Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Hammarlund HQ-170A burning up resistor R9  (Read 8771 times)
K3STX
Member

Posts: 1079




Ignore
« on: August 29, 2012, 06:06:50 AM »

My HQ-170A has got a real problem. All was fine one night listening to the rig, then I smelled smoke and turned off radio. Took it apart, the 1K resistor R9 was burnt up. I replaced it with a new resistor, and upon power up this one also began to smoke. I took out all tubes, STILL blew another new resistor R9 (it has selenium rectifiers, so I do not remove them). Used a dim bulb tester with a 150 watt bulb with NO TUBES in the rig, it lit up pretty bright, leading me to suspect shorted filter caps in the can electrolytic assembly C82.

But the other 1K resistors on the same circuit as R9 (namely R12 and R17) don't burn up, only R9. Am I correct in suspecting the filter caps? Last night I replaced only C28A and it did not help (still smoking R9), but I did not finish C82A, B, or C yet. I would LOVE to measure voltages, but I can't turn the rig on with this problem. Any thoughts would be great, I am certainly not an electronics pro (or even much of an amateur). FWIW, the schematic is at the back of the HQ-170A manual, I have a hyperlink to it on my website:  http://k3stx.com/ under the BOATANCHORS TAB along the top.

I would love to get this going again.

paul
Logged
W9GB
Member

Posts: 2659




Ignore
« Reply #1 on: August 29, 2012, 06:45:48 AM »

Paul ---

It appears you have the schematic for the HQ-170A and can read schematics and follow along.
Quote
I would LOVE to measure voltages, but I can't turn the rig on with this problem.
SURE YOU CAN.
YOU NEED TO ISOLATE that High Voltage, B+ lead from the radio's B+ rail to various circuits.

SAFETY ADVISORY:  You are dealing with B+ HV in the 250 to 400 VDC range ....
that can give you a severe shock OR Stop your heart.

1.  UNKNOWN RADIOS -- I ALWAYS start with Power Supply section.
WHY?  To ASSURE the all DC output voltages are stable (VOM, VTVM) and minimal ripple (Scope).
I normally disconnect that power supply section (voltage wires/rails) from the radio -- then test for proper operation.
DO NOT 1/2 rebuild or shotgun replacements in main power supply (foolish) -- DO IT RIGHT

2. R9 is a 1K resistor (1/2 watt) that taps to the radio's MAIN B+ Voltage Rail from the Power Suuply.  

3. WHAT does R9 feed with this B+ voltage ??
It feeds T1 (after the first mixer) and an S2 WAFER (F, FRONT) of the S2 switch.

4. WHAT causes a 1/2 watt resistor to be destroyed???
 E = I x R (Ohm's Law); P = I x E (Power Law)
http://en.wikipedia.org/wiki/Ohm's_law

EXCESSIVE CURRENT (I) or VOLTAGE (E)
A short (to ground) at T1 or a short/grounding problem at S2 can cause excessive current draw.
====
Quote
I would love to get this going again.
DO NOT get in a hurry .... you have to be patient, since you re a Novice at these repairs AND you are learning (education) at same time.
« Last Edit: August 29, 2012, 07:26:20 AM by W9GB » Logged
AB1MN
Member

Posts: 48




Ignore
« Reply #2 on: August 29, 2012, 06:50:53 AM »

Looking at the schematic, R9 supplies B+ to the 1st mixer so the possible problem causes are fairly limited.

Some thoughts for troubleshooting:

With power off: Disconnect one end of R9 and measure the resistance from Pin 5 of V2 to ground. With R9 disconnected, the resistance to ground should be infinite (or at least extremely high).

If not, remove V2 to eliminate an internal short in the tube as a possible cause (unlikely, but this is easy to do).

If you measure zero ohms, or close to it at pin 5 of V2, the possible causes are:
the wiring to S2, or S2 itself - while measuring the resistance, rotate S2 through all of its settings and see if the short goes away,
C11, the .01uf bypass cap (disconnect one lead while measuring the resistance the resistance to ground),
T1, the bandpass transformer, or
something in the associated wiring that is shorted to ground (check the shielded cable to pin 5 of V2).

If the problem is not C11, I would be suspicious of T1. You can disconnect the leads that go to the primary side of T11 (pin 5 of V2, R9 and C11, and S2) and measure the resistance of each pin on T1 to ground to see if there is an internal short to ground.

While the various leads are disconnected, you can also measure the resistance of each lead to ground and this should help isolate the problem.

The good news is there are not that many components that could cause R9 to burn and that should help in isolating the problem.

Good Luck,

Bob  AB1MN
Logged
AC5UP
Member

Posts: 3956




Ignore
« Reply #3 on: August 29, 2012, 07:15:32 AM »

Think in terms of current flow.

