HP 608C Repair.

[neo]

Why do i need a 100x probe exactly? My scope has a maximum input voltage of 500V ac coupled, and even if there is 500V present then with a 10x probe it becomes 50V, well within spec.
Am i missing something? 
*NOTE*  scope rating at 1 kilohertz or less, can't find a rating beyond that and the manual don't say.

Hmm, I know what you mean, but I've discovered that a local company to me that specialises in sourcing hard to find parts actually has a tester for their own use and will also test for customers. Try looking on line for companies that sell tubes in your area, they will also more than likely be able to test them for you.

I don't even know how to begin looking for that... yes the google-fu is weak in this one. Just try searching for tubes by location perhaps?

[amspire]

All of which would be easier with the correct manual, all i can find is the 608D.

I think the only differences is that the 608D has the crystal oscillator and headphone amplifier that is not in the C version, and the output stage is different. Perhaps the 608C does not have the buffer stage - you will work it out.

Edit to note: I'm not relying on the meters to tell me if i have an output, i put the output on my scope and got nothing.

Just be careful with the scope. You can destroy the scope input very easily especially if you are using x1/x10 switch-able probes.

Probes like this make your scope pretty safe:
https://www.aliexpress.com/item/100X-P4100-Oscilloscope-Probe-High-Voltage-2KV-Oscilloscope-Scope-Passive-Clip-Probe-Kit-100MHz-With-Marker/32850502604.html
The maximum voltage that ever reached the scope inputs is 2.25V with this oscillator - that is very safe.

To test the oscillator, you should be able to put the scope on the band switch contact going from the oscillator stage to the grid of the next stage. If there is a signal there, then you have a problem in the amplifier stages. No signal and the oscillator has a problem.

The oscillator valve gets its filament voltage from that DC mod circuit, and that could fail, particularly if they used a germanium PNP power transistor. The voltage on the transistor emitter should be about 9.3 to 9.7 volts DC. If it is 12-16V, then the transistor is shorted and the filament in the oscillator valve may have been damaged. Hopefully it is OK - that will be a really hard valve to replace:
http://www.r-type.org/exhib/aaa1026.htm
The valve came out at the same time as the oscillator, so the valve manufacturers may have designed it to HP's spec. Definitely not a standard valve.

Be really careful probing anywhere near the valve or the DC filament circuit. One slip could destroy the valve.

[amspire]

Why do i need a 100x probe exactly? My scope has a maximum input voltage of 500V ac coupled, and even if there is 500V present then with a 10x probe it becomes 50V, well within spec.
Am i missing something? 

If you never make a mistake, and if the x10 switch never gets accidentally knocked to x1, you probably will not destroy your scope. Put some tape around the switch so it cannot ever go to x1 and test it.

If you use x1/x10 switchable probes near high voltage without disabling the switch, you will one day make that 1 second mistake.

[tautech]

Why do i need a 100x probe exactly? My scope has a maximum input voltage of 500V ac coupled, and even if there is 500V present then with a 10x probe it becomes 50V, well within spec.
Am i missing something? 

If you never make a mistake, and if the x10 switch never gets accidentally knocked to x1, you probably will not destroy your scope. Put some tape around the switch so it cannot ever go to x1 and test it.

If you use x1/x10 switchable probes near high voltage without disabling the switch, you will one day make that 1 second mistake.

Yep, do yourself a favor and limit usage of 10x probes to less than 200V.
With 100x probes; voltage rating, frequency derating, scope and personal safety are much enhanced.

[neo]

Why do i need a 100x probe exactly? My scope has a maximum input voltage of 500V ac coupled, and even if there is 500V present then with a 10x probe it becomes 50V, well within spec.
Am i missing something? 

If you never make a mistake, and if the x10 switch never gets accidentally knocked to x1, you probably will not destroy your scope. Put some tape around the switch so it cannot ever go to x1 and test it.

If you use x1/x10 switchable probes near high voltage without disabling the switch, you will one day make that 1 second mistake.

How fortunate then that my good probe isn't switchable and is 10x only. Thanks for all your help up to this point, i'm not a stranger to high voltage but neither am i an expert so your help and input is valued. Especially since my knowledge of vacuum tubes is iffy.

Getting a 100x probe would mean putting this thing back on the shelf for a couple of months, for financial reasons I'd rather not explain. So unless you say that using a 10x probe, as a last resort, to fix this thing is suicide (scopeicide?) then i will use the 10x, as a last resort.

