Comet Pro Help Needed

[Joe Connor]

I need some troubleshooting help on a basket-case Hammarlund Comet Pro that I'm trying to restore. Here's the schematic:
http://www.nostalgiaair.org/PagesByModel/994/M0008994.pdf

   Mine is the early model with the only gain control a pot that varies the voltage on the cathodes of the two 58 IF tubes. This one was a real mess when I got it but I love these old Comet Pros so I want to get it performing well.

   Here are my symptoms: There is low distorted audio when the gain control is turned fully counter-clockwise. As I turn the control clockwise, the audio gets louder but gets very distorted. Right as it gets to full volume, the audio gets clear but is very, very loud. I can't get clear audio at a lower volume.

   I have recapped the set (except for the 1 mfd cap right after the second detector). I disconnected the gain pot and tested it. It it turns from 0 ohms to the full 12K ohms. (I used a DMM so I'm not sure just how smooth the change in resistance is). The gain pot causes the cathode voltage on the IF tubes to vary between 0 and 13 V. All voltages in the set are in the ballpark. The resistors all test within 20% or so of spec.

   Does anyone have any ideas?

   As always, thanks for your help.


                                      Joe Connor

[AB2EZ]

Joe

In the schematic, there is a 0.1uF capacitor from the 1st i.f. tube's cathode to ground. Try adding (don't worry about the existing triple capacitor) a .1uF capacitor from this tube's cathode pin to the nearest ground.

Reason: When the receiver is set to full volume, there is a 200 ohm resistive path from the 1st i.f. tube's cathode to ground.

If the 0.1uF RF/IF cathode bypass capacitor (-j3.4 ohms at 465kHz) is not doing it's job, the RF/IF impedance from the cathode to ground will increase when the volume control is turned to a lower volume setting. This will result in changes to the intended feedback ratio from cathode to grid of the 1st i.f. tube... i.e. the effective transconductance of the tube (unintentionally operating in a cathode follower configuration) will be reduced v. its intended design value. Also, since there is a 0.1uF capacitor connecting the plate to the cathode, if the cathode bypass capacitor is not doing its job, there will be much too large a feedback ratio from plate to cathode, and the primary of the plate's tuned load circuit will not have a low impedance path to ground.

Stu

[W7TFO]

Hi Joe,

My Comet is a little later production than yours, but will sometimes suffer if the station received is swamping the first RF tube with too much signal.

Jockeying them around in sockets to select the best operation helped, along with making sure none of them are gassy, and having good bypass caps as Stu mentioned.

73DG

[Joe Connor]

Thanks for the help. I think I've got the problem solved.

I spent so much time double- and triple-checking my work that I neglected to look at what the prior owner did. For some reason, he used a heater tube pin on the IF for a tie point for the wire coming from the gain pot to a 200 ohm resistor that went to the IF cathode. Once I removed them from the pin and connected them in space, the gain control worked like a charm.

   This radio has been quite an adventure. When I bought it, I changed the filter caps and fired it up. After a little fiddling, I got it working but the gain control was screwy. Before I got a chance to check that out, the power transformer went up in smoke. Ouch. I got it rewound (thank you, Gary Brown), replaced all the caps and had to wrestle with the gain-control problem. I could see that the prior owner had done work on this set (and not to the highest standards of neatness). For example, he installed a toggle switch on the front panel between the grid cap of the first detector and ground. Now, I'm going to go through and check out the rest of his handiwork. I also have to get a new audio output transformer. The prior owner replaced it, probably because he didn't want to deal with the 4,000-ohm output impedance of the original. Somehow, he ripped some of the connection lugs off the replacement so it's essentially useless now. However, the set is very sensitive through headphones and I am picking up all kinds of stations on the BCB.

                                Joe Connor