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Author Topic: Hammarlund HQ-170A burning up resistor R9  (Read 5973 times)
W9GB
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« Reply #15 on: August 30, 2012, 07:20:41 AM »

Phil Nelson's restoration of a Hallicrafters SX-42 ...
Took weeks as he chased the root cause of a short on its Multi-wafer band witch.
http://www.antiqueradio.org/halli07.htm

Carbonized tracks or burnt phenolic on the wafer can easily create a short situation.
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AB1MN
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« Reply #16 on: August 30, 2012, 08:09:14 AM »

You should see a short between v2 pin 5 and the junction of R9/C11. This is the B+ path to the plate of V2. What you are looking for is the cause of a short between this path and ground.

The three wires to S2F(Front) are all connected together for DC through T1 so unless the short is in the switch itself, disconnecting the wires will not isolate the problem.

Also, be extremely careful when prying on a switch as you certainly don't want to damage it and create another problem.

I would suggest disconnecting the three wires on the primary side of T1 and repeating the resistance checks.

  Each of the connections on the Primary side of T1 connects to S2F (Front), and additionally,
  One wire goes to V2 Pin 5
  One goes to only to S2F
  One goes to the junction of R9/C11

After disconnecting these wires from T11, if you have a short between any of the pins on the primary side of T1 and ground, the problem is in T1 itself. Because all of the primary side connections are connected internally by the windings, if there is any short internally, they will all show a short to ground. If no short to ground, then the problem is not in T1 (good news). I don't have this radio to look at, but it may be possible to remove the shield can while leaving T1 installed in the radio to see if something is shorted to the shield.

I have an HQ-180 where one of the IF transformers had a short between the primary coil and shield can causing exactly the type of problem you are experiencing so keep this in mind as a possibility.

At this point, you will have also isolated the wires that go to S2F and you can meter them for a short to ground as well. This should help eliminate S2F as a potential cause (or as the cause).

Good Luck,

Bob
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K3STX
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« Reply #17 on: August 30, 2012, 08:30:37 AM »

Unless it's a be-yatch to do, I'd break the connection between wafer F and the junction of C11/R9. If the short stays on the switch side that's a clue, if it stays on the T1 side that's another clue. If it's on both sides that's a clue I don't want but still a clue.............

Wafer F is the NEAREST to the knob, go figure!?

I will do exactly what you say. With the connection between wafer F and the junction of C11/R9 broken a short between V2pin 5 and ground means something T1 is bad. No short strongly implicates the switch.

p
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W4HTH
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« Reply #18 on: August 30, 2012, 09:07:29 AM »

write all tests and measurements down for future reference. having them on paper or in a folder on the computer beats relying on my failing memory 6 mo. down the road [been there done that]. jim w4hth.
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KQ6Q
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« Reply #19 on: August 30, 2012, 08:34:57 PM »

If this problem is still unsolved,consider the possibility that a wrong tube is in the mixer socket, which could be causing the short. try removing the tube and check the resistances again. check the tube designation and make sure you have the correct tube in the socket.

Fred, KQ6Q
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K3STX
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« Reply #20 on: August 31, 2012, 01:07:30 PM »

UPDATE:

I think it is my switch, but I still would like advice, hard to figure out how to read the schematic with the switch positions.

I completely disconnected all wires going to S2F(FRONT) AND disconnected wire from R9/C11 to T1. From V2pin5 there is NO SHORT, I think T1 is fine.

With all wires going to S2F(FRONT) disconnected and all else intact (reconnect R9/C11 to T1) and applying power R9 does NOT smoke!!! That is great. Then I hook one wire from S2F to V2pin5, apply power and all is fine.

If I connect S2F(FRONT) wire to junction of R9/C11 >>>SMOKE!
If I connect S2F(FRONT) wire to T1 >>> SMOKE! (THIS IS TRUE EVEN WHEN R9/C11 IS LEFT OPEN AND FLOATING).

I again did resistance measurements to ground at S2F(FRONT) with ALL SWITCH WIRES DISCONNECTED.
The lug that goes to V2pin5 is open circuit, hooray.
The lug that goes directly to T1 shows 4000 ohms at all positions.
The lug that goes to R9/C11 is open on 160/80M but 4000 ohms at 40-6meters.

Shouldn't S2F(FRONT) show an OPEN circuit on all bands at every lug at all times (with no wires attached)?

Sorry I am so dense. Electricity is easy for you, molecular biology is easy for me  Smiley.

paul
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AC5UP
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« Reply #21 on: August 31, 2012, 03:49:40 PM »

The switch lug on S2F (front) that connects to the junction of R9 and C11 should never show a DC path to ground.

