Stange.  Would someone really price this at US$1?  Maybe US$1000. 

With only one three-turn inductor in the pi network, that's definitely a CB amp.  I see items like this on e-Bay all the time, although the construction on this one appears to be better than average.

I cannot now, nor will I ever, understand the mindset that drives someone to put the time, effort and money into something that's illegal, dangerous and devoid of any real on-the-air benefit. 

Well, if I lived in the area, I'd grab it for the parts.  It could be easily converted to a single-band, two-tube, legal-limit amateur amplifier.  A nice follow-on project would be conversion to several bands, which could be financed by selling off some of the spare tubes.

-Dave

OK, it sounds like my amp's performance is nominal for an SB-200 wired for 120 VAC.  I'm sure some of you fired up your equipment to provide the data.  Thanks--much appreciated.

I'll be remodelling my kitchen this year and will have it rewired for 20A on a separate circuit.  While the electrician's at it, I may have him drop a 240 VAC line to my shop that doubles as my shack.

-Dave, K3WQ

Yes, that's another way of putting it.

I can certainly wire the unit for 240 VAC and (inconveniently) get that voltage to the shack.  However, before I attempt to do anything like that, I'd like a sanity check from anyone else running an SB-200 from a 120 VAC line.


OK--someone gets only a 100 VDC drop when using a 120 VAC line.  I assume this is under a CW signal.  Very helpful.  Thanks!

I'm running an extension cord to a plug that, I believe, is on a fairly unencumbered circuit.  It's a beefy cord, but could still be the cause as it's probably close to 20 feet long.

-Dave, K3WQ

I just finished refurbishing an SB-200 linear amplifier I purchased through eBay.  I replaced out-of-spec resistor and caps, touched up various solder joints, installed the Harbach power supply and soft start circuit, and installed a new set of 572B tubes.  It generally behaves as expected; however, I notice considerable sag on the high-voltage (HV) supply.  The idle voltage is 2300 to 2400 VDC, but when keyed (in CW) it drops to 1800 VDC, so the RF output power is lower than expected.

The unit is wired for 120 VAC, but I suspect my mains are not sufficiently “stiff” because the line voltage drops about 8 VAC when keyed. 

QUESTION: What amount of HV sag is considered acceptable? 

-Dave, K3WQ

Oops--I was referring to the TH-79A, not the TH-27A.  Sorry.  I think I'm mildly dyslexic. 

Some of the advice might still apply.  In any case, be very careful when you disassemble your HT, as it probably has lots of small parts that are virtually impossible to identify.

Here's something to look for if you dare to disassemble the unit and remove the printed circuit board that controls the display and key pad:

At the top of the display there are two clear rubber cubes sitting within square loops.  These contain flexible conductors that make the connections between the display circuit board and the speaker.  It's possible that one or both have corroded or have become unseated.  Remove them, clean the contacts on the speaker and the board and reinstall them.  (Use a Q-tip and rubbing alcohol.)  The other thing you can do is replace the cubes with wires soldered in place. I used thin insulated wire for wire wrapping (two pieces, each about three inches long).

Incidentally, to remove the display circuit board from TH-79A chassis you must pry off a plastic plate that holds the key pad in place.  This is illustrated in the service manual that is available in PDF format.  Do a web search on "TH-79A service manual" to locate a copy.

I just fixed the same display problem with my TH-79A, and although this thread is old, I thought I'd add a few helpful comments.

First, KE3WD offers good advice concerning the maintenance of flex cables and touch-pads. That "goo" K6DMA encountered might not have been applied during manufacturing. Rather, it could have been a byproduct of the rubber deteriorating. I've encountered this problem with touch-pads on TV remotes. In any case, clean off the goo with a Q-tip and rubbing alcohol. And while you have the radio apart, take the opportunity to clean all touch-pad contacts.

There's a trick to removing the display circuit board from TH-79A chassis that involves prying off a plastic plate that holds the key pad in place.  Disassembly of the unit is detailed in the service manual that is available in PDF format.  Do a web search on "TH-79A service manual" to locate a copy.

To remove the display from the circuit board, you have to unsolder two vertical posts in the corners of the board near the key pad. 

At the top of the display there are two clear rubber cubes sitting within square loops.  Do not loose these!  They are actually comprised of a metal contact sandwiched between two rubber pads and are used to make the connections between the display circuit board and the speaker.  I lost one, so I was forced to solder wires between the board and speaker.  I used thin insulated wire used for wire wrapping.  In retrospect, this makes for a more robust connection, but installation is a tedious procedure.