Electronics Forum > Test Equipment
B&K 2040 Repair video
The Radio Shop:
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.
The Radio Shop:
schematic
The Radio Shop:
schematic
The Radio Shop:
--- Quote from: Handy Andy on March 31, 2015, 12:58:49 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.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
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...
--- End quote ---
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 <+:
--- End quote ---
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.
The Radio Shop:
--- Quote from: Handy Andy on March 31, 2015, 08:15:29 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 <+:
--- End quote ---
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....
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