HP 608C Repair.

[amspire]

A fuse and a thyristor latch circuit to clamp down on the voltage is not a bad idea. Protecting those filaments is key to keeping that oscillator alive. A circuit consisting of a thyristor from emitter to ground, a resistor from gate to ground and a zener to the output would do the job. A capacitor from gate to ground can be used - you do not need instant response.

[neo]

A fuse and a thyristor latch circuit to clamp down on the voltage is not a bad idea. Protecting those filaments is key to keeping that oscillator alive. A circuit consisting of a thyristor from emitter to ground, a resistor from gate to ground and a zener to the output would do the job. A capacitor from gate to ground can be used - you do not need instant response.

Could i trouble you for a schematic of that?

[amspire]

http://axotron.se/index_en.php?page=26

I just picked of a ST TN1610H-6T from Digikey (85c price)

The values of parts is not critical. If you go for the thyristor-zener-resistor-capacitor circuit, the resistor can be 1K. The capacitor could be 10nF. If the thyristor switches on at 1.3V, and you want to stop greater then 10V, then you want a 8.7V zener. Get both a 8.6V zener and a 9.1V. Try the 8.6 first. If there are spurious latches, then use the 9.1V.

It might be better taking the anode of the SCR to across the 1000uF capacitor. Shorting the emitter might just cause the transistor to current limit instead of blowing the fuse.

The TL431 based circuits can be set to trigger within 1% of a voltage you set while the one above is within about 10%.

[neo]

http://axotron.se/index_en.php?page=26

I just picked of a ST TN1610H-6T from Digikey (85c price)

The values of parts is not critical. If you go for the thyristor-zener-resistor-capacitor circuit, the resistor can be 1K. The capacitor could be 10nF. If the thyristor switches on at 1.3V, and you want to stop greater then 10V, then you want a 8.7V zener. Get both a 8.6V zener and a 9.1V. Try the 8.6 first. If there are spurious latches, then use the 9.1V.

It might be better taking the anode of the SCR to across the 1000uF capacitor. Shorting the emitter might just cause the transistor to current limit instead of blowing the fuse.

The TL431 based circuits can be set to trigger within 1% of a voltage you set while the one above is within about 10%.

I replaced the transistor and left the emitter unconnected, the V out into a 700 ohm load hovers around -9.5 to -9.7 VDC. Referenced to the chassis.

[amspire]

 

[neo]

I'm still at a loss understanding that fuse, maybe i'm a bit dim. Seems to me it would be just as effective if i added a half amp to the load and then used that as the value for a slow blow fuse? 

[neo]

and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

[Specmaster]

and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

Yes to both your questions and as regards to the 7 ohm being there on the schematic, that might be because you don't actually have the correct schematic do you? If so than that might well be correct.

[neo]

and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

Yes to both your questions and as regards to the 7 ohm being there on the schematic, that might be because you don't actually have the correct schematic do you? If so than that might well be correct.

It is a 608C service note i got it from.

[Specmaster]

and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

Yes to both your questions and as regards to the 7 ohm being there on the schematic, that might be because you don't actually have the correct schematic do you? If so than that might well be correct.

It is a 608C service note i got it from.

Well in that case it will limit the current to the filament as well as the voltage but on your model I'm afraid I'm batting in the dark.

[neo]

          

Something weird is happening.
small is 400R, across voltmeter
large is 5R, resistor not filaments and only applied to the output.

Emitter, small load; 5.14
Emitter, large load; .070
Collector, small load; 5.51
Collector, large load; no change
Base, small load; 5.20
Base, large load; -.05

[amspire]

and on the note of confusion, why does the schematic show a 7 ohm resistor in series with the filament and it's not there in my unit? Besides that, wouldn't that just form a voltage divider and limit the filament current?

Yes to both your questions and as regards to the 7 ohm being there on the schematic, that might be because you don't actually have the correct schematic do you? If so than that might well be correct.

It is a 608C service note i got it from.

Well in that case it will limit the current to the filament as well as the voltage but on your model I'm afraid I'm batting in the dark.

The service note actually explains that it should be chosen to get the filament voltage down to 6.5V for the 608C.

I think someone installed the mod, and didn't bother to put a resistor in.It wouldn't have been provided with the mod hardware as it is a select on test.

6.5V is a very normal sounding filament voltage so I would go with that. 9V sounds high. You always want to start with the lowest voltage as it will prolong the life of the oscillator, so I would add the resistor.

My guess is about 3.9, 4.3 or 4.7 ohms 2W. I find 7 ohms hard to believe, unless the mod only does one valve in the oscillator section. We do only have the 608D schematic.

