HP 608C Repair.

[neo]

AHA! A change! the output meter now responds...... but wait there is no output

Since the output meter is an RF indicator there is RF present which makes me question how you concluded that there is no output...?

Using a 10X probe i put my scope on the output connector and got nothing.

It sounds like like the output stage is where the problem is then to my mind. Can you detect any sign of a signal prior to the output stage at all?

I'm actually having trouble actually physically finding where the signal comes from, yeah i'm an idiot but tracing the cable lead me to mechanics not circuitry.

[neo]

OK having figured out what i'm looking at, i'm left wondering which would be a good spot to probe? The attenuator is deeply buried circuitry.

[amspire]

It sounds like like the output stage is where the problem is then to my mind. Can you detect any sign of a signal prior to the output stage at all?

I'm actually having trouble actually physically finding where the signal comes from, yeah i'm an idiot but tracing the cable lead me to mechanics not circuitry.

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

[neo]

It sounds like like the output stage is where the problem is then to my mind. Can you detect any sign of a signal prior to the output stage at all?

I'm actually having trouble actually physically finding where the signal comes from, yeah i'm an idiot but tracing the cable lead me to mechanics not circuitry.

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

Infinite in all ranges.


On the attached picture, the connection under the red circle was where i probed. Set to 12.5MHz and got 12.45MHz, it changes frequency as i adjust.

I think i have just narrowed the problem down to a bit of circuitry i'll have to take the thing to bits to get to.

[amspire]

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

Infinite in all ranges.

.

Looking again at the circuit and the attenuator is capacitively coupled so you will not see resistance.

They are easy to damage, which is why the service note mentions how to replace them.

To check it, you would need to feed in probably 10MHz or more from a 50 ohm source and see if the source voltage halves as it should. If you do this, Watch The Level!. Accidentally connecting RF transmitter outputs to these generators is exactly how these attenuators get blown.

It is not the easiest thing to check as the RF gets via a pickup coil - there are no input connections. To do the ranges, a switch seems to physically switch different output transformer coils.

[neo]

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

Infinite in all ranges.

To check it, you would need to feed in probably 10MHz or more from a 50 ohm source and see if the source voltage halves as it should. If you do this, Watch The Level!. Accidentally connecting RF transmitter outputs to these generators is exactly how these attenuators get blown.

It is not the easiest thing to check as the RF gets via a pickup coil - there are no input connections. To do the ranges, a switch seems to physically switch different output transformer coils.

Yeah... i don't know how to do that. 

That switch would the range selector.

[amspire]

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

Infinite in all ranges.

To check it, you would need to feed in probably 10MHz or more from a 50 ohm source and see if the source voltage halves as it should. If you do this, Watch The Level!. Accidentally connecting RF transmitter outputs to these generators is exactly how these attenuators get blown.

It is not the easiest thing to check as the RF gets via a pickup coil - there are no input connections. To do the ranges, a switch seems to physically switch different output transformer coils.

Yeah... i don't know how to do that. 

That switch would the range selector.

OK. We can just improvise. Do you have a digital oscilloscope? Attach a 470 ohm resistor to the output and attach the oscilloscope across the output. Set the scope to a single trigger on 500mV and touch 1v to the resistor. You should get an instant jump to just under 0.1V and then there should be a curve up to 1V that takes about 10uS. If you see that, then the attenuator probably works.

Might have to take a few goes to capture it.

If the voltage shoots straight up to 1V in nanoseconds, then things do not look good for the attenuator.

[neo]

If you put an ohm meter across the output, what is the resistance? Try different ranges. Powered off of course.

Infinite in all ranges.

To check it, you would need to feed in probably 10MHz or more from a 50 ohm source and see if the source voltage halves as it should. If you do this, Watch The Level!. Accidentally connecting RF transmitter outputs to these generators is exactly how these attenuators get blown.

It is not the easiest thing to check as the RF gets via a pickup coil - there are no input connections. To do the ranges, a switch seems to physically switch different output transformer coils.

Yeah... i don't know how to do that. 

That switch would the range selector.

OK. We can just improvise. Do you have a digital oscilloscope? Attach a 470 ohm resistor to the output and attach the oscilloscope across the output. Set the scope to a single trigger on 500mV and touch 1v to the resistor. You should get an instant jump to just under 0.1V and then there should be a curve up to 1V that takes about 10uS. If you see that, then the attenuator probably works.

Might have to take a few goes to capture it.

If the voltage shoots straight up to 1V in nanoseconds, then things do not look good for the attenuator.

Unfortunately i don't have a digital scope, i do have a (very nice) analog one, and if i understood your instruction clearly then the attenuator would appear to be blown. I tried a 10k just to be sure, still went high fast.

EDIT, the two labels i forgot to edit are Scope Probe and RF out

[ArthurDent]

The coils for the different frequency bands are on a turret and these coils are switched in front of an output port that is just a hole in the cast osc/amp assembly. On the outside of this hole is a cylinder that the attenuator slides in and out of. There is also a fixed coil on the inner end of this cylinder that runs across the opening and that coil may even be a straight piece of wire and is the pick-up coil for the meter circuit. The attenuator has a one-turn coil that is moved in and out on a smaller concentric metal tube.

These two coils are inductively coupled to the switched turret coil. The fixed coil’s output is rectified to drive the meter and the movable coil is moved in and out to determine the attenuation by varying the inductive coupling between the turret coil (primary) and movable coil (secondary). The movable coil is capacitively coupled to the output as previously pointed out (on the C, not the D model). The output impedance (or A.C. resistance) is 50 ohms but you can’t see that on a DMM which measures D.C. resistance. Amspire has presented a great way to check it but you’d need a second signal source of greater than 10Mhz to do that.  If you don’t have access to another source there is another way to troubleshoot the problem.

