Mohican question, capacitors.

[Geoff Quickfall]

I just picked up an HP 1202a and a newer Hameg 203-7. The Hameg is 60 mHz (corrected) and I have 4 new 100 mHz probes.

I cannot see why the 100 mHz probes cannot be used on the Hameg. The 100 look more capable with higher tolerances for voltage. Is that true or will they distort the results.

Geoff

On another note, I need a 6 inch oval 34 ohm speaker for an Heathkit Mohican short wave. Will a lower homage speaker with an inline resistor in series and totalling 34 ohms work. Keep in mind this is a low wattage receiver and heat generation by the resistor would not be great.

I know it is the coil that should be 34 ohms.

[Dave Frederickson]

Geoff,

I see no reason why the 100 MHz probes shouldn't work. Just remember to compensate the probe by connecting it to one of the CAL terminals and adjusting the variable cap in the probe connector, usually visible through a small hole, until the square wave corners are square and not rounded.

Regarding the speaker with 34 ohms of impedance, adding a resistor won't accomplish your goal. What might work is to add an inductor which together with the impedance of the speaker will total 34 ohms.

To help, the formula for inductive reactance X(L) = 2 * PI * f * L, where L is the inductance of the speaker coil and f is 400Hz, the frequency at which speaker impedance is calculated.

The impedance, Z, is SQRT(R^2 + X(L)^2) where R is the DC resistance of the inductor or coil.

Yes, it does look like the formula for the hypotenuse of a triangle, because the DC component is one side, the AC component is the other side, and the impedance is the hypotenuse.

I think.

Dave

[Paul Berger (Canada)]

(01-20-2016 11:41 PM)Geoff Quickfall Wrote:  I just picked up an HP 1202a and a newer Hameg 203-7. The Hameg is 60 MHz and I have 4 new 100mHz probes.

I cannot see why the 100mHz probes cannot be used on the Hameg. The 100 look more capable with higher tolerances for voltage. Is that true or will they distort the results.

You are right 100MHz probes should not be a problem on a 60MHz scope one of the issues you may encounter is the probes need to be matched to the input impedance of the scope. Most scopes have a 1 megohm input impedance but there are exceptions. Most probes also have a trimmer capacitor in them to reduce distortion.

Quote:Geoff

On another note, I need a 6 inch oval 34ohm speaker for an Heathkit Mohican short wave. Will a lower homage speaker with an inline resistor in series and totalling 34 ohms work. Keep in mind this is a low wattage receiver and heat generation by the resistor would not be great.

I know it is the coil that should be 34 ohms.

Putting in a series resistor would protect the output transistors from overload, but I would expect less volume and likely some distortion. For instance if you used an 8 ohm speaker and a 26 ohm resistor more than 3/4 of the output voltage will be dropped in the series resistor. The rating on a speaker is not straight DC resistance it is AC impedance at a specific frequency.

Paul.

[Geoff Quickfall]

Fantastic info guys. Thanks!

Newbie with the Oscilliscope, hence the questions! Read both the scopes manuals and see the setup and calibration pages. Will work through back to front and a couple of Hameg tutorials.



Off to Hong Kong, I know s vintage electronics shop in MonKok including the electronics second hand market. Might be able to salvage or find a speaker there, otherwise one inductor coming up!

Cheers, Geoff

[Dave Frederickson]

Here's the manual for the Heathkit Mohican. I see that the speaker is 4"x6" oval, 35 ohms. Well that changes everything! The part number is 401-33.

[Geoff Quickfall]

Dave,

Two things, firstly; after searching hours for every variation of Mohican, manual, user manual, Heathkit m annuals, I could only find a few instruction pages. Boom, you find the manua! thanks!

Secondly, yes 35 ohms :-) not 34; now to find it! Still I will see what I can find, there is a speaker shop in MonKok that may have the appropriate version, or at least the correct size.

Best regards, Geoff

[Dave Frederickson]

Hi Geoff,

Okay, here's a whacky idea. Find two 16 ohm speakers, remove the coil from one and wire it in series with the other, giving you a 32 ohm impedance speaker. Close enough to 35 ohms.

Dave

[Garth Wilson]

Speakers' impedance may be partially capacitive or inductive depending on the frequency of a test tone relative to where the speaker's several resonances and anti-resonances are, and the resistive component won't be the same from one frequency to another either. I've seen Nyquist plots of speaker impedance that were more or less a spiral centered on the X axis as you sweep the frequency range. As far as cutting the cone off a coil, that will change the coil's electrical properties. The best solution would be a transformer. Otherwise, putting a resistor in series may be a good idea. Going straight into the 8Ω speaker with no resistor shouldn't be hard on the amplifier as long as you're not requiring much power out of it.

[walter b]

(01-20-2016 11:41 PM)Geoff Quickfall Wrote:  The Hameg is 60 MHz and I have 4 new 100mHz probes.

I cannot see why the 100mHz probes cannot be used on the Hameg.

Geoff, please note there's a factor of 1 American billion (i.e. 10^9) between MHz and mHz. You know me - ISCNR. Regards to your grandpa (the one we ate last time).

d:-)

[Katie Wasserman]

(01-21-2016 03:52 AM)Geoff Quickfall Wrote:  Secondly, yes 35 ohms :-) not 34; now to find it! Still I will see what I can find, there is a speaker shop in MonKok that may have the appropriate version, or at least the correct size.

32 ohm speaker were quite common and will work just fine in your receiver. Finding the exact size might be more of a challenge, check ebay for some possibilities, here's one.

