Looking for help with Hewlett Packard 606A

[IsquaredR]

I am a newbie to this forum but not new to electronics.

My HP 606A signal generator has had a myriad of problems that I inherited when purchasing it. I have replaced its electrolytic capacitors as well as most of its valves. I've got the signal generator working very nicely on Bands 2 through 6 but when I switch to Band 1 (50 - 170 kHz) V3, V4 and V6 draw very high current. On Bands 2 through 6 the B+ bus current is 25 milliamperes but when I switch to Band 1 it jumps to 125 milliamperes. Although I must do it quickly and only momentarily, when i switch to Band 1 the voltages on the grid of V6 jumps from -16 VDC to +1 VDC' on the V6 plate the normal plate voltage reads 115 VDC but jumps to 200 VDC on Band 1. V8 typically has 87 VDC on its plate but switching to Band 1 increases to 160 VDC. On the grid (pin 7) the voltage is a respectable -16 VDC but when switching to Band 1 jumps to 0.
Again, my measurements have to be quick and I do not have the luxury of time to allow the voltages to stabilize so some of this data is probably skewed.
I am most interested in your ideas and wonder if anyone might have any data as to the coil resistances for the Band 1 oscillator coil? I have unsoldered and re-soldered just about every component I can figure might be causing this high current (loss of oscillator signal?) and suspect the turret coil
may be culpable although I show continuity on each winding when measuring the contacts. Thanks so much.

[Radio Wrangler]

This signal generator employs closed-loop control of the output amplifier. This is used to apply modulation, and to give quite precise control of the output level.

If the oscillator itself doesn't run, the output detector sees too little (none!) output and in an attempt to increase the level, via V7A&B and V8A&B it turns V6 pretty much all the way off to move V4 and V5 further up the steeper part of their characteristic to get more gain. This takes more HT current.

All you've described sounds to be the consequence of the oscillator not running.

The only thing in the oscillator used in range 1 that isn't used in the other ranges is the coil assembly in the turret and it's associated contacts.

I wondered about a coil open due to moisture and green-spot corrosion, but if the coils show continuity on an Ohmmeter, then that's out.... maybe the contacts? otherwise, you're looking for a shorted turn.

An outside chance might be a open-circuit decoupling capacitor reducing the gain at low frequencies.

Your voltages and currents sound reasonable.

David

[IsquaredR]

Thank you David. I really appreciate your response. I too am afraid I might have a bad coil but I would also like to somehow get in there and check the contacts which seems nearly impossible except for totally dismantling the signal generator. I'll keep the forum posted on progress.

[Radio Wrangler]

Use the Ohmmeter connected to the stationary electronics and move the turret. A band that works ought to let you check your test connections and choice of pins, then when the turret is brought round, you should see continuity again. If not, then it's mechanical engineering time, or the dreaded coil.

You need to get the coil dead right because the tuning of the oscillator and the tuning of the power amp output track.

David

[IsquaredR]

Thank you, David. That was a superb suggestion. I did exactly as you described and
was able to verify that all but two of the contact sets on the 50-170 KC amplifier coil are good. The reason I cannot verify the contacts for amplifier windings 1-8 and 3-7 is due to the fact that my amplifier coil winding apparently is open on those. I suspect strongly that my amplifier coil on winding 1-8 (and 3-7 which is part of 1-8) is the reason I have lost my signal on Band 1 and it drives the tubes into drawing high over-current.
I do not believe the information is readily available but in the event that at some future date someone is searching on this forum for information about the Band 1 turret coils, here are the resistance measurements I made for each winding. (Unfortunately I cannot say the resistance on windings 1-8 and 3-7 for obvious reasons but when I obtain a replacement coil I will recheck these measurements and post them here). Note that no attempt to isolate the windings from other turret components (i.e. resistors, capacitors et cetera) so these measurements are probably affected by those components; however when making the measurements I was only interested in finding what values to look for to confirm mechanical contact integrity.

By the way, when I obtained this HP 606A as a restoration project the seller told me that he left the output switch in the 3 volt position OVERNIGHT (!!!). Evidently one can derive from this that the warnings in the manual as to the 3 volt position are there due to the deletrious effect on the amplifier coil.

Oscillator Coil - Band 1 (50-170 KC)
Winding
1-2 14.85 ohms
2-3 9.55 ohms
3-7 9.75 ohms
7-8 14.9 ohms
4-6 0.7 ohms
6-5 0.65 ohms
1-8 48.65 ohms
4-5 1.25 ohms

Amplifier Coil - Band 1 (50-170 KC)
Winding
1-2 290 ohms
2-3 7.8 ohms
3-7 Open (Defective)
7-8 293 ohms
6-5 1.1 ohms
5-4 2410 ohms
1-8 Open (Defective)
6-4 2410 ohms

[IsquaredR]

Is there anyone who might be able to measure the resistance of the winding between pin 3 to pin 7 of the amplifier coil for me? Thank you.

[factory]

Would resistance measurements from taken a HP 606B be of any use?