Seeing that the meter circuit and the attenuator get their signal from the same coil inside the osc/amp assembly, if one of them works properly the problem is outside the assembly. If you can adjust the output meter level to the set line and you still have no output, the attenuator has to be defective. Depending on the amount of use even the coax may have been flexed too many times but I’d be looking at the resistors and coil inside the attenuator.

[ArthurDent]

Oh, one other way to check out part of the attenuator is to use a component tester or DMM with a capacitance range. It appears the capacitor in series with the resistor/coil is .01 microfarad (10nF) if I read the drawing in a previous post correctly. If you can't read that capacitor you have an open circuit. I tried reading a .01 through a 100 ohm resistor with both instruments and it does work.

[neo]

At this point i am fairly sure the attenuator is blown, not sure what caused it because it worked when i got it though it does appear to be blown now.

So under my assumption that it is blown, do i take it apart? Sub in new parts? Give up and accept defeat? What should i do? 

[neo]

OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

[Specmaster]

OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

Looking more and more likely you may have a dodgy tube there, I'd personally at this juncture be looking for a way to test them. At least then you could eliminate them from the equation if they turn out to be OK, one more suspect ruled out of contention. Caps, resistors, transistors and coils etc can be checked normally using the tools you already have, but tubes are not that easy.

[neo]

OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

Looking more and more likely you may have a dodgy tube there, I'd personally at this juncture be looking for a way to test them. At least then you could eliminate them from the equation if they turn out to be OK, one more suspect ruled out of contention. Caps, resistors, transistors and coils etc can be checked normally using the tools you already have, but tubes are not that easy.

Two problems,
I've made a mess of the attenuator probe.
No tubes in the way, i have oscillation but not an output. The output is there but not going through the attenuator probe.

[Specmaster]

OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

Looking more and more likely you may have a dodgy tube there, I'd personally at this juncture be looking for a way to test them. At least then you could eliminate them from the equation if they turn out to be OK, one more suspect ruled out of contention. Caps, resistors, transistors and coils etc can be checked normally using the tools you already have, but tubes are not that easy.

Two problems,
I've made a mess of the attenuator probe.
No tubes in the way, i have oscillation but not an output. The output is there but not going through the attenuator probe.

You proved that the output was leaving the tube then?

[neo]

OK I may or may not be losing my mind. Capacitor now a bit ugly but still functional and i'm at a loss with fixing this thing.

Looking more and more likely you may have a dodgy tube there, I'd personally at this juncture be looking for a way to test them. At least then you could eliminate them from the equation if they turn out to be OK, one more suspect ruled out of contention. Caps, resistors, transistors and coils etc can be checked normally using the tools you already have, but tubes are not that easy.

Two problems,
I've made a mess of the attenuator probe.
No tubes in the way, i have oscillation but not an output. The output is there but not going through the attenuator probe.

You proved that the output was leaving the tube then?

I probed the connection in the red circle of this picture and got the frequency.

[ArthurDent]

The suggestion of removing either of the two 5675 pencil triodes from the osc/amp assembly is a very bad idea and as I previously stated should be a last resort. First, there is probably a 0% chance you can find a highly specialized tube tester that could test them, they aren’t your usual plug-in tubes. Second, in circuits like this, manufacturers generally will say not to even test tubes because if they work in the circuit that is all you’re interested in. Plus most tubes will work well outside of what a tube tester will say is weak or bad.

As I said before, the best way to check the tubes is to check the supplies feeding them plus, in this case, the R.F. output meter. The question I’ve asked before is whether the output to the meter can be adjusted to the set line and is that reading stable. The manual states the meter is reading the voltage at the input to the attenuator so if the meter indicates proper output, forget the tubes. If the meter indicates properly the problem is external to the osc/amp assembly and messing with it is a good way to really screw the HP 608 up beyond repair. The HP608 was calibrated after the tubes were properly installed and even moving some of the connections at these higher frequencies could really change calibration. Checking the filament voltage and current then checking the B+ will check the inputs to the tubes and the meter will verify the output.

[ArthurDent]

If the 5675 tubes need to be replaced this could require recalibration of the signal generator. The tubes are selected for specific characteristics from a number of those available. If you just buy one it might not be within the circuit specs.

[Cyberdragon]

Did you check L17 and R58?

[neo]

LET ME BE ENTIRELY CLEAR!
I've found the problem! I know what the issue is but not what is causing the issue. I don't want to and i have not once thought of removing the the tubes. The issue is not in the tubes!

THERE IS OSCILLATION!

The problem is exactly this,it uses inductive coupling and an attenuator probe in a metal tube to pick up the frequency. For some reason it is never picked up.

EDIT to add;
Sorry if this was over the top.
It might not help that to me this method as well as most VHF and up is perceived as a kind of black magic. 

[Specmaster]

Do you think the message has sunk home yet?

Can the attenuation probe be dismantled at all to gain access to the components inside or does it have to replaced as a sealed unit? 

[neo]

It, as far as i can tell, doesn't appear to be something that can be disassembled. Unless the variable capacitor in the thing is in the order of a few puff then i think it went short.

[Cyberdragon]

It, as far as i can tell, doesn't appear to be something that can be disassembled. Unless the variable capacitor in the thing is in the order of a few puff then i think it went short.

Highly unlikely, it's a tiny mica cap, which won't short unless there's corrosion. Did you actually try pulling (hard) on it from the end where the components stick out? Is there any solder around the rim you can melt off?

[neo]

There is nothing to pull on.

[Cyberdragon]

There is nothing to pull on.

The end comes off somehow (look at the pic above). There should be a seam. It might be covered in solder though.