Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Oct Sat 21, 2017 3:51 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 31 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: HQ-140X Det/AVC voltages
PostPosted: Oct Thu 12, 2017 11:23 pm 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Manual says voltages on Det/AVC/Limiter tube (6AL5 is V7) using a VTVM with RF gain MAX and AF gain MAX, MANUAL setting with no antenna:
pin1 -0.2V
pin 2 -0.4V
pin 7 -0.2V

I get these values with RF gain/sensitivity at MINIMUM; when I go to MAX sensitivity I get

pin1 -7V
pin 2 -5V
pin 7 -2.5V

What gives? FWIW, the voltages read more or less correct until 80% of MAX, then they just spike up. So for the first 80% all is good (I will just use it that way). Maybe pot is bad?

Also find when BFO is engaged 12AU7 pin 7 measures -30 volts (not the expected -13V). Independent of RF gain.

And yes, my VTVM is calibrated and I am using a 1 meg resistor in tip. But this is not a VTVM thing, I get same crazy readings using my DMM. Radio WORKS fine, I am just curious what could be wrong here. All other POSITIVE voltages are just fine.

paul


Last edited by K3STX on Oct Fri 13, 2017 4:39 am, edited 2 times in total.

Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Thu 12, 2017 11:51 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
Paul,
How noisy is the receiver with the antenna disconnected? It is developing -2.5 volts of AGC with no antenna connected which doesn't sound right, I would expect far less as suggested in the manual voltage table. Perhaps a noisy tube or a stage is oscillating.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 1:14 am 
Member
User avatar

Joined: Sep Wed 10, 2014 2:01 am
Posts: 1390
Location: Costa Mesa, California
I agree with Rodger. It sounds like something breaks into oscillation when the RF gain is turned up above a certain point. This is not uncommon--especially if work has been done around the mixer tube circuit or other amplifier circuits. I have had this happen to me and resolved the issue by re-routing leads and components close to the chassis and around the other side of the tube socket. This is where a jiggle stick can come in handy. Get the radio right to the point before oscillation or right after and try poking things--especially connected to the tube grids.

Norm

_________________
KK6IYM


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 2:45 am 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Funny you both should mention oscillation. I had NOT speaker connected to there radio (and no antenna), and when I increased the RF gain control to the point where I started getting crazy high readings (at about 80% of max) I began to hear a BUZZ out of the radio!! I was trying to hear where it was coming from (radio on its side), but I am a scaredy-cat and didn't feel like putting my head right up there next to parts to find out where the noise was coming from. But it was certainly strange.

I have been hooking leads on and off of the grids of this radio while learning how to do oscilloscope alignment with sweep. Maybe something got loose. Or maybe a bad tube. I will check around. What I decided to do last night it just keep RF gain at 75% and THEN do my sweep IF alignment with scope and RCA WR-50B. It worked great; beautiful to peak the IF to symmetry.

paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 3:08 am 
Member
User avatar

Joined: May Fri 04, 2012 10:39 pm
Posts: 1210
Location: Hidden Valley, AZ
Running a tube-type audio amp without a load (speaker) at high levels can ruin the output transformer.

What you most likely heard was it singing.

DG

_________________
There's a reason it wasn't Werewolves of Lubbock....


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 3:21 am 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Hmmm, OK, I will try that. Manual never mentioned that. Of course, manual doesn't mention LOTS of things (and Hammarlund manuals are notoriously terrible with lots of mistakes). But I sort of doubt this is the problem.

NOTE: Just tried it, no difference. Still problem. But now increasing RF gain blew my ears off (with antenna grounded).
And I can see VR tube increase in the "blue" as I get past that 80% point. I will start poking around.

paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 7:25 am 
Member
User avatar

Joined: Sep Wed 10, 2014 2:01 am
Posts: 1390
Location: Costa Mesa, California
When I say that some part of the circuit breaks into oscillation, I mean that there is a feedback loop in operation. Any tube with a grid is essentially an amplifier. If there is a path to feed the signal from the amplification process back to the grid, then the tube has a runaway loop situation that is an oscillator out of control. This is why the leads and components connected to the grid need to be arranged such that the signal can't get back into the grid path after it appears on the plate (or cathode).