[neo]

Just be careful with the scope. You can destroy the scope input very easily especially if you are using x1/x10 switch-able probes.

I thought the output was limited to a volt? 

[tautech]

Neo, I have in the past used 10x probes on mains but I'm reluctant to do so now.

If you take a small step back and think before you connect you'll be OK if you know what you're doing.
Get the DMM out for a quick sanity check rather than risk your scope or yourself. 

[neo]

Neo, I have in the past used 10x probes on mains but I'm reluctant to do so now.

If you take a small step back and think before you connect you'll be OK if you know what you're doing.
Get the DMM out for a quick sanity check rather than risk your scope or yourself. 

Yes, exactly the words i was looking for. Save the oscilloscope for a last resort option, and only if the point in question has already been examined and found to be within bounds.

Use anything but the scope on the inside of the 608C unless i have to. Just a little bit of common sense goes a long ways.

"What does this do?" Famous last words.

[amspire]

Make up a little probe like this:

For the input capacitor, use something like 30pF to 100pf instead of 100nF. Try and find a 500V capacitor if you can. A 50V capacitor is not enough.

or this:
http://www.electroschematics.com/659/rf-probe-circuit/

Use common signal diodes such an 1N4148 or 1N914. Nothing special as long as it is a fast low power diode.

The construction can be quite crude - parts soldered together and wrapped in tape. Just make sure the input capacitor that takes all the DC voltage cannot short out on anything.

In both cases, the negative rail has to connect to the chassis. A clip on the probe end would be handy. Keep the probe end short if possible.

Connect it to your multimeter on the volts range.

If RF is present, you will get a voltage out. If it is not, you will get zero volts out.

For detecting Audio signals in the 608C, it should be OK to use the AC voltage range of your multimeter. It probably has a frequency range too that you can use to check the 400Hz/1000Hz.

[Specmaster]

Neo, I have in the past used 10x probes on mains but I'm reluctant to do so now.

If you take a small step back and think before you connect you'll be OK if you know what you're doing.
Get the DMM out for a quick sanity check rather than risk your scope or yourself. 

Yes, exactly the words i was looking for. Save the oscilloscope for a last resort option, and only if the point in question has already been examined and found to be within bounds.

Use anything but the scope on the inside of the 608C unless i have to. Just a little bit of common sense goes a long ways.

"What does this do?" Famous last words.

That sounds like good common sense to me

[Tomorokoshi]

The RF band switch is critical - if the contacts need attention, that will kill the oscillator.

Check the contacts of all rotation switches to see if there is black silver tarnish on them. Switches of that vintage tend to be silver plated. They go high-resistance / open-circuit if that's the case. Inspect the wiper and the contacts.

With the unit unplugged and discharged an ohmmeter between the wiper and the contact will usually work well. Check at the wires or terminals. If a connection is suffering from tarnish turn the knob a little, and the resistance will jump all over. I like analog meters for this type of work.

[neo]

The RF band switch is critical - if the contacts need attention, that will kill the oscillator.

Check the contacts of all rotation switches to see if there is black silver tarnish on them. Switches of that vintage tend to be silver plated. They go high-resistance / open-circuit if that's the case. Inspect the wiper and the contacts.

With the unit unplugged and discharged an ohmmeter between the wiper and the contact will usually work well. Check at the wires or terminals. If a connection is suffering from tarnish turn the knob a little, and the resistance will jump all over. I like analog meters for this type of work.

How do i hold two probes in place and turn the knob? None look tarnished, testing their connections now.

[Specmaster]

The RF band switch is critical - if the contacts need attention, that will kill the oscillator.

Check the contacts of all rotation switches to see if there is black silver tarnish on them. Switches of that vintage tend to be silver plated. They go high-resistance / open-circuit if that's the case. Inspect the wiper and the contacts.

With the unit unplugged and discharged an ohmmeter between the wiper and the contact will usually work well. Check at the wires or terminals. If a connection is suffering from tarnish turn the knob a little, and the resistance will jump all over. I like analog meters for this type of work.

How do i hold two probes in place and turn the knob? None look tarnished, testing their connections now.

Get a pair test leads with croc clips?

[neo]

The RF band switch is critical - if the contacts need attention, that will kill the oscillator.