The switch lugs on S2F (rear) will show a path to ground on both lugs, one through L8 and the other direct.

Or, to simplify:  Pin 5 of V2 (the 6BE6 plate) should have high resistance to ground (7000 Ohms) or 250 VDC according to the tube socket voltage and resistance charts in the book. With everything connected as original (including R9) you'll show a near dead short to ground on Pin 5 and that's exactly what's wrong. Disconnect the wire on the lug between the switch S2F (front) and the junction of R9 and C11. Only that wire. If the short goes away the switch is shorted, and with that connection open you can readily test the resistance to ground on the hot side of C11 to make sure it's at 7000 Ohms or better. If so, then T1 is OK. If you do a resistance check on the disconnected S2F (front) lug and it shows a short to ground you've confirmed the switch is shorted to ground.

The book gives you two pages worth of "normal" readings relative to ground for voltage and resistance on all the tube pins. Which should make it easy to determine where your radio deviates from a good working example of that chassis.

I get the feeling you can't see the trees in the schematic due to the forest. You don't have to fix the entire radio, only the B+ line that feeds the plate of V2 through R9. Divide, simplify, isolate, diagnose and repair. You keep mentioning "all wires disconnected". That's bad trouble shooting technique as you've multiplied the possibilities of any one change by the number of items disconnected and added noise to the process. Better to change only one thing at a time and evaluate that change in terms of better or worse. Then you determine why that affect and if the test points you to an Abby Normal part.

All you need to know at this point is whether T1 or S2F is shorted to ground. One of them will be. When you figure out what's shorted and repair same you'll have fixed the radio.
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K3STX
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« Reply #22 on: August 31, 2012, 06:02:15 PM »

Quote
The switch lug on S2F (front) that connects to the junction of R9 and C11 should never show a DC path to ground.

I realize that. It shows a resistance of 2000 ohms to ground even when all wires are removed from S2F(front). To me that suggests something is wrong with the switch, no? But how can it be 2000 ohms? I would have though completely OPEN (as one would expect from a new switch) or completely SHORTED. Not something in between?!

Quote
The switch lugs on S2F (rear) will show a path to ground on both lugs, one through L8 and the other direct.
Yep, I got that.

Quote
Or, to simplify:  Pin 5 of V2 (the 6BE6 plate) should have high resistance to ground (7000 Ohms)

It reads 2000 ohms. V2 pin5 is attached to one of the lugs of S2F(front). Curiously, the resistance of EVERY lug of S2F(front) to ground is 2000 ohms regardless of the band chosen.

Quote
Disconnect the wire on the lug between the switch S2F (front) and the junction of R9 and C11. Only that wire. If the short goes away the switch is shorted,

I have done this, the short does not go away, still shorted. But since the other two lugs of the switch are still in the circuit, they are attached to T1 and since R9/C11 also attach to T1 isn't it possible the switch is still at fault?

That is why I thought I would completely remove the S2F(front) from the equation by removing those three wires. When I do this, the short is gone. In my simple mind this says T1 is fine and the problem is the switch.

Quote
You don't have to fix the entire radio, only the B+ line that feeds the plate of V2 through R9. Divide, simplify, isolate, diagnose and repair. You keep mentioning "all wires disconnected".

I think I am only focused on this region of the radio. The "all wires disconnected" were the three to the switch; I was trying to isolate the switch.

I really appreciate all your help. I am learning alot, but I am afraid I am frustrating all my friends here due to my inability to comprehend simple instructions  Grin

paul
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AC5UP
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« Reply #23 on: August 31, 2012, 07:00:23 PM »

If you're telling me the lug on S2F, when not connected to anything, gives you 2000 Ohms to ground then the switch is partially shorted.

At this point you bring out the flashlight, Q-Tips, and whatever else might help you take a very close look at the switch contacts for a carbon arc, conductive crud, or bent contact finger on the wafer. From what you described in your original post the radio failed spontaneously which is more consistent with a mechanical short than a carbon path, but for all we know R9 may have been running hot until calling it quits with a puff of smoke. OTOH, if a replacement R9 pops open within a few seconds that tells me there's a solid short and I'd be looking for a bent or broken contact finger on the wafer. Either way, once R9 opened the 1st mixer stopped mixing and you stopped listening to the radio.

                             (?)

Give the switch wafer and rotor a thorough inspection.
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W9GB
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« Reply #24 on: August 31, 2012, 07:40:11 PM »

Quote
Give the switch wafer and rotor a thorough inspection.
You will need a Magnifying glass and Excellent lighting ...
some spots of switch are hard to see.