Despite the schematic on your web site due being a bit difficult to follow thanks to the three page split, follow the B+ up from the power transformer. Selenium rectifier, two filter condensers, then a 0A2 voltage regulator tube. There are 1k resistors feeding the plate of the 6BZ6, the first two 6BE6's, and the 6BA6 (R97, R9, R12, R17) from the regulated B+ line. Of those four resistors only R9 has a smoking problem and you'll notice that in three cases the B+ runs through the primary of an RF transformer to the plate of the tube. This is very normal and you can use the plate circuit of V3 and V4 as something of a cross reference for V2.

So........ What are we looking for?

At some point UPSTREAM from R9 there's a very good path to ground. Good enough to draw enough current through R9 to smoke it muy pronto. That's actually a good thing because a solid short can be easy to spot with an Ohmmeter. Prime suspects would normally be a hard grid short within V2 but since you pulled the tube and it made no difference that's ruled out. Second suspect would be a shorted C11 or C134 which are likely ceramic disk condensers. It's rare they go bad and that brings us to the bad news part of this exercise... What's the only other component that could develop a hard path to ground?

T1, the 1st mixer transformer. If that dude is shorted internally it will do exactly what you describe.

Further, look for a short to ground on the pin C11 and R9 connect to and hope you find one. If the primary winding is shorted to the inside of the can it's possible you could pull it apart, slip some paper or plastic between the two and get off lucky. If not, the primary winding could be toast and then you'll need to inquire about a replacement from Unobtanium Parts of Zeta Reticulae or some other planet 60+ light years away because it could still be the 50's there....

In any case, the power supply filter condensers are not your problem. You have voltage feeding R9 and more than enough to smoke it, so look for a DC short to ground north of R9. Remember that DC cannot travel through a condenser unless it's shorted.

BTW: If that radio were mine the selenium rectifier and any paper tubular condensers would be gone, gone, gone. When a selenium rectifier fails it can release a noxious smoke that is best avoided by an upgrade to a modern silicon diode.
Logged

Never change a password on a Friday                
W9GB
Member

Posts: 2659




Ignore
« Reply #4 on: August 29, 2012, 07:29:57 AM »

Quote
Further, look for a short to ground on the pin C11 and R9 connect to and hope you find one. If the primary winding is shorted to the inside of the can it's possible you could pull it apart, slip some paper or plastic between the two and get off lucky. If not, the primary winding could be toast and then you'll need to inquire about a replacement from Unobtanium Parts of Zeta Reticulae or some other planet 60+ light years away because it could still be the 50's there....
IF you do find T1 shorted .... It can be rewound.
Antique Radio Restorers do this with older radios .... AND even Kenwood Hybrid radios from 1970s ...
 When original part is no longer made or available.

I can supply DIY links and references, if that is necessary.
http://www.qsl.net/k5bcq/HK/HK.html#REPI

Harry, SM0VPO
http://www.sm0vpo.com/blocks/if-txfmr_tube_00.htm

Make Your Own IF Transformers
http://www.angelfire.com/electronic/funwithtubes/IF_Can-1.html

« Last Edit: August 29, 2012, 08:01:34 AM by W9GB » Logged
K3STX
Member

Posts: 1079




Ignore
« Reply #5 on: August 29, 2012, 08:19:59 AM »

you guys are the greatest. I went from being somewhat depressed to now actually pretty excited to get in there and trace the problem. I am a complete novice, but I really wonder about C11. When R9 is smoking I also some "sparking" at one of the switch positions on the bandswitch, I will have to try to figure out which one. But problem was on more than one "band".

Thanks, I'll get back on this horse later tonight.

paul
Logged
W9GB
Member

Posts: 2659




Ignore
« Reply #6 on: August 29, 2012, 10:29:19 AM »

A Q-Tip and some De-Oxit .... will clean corrosion and dirt from brass contacts of S2
Logged
K3STX
Member

Posts: 1079




Ignore
« Reply #7 on: August 29, 2012, 11:01:58 AM »

i was just wondering if the bandswitch itself could have been the problem. But I will look into all of this.

p
Logged
AC5UP
Member

Posts: 3956




Ignore
« Reply #8 on: August 29, 2012, 12:26:14 PM »

...notice the way S2-F (front) is wired and its purpose. Assuming the schematic uses standard notation, it shunts T1 when the bandswitch is in the full clockwise position. Otherwise the S2-F connection to the junction of R9 and C11 floats open. If the bandswitch is smoking R9 an Ohmmeter should tell the tale as the switch is rotated. Does K3STX recall changing bands immediately before R9 became a heating element?