There are some slight circuit differences in the turret assembly of the 606B, the amplifier turret has some extra components are fitted to the secondary coils of first three ranges and two connections are swapped on the secondary coil of the fourth range.
The oscillator turret looks the same apart from the variable capacitors being all the same value on the 606B (the parts list gives a different value to the circuit diagram though).

David

[IsquaredR]

Thank you, David.

Yes it would be helpful and I appreciate your offer to provide the resistance between pins 3 and 7 on the HP606B band 1 coil. I checked the parts list for the '606B versus '606A and see
they changed the part numbers. But everything else looks the same (sans the new tap and the RC combo added to the secondary). The rest of the parts on the primary of that transformer seem identical. If I could obtain the resistance I might be able to roughly derive coil turns information torepair my coil.

I appreciate your help.

Ernie

[factory]

Hi Ernie

I have taken the measurements earlier today & will let you know the results later, as I have just typed it all out and lost it all when I had to re-log in, I must be tried as I usually copy it first.

David

[factory]

Hi Ernie,

It took longer than I thought to clear enough bench space, as it was in quite a mess.

The part numbers for the complete turret assemblies did indeed change for the 606B, that will be due to the minor component changes. Also there are no new taps on the secondary windings of the coils, there is an inductor fitted as well as the resistor & capacitor. The resistance readings will be unaffected due to the capacitor being in series with the inductor & resistor.

The coils fitted to my 606B should be mostly the same as the 606A, the 606A manual gives individual part numbers for all 12 coils, but the 606B manual lists them as N.S.R. (not separately replaceable). The letters printed on mine match up with the 606A manual, apart from coil T210 M which is H in mine (the manual shows H in the picture though) and the two thickest coils which I can't see any markings on.

Both the manuals don't have any diagrams for the numbering on the actual switches, so here is what I figured they are;

1 - - 8
2 - - 7
3 - - 6
4 - - 5



I have now noticed that mine has nothing connected to switch contact 1 for the amplifier turret, clearly something I missed when correcting all the bodges in the RF section from a previous owner, still haven't done the power supply section yet (somehow it works though).

Here are the resistance readings of coil 1 (50KC-170KC band) for the amplifier turret on my 606B;

1-2 296Ω
2-3 7.8Ω
3-7 7.7Ω
7-8 295Ω
6-5 0.8Ω
5-4 2.4KΩ
1-8 605Ω
6.4 2.4KΩ

The values are very close to those you got for the good windings on your 606A, looks like windings 1-2 & 7-8 and 2-3 & 3-7 should be the same for each pair.



Hope this help with the repair of your 606A.

David

[IsquaredR]

David, thank you so very much for your most comprehensive measurements and
supplementary information. I too looked at the 606B diagram and concur with your
belief that the coil between the two versions of the signal generator are the same.

I thought it was worth it to try a "quick and dirty" work-around for the missing coil
winding. Although I certainly concede that the inductance would be unsuitable I bridged
the 3-7 winding with a 7.5 Ω wirewound resistor just to see if there was any effect to the high current drawn on the B+ bus or any indication of RF "seeping through." There was not. Since my Band 1 is not working anyway my next step is to remove te coil, unwind the portion with the 3-7 winding and see if I can locate the breach, which I can then mend.
Your information was so helpful I printed it out in hard copy and am adding it to my
HP 606A manual. I am grateful to you for your immense help.
Kindest regards, Ernie

[factory]

Glad I could help.

I thought I would add a couple of pictures of the band 1 (50KC-170KC) amplifier coil A, there are quite a few solder joints that still have flux residue on mine.

Originally Posted by factory

I have now noticed that mine has nothing connected to switch contact 1 for the amplifier turret, clearly something I missed when correcting all the bodges in the RF section from a previous owner, still haven't done the power supply section yet (somehow it works though).

I had another look today and I don't think anything is missing from switch contact 1 on my 606B, the connection going to the big variable capacitor is hidden, I'm still puzzled why the side with the PCB has been soldered to at some point though.

David

[IsquaredR]

David, thanks for your photos of the coil and thoughts.

Like yours, my 606A also has lots of remaining flux on connections and
I believe them to be original and not left-overs from a prevous owner's repair work. It sort of surprises me to find this in an HP.

I dug a little deeper into my amplifier coil for Band 1 and studied the coil. The more I looked at it the more I thought it folly to think I could unwind, repair and rewind the 3-7 winding. Wow! That's some basketweaving! So I think my only recourse is going to be to find a parts donor from which to harvest the coil.

My original reason for digging into my 606A was to replace its capacitors and clean up some of the previous owners sloppy work; which I completed. Since I had replaced almost every valve in the signal generator I thought it prudent to grind through the alignment procedure. When I got to the first section dealing with Band 1 was when I discovered the problem. It was drawing splendidly high current on the modulator tube and one of the 6CL6 power amplifier valves. It was unlike any light show I had ever seen and continued for probably 10 seconds AFTER I removed all power. It was pretty dramatic. Obviously both of those valves were ruined and one of the cathode resistors on the power amplifier valves actually split in half. Since then all troubleshooting has been restricted to 3-5 second intervals on Band 1 which has been quite a challenge.

But one must persevere.