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Mohican question, capacitors.
01-21-2016, 10:28 PM (This post was last modified: 01-21-2016 10:30 PM by Geoff Quickfall.)
Post: #21
RE: Oscilliscope question
HeathKit was a dream at that time although I had a lloyds shortwave receiver which did well. Still have it, it's in the garage. Did a lot of soldering and wiring back then but never the kits.

It just never looked techy/military enough. :-)

So now I can get heathkit.

1) It is disassembled down to the boards.

2)Face, glass bezel, tuner cords, transistors marked and removed.

3) Speaker out, bad solders cleaned up and repaired.

4) Tuners cleaned, as well as selectors.

5) Boards washed and cleaned including transistor sockets.

6) deoxit contact, volume and tuner preservers to be added. Deoxit gold for the power socket and transistor sockets. New tuner cord to be added.

7) Wax the case an metal front fascia.

8) move the handle from the side to the top.

At step 5 and house chores todo! So on hold.

Then reassembly with new caps, speaker and mounting bracket.

Should be done next week.

cheers, geoff
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01-21-2016, 11:51 PM
Post: #22
RE: Oscilliscope question
(01-21-2016 08:20 PM)Dave Frederickson Wrote:  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

LOL!! It's all about passion isn't it? After all, the keys are most likely needed to go to work...

--Bob Prosperi
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01-22-2016, 02:10 AM
Post: #23
RE: Oscilliscope question
They seem to be very slowly coming back to life. http://www.heathkit.com/

In the early 1980's I had over 20 Heathkit products. I still have many of them, and still use some of them. I gave a few away.

http://WilsonMinesCo.com (Lots of HP-41 links at the bottom of the links page, http://wilsonminesco.com/links.html )
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01-22-2016, 02:44 AM
Post: #24
RE: Oscilliscope question
(01-22-2016 02:10 AM)Garth Wilson Wrote:  [Heathkit] seem to be very slowly coming back to life. http://www.heathkit.com/

Sure enough!

Geoff, if you check here you'll discover a vintage GC-1A manual in "very clean" condition for $20 USD!

Dave
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01-22-2016, 04:15 AM
Post: #25
RE: Oscilliscope question
(01-21-2016 08:20 PM)Dave Frederickson Wrote:  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

Small world. It was very close to that address that I bought my first computer in late 1979. It was an Ohio Scientific Challenger 1P for $350. The store was an Ohio Scientific dealer (at least I didn't see any other brands of equipment there).
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01-22-2016, 05:39 AM
Post: #26
RE: Oscilliscope question
(01-22-2016 04:15 AM)Steve Simpkin Wrote:  
(01-21-2016 08:20 PM)Dave Frederickson Wrote:  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

Small world. It was very close to that address that I bought my first computer in late 1979. It was an Ohio Scientific Challenger 1P for $350. The store was an Ohio Scientific dealer (at least I didn't see any other brands of equipment there).

It is indeed! The first computer I bought was a SWTPC 6800 around '77, another kit!
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01-22-2016, 08:03 AM
Post: #27
RE: Oscilliscope question
(01-22-2016 02:44 AM)Dave Frederickson Wrote:  Geoff, if you check here you'll discover a vintage GC-1A manual in "very clean" condition for $20 USD!

Dave

Thanks Dave, picked one up two weeks ago from them, waiting on the snail mail. In the meantime I have been using the one you linked to previously. Got the front three facia plates off, marked and removed all transistors for cleaning and testing.

Will post a picture when it is fully restored. Those knobs are beautiful quality!

Geoff
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01-22-2016, 09:30 AM
Post: #28
RE: Oscilliscope question
Hi Geoff,
Out of curiosity do you know the IF filter frequency on that unit ??,
Never had chance to work on much Heathkit gear, but always interested in what these guys spec'ed' up.
One my hit list would be the Heathkit GC-1000 clock.
Ray
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01-22-2016, 10:29 AM (This post was last modified: 01-22-2016 10:32 AM by jebem.)
Post: #29
RE: Oscilliscope question
(01-22-2016 09:30 AM)vk6ti Wrote:  Out of curiosity do you know the IF filter frequency on that unit ??,

Accordingly to the manual indicated by Dave, it should be 455 KHz.

Now, what better equipment than an Heathkit IGW-102S to do the receiver alignment?