One crude diagnosis technique is to let the receiver break into oscillation and then start pulling tubes from the RF amplifier, mixer, and IF sections one at a time. When the oscillation stops--you have the likely culprit.

Norm

_________________
KK6IYM


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 11:53 am 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
Check the screen bypass capacitor connections in the RF and IF sections to make sure one of those wasn't disturbed when you were unhooking leads.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 1:27 pm 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Norm hit it on the head. I pulled the mixer tube and this "problem" went away, could go to max RF gain and voltage on pin 1 V7 stayed at -0.2 volts. Replacement of mixer tube did not help. I will poke around, thanks guys.

paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 1:36 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
Paul,

Did you also try pulling the RF amp tube? It could appear in that stage.

Note that tube pulling helps provide a clue to the problem area however pulling a tube downstream from the oscillation source may cause the problem to go away since that tube is now isolated from the gain chain even though it is still oscillating while pulling one upstream from the culprit may also kill the oscillation since the noise needed to excite the oscillation event has been removed.

If the RF stage is at fault often changing the antenna trimmer setting will have a major impact upon the oscillation.

A missing/open bypass on the AGC bus is another prime suspect when you have feedback.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 3:17 pm 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Yes, i did pull the RF amp tube and that did not fix my problem. I can go back later and try pulling more tubes along the IF chain, but now I think I know what I am looking for.

paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 3:40 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
OK Paul,

The mixer can go unstable but since the mixer is neither on the RF gain nor AGC buses reducing the RF gain with it in place shouldn't have cleared a mixer instability problem. The mixer is the primary noise contributor in the pre-detection portion of the receiver so the noise generated in this stage may be what is providing enough signal to allow the later unstable stage to break into oscillation; in receiver design the RF stage needs enough gain to overcome mixer contributed noise which is how the RF stage sets the basic noise figure for the receiver.

So in short it could be the mixer stage but I would not be surprised if a later IF stage is at fault. You can test this by pulling the mixer tube and then coupling a small amount of RF (at 455 Khz.) into the primary of Z1 via pin 5 of the V3 socket. Use a coupling capacitor to isolate your signal generator from B+ and start with a very low level signal from your signal generator and then slowly increase it and see whether the set suddenly takes off with oscillation in a later stage.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 8:53 pm 
Member
User avatar

Joined: Sep Wed 10, 2014 2:01 am
Posts: 1390
Location: Costa Mesa, California
Many times components at tube bases share common grounds. If the ground is not good, there could be problems. With the old large paper caps, most of the by-pass leads ended up away from the tube at another ground location. With new small caps, people like me tend to find convenient grounds closer to the tube base. This means components start to share grounding points. You can have a cathode by-pass cap sharing a ground with a grid resistor and the ground may not be making good contact. It is another thing to look at and consider.

Norm

_________________
KK6IYM


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Fri 13, 2017 9:22 pm 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
W7TFO wrote:
Running a tube-type audio amp without a load (speaker) at high levels can ruin the output transformer.

What you most likely heard was it singing.

DG

I did not realize that. Are you saying that when I am troubleshooting voltages I should always have a speaker attached even when not "listening" to the radio?

paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sat 14, 2017 1:15 am 
Member
User avatar

Joined: Sep Wed 10, 2014 2:01 am
Posts: 1390
Location: Costa Mesa, California
I find that contradictory to many alignment procedures that call for the AF gain at max. This is either going to get the neighbors riled up or ruin my output transformer. I don't ever follow those alignment instructions as I like to have a speaker hooked up to hear what is happening. I set the AF gain at a comfortable listening level and peak the output meter.

Norm

_________________
KK6IYM


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sat 14, 2017 3:24 am 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
I agree with you Norm on the audio gain at max issue. The idea behind that was to make sure that gain compression didn't occur in an earlier RF or IF stage during alignment so the output stage runs at maximum gain with just enough drive from the signal generator to provide a usable reading on the output meter. But you can reach this requirement with far less than maximum audio gain.

