Author Topic: B&K 2040 Repair video  (Read 19301 times)

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Offline The Radio Shop

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Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #1 on: August 09, 2014, 08:41:32 AM »
Here the the circuit that is bad.

Offline The Radio Shop

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Re: B&K 2040 Repair video
« Reply #2 on: August 17, 2014, 10:04:02 AM »
The next 2040 I am going to attempt to fix is in bad shape.
The first issue is the PLL is not operating properly. Very little signal out and it does not change when you select different frequencies.

First thing we will do is check the power supply and verify we have the correct voltages.  Since the 74xx chips needs 5 volts to operate we will have to verify that they are spot on and have little ripple.
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Re: B&K 2040 Repair video
« Reply #3 on: March 29, 2015, 09:03:58 AM »
Another 2040 here in the shop has bit the dust. This is the one I repiared the meter error months ago.
It has the deaded PLL unlock symptom. You turn unit on set to generate and the radio under test hears nothing. A few minutes later the radios hears a signal sweeping the band. So the PLL is running wild and is not locked on one frequency.

My weakness in radio has always been the PLL circuit.  So I have a lot to learn. There has been others that have helped me through it to understand more about how PLL works. The PLL in these type of generators are a bit different than you ham or cb radio. These unit run a 27 MHz signal at the VCO's output.

The stock service material available for the 2040 is not bad. However they did not give you a block diagram, there are no frequency path on the schematic, and most circuit blocks are not labled what they are. There is a copy of the manual with some of this info available. The stock manual does provide a voltage chart.

So at the momnet I have 3 of these units. My 2040 on the main bench is in pretty good shape and works fine. Now the unit on my other bench has failed. This one is the one I replaced an opamp in and is a very clean and has never been worked on that I can tell. The third unit is one I bought that was non working. Someone had modified it for extended frequency coverage. Has a lot of hack jobs done to it but it is doing the same as the other unit.

So what I am going to do is take my working unit and open it up. Go through it with a volt meter and scope noting readings onto the schematic.  In all hope we can find out more of what goes on in these old test gear.
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Re: B&K 2040 Repair video
« Reply #4 on: March 29, 2015, 09:07:42 AM »
The manual listed below is the one that has the circuit lables. It is what I will be using to make the test notes.

I like all that want to join in please do. But lets keep the thread on topic please.
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Re: B&K 2040 Repair video
« Reply #5 on: March 29, 2015, 08:25:33 PM »
What I found today


It has a 10.240 and a 10.243 oscilator.
I figured out say channel 20 you take 27.205 subtract the doubler frequency of 20.48 equals the clock frequency of 6.725.
The VCO runs at 27 meg signal.  Have not figured the doubler signal yet.

What I did find on the bad unit is at Q307 there should be 27.205 MHz while on channel 20.  But I am getting 24.7526 MHz there.
Also IC304 should have 6.725 MHz on pin 12 and 13.  12 and 13 are tided together. I am getting 8.889 MHz on 12 and 4.279 MHz on 13. Not sure how this is happening unless there is an issue with IC304.
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Re: B&K 2040 Repair video
« Reply #6 on: March 30, 2015, 04:48:49 AM »
schematic
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Re: B&K 2040 Repair video
« Reply #7 on: March 30, 2015, 04:49:05 AM »
schematic
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Re: B&K 2040 Repair video
« Reply #8 on: March 31, 2015, 05:44:51 AM »
Greetings!

Man I'd just hate to have you start dropping in parts, but I've gone over both schematics. The programmable dividers...how many clock cycles to the carry before the "overflow - carry" onto the next divider? I see Pin 11 looks to be the reset so the pulse train starts back at zero back at IC305.

Something I really do not want to do with this old thing.  The way you have to lift the board out to get under it can cause lots of problems. The board is double sided with plated feed throughs which opens another can of worms.  A lot of care had to be taken not to damage things.

Quote
Now, if the PLL is going on out lock, how is the health of Q304 and Q307 - now I do see the Varactor on Q307 as well as several electrolytic caps - it's why I'm asking about their health. I'm under the belief that if both Xtals - going through the phase detector and the separate frequency divider - if they don't match then the unit is sensing this and shutting down.

I scoped both Q304 and Q307 this past sunday. And compared the readings to my working unit while having both units set the same. Even verified with diferent scopes to see what is happening.  On the gate of Q307 I was getting the signal of 24.7526 (selector on 20) I removed the coax at node "P" at the gate of Q307, At the gate end I had no reading on the conter. On the coax end I had the 24.xxx signal.

Quote
I see a tie-in to IC308 and IC 310 - both getting input from the Master oscillator. You're getting 24.7xxx and missing 6 MHz from it. Only wonder if the output of the master Oscillator has weakened to a point where clock tracking is affected by power supply noise. IF you need 6MHz, it's gotta' find the timing by counting off the two oscillators shift off a signal then continue the run - to me - my biggest headaches were from the green mylar Chiclets that "supposedly" filtered the TTL noise out and reduced ripple and the spike noises back then in the Personal PC 6502 series chips - a bad clock wasn't really the problem - it was the power supply sourcing then these things "paused" to handle interrupts - sometimes they'd miss and you'd get the lockup from unexpected address "garbage" - it missed a pulse train due to interpreted a spike as a clock pulse.