[Geoff Quickfall]

Arghh Walter, typo, MHz.

Cheers!

[Geoff Quickfall]

Katie, thanks,

The cabinet has 4 welded studs for a 4 by 6. The previous owner stuck a 4 by 4 eight ohm on by two screws, not up to my OCD standards.

32 sound fine, I will try to source that.

Geoff

[Katie Wasserman]

Here's a modern 32 ohm one, that according to the part number of 6.5". It might be a decent fit.

[jebem]

Nice radio!

Finding high impedance speaker drivers with that sizing is not an easy task anymore.
Good hunting!

The impedance here is that critical for this application.
35ohm with 20% tolerance is not an issue on that output circuit.

By the way, make sure the output coupling capacitor is is good condition BEFORE connecting a new speaker (at least it must not be in short circuit on DC - a common multi-meter in DC resistance can test for this - or else the speaker will most probably burn very fast, considering its low power handling).

Meanwhile, if you don't mind a little diy , this is what I would do:

Get a set of four common easy to find small low power (0.5Watt or less) 32ohm speaker drivers.
These are very common on 4cm diameter sizes. See here and here or here.

Create two pairs of two speakers in series and connect them in parallel for a total of 32 Ohm.

Make a wood plate buffer with 4 x holes for the speaker drivers.
You will end up with a bigger sound than what it seems possible from such small drivers.
However the audio bandwidth will be somewhat limited specially on the bass but I guess that is not an issue on these radio applications.

See the draft picture for electrical details.

[Dave Frederickson]

After examining the audio amp circuit on p.9 of the manual I'd like to offer another solution. Replace C59 with an alternate value cap to maintain the RC time constant of the cap and speaker impedance. So for a 16 ohm speaker, the alternate value would be (50uF * 35 ohms)/16 ohms = 109uF, so a 100uF cap would come close to maintaining the original RC time constant.

Note that the phone jack expects headphones of the same impedance as the speaker.

Dave

[Geoff Quickfall]

Wow,

Solutions every where,

Firstly, all the caps are being replaced, SOP (standard operating procedure) for the radios that I do.

Secondly, looks like a 4 by 4 32 ohm speaker mounted on a wood or metal 4 by 6 bracket which I will make here. The bracket will have some spacers at the four screw points so the bracket can accept 4 small nuts and bolts. Lots of room under the cabinet ceiling.

Jig saw and metal cutting blade with a used aluminum clip board as a metal source for the bracket.

So I think I have Jebems' bracket idea, Katies'speaker sourcing, Daves' manual plus capacitor recommendations and Walters speeling corrections. Walter, damn auto correct keeps switching mHz to MHz and spelling to speeling!

Thanks All!

P.S. MonKok on Saturday not today, oops, misread my schedule, at least I didn't miss the flight!

[Geoff Quickfall]

Dave,

My first headset for my shortwave receiver and transistor radio used when mowing the lawn in 1969 (I was twelve) was fashioned out of two transistor radio speakers and a coat hanger with cotton batten for padding.

Looked goofy but they worked!

Today is disassembly and washing day, maybe a few caps will be replaced. The cabinet is going to be washed, cut polished and waxed. No chips or scratches on it. The plastic reverse painted dial will be plastic polished on the front only, of course to remove fine scratches. Knobs will be polished and the red paint reapplied to the index triangles.

Nice eye candy, especially sitting beside my restored tubed HealthKit signal generator circa 1947. Will be using deoxit on all transistor connection sockets and of course the tuner contacts, volume control and switches after the cleaning.

Geoff

[jebem]

If I may, and for the sake of documenting as many aspects as possible about this issue:

Why did the radio designers used high impedance speaker drivers (like 35 Ohm here) in the 60's radios?

It's about saving money.
They traded the audio output transformer for a high impedance speaker, and at the same time they kept using small current Germanium transistors.

The used 2N407 with its maximum current of 70mA is not really a power transistor able to drive direct speaker loads as we see as the standard design since the mid 70's.

Remember, in the 60's it was a standard practice to use two audio transformers, one driver (as used in this radio), and one output (missing in this radio design).

So the obvious solution here was to use a high impedance speaker driver, in order to maintain the output transistor's current as low as possible and under its admissible operation maximum ratings.

If they had employed a output power transformer, it would be able to use low impedance speaker drivers, without overloading their choice of output transistors.

They also could have used a low impedance speaker driver if they had selected Germanium power transistors.
However that solution would be more expensive than using an output transformer (Remember, in the 60's the Germanium transistors were replacing the valve/tube technology but they were expensive specially the high current power versions).

So, on this specific radio design:
The issue I see on lowering the speaker load is that the output circuit will be current overloaded and this usually will result in an large increase in distortion.
That output circuit was not designed to work with low impedance speakers.

[Geoff Quickfall]

Katie,

Looks like the tv speaker is the best bet, I can flatten the connecting flanges and mount on the bracket I will fabricate.

It is on the watch list and I will keep searching.

Thanks!

[Dave Frederickson]

(01-21-2016 06:07 PM)Geoff Quickfall Wrote:  My first headset for my shortwave receiver and transistor radio used when mowing the lawn in 1969 (I was twelve) was fashioned out of two transistor radio speakers and a coat hanger with cotton batten for padding.

In '69 I was thirteen and assembled my first Heathkit, coincidentally, a shortwave radio. I remember the drive to Heathkit at 330 E. Ball Rd. in Aneheim. Strange I remember that address but forget where I put my keys.

Dave