Check the last 8 pictures from my specimen - mine was made in Canada, even has s/n on the back, this one could not be a kit.
Works nicely, just needed calibration as usual after all these years (I believe it was made in 1968 because one capacitor has got a date of 1967 week 52).
The previous company owner has installed a BNC connector to make the equipment more usable.

Jose Mesquita
RadioMuseum.org member

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01-22-2016, 06:13 PM
Post: #30
RE: Oscilliscope question
Hi Ray,

No Sydney's for awhile, maybe feb or later. Up for a meet? Will let you know and send me the dates you expect to be in Sydney. Also, the GC-1000 is a neat system. I have two, one for spares and the other running. Someone out there makes EPROM replacement chips for the synching receiving and our Jim Johnson is the moderator for a the GC-1000 Yahoo forum.

Jebem:

I use a 1957 HeathKit RF Signal generator similar to this one:

http://biggerbids.com/auction-image-gall..._id=724565

The Mohican:

Today, polish case, new speaker wire with two pin plug halfway, just in case I have to get back into the radio, saves soldering. I will temporarily install the 8 ohm speaker for testing purposes.

Spent quite awhile reading up on speaker impedance, watt ratings, the theory and etc. Sure didn't teach me this in Botany/forestry or palynology at university!

The Mohican, GC-1000, the signal generator on a shelf together along with the HP 59309a (http://www.hpmuseum.net/display_item.php?hw=741) and my HP 1202a oscilloscope make for interesting shelf mates.

?????

Now for another Mohican question. The speaker wire is a two part shielded wire. The outer braided shielding is for side of the speaker conducting wire, the inside is the second. It does not appear to me that the shielding is functional. Secondly, none of my other shortwave or tube radios (zenith transoceanic 7000-3000, Grundig 4035 and others) have shielded speaker wire. The wire was in bad condition (really sloppy solder joband to be expected on the braided side) and needs to be replaced. After reading about shielded speaker wire for sound systems (different application) is the shielded cable required or can standard speaker wire work? Even the GC-1000 does not have shielded speaker wire and no transient hum from the ac circuitry is picked up.

Geoff
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01-22-2016, 06:37 PM
Post: #31
RE: Oscilliscope question
(01-22-2016 06:13 PM)Geoff Quickfall Wrote:  can standard speaker wire work?

Fig. 29 of the manual illustrates the speaker wiring using a shielded cable. Any wire will work, but Heathkit probably used shielded cable to minimize hum.

Dave
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01-22-2016, 07:00 PM (This post was last modified: 01-22-2016 08:01 PM by Geoff Quickfall.)
Post: #32
RE: Oscilliscope question
Thanks Dave,

Will try standard wire and if there is a hum will replace it. Funny thing is, under the board it is exposed for an inch and a half. Guess they thought the case shielded it from the AC side.

Geoff

P.s. Would it not be better to have the wire be a three piece part, two internal for the speaker and one shielded surround which would be grounded to the chassis by circuit and the second grounding point at the chassis by the speaker?
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01-22-2016, 08:46 PM (This post was last modified: 01-22-2016 09:08 PM by jebem.)
Post: #33
RE: Oscilliscope question
(01-22-2016 07:00 PM)Geoff Quickfall Wrote:  P.s. Would it not be better to have the wire be a three piece part, two internal for the speaker and one shielded surround which would be grounded to the chassis by circuit and the second grounding point at the chassis by the speaker?

Hi, Geoff,

It was common in those days to connect one of the two speaker coil terminals to its metal frame (those speakers had in fact three terminals, where one of them was riveted to the metal frame - example in a Toshiba receiver that I restored recently).

The shielded cable would then have its shield connected to the speaker metal frame connector, and at the other side of the cable its shield would be soldered to a chassis ground terminal.
This arrangement only makes sense when the speaker have a large metal area and it is close to sensitive components.
Also the radio enclosure, being metallic, should form a single ground surface (or shield).
This is not only about hum (audio interference) but also about RF interference caused by people touching the metal parts of the radio, for instance.

Killing hum/RF interference is kind of a art on its own.
Too much shielding sometimes leads to great hum, for instance when we create ground loops (without even knowing it).
Basically experimentation is the key, along with some best practices.