I will go back and revisit this area and take more readings.

Quote
So, what is the general health and output signal level of Q304 and Q307 into the counter IC's? (If the coax jumper can be removed, verify high - impedance across the input - no shorts only to the respective ends of the coax jumper at each end - no kinks...
The coax is very easy to remove. It is a good way to isolate stages and do test. I will revisit this area also.

Quote
I'm thinking signature may be needed to see if 6MHz pulse train never gets summed...referring back to the PLL getting timing - but the clock interrupt causes the missing pulse train - especially if the frequency of the 24.7xxx really bounces by proximity of your hand or lead capacitance from the probes - it would explain how you're pulling the clock pulse for timing but the shift is too slow and gets missed - that may be a noise issue onto poor signal level onto something even more exotic..

Regards!
:+> Andy <+:

Yep. It does not take much to load these circuits down. Again this is one good reason that I have one working unit here. I plan this week to take lots of measurements both voltage and scope and add them to the overall schematic. Those caps you are seeing on the schematic are Tantalum. No electrolytic on the PLL board.  Most of the time when a tantalum fails it leaves a horrid explosion crater at ground zero.

I have a feeling this is going to be a long repair. But once we understand more of what is going on in this maze of discrete logic the more we can find.

In the snippet below you can see R385. I am going to pull R385 end up near IC316. I will then attach my variable power supply at that point and see if the VCO is actually tuning. From what I see here we should have a tuning voltage of 1 to 6 volts DC. On channel one I measured 14 volts at that end. Very high.  This tells me there is some out of lock error there. I just have to verify more on the working unit.  For those that need to know the varactor diode and be replaced by one from a cobra 29.

Wished I had all my notes here at work that I written down over the week end. I plan on putting this on the schematic also.
What we need in more theory :)
I think this train ride is going to be wild.
Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #9 on: March 31, 2015, 08:29:35 AM »
Greetings!

Ok, I'll back off a little - you looked like you needed a second, third or fourth opinion on some stuff but by the way it looks - reading by your experience - this is going to be a tough dog to teach...

Regards!
:+> Andy <+:

No need to back off. All good.  I just need to verify things. Plus at work so my replies from here are short, quick, then back to work. Lot of wondering eyes here.
I will say this unit has one of the strangest PLL circuits I ever ran across. And this is only the second one I have had my hands on that has a PLL issue.  The other dozen or so I worked on were audio board issues, low output, attenuator problems. I think once we have a schematic with more data on it, waveforms and voltage measurements, things will become more clear.

Lots of work here to do....
Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #10 on: March 31, 2015, 04:19:12 PM »
Hmmm, I quoted a post and things when hay wired, never seen this happen before.
Let me see if I can fix it.

K got it fixed and here is my reply


Strange, My eyes must be going on me. I never seen that varactor diode there. I knew there was one at 306. Not sure why they put a varactor off the crystal like that. interesting.
Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #11 on: April 01, 2015, 09:05:35 PM »
While doing some testing tonight I found a little something that is not correct.
The signal of 10.240 enters IC308 pin 1, This is a divide by 16 on the output pin 11. Should be 0.64MHz.  But I am getting 60.28KHz.

First pic is area on schematic.
Second is pin 1
Third is pin 11
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Re: B&K 2040 Repair video
« Reply #12 on: April 02, 2015, 05:42:25 AM »
Hmmm, You may be on to something Andy.  :D
I called myself checking the supply rails, but fail to check the health of them. Also I checked the supplies at the source and not at the PLL board.
Rats, another reason why I should invest in a Constant Current Dummy Load. Thought I would get by building that little 2 amp supply. I may be seeing a used Agilent in my future :)


When I leave the salt mine today I may pull the 5 volt rail and attach my DC 5 volt line to the input of the PLL board. Then take another reading and see what I find.

Well looks like I may have to take a peek under that bed. All the supply filtering is done on the audio board. And is a pain to get into.
I will let you know what I find.
Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #13 on: April 02, 2015, 06:27:40 AM »
For those playing along at home here is the power supply schematic.
Buddy kc4umo

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Re: B&K 2040 Repair video
« Reply #14 on: April 02, 2015, 03:04:50 PM »
I have not even started the yard clean up yet. I have 4 days off after today. Perhaps I will begin......But who knows :) Rain coming.
I live on a sand dune. Yes, sand dune. I am 14 feet above the main road. There are many sand ponds around my place. So that means lots of skeeterz.

 Not sure if I will get anything done in the shop tonight. But I want to pull out the Tek And the CS-61 scope and look at these signals again. Sometimes a DSO will pick up artifacts and put you on a hunt. And I have to agree that these logic devices do not like noise but pure sign waves. Really makes me wonder what B&K was thinking when they designed this piece of gear. Does not follow their usual quality. But since it was aimed for the cb market....They were just testing the waters and their competiters, Sencore.

I really am starting to question the 5 volt rail more. So will check ripple on that and see what it looks like.
C2 and C10 make be some real killers.
Buddy kc4umo