1st timer board install help

Flames1950 · Post by **Flames1950** » Tue Jan 25, 2005 2:53 pm

I don't know how the Park came stock, they're hard to come across and schematics aren't terribly common. But for the Marshall circuit you'll need to add a jumper there, you're not moving any signal at that point.
Once the jumper is added you'll hopefully make some noise....

clarkydaz · Post by **clarkydaz** » Tue Jan 25, 2005 3:31 pm

i just jumpered pins 1 and 7 with the bare wire, its still the same. no reading!?

Post by **VelvetGeorge** » Tue Jan 25, 2005 4:25 pm

Just to be sure, do you have this buss wire installed between the two 10K resistor terminals?

George

Flames1950 · Post by **Flames1950** » Tue Jan 25, 2005 7:10 pm

I got excited about finding something amiss and glossed over the post about having 192VDC at the bottom end of that 10K and nothing at the top going to your tube. (You'll need the jumper anyway.)
Are you sure the resistor at that spot is a 10K, and not a 100K or 1meg? I'll confess that my eyes can barely tell the difference between the third band being orange for the 10K or yellow for the 100K, I've inserted the wrong ones many times without noticing until it's too late!! The 1meg would be a green band and that should be noticeable. Make sure it's a 10K, the bands should be brown, black, orange (and a fourth tolerance band.) If it's the right resistor go through the solder joints at the end orf the resistor and the tube connections I had you check to make sure you've got contact. We know there's voltage up to this point, so there's something right here stopping it. (You might try lowering the range of the voltmeter to see if there is a very small voltage present since you're showing zero, but don't forget to bump it back to 1000VDC when we get moving again!!)
You can also put one lead on one end of the resistor, and the other lead on the other end of the resistor. This will show you how much voltage is being "dropped" across that resistor, and it definitely should NOT be the full 192VDC!!

Flames1950 · Post by **Flames1950** » Tue Jan 25, 2005 7:14 pm

I double checked your pics but I can't tell if the color bands on that resistor are correct or not.
Something else that's been wearing on my mind is that if the Park did not have the pin 1 to pin 7 jumper, the Park preamp is not arranged the same as the Marshall, and we may have wires out of place for the Marshall circuit on these first two preamp sockets. If you can get another pic detailing the preamp tube sockets and that end of the board ready to go we may need it.

clarkydaz · Post by **clarkydaz** » Wed Jan 26, 2005 7:35 am

it seems the resistor is 100k, not 10k. (this is what george sent me in defense! he, he). you are right about the wiring on the parks,

on v3, the yellow wire was connected to pin 3 and joined to pin 8. i reverted it the other way, per marshall

on the power tubes the yellow wire was at pin 4, and chained, not pin 6 as marshall.

i happen to have pics of the 100 watt park bass, same model, stock. maybe it will make more sense to compare them with my preamp tubes at the moment? if i can send someone the pics again...?

Flames1950 · Post by **Flames1950** » Wed Jan 26, 2005 8:17 am

You can e-mail them to me at chevysix235@aol.com if they're not too huge. I'm on dial-up, yuck.
The 10K/100K thing nipped me in the tush when I reworked my '78's original PCB with SoZo and Mallory caps, I had to put a 10K in the board at about the same spot you added one, and put in the 100K instead. It was not an overwhelming experience when I fired her up. Even more irritating when you're working PCB instead of the PTP. I cheated and managed to get the right resistor soldered in.....from the top side of the PCB. No fun.