Jose Mesquita
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01-22-2016, 10:10 PM
Post: #34
RE: Oscilliscope question
Thanks Jebem.

Interestingly this is a two part shielded cable, one for each speaker terminal. I understand the RF side of the shielding. Also that shielding can form a capacitor and on an ac powered speaker system can wreak havoc on the amp.

Off to electronics shop Monday for the four electrolytic caps and what the heck, some shielded speaker wire.

Our hand microphones jacks on the B777 are situated by the 120v 60Hz power wall plug. When a cheap wall wart is inserted the mic's are erratic. Aviation bulletin, " when wall wart adapters plugged into wall plug hand held microphones cannot be used". Due of course to stray RF emanating from badly shielded wall warts.

Geoff
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01-22-2016, 10:50 PM
Post: #35
RE Mohican capacitor question? Dave, Jebem?
Looking into the three electrolytic caps in the radio.

-On the GC1a circuit board schematics page 38 are two electrolytics; a 50 mFd-15v and a 150 mFd-15v. On my board these are 100 mFd replacing the 50 mFd and 250 mFd replacing the 150 mFd.

-Referring to figure 25 and page 54; fourth paragraph down on the left the capacitor should be 50 mFd but mine is rated at 220 mFd.

Of course these are outside a general rule of thumb for replacement values of 10%; ie 100 mFd could be replaced with a 110 mFd with no adverse affects as long as the voltage is the same or greater.

Looks like mine was recapped but with grossly wrong values or does someone know something the manual doesn't? These seem to be outside the 10% range.

But the way, I do have the GCA1.

Cheers, Geoff
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01-22-2016, 11:20 PM (This post was last modified: 01-22-2016 11:30 PM by Katie Wasserman.)
Post: #36
RE: Mohican question, capacitors.
It looks like the 150 uF is used as a bypass cap for the common emitter circuit driving the output transformer.

One of the 50 uF caps connects the volume control to the base of the same transistor, I think that's the one you're referring to on the PCB. The other 50 uF cap connects to the headphone jack and is the final output capacitor. These are both in the signal path.


The 150 uF cap can be a larger value without causing any issues. It will help drive the speaker at lower frequencies.

The 50 uF values can be a larger value but you might find that you're hearing more 60Hz hum with a larger values. They should both be replaced with new 50 uF one, as specified.

Note the polarity of both capacitors on the PCB, it's important to get them right. The output cap seems to me that the schematic has it shown backwards. It probably won't make much difference but ideally this one should be a bipolar cap, I think.

-katie

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01-22-2016, 11:52 PM
Post: #37
RE: Mohican question, capacitors.
Thanks Katie!

I will recap all three too the book values. Just wanted to know if there was a mod I didn't know about. I will replace them as they are inserted as far as the polarity goes as the radio was working prior to my refurbishment.

Let me check the polarity of the caps versus the wiring diagram, remember the radio is positive ground, would that make a difference in the diagram polarity alignment?

Geoff.
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01-23-2016, 12:13 AM
Post: #38
RE: Mohican question, capacitors.
(01-22-2016 11:52 PM)Geoff Quickfall Wrote:  remember the radio is positive ground

The negative terminal of the battery goes to pin 6 of the Power Socket, which goes to ground. Why do you say it's positive ground?
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01-23-2016, 12:14 AM
Post: #39
RE: Mohican question, capacitors.
Hi Jebem,
Thanks for that, apart from the double conversion types, some of the older ones
still went 500 kHz, ( so they could pick up distress signals through IF's no matter what you had on dial ), but I have only come across it once,

Hi Geoff,
Just got back from Sydney last week, so may be next time, I won't be back there for 3-4 months or so, still like to have a look at that HP41CL you had last time.
Re speaker, thought about an impedance transformer ?, sometimes gets you out of trouble or suitable for testing purposes till you find the correct item

Did you have success on the gold trace and the HP-01 boards ? good luck if you did :-)
Cheers
Ray
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01-23-2016, 12:25 AM
Post: #40
RE: Mohican question, capacitors.
Dave,

Researching the Mohican on line I came across the following:

http://www.antiqueradios.com/forums/view...5&t=187634

Second post from the top. A few other sources also.

Ray,

Sydney at the end of February. Will bring the 41cl next time I see you. Haven't used the gold trace paint yet, expensive stuff!

Cheers, Geoff
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