HP 200CD oscillator repair

[pompeiisneaks]

I got a free one, and I've replaced the electolytics as it was super crusty.  Now I power it up, and nothing seems 'wrong', but it won't do anything either.  So I get some voltages, and at the rectifier tube input the power transformer is outputing over 300VAC.  I touch the output side and get 90VDC.  ????

It's got a 5AR4, so I figure maybe there's something bad.  I don't have one, but I do have a 5R4, and I know it will drop a bit more voltage but should still work... nope, I get now about 40 VDC instead?

Any ideas what would make me lose about 200V across the rectifier tube?  This is even with all other tubes removed, it does nothing. 

Here's a schematic: http://www.hparchive.com/Manuals/HP-200CD-Manual-SNP_605.PDF

Any ideas what to check?  The schematic is in there, page 24.  I replaced C12, C13A/B/C,  & C14  I get the same 90VDC on the lead of C14 too, but I'm not sure what else could be sucking down the voltage that far?   could it be the choke?  Any other suggestions/ideas?

~Phil

[shooter]

It's got a 5AR4

pg 24 has 2 diodes, cr1 & 2
wire it like that, don't hook up anything to the anodes except meter an see what you get

[sluckey]

pg 24 has 2 diodes, cr1 & 2
wire it like that, don't hook up anything to the anodes except meter an see what you get

Please note that the diodes are drawn backwards on that schematic. 

[shooter]

diodes are drawn backwards

I've been stuck in traffic n just kept thinking, something wasn't right about that circuit, now I'm happy

[pompeiisneaks]

Yeah later generation swapped the tube rectifier fir solid state.  Maybe i'll just use the socket as the connect points

[sluckey]

I recommend you get the correct manual for your serial number before you start modifying it. If you can't find it I may have it. What is your serial number?

[pompeiisneaks]

I'll get it when I get home, but I've already removed the Ecaps and replaced them, but from my view of it, it was definitely identical in the areas I checked.  All the points connected to the same things in theory (Except the rectification being tube instead of SS). 

Thanks, I'll let you know when I get it.

~Phil

[pompeiisneaks]

So I'm not finding the right one, I've found older ones with the rectifier tube but they have the wrong other tubes, the serial number: 229-40538

seems either the ones I find are for serial numbers like 5000 or 8000 range or they're way over the one I need.

Mine uses a 5AR4 rectifier, and a pair of 6AU6's for the first part and the cathode followers are EL86/6CW5's

Should I be okay to still solder in some Diodes?

~Phil

[pompeiisneaks]

So I put the diodes in, and it still sits at about 32V at the other side of the diodes, that means something in the chain after it is really pulling down the voltage heavily right?  What do I even look for at that point?  one of the smaller caps?

~Phil

[sluckey]

Just put your 5R4 back in it and remove the diodes. Then drain the filter caps. Then use your ohm meter to check resistance from positive to negative of each filter cap. Notice that the cap can (C13A,B,C) negative terminal does not connect to ground! Also check positive to negative terminals of C11 and C14.

When you replaced the cap can C13 you didn't connect the negative side of the caps to ground did you? If you did that will kill the negative bias for the output tubes and cause them to draw excessive current (probably destroy the EL86s) and load the power supply way down like you are seeing. As a quick check, remove V2 and V4 (label the tubes so they can go back in the same socket). Now check the B+ voltages. I bet they will be OK.

I have the exact manual you need. Let me know when you get it. Here's the link...

     http://sluckeyamps.com/HP_200CD_SN_229.pdf

[pompeiisneaks]

I was very careful to keep the negative ground connected to the same potential as the existing one, it was tied into the 10k resistor that went to ground.  The center tap, and the other lines all tied to that 10k except the C15 40uF one.  I have all the tubes out and still get the voltage at 30V after both tube and SS rectification.  I've got the file downloaded.

~Phil

[sluckey]

Check all caps and resistors in this pic. Since you already replaced C13 and C14 my next suspects are C10, C11, and R40.

R40 is used to develop the negative bias. Measure voltage across R40. If it is considerably higher than -20V then it has likely increased in value. The higher the voltage across R40, the lower your B+ out of the rectifier will be.

[pompeiisneaks]

you 'may' be onto something there... I measured the voltage drop across that 10k resistor, and I think -435V MAY be wrong, thus the 'quotes'

I'll go buy another 10k 10W at the local store and drop it in, see what I get.

~Phil

[sluckey]

I'll go buy another 10k 10W at the local store and drop it in, see what I get.

Only takes 10 seconds to check that resistor. If it measures 10K (+10%) leave it alone.

But if you really have -435v on that resistor, then the resistor is probably open or very high resistance.

[sluckey]

BTW, I own two of these and one is a serial #229. So I can make some comparison checks if you like. Also, when I said put your 5R4 back in I really meant to say "put your 5AR4 back in".

