[Heath] HW-16 update and additional question (11 MAR 14)

[Kenneth G. Gordon]

On 11 Mar 2014 at 1:13, John Klingelhoeffer wrote:

> 
> Thanks to all who responded and gave comments and 
> suggestions to the questions regarding the HW-16 
> refurbishment. Most were very helpful. I still have a 
> locked slug in the 15M het oscillator coil form in the 
> receiver that I may have to break out and retune with an 
> external capacitor.

Well, actually, Radio Shack has (or had?) packages of various slugs. I have 
repeatedly found good replacements in that package for slugs I had to 
destroy to remove.

> Silicone spray was not the answer. 
> That 15M LO level is a lot lower than 40 and 80 M.

Yes. That is common, but at the TP, they should all be about the same in 
that rig.

> On 
> the transmitter side, I seem to get 50-60 watts out on all 
> three bands so I'm happy, happy, happy about that. 

Good.

> The comments I have seen previously on the reflector 
> about carbon composition resistors increasing in value 
> on this old equipment is very important to heed. So far, of 
> those removed or tested, all have been 15-20% high in 
> value. Some also had very little 'headroom' in their 
> dissipation.

Yes. I agree. 2 watters should be replaced with modern 3 watters, or even in 
some cases, 5 watters.

> The order to the electronics parts place is 
> already in process. 
> 
> Okay, so I won't say this is the last problem I'll post, but it 
> is so far a vexing one. At even moderate audio levels, a 
> strong station will cause something - I think the audio 
> output tube - to regenerate the received tone. It sounds 
> like the tube is oscillating in synchronization with the 
> incoming CW and producing a strong keyed tone - sort of 
> like a locked oscillator. It ends up being a LOT louder 
> than the signal level.

Hmmmm......I have experienced that at times too.
 
> The troubleshooting instructions in the manual talk about 
> the output transformer plate (blue) lead being too close 
> to the PCB (and hence the grids of the audio amp tube) 
> and causing 'howling'. I would not characterize this as 
> howling (unrestrained audio feedback) - the oscillation 
> only happens when the strong CW signal is there and 
> ceases between dits and dahs. It's a keyed audio 
> oscillation.

What is happening is that the stage or stages are on the edge of oscillation, 
and the keyed signal being received "triggers" the oscillation off.
 
> Just in case, I moved the blue transformer lead all the 
> way to the edge of the chassis per the troubleshooting 
> hint, and I also shielded it with the braid from of a piece 
> of RG-59 cable. However, no joy. Still as bad as ever. 

OK. Then IMHO, that is not the problem.

> Also tried to replace the audio tube cathode electrolytic 
> capacitor since I wasn't sure I'd changed that recently. 
> No change. I performed the tube socket voltage tests and 
> all were within a couple of percent. 

Sounds to me as though you are somehow getting feedback from the audio 
output back into the audio PRE-AMP. 

Phase from input of an amp to its output is usually 180 degrees out of phase 
(except for inter-electrode capacitance, which is only a factor at RF). 
However, the preamp does the same thing, so that if you are getting 
oscillation in the audio stage (which in the HW-16 is ONE tube.) in my 
opinion, the feed back must be getting into the audio PRE-AMP.

> Until this gets fixed, I'm not going to attempt any of the 
> increased gain mods for the IF and RF amps as they 
> would be counterproductive.

Indeed.

> But, it needs to be fixed else 
> the radio will be pretty useless. Anyone seen this before 
> and found a fix for it?

I have a similar problem in one of my own HW-16s, but in that one I have 
added an Autek QF-1 audio filter between the detector and the input to the 
audio preamp. At the 80 Hz selectivity point of the filter, with the audio gain 
up too high, I get the same sort of thing you are experiencing, and it is quite 
loud too.

> I don't remember this sort of thing 
> happening on the one I assembled and used back in 
> 1968-1970.

More than likely it never did that. It seems that all HW-16s are "unique" in 
certain respects. 
 
> 
> Should I just take a piece of insulated solid #16 buss 
> wire, bring it up absolutely vertically from the PCB plate 
> connection, and then route it over to the vicinity of the 
> transformer before connecting to the blue transformer 
> wire on a terminal strip under the transformer mounting 
> screw?

Do you mean, to make certain the audio transformer is grounded?

> I can't think of any way of getting it any more 
> quickly away from the PCB and putting space between it 
> an the tube grids. I could use Teflon coaxial cable to 
> make this a shielded run as well. Other thoughts?

My thoughts are that that isn't the problem. I think it is more subtle than that. 
It may take some looking around with a scope.

I believe that there is at least one of the original Heath Company designers 
on this forum. Maybe he'll jump in here.

Ken W7EKB