[neo]

Well it kind of looks like it is only for one, each filament connection is brought out individually and this only goes to one. Hows that account for the voltage oddities?

[amspire]

         

Something weird is happening.
small is 400R, across voltmeter
large is 5R, resistor not filaments and only applied to the output.

Emitter, small load; 5.14
Emitter, large load; .070
Collector, small load; 5.51
Collector, large load; no change
Base, small load; 5.20
Base, large load; -.05

Do you mean negative voltages? There shouldn't be any positive voltages.

The mod is powering just the oscillator - that does make sense. Check the other filament supplies in the oscillator section are 6.3V AC.

Did you put the 1000uF C75 back properly? The negative rail of the capacitor has to go to the collector. -5.51 on the collector is definitely wrong. I think it is something more. I think there is either something funny about the bridge rectifier, or there is not 12.6VAC going into the bridge rectifier. Until you have proper fullwave power going to C75, the transistor cannot work.

The base bias to the base of the transistor is from -165V through a 18K resistor (R127). If that is missing, you will not get the -9V across the zener. If  you short the base to ground with a meter current range, you should get -165/18K = -9.2mA. The power transistor will need a gain of at least 40 at about 300mA.

If it is all too much, scrap this bodgy mod and get a LM337 TO3 adjustable negative voltage regulator. Make it adjustable with a potentiometer. See the data sheets.  C75 + the bridge rectifier remain. You have to sort out the 12.6VAC/Bridge rectifier/C75 issues before anything else. That should be simple - just three things to check.

[neo]

I'll check again, i referenced them to the chassis.

[amspire]

I'll check again, i referenced them to the chassis.

The chassis is fine. All the voltages should be negative compared to the chassis.

[neo]

I'll check again, i referenced them to the chassis.

The chassis is fine. All the voltages should be negative compared to the chassis.

I had the leads reversed, my mistake. Yes they are all negative, the one marked as negative is positive.

[neo]

FL3, 226.4 DC+ 0VAC
FL2 165.44 DC- 0VAC

My mistake assuming FL meant filament, so yeah this mode powers the whole oscillator section apparently.
Completely unloaded I.E Emitter not connected,
Emitter, -45
Collector, -54
Base, -45

700R load on the Emitter,
Base, -9.65
Collector, -18.71
Emitter, -9.5

400R load on Emitter,
Emitter, -5.2
Collector, -14.6
Base, -5.23

[neo]

Between the negative of the bridge rectifier and the collector is a 51 ohm resistor, not on schematic.
The negative end of the bridge with

400R, -14.25, .027VAC
NC  , -10.8 , .4VAC
700R, -18.64, .26VAC

[amspire]

Where is the 9V zener?

51 ohms to the collector? That is crazy. I think you have to draw this circuit from the transformer to the valve filament.

Before this mod, the 608D used a multivibrator valve circuit to generate the filament current (V17). Is that circuit still present?

Have you looked for R127 to the base?

[neo]

Where is the 9V zener?

51 ohms to the collector? That is crazy. I think you have to draw this circuit from the transformer to the valve filament.

Before this mod, the 608D used a multivibrator valve circuit to generate the filament current (V17). Is that circuit still present?

Have you looked for R127 to the base?

I drew it out when i saw that 51 ohm resistor, all is exactly as it should be with the exception of that one resistor.  I didn't check the transformer connections, didn't suspect them, but i can if you like.

[Specmaster]

Is the schematic of the mod as provided by amspire correct to as per your unit?

I'm trying to follow your measurements and descriptions but they don't seem to be relevant to that schematic in my opinion, so are we all talking about the same thing or do you actually have a slightly different circuitry there? Perhaps if you do have something different, it might be a good idea to draw it out and post here so we can actually see what you can see.

Another thing is could you also clarify just what the voltages are the red and green, red is connected to the left of R125 (7 ohms) and green (FL3) is not shown connected to anything on this drawing?

I also take it that the junction of CR18 and CR17 is connected to the filament of the tube and the other connection to the tube from the junction of CR19 and CR16 which is DC, so are all the other voltages here AC?

[neo]

I've said what was on the FL's, red is filament green is FL3 which is 226V.
As I've also said, though just to be entirely clear. The schematic is almost dead on but some of my circuitry is physically different.

One end of the filament is the chassis, which the junction of CR17 & 16 are tied, all voltages unless otherwise marked are to be assumed to be DC.

[neo]

OK this is odd, across CR20 i get -6.8V, i move the ground clip to the chassis it somehow becomes -45V 

[neo]

Someone was messing around in here long before me. This is all kinds of screwed up.