You definitely want either a speaker or sufficiently large power resistor of the proper impedance connected to the audio output at all times to avoid damage to the output transformer. With no load on the secondary extremely high voltages will be built up in the transformer that can cause insulation punch through. Some of the vintage receivers have protective covers over the audio output terminals because the high impedance terminals available on some receivers will produce significant voltage across them, for example 10 watts audio output across the 5,000 ohm tap available on some receivers will result in over 220 volts.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sat 14, 2017 3:18 pm 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
Maybe my problem is that I am measuring voltage at wrong point. I am using VTVM on pin 2 of V7 (6AL5). But even when cruising BCB when I come across a strong station with RF gain at half max my voltage on this pin goes to maybe -20 volts; it doesn't stay at -0.4 volts. And RF gain control can vary this voltage.
Why would the same also not be true during alignment? Changing strength of "signal", either by tuning dial or injecting 455 kc into IF with dig generator would change the voltage, wouldn't it?

Radio SOUNDS good, I wonder if what I am seeing is normal but I am connecting something incorrectly. Do other owners of HQ-140X not find that RF gain control affects voltage on pin 2 V7?

Paul


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sat 14, 2017 3:58 pm 
Member
User avatar

Joined: Jun Sun 19, 2011 2:31 pm
Posts: 4583
You are measuring developed AGC voltage when measuring at that pin so it will be affected by signal strength (which is itself affected by the RF gain control setting). Monitoring the voltage here for alignment is fine as long as you keep the signal generator to the point where gain compression doesn't occur, basically keep the signal generator input low enough just to have a significant rise in voltage above the no signal level. Many later receivers included a procedure for using the built in calibrator and S meter (measures AGC level) for the RF and IF calibration.

The concern with your receiver is you were reading a large increase in this voltage at full RF gain with no antenna attached which should not happen.

Rodger WQ9E


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sat 14, 2017 5:29 pm 
Member

Joined: May Sat 12, 2012 1:33 pm
Posts: 1330
Location: Rochester, NY.
I had a National NC-98 that went into oscillation as the RF gain past the 75% position to full, even without an antenna. Stations had a 'heterodyne' like the BFO was on, but the BFO was off.
The S-meter pointer would stay up even when tuning between stations in dead air.
AGC voltage would go way negative when the 'howl' started. De-tuning the IF transformers would ease the heterodyne out and stabilize operation, but at the loss of gain and fidelity.
It seemed strongest at the 2nd IF amp area, but that may have been from the cumulative RF/IF amplification up to that point.
I had just completed a wax cap and high-Ω resistor service when this anomaly started. This is when I found out how important the original lead dress and original harness routing really was.
I found some under-chassis photos online in order to get the wiring and components back to how they looked from the factory:
http://n9dd.net/images/nc98-1.jpg
See if you can find an underside image of an HQ-140X to compare yours. Best of luck to you.


Top
 Profile  
 
 Post subject: Re: HQ-140X Det/AVC voltages
PostPosted: Oct Sun 15, 2017 2:50 am 
Member

Joined: May Tue 16, 2017 3:03 pm
Posts: 86
Location: Potomac, MD 20854
rsingl wrote:
Monitoring the voltage here for alignment is fine as long as you keep the signal generator to the point where gain compression doesn't occur, basically keep the signal generator input low enough just to have a significant rise in voltage above the no signal level.

Rodger WQ9E


Just to make sure understand, I DO try to keep the sig generator input very low but PART OF "keeping it low" was to ALSO keep the RF gain low. I take it this is NOT the way to go? (My dog generator only has LOW and HIGH and an attenuator. So it is pretty loud.

But my RF gain should be MAX according to this discussion; i have to figure out how to stop "nonsignal" high voltage.

Paul


Top
 Profile  
 
Post New Topic Post Reply  [ 31 posts ]  Moderator: Sandy Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: WoodchuckTN and 10 guests



Search for:
Jump to:  




















Privacy Policy :: Powered by phpBB