Check the contacts of all rotation switches to see if there is black silver tarnish on them. Switches of that vintage tend to be silver plated. They go high-resistance / open-circuit if that's the case. Inspect the wiper and the contacts.

With the unit unplugged and discharged an ohmmeter between the wiper and the contact will usually work well. Check at the wires or terminals. If a connection is suffering from tarnish turn the knob a little, and the resistance will jump all over. I like analog meters for this type of work.

How do i hold two probes in place and turn the knob? None look tarnished, testing their connections now.

Get a pair test leads with croc clips?

Good idea, but the way the contacts work it isn't exactly feasible. Nothing to clip on to.

[Specmaster]

Solder some temporary tags on it?

[neo]

Solder some temporary tags on it?

It is a thought, though for now i think i have checked the contacts well enough. No visible tarnish, no noticeable erratic nature to them. Besides all else they were in a sealed case for the years.

So moving on to basic voltage checks before i start pulling resistors to test.

[Cyberdragon]

Don't remove resistors unless you have to, you'll make a mess. Check them first, if they are higher then rated (by tolerance band) then they are obviously bad. Only remove them to check if they read much lower than marked.

You can use a scope if your B+ doesn't exceed 300 volts (the max for most 10x probes).

[neo]

Don't remove resistors unless you have to, you'll make a mess. Check them first, if they are higher then rated (by tolerance band) then they are obviously bad. Only remove them to check if they read much lower than marked.

You can use a scope if your B+ doesn't exceed 300 volts (the max for most 10x probes).

Thank you, though the way the resistors are soldered in there is no way to lift an end and check the value. To do that, check the value, they pretty much have to be removed.

[neo]

I checked the

heater V, 6.32   VAC
+225   V, 226.7  VDC
-165   V,-165.6  VDC


By all logic this thing SHOULD work.... so why doesn't it?

Also oddly the transistor is 2n1554, i thought it was meant to be a PNP?

Short of taking this thing to bits i am running out of things to check, should i check the resistors now?

[Specmaster]

Don't remove resistors unless you have to, you'll make a mess. Check them first, if they are higher then rated (by tolerance band) then they are obviously bad. Only remove them to check if they read much lower than marked.

You can use a scope if your B+ doesn't exceed 300 volts (the max for most 10x probes).

Thank you, though the way the resistors are soldered in there is no way to lift an end and check the value. To do that, check the value, they pretty much have to be removed.

Personally, I'd be inclined to snip the longest leg of the resistor through, measure the values and if they are within spec, make a solder bridge across the cut, otherwise replace them.

[amspire]

Also oddly the transistor is 2n1554, i thought it was meant to be a PNP?

It is - a 90W Germanium PNP transistor.

That definitely needs checking. If you look at the circuit, there is a 7 ohm power resistor on the emitter of that transistor. If you can read the voltage to both sides of that resistor it will check that the DC regulator is working and if current is flowing to the oscillator valve. If there is about 9.4 volts on one end and something less on the other end, then it is working. If it is 9.4V both ends, then we have a problem.

If you use a multimeter to check the junctions of the 2N1554, you will get a lower voltage drop then usual - perhaps 0.2V for a power transistor. Much lower then silicon.

[neo]

Also oddly the transistor is 2n1554, i thought it was meant to be a PNP?

It is - a 90W Germanium PNP transistor.

That definitely needs checking. If you look at the circuit, there is a 7 ohm power resistor on the emitter of that transistor. If you can read the voltage to both sides of that resistor it will check that the DC regulator is working and if current is flowing to the oscillator valve. If there is about 9.4 volts on one end and something less on the other end, then it is working. If it is 9.4V both ends, then we have a problem.

If you use a multimeter to check the junctions of the 2N1554, you will get a lower voltage drop then usual - perhaps 0.2V for a power transistor. Much lower then silicon.

I cant find that resistor, it looks as if it just goes straight from the emitter through.

EDIT: to fix a brain fart.

[amspire]

I cant find that resistor, it looks as if it just goes straight from the collector through.

Do you mean the emitter? 

[neo]

I cant find that resistor, it looks as if it just goes straight from the collector through.

Do you mean the emitter?

I did, thank you for catching that. Besides no resistor there is also no voltage on the emitter of that transistor.

[neo]

1.5   VDC on the base
5.8   VDC on the collector
.075  VDC on the emitter
.15   VAC on the emitter
.06   VAC on the collector
1.2   VAC on the base