Flames1950 · Post by **Flames1950** » Wed Jan 26, 2005 9:13 pm

Hey dude, most of the preamp wiring looks like it's going where it needs to be. If you've added that jumper and changed out the 100K for a 10K there's only one other thing I see that's obvious.
Right now one half of you first 12AX7 tube is not hooked to a cathode resistor/cap. If you're plugging into either input of the I (Bright) channel you're not going to hear anything. Since it looks like you were going for the Bass circuit (I wasn't sure from your earlier post if it was the board or the head that was a Bass) move the second yellow wire from the left (looking at the head like your pics of course) over and connect it to the same spot as the first yellow wire, off the 820ohm/330uF resistor/cap combo. Bass amps are a shared cathode circuit, so both the yellow cathode wires will come off the same point. Although if it's a Bass circuit the coupling caps to the power amp are a little small.....if it's a Lead circuit you're missing a 2.7K and a .68uF cap for that spot.
I can"draw" on your pic to show this better if the description isn't enough, and either e-mail it or reduce it in size and post it here. I'm just running behind tonight, it was my Grandma's 103rd birthday so the family was at the nursing home for dinner. Otherwise I'd have gone ahead and done it already.
P.S. if you re-measure all the voltages we've already measured with the correct 10K resistor in place, have the voltages all gone up some now that we reduced the load on the B+ line by getting rid of the 100K resistor? I'm hopeful that they will all bump up a little closer to standard now.

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 10:10 am

i replaced the resistor with a 10k, but i get a weird burning smell and i noticed in a matter of minutes the orange band had darkened. i originally thought i had the wrong resistor next to the bias pot also, but the same thing is happening. maybe the original resistors here were correct, but are taking more than they should?

even with the replaced resistors i still get the same voltage, and no reading at the top of the 10k resistor leading to v2.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 10:45 am

What are you running for tubes in there, are they original as far as you know? Let's pull all the tubes out of the amp for all testing from here, so that you aren't drawing current through the B+ line. Something's drawing too much if your resistors are going, and yanking the tubes should stop this while we figure out what.
Once the tubes are yanked, you'll have to (AGAIN. I know

) re-measure the voltages and see if we can find voltage past that phase inverter slot now. All your voltages will jump up quite a bit with the tubes out but you won't be pulling current so don't panic. Expect maybe 420-440 on the power tube socket pin 3's and an increase in all the spots you were getting 192-194VDC. You may also notice with no current flowing that you read extremely high on the preamp tube sockets; with no load the voltage doesn't always drop like it would with tubes in (I think on my '78 I read around 100 volts too high without the tubes in, but without current flow you don't get a real voltage drop across the resistors, no work is being done.)
With the tubes pulled, are we getting anything past that 10K resistor?
If we don't I'm not sure what the next step is going to be, from the pics I can't see anything other than what I posted above that should be causing you to drop all your voltage at that resistor. I'll keep looking though, but I don't have the pics you sent yesterday here at work, they were too big to spend the time to download here!! I'll post them up at some point, Dax was interested in seeing the Park guts too.

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 10:47 am

P.S. check the fuses and make sure they're the proper rating (usually 500mA for the HT fuse and 2-3 amps for the mains.)
If someone in the past had overfused that amp and you weren't aware of it you'll want the correct fuses installed, we don't want to keep letting things draw too much current if it's overfused!!

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 10:59 am

yep, the fuses are fine. is there much left to go at? i'm starting to think i'll have to let someone have a look at it now, its seeming a bit more than a quick fix?

Flames1950 · Post by **Flames1950** » Thu Jan 27, 2005 11:02 am

clarkydaz wrote:yep, the fuses are fine. is there much left to go at? i'm starting to think i'll have to let someone have a look at it now, its seeming a bit more than a quick fix?

Well, let's pull the tubes and measure, and take care of the first preamp tube wiring problem I posted a couple of posts up, and hope we find something. But it's getting down to having to be able to see it in person to make any more calls on ideas.
I'm assuming the 10K you replaced in the middle of the board was the same size two-watter? I know, but I just thought I better check.

Again, I'll keep staring at those pics tonight too.

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 11:12 am

yes, they were the 2 watter. i actually tried 2 different types, but they both started burning. when i put the ones george used it went ok again. may have been a mistake working on a park, dunno considering all the subtle changes in there it seems, especially for a newbie.

clarkydaz · Post by **clarkydaz** » Thu Jan 27, 2005 11:23 am

sorry, should haver stated that all voltage measurements i have taken so far have been without any tubes present, missed that post! i shall try the yellow wire move you mentioned, though i remember trying all inputs, all being dead