[pompeiisneaks]

Oh I did measure it, when warm it was like 3.3M, and then started dropping like a rocket down to something around 150k, but that was hard to get, because as I had pressure on the pin it kept going up and down, so I'm pretty sure it's shot :)  Yeah I put my 5AR4 back in, doesn't a 5R4 drop more voltage?   At any rate, I'm pretty sure the old rectifier tube was fine, and it's back in, and the diodes are out.

~Phil

[sluckey]

Well, that's why your B+ voltages are very low. Go get a replacement. Parts list calls for 10K @ 10 watts, wirewound. Should be easy to find. If not local, then AES has one for $1.30.

[sluckey]

My working sn229 is on the bench. I've made a couple checks for comparison. Here's what I got...

First, you must disconnect one end of R40 to get a valid reading. I suggest removing the single wire from the bottom end. Resistance measures a solid 10K. Put the wire back when you're done with this check.

Voltage checks are referenced to chassis ground. Choke has 225V on one side and 195V on other side.

Top of R40 measures -180V. Yes, that's one hundred eighty volts.

[sluckey]

One more thing... I have the exact resistor you need. You can have it if you want it.

[pompeiisneaks]

I found one at the local shop, different brand, not ohmite, milwaukee or some such, but wire wound with the center hole, so I should be able to just swap it out easily.

~Phil

[pompeiisneaks]

Hmm removed the ground leg, measured and it's exactly 10k... maybe one of those mica 100pF caps?  I could replace those both, I bought them as well. 

~Phil

[sluckey]

I really don't suspect those 100pF caps, but could be. I would think a simple resistance check across them would reveal a short.

Measure resistance across your choke. Mine reads 246Ω.

Measure resistance from bottom of R40 to chassis. Should be zero ohms.

[pompeiisneaks]

One of the two measures .2 ohsm, it's a short circuit.  It also is grounded to the socket and when I tried desoldering it, the ground tab just came off, probably due to heavy current flow.  I'll replace them both since I have them off.  My choke measures 226.3 Ohms

[pompeiisneaks]

Ok replaced them both and now get 380VDC at output of rectifier and the negative bias there at that 10k seems to be around -9VDC minus tubes, do you think I'm good to put the tubes back in now?

~Phil

[pompeiisneaks]

I decided to risk it, and bam it's perfect, exactly what it was, left the original resistor in, but replaced the two 100pF mica caps to ground from pin 7 of the output tubes and in business!  The negative bias on mine also is about -180VDC with tubes in.  Thanks Steve!!

~Phil

[sluckey]

Good deal. Hopefully that's all that was wrong with it. You can use the maintenance section of the manual to verify it's working OK. I would not be too concerned if the frequency dial does not track exactly, but verify that the frequency range works for all switch positions. A common problem with these units is the small frequency knob will slip, skip, or even be completely frozen. The oil usually has turned to gum or glue. So, lubricate with light machine oil to fix minor problems. I've actually had to use liquid wrench penetrating oil to free up frozen knob shafts. If this is the case just keep the penetrating oil on the shaft and maybe put a small vise grip plier on the shaft to help rock the shaft back and forth until the shaft turns freely. Be patient. This could take a couple days.
 

[pompeiisneaks]

So far it moves well, except it does seem to want to bind up at a certain point, but it never binds, just gets a bit stiffer to rotate, I think the main wheel may be warped a bit or have slid on the shaft to an angle. 

I bought a 600 ohm resistor to do just that part of the calibration etc.

~Phil

[SoundmasterG]

I recommend you get the correct manual for your serial number before you start modifying it. If you can't find it I may have it. What is your serial number?

Hi Sluckey,


I've got one of these HP's also that needs a rebuild. Any chance you can look through your manuals for these and see if you have the correct one for my serial number also? I can get you the number in a couple hours. It would be great to have the correct manual for it!


Thanks!

Greg

[sluckey]

I can do that.

[SoundmasterG]

I can do that.

Hi Sluckey,


Thanks a lot! I finally got the time to look at it and get the serial number. It is 333-50033.


Greg

[sluckey]

This manual is for serial numbers 605 and earlier. Refer to Appendix C for the change needed for your serial number 333. Change 8 is the only change that applies to you. It's a simple power supply change.

PDF is 13MB. Let me know when you have downloaded the manual.

...Steve

     http://sluckeyamps.com/HP_200CD_SN_605.pdf

[SoundmasterG]

This manual is for serial numbers 605 and earlier. Refer to Appendix C for the change needed for your serial number 333. Change 8 is the only change that applies to you. It's a simple power supply change.

PDF is 13MB. Let me know when you have downloaded the manual.

...Steve

     http://sluckeyamps.com/HP_200CD_SN_605.pdf

Hi Sluckey,


I've downloaded the manual. Thanks very much for the manual and the advice!

Greg