Pages 26 and 27 of the user guide mention how the heterodyne scheme differs on the lowest two bands and how S2-F controls this. Before troubleshooting the bandswitch I'd recommend reviewing these two pages as the 1st mixer circuit employs an interesting design concept..........  Tongue
Logged

Never change a password on a Friday                
N4NYY
Member

Posts: 4820




Ignore
« Reply #9 on: August 29, 2012, 05:26:44 PM »

I just googled it ans it looks like a great looking radio. Did them and Hallicrafters compete against each other? A lot of their equipment looks similar to each other. Not really familiar with Hammarlund.
Logged
K3STX
Member

Posts: 1079




Ignore
« Reply #10 on: August 29, 2012, 08:35:22 PM »


With power off: Disconnect one end of R9 and measure the resistance from Pin 5 of V2 to ground. With R9 disconnected, the resistance to ground should be infinite (or at least extremely high).

If not, remove V2 to eliminate an internal short in the tube as a possible cause (unlikely, but this is easy to do).

If you measure zero ohms, or close to it at pin 5 of V2, the possible causes are:
the wiring to S2, or S2 itself - while measuring the resistance, rotate S2 through all of its settings and see if the short goes away,
C11, the .01uf bypass cap (disconnect one lead while measuring the resistance the resistance to ground),


Well... with V2 removed and R9 disconnected I measure a direct SHORT to ground at pin 5 of V2!!!

From V2 pin 5 to the front of S2 I get a SHORT!! Even when I rotate the switch through the bands. I checked C11, it is fine (no short). I am thinking of S2. I will study the switch, but if I kind of pry the brass "ring" at the front of S2 a bit the direct short goes away. I have to study the switch and see exactly what "prying" it off is doing. Maybe something slipped somewhere on the switch and the brass ring on the front of S2 is shorted. The little brass "ring" on the rotary switch should not be to ground at every position, should it?

And I think Hammarlund WAS a direct competitor of Hallicrafters. This old Hammarlund, when it was working, could hear just about as well as my modern Kenwood TS-850S.

paul
Logged
K3STX
Member

Posts: 1079




Ignore
« Reply #11 on: August 29, 2012, 08:52:02 PM »

Otherwise the S2-F connection to the junction of R9 and C11 floats open.

The S2-F connection to the junction of R9 and C11 reads a direct short at every position on the bandswitch.

Actually, looking at the switch it almost appears that there is a direct short between the FRONT brass "ring" and the REAR brass "ring" that are flanking the insulating wafer in S2. When I pry the front brass ring out a bit, the short goes away. Hmmm

p
Logged
AC5UP
Member

Posts: 3956




Ignore
« Reply #12 on: August 30, 2012, 05:55:08 AM »

Long ago and far away before diode matrix and TTL logic became common there were some very sophisticated switching arrangements built around rotary wafer switches. Not unusual to see a jumper between one side of a wafer to the other, nor from one wafer to another. The sophistication of the circuit often reflected the cleverness of the designer and some of those folks were very talented.

So........... If you can jigger the rotary switch to relieve the short that (probably) means T1 isn't shorted. This is both good news and bad news. Working on a bandswitch is no walk in the park but if you need to replace a wafer you're looking at more time than money for a part that's easier to find. Best advice is to test the switch until you can pinpoint the short, photograph the switch from all angles with a digital camera before you change anything, then figure the least intrusive way to repair or replace it.

Point being that multilayer rotary switches like that can get very confusing very quickly and going in on a hunch isn't the fast lane to Mission Accomplished. Better to get it right the first time..............  Cool
Logged

Never change a password on a Friday                
K3STX
Member

Posts: 1079




Ignore
« Reply #13 on: August 30, 2012, 06:15:23 AM »

Looking at the schematic (and the switch) it appears that there are THREE wires going into/out of S2 (FRONT). NONE of these lugs on the front of S2 look like they should be grounded (to me). I will unsolder all three wires from the solder lugs and then see if there is a short to ground on any of these three lugs. Sound reasonable?

I too thought that if T1 was shorted there would be NOTHING I could do to the wafers in S2 to prevent the short from V2 pin5 to ground.

Unfortunately, really looking at S2 means removing chassis from front panel, HERE WE GO!!

paul
Logged
AC5UP
Member

Posts: 3956




Ignore
« Reply #14 on: August 30, 2012, 06:45:45 AM »

Looking at the schematic (and the switch) it appears that there are THREE wires going into/out of S2 (FRONT). NONE of these lugs on the front of S2 look like they should be grounded (to me). I will unsolder all three wires from the solder lugs and then see if there is a short to ground on any of these three lugs. Sound reasonable?

    Nope.    The radio played for 60 plus years with those wires exactly where they are today.

What you may not realize is that on multilayer rotary switches the individual wafers are usually noted on the schematic by a letter and front or back side. In this case switch 2, wafer F. Which wafer is F? The first wafer on the switch (nearest the knob) is normally wafer A, followed by wafer B, and then we move down the stack to wafers E and F.

As mentioned previously, rotary switches can become very confusing if you lose your reference point(s) between the schematic and physical switch. 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.............
Logged

Never change a password on a Friday                
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!