[Heath] Final transistors for SB 104A

[JF]

On 11/24/2014 1:40 PM, John King via Heath wrote:
> I have learned that the 2n6456 final RF transistors are no longer available and I NEED to replace the four (4) in a Heath SB 104A. Has anyone found a suitable replacement transistor for this application? I have thoroughly checked with RF Parts and they no longer stock those transistors.
>
> Does anyone have experience substituting the final transistors in the SB 104A? If so what did you use as a replacement  and do you know where they can be found? Thanks and 73, John, K5PGW

A caution about replacing these finals -  a 1975 Heath service 
bulletin said:
"After power has been applied and an output transistor has been found 
electrically damaged, all four output transistors should be changed.
In cases where physical damage has occurred such as the stud being 
broken off during construction and power has not been applied, only 
the one damaged transistor can be replaced. It must be replaced with 
one from the same vendor. No other matching considerations are required."

Also, a thermal grease should be used when mounting.
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That said, there are some 2N6456 substitutes:

(1) RF parts still stocks  the CD-2664A at $42.50 each, which 
apparently were sometimes used by Heath in place of the 2N6456.
           http://www.rfparts.com/cd2664a.html
In regard to this, on another forum an SB104A owner said:
"The 4 PA transistors (#417-831) were:  either CTC CD-2664A or 
2N6456, which, according to the manual, had the same Heath number but 
slightly different mounting steps."
I do not know what the "slightly different mounting steps" are,  
possibly the mechanical layout is a little different.
Another equivalent number may be the PT5767, which crosses to the 
NTE325 like the 2N6456. Note the caveat below.

(2) NTE325 transistor, listed by NTE as equivalent to the 2N6456, 
with the caveat that "Minor mechanical differences exist, but device
will fit most applications".
You would have to check the datasheets to find out what the 
differences are, which might be anything from slight size differences 
to different pin-outs. The NTE325 goes for around $55-$60 from 
various places.
The 2SC1177 may be similar to the NTE325, perhaps with different 
mounting, I'm not sure.

(3) Other hams have said that the Motorola MRF455A & similar might be 
used, but they may require some mechanical modifications to wiring 
and/or fit. Like wise for the MRF 455A and the MRF450A-MP.

(4)  There are some new 2N6456's listed on eBay and "ebay.co.uk" by 
G4IUG for about GBP 5 each (US $7.86 ) plus delivery; if I were you I 
would write the seller post haste to verify that he will ship a 
matched set of 4 to the U.S.  The title of the listing is:

  "TRW 2N6456 PT5767 HF RF Output Final PA Transistors for Heathkit 
SB-104 HW-104"

and the eBay-UK url (one long line) is:

<http://www.ebay.co.uk/itm/TRW-2N6456-PT5767-HF-RF-Output-Final-PA-Transistors-for-Heathkit-SB-104-HW-104-/251718554226?pt=UK_Mobile_Phones_Communication_Ham_Radio_Amplifiers&hash=item3a9b985272>

These also appear on eBay's U.S. site at:
<http://www.ebay.com/itm/TRW-2N6456-PT5767-HF-RF-Output-Final-PA-Transistors-for-Heathkit-SB-104-HW-104-/251718554226?pt=UK_Mobile_Phones_Communication_Ham_Radio_Amplifiers&hash=item3a9b985272>

73
John   KE5ZB

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