Reaching into your microbiology ... You are looking for AB-NORMAL ...
unfortunately unless you know what factory/normal should be .. You may overlook the obvious.
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K3STX
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« Reply #25 on: August 31, 2012, 08:01:38 PM »

Correct; each of the lugs on S2F(FRONT) when none of them are connected to ANYTHING, shows 2000 ohms. Each of the three lugs, the same 2000 ohms.

I guess I should be able to inspect/clean this switch until I do NOT get 2000 ohms, I should read an open circuit if all is good, correct.

Quote
unfortunately unless you know what factory/normal should be...

Truer words have never been spoken. I will look at some of my other radios. Thanks guys. I think I will put this project aside for tonight and get on the air.

paul
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K3STX
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« Reply #26 on: September 03, 2012, 06:04:37 PM »

Well ... ... ...

I suspected the bandswitch, and looking closely I noticed that one the little contacts from S2F(REAR) seemed to be really sticking down, almost THROUGH the little opening to the other side of the wafer. I pulled it UP a bit, and now my partial short went away! S2F(REAR) is grounded, I suspect it was touching the S2F(FRONT) rotor. Don't know how THAT happened.

I got lacquer thinner and Q-tips/cloth and toothpicks/cloth to run all the contacts on S2F (front and back). There was lots of black crud.

So I put it all together (rehooking wires to switch with jumper cables (no tubes yet) and fired it up on Variac slowly. At about 90 volts I heard a little zap on the switch that lasted one flash, but that it is. I chalk that up to burning Q-tip fuzz! But 110 volts, no problem. Then put the tubes in, fire her up, and with Variac at 80 volts heard stations. Up to 120 volts, no problems!! So I think I fixed it! Everyone soldered in, all good.

But was the lacquer thinner to clean the switch a good idea? It is still upside down, should I buy some De-Ox-It to remove any lacquer thinner residue or am I good to go? It is still upside down.

Thanks to everyone for their help, I learned alot.

paul
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W9GB
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« Reply #27 on: September 03, 2012, 07:25:21 PM »

Paul --

Lacquer thinner is flammable,
 so the zap may have been Cotten (Q-Tip) with some residue lacquer.

In tight spaces, I use foam cleaning tips (used to clean VCR heads), so no Cotten is left.
I am not surprised of the black dirt, dust and oxidation, common with older radios.

Check that switch on all of its settings over the next couple of days.
Easier to clean a switch, then rewind an IF transformer.
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AC5UP
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« Reply #28 on: September 03, 2012, 09:09:44 PM »

But was the lacquer thinner to clean the switch a good idea? It is still upside down, should I buy some De-Ox-It to remove any lacquer thinner residue or am I good to go?

The lacquer thinner is OK as it's an effective cleaner and volatile enough that any residue will evaporate within a matter of days, if not hours. The black coating you cleaned off is probably light oxidation and very common on switches that old. As long as it's not bridging the phenolic from one contact finger to the next it shouldn't cause any harm.

With that said, if it were mine I'd go through the switch with CRC 2-26 and Q-Tips to clean all the wafers. I'd be looking for any stray cotton wisps left behind and remove same. DO NOT SPRAY THE SWITCH when doing this! Instead, spray the Q-Tip then swab the switch rotors on both sides. Rotate the switch to work in the cleaner and repeat the process until the Q-Tips come up clean. The goal here is to both clean and lubricate the switch contacts as they can stick when too dry and snag a contact finger. I have a hunch that's what happened to your switch. Finish the exercise with a drop or two of light machine oil on the ball detent mechanism (the part that makes the switch click into each position) and the switch shaft.

You should end up with a switch that's noticeably smoother to rotate and feels "crisper" when it snaps from one position to the next.

You will see replies advising that Deoxit is the only cleaner to use and while I agree it's a good product I think it's overpriced and overkill for some applications. Just my opinion, and I've had excellent luck with CRC 2-26 as both a cleaner and lubricant. http://www.crcindustries.com/ei/content/prod_detail.aspx?PN=02004&S=Y

I bought a bottle of Deoxit years ago and it has gone a long way as I use only a thin wire or small screwdriver blade to transfer small droplets to the work. No spray, no overspray. There can be too much of a good thing and all I want is to wet the contact surfaces without soaking the phenolic.
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KC4MOP
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« Reply #29 on: September 06, 2012, 11:26:56 AM »

This thread actually has a nice ending and a solution. How many internet threads will someone search, using google, and pages of replies and frustration and never a solution???
And K3STX bravo to you for having an analytical mind to 'tune in' to the advice here and coming to a conclusion for your radio.

Fred
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