Mohican question, capacitors.

[Katie Wasserman]

(01-22-2016 11:52 PM)Geoff Quickfall Wrote:  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?

It sure would. There's even a note on the schematic: "all voltages are negative with respect to chassis ground". So yes, the polarity on the electrolytic capacitors are correct on the schematic, my mistake.

[Geoff Quickfall]

Thanks Katie,

Shopping list is ready, now to restring the tuner and bandwidth tuner.

Geoff

[jebem]

To complement what Katie have said:

Looking into the page 12 of the schematics indicated by Dave, we have the following eight electrolytic caps:

AVC (aka AGC) automatic volume control (page 8) and RF OSCillator stage (page 7)
- 2 x 10uF at 10V (C27 and C48 in the schema)

Power supply lines decoupling and filtering (page 9):
- 1 x 100-100-100uF at 15V -> this is a can with 3 caps, each one of 100uF at 15V. (C56, C57, C58)

Audio amplifier and output power stages (page 9):
- 2 x 50uF at 15V (C54 and C59)
- 1 x 150uF at 15V (C55)

This would sum up to a total of 8 polarized caps.

Concerning their capacitance values, electrolytic caps have an extremely high variance, even for recent production. Minus 20% and plus 50% are common and should not be an issue.

Some of the specified values are not standard anymore.
50 uF can be replaced with the now standard value of 47uF.
Concerning the working voltages, 15V should be replaced by 16V or higher.

Concerning the power supply:
It is positive ground. Why?
Because those Germanium PNP transistors have their emitter connected to ground (through a bias resistor which is AC bypassed by caps).
A PNP transistor operates with a positive voltage applied to the emitter (E) in respect to its collector (C).

[Geoff Quickfall]

Jebem:

Got the list of caps from the parts page and locations. This version has the AC option with no battery box, so I have to add a 3000mFd to that list, size of a c cell. Thanks for the update and the time you have taken Jebem.

I will try the electronics section in Hong Kong Monday for all the caps. It does appear that this was recapped although as stated earlier, their values are even greater then 50% over. I have re-capped radios before and see the standards have changed 50 versu 47. With the wide variance of values at the factory, why worry about 4700mFd versus 5000mFd let alone 47 versus 50. My OCD likes the 50s divisions :-)

I am checking Mouser and Digikey for the can with three 100mFd. I have also seen this can replaced and on some circuit boards for the Mohican, labelled for three discrete 100mFd caps. I like the can option, tidier ( see above and OCD).

Thanks for the transistor versus positive ground info. I was wondering why it was designed with the positive ground.

I was trying to source the transistors (obsolete) but did find a Russian source (eBay SOVCOM) and awaiting a response. Researching radio forums the (SOVCOM) eBay store was pointed out for Zenith transoceanic replacements with great success. Would you know a source?

At the moment all transistors appear to be working form the initial test (turned on the radio when I purchased it) by cycling through all the bands and it was receiving on all bands. I haven't tested them separately but may get a cheap Chinese tester on Monday (watched a couple of YouTube transistor testing videos last night).

Thanks again all!

Dave,

still awaiting the manual form HeathKit but your Adobe version is a life saver, the radio is looking like New Old stock for fit and finish. Just awaiting the power supply cap replacement and new ac cord.

Geoff

[Geoff Quickfall]

Okay,

Searching the web and Gigi is applying itself in full force: what is the term for a multiple capacitor in a single can.

I am trying to locate a replacement for an:

MEC Nashville 885-0-35 100/100/100 mFd can in the Mohican, anyone got a source for an equivalent?

Geoff

[jebem]

(01-23-2016 07:31 PM)Geoff Quickfall Wrote:  Okay,

Searching the web and Gigi is applying itself in full force: what is the term for a multiple capacitor in a single can.

I am trying to locate a replacement for an:

MEC Nashville 885-0-35 100/100/100 mFd can in the Mohican, anyone got a source for an equivalent?

Geoff

Hi, Geoff,
Thank you for sharing your other excellent radio's hobby here.
Radio is a cherish theme to many of us here, myself included.

Edit: I found the correct designation, I believe:
It is a Multi Section capacitor

Concerning that 100+100+100 uF capacitor, well, they don't make them anymore as far as I know.
It was common in the good old days to have two, three and even four caps in the same can, specially on tube/valve radios.
In fact it is still easy to find them as twins in the can for high voltage (above 300VDC) to be used to restore old tube radios.

I understand perfectly your restoration choices.
We should maintain the original design and technology as much as possible.
Replacing original components by modern ones is kind of misrepresent the original product.

So, I only replace components in vintage equipment when it is really necessary.
In this case, a capacitor meter is of great value. Even many multimeters can measure capacitors.

One idea shared by a fellow in other (radio) forum where I'm a member, in order to preserve the original spirit and fix the faulty capacitors, goes like this:
- Strip/remove the old contents from the can (foil plates, dielectric and electrolytic)
- Save the can only to insert modern, discrete capacitors inside and seal the can once done.

One example of this cap rebuild here and here and also here.


Edit2:
Sorry, I missed your question on the Germanium transistors.
Should you need to replace any of those transistors, it is easy to find equivalents, for instance on eBay that's where I go for this stuff, including Russian/Ukraine/Bulgarian sellers).
Russian Germanium transistors are of good quality and will outperform easily the original 2N series used in this radio; however they are not the original ones, so again, I try not to install substitutes on vintage equipment in order to keep their value as high as possible.

One word about testing Germanium transistors, if I may (I know you are aware of this, but others my find this info useful:
Most of the he usual modern multimeters are not adequate to measure Germanium semiconductors, as they are designed to test Silicon semiconductors that have a junction barrier voltage of about 0.7V, while Germanium has only 0.2V.

[Geoff Quickfall]

Oops,

I ran into that on my Grundig 4035 can capacitor. Discussed recapping and saving the can. Fortunately the can tested okay.

Now back to the Mohican.

Geoff