ones.

But it wouldn't start, so the next day I drained the fuel or what was left of it, pulled off the carburetor, blew out the jets and needle and seat, and put it back together.  It started on the first pull but  there was plenty enough emulsified oil and water in the crankcase to drain out and flush with a bit of gasoline. Then I put the air nozzle in and let it have a good proper blow job to air things out.

This is a pretty typical Briggs carburetor. It's got only three passage ways. Carefully detach it from the governor and choke linkage. Remove the bowl and clean it. Take some carburetor spray cleaner and blow out the main jet and idle jets and the fuel inlet passage where the needle and seat goes. check the operation of each passage with a bit of compressed air. Make sure the float is not sunk and the needle shuts off fuel when the float's up. Clean out the sediment well in the fuel bowl drain plug.

That's all there is to it.


Fresh oil-about a pint of detergent 30W- and fuel, and all seems well. I've got a good solid mower for nothing except an hour's worth of work. The drain plug is a 3/8 square pipe plug that sits in the bottom of the crankcase, or you can tip the mower on its side and drain the oil through the filler neck.

While all this was happening the other mower, the Snapper Ninja self propelled number that has been my workhorse for the last five or six years-it came out of an auction for twenty five bucks and it is at least 25 years old- started having problems with the transmission drive.

So I got out the parts manual and ordered some transmission bearings, a drive belt, an idler pulley and a drive disk from Snapper-which happens to be the cheapest source of parts I found.

After cleaning off the rust and dead grass and treating the deck with some rust stopping compound, I reassembled it yesterday and today with some new hardware, and while I was doing that I cooked up a good workaround that had never occurred to me before.

Clean off the corrosion and dead grease that coats everything. Buy your replacement carriage bolts at the hard ware store for a quarter instead of paying $2.35 for one plus shipping.

This mower has two springs that tension the drive mechanism components. Once the drive is assembled you gotta get the springs on but it proved difficult-until I wrapped some safety wire around one end of the spring and grabbed it with my safety wire twisters and pulled it through the hole. Nice.

Take a good look at the springs number 39 and 54. While you're at it look carefully at 73, which is a twisted piece of coat hanger wire that some people want twelve dollars for.

That went swimmingly, and the drive mechanism is functional and working smoothly.

Tomorrow I've got to see about changing both inner tubes on the snow blower's tires, change the oil and drain out the old gas and get it ready for winter. Then it's time for the pressure washer, the string trimmer, the chainsaw and likely the leaf blower too.

Converting a 50 watt Marshall Plexi From 6550s to EL34s The Right Way

A vintage Marshall "Plexi" arrived here that was in bad need of some refreshment. These are pretty much the ultimate rock and roll amps-loud, brash, edgy and in your face. So they are well worth the effort to bring them back to life.

In no particular order it needed a fresh suite of electrolytics and some general freshening up before retubing and troubleshooting.

It was an ebay find I think.

As part of my standard procedure I noted that someone had "converted" it in a prior time to use EL34s but it didn't have any bias voltage, which makes me think that's why whoever had it gave up on it a long time ago. They didn't "convert" it either-just some half assed hack job.

Amps that were destined for the USA were equipped with 6550 power tubes by the distributor in the US which was, I think, Unicord. They changed the sonic signature of the amp, and it didn't sound very British any more.

There are four steps that must be taken to effect the conversion.

First, the bias splitter resistors should be changed from 150k to 220k metal film resistors. For some unknown reason the board was marked 82k.

This is relatively easy to desolder as all these things are by lifting up the board and putting a small light inside, like one of those LED lamps you strap on your forehead. Then you can see what you're doing.

 The 47k resistor should be changed to 22k. Don't bother with mickey mouse scabbing resistors on top of one another as some folks suggest.

It looks crappy. Do it right.

The two 10uf or 8uf 160v electrolytic capacitors need to be replaced. They do not last forever.

Now here comes the tricky stuff.

Install a 220k bias level resistor as shown on the schematic coming from the high voltage tap on one side of the power transformer to the  bias supply diode. Set the bias pot to the lowest setting. You can set up your bias checker with a set of power tubes, make everything ready and briefly throw the standby switch on, take a quick reading and then put it back on standby because you're probably drawing too much current.

Now, take two test leads and parallel the 220k resistor to reduce the bias level down to about half. Then you can see whether you're close enough to do a fine adjustment with the bias pot. If you are, measure the paralleled resistance and find something relatively close.  I ended up with a 100 k resistor. Or you could start with a 100k resistor, or you could fine tune it with a pot and then measure the total paralleled resistance.

The last step is to relocate the negative feedback wire from the 16 ohm speaker out[ut tap to the 8 ohm speaker output tap-which hadn't been done in this amp.

Once you've done all this check and check your voltages again before you button it up, and do a sound check.

You might find, as I did, that this amp has an oscillation problem which is likely how it got shelved to begin with.

If you're ready to button it up, clean off all the flux and schmutz with some anhydrous isopropyl alcohol and an acid brush cut off to about 3/8 inch. It makes a dandy cleaner.

Rock on.