208p build thread (papz's PCBs)

[papz]

The Alps sliders and any part can be removed cleanly and easily with a good desoldering tool. I use a Hakko FR-300 and I'm super happy with, its currently available successor, the FR-301, should be even better.

[tele_player]

Yes, a good desoldering tool would help. Unfortunately, I only have an old blue plastic Edsyn SOLDAPULLT, and some desoldering braid.

[papz]

The time has come to get a Hakko then.
It makes the work really easier and saves a lot of time, stress, PCBs and parts.
Definitely my best tool purchase of the past decade.

[AlanP]

Just to check, as long as I have the correct C&K on-on-on switches, they can be inserted either notch-up or notch-down? They aren't orientation-dependant (like the on-off-momentary switch), I mean?

[papz]

Yes you can insert them as you like.

[papz]

Maybe this will help a builder someday.

There was a hard to find short between quiet ground and -15V on the motherboard of a 208 I'm currently building.
It was caused by a metal tab below the CO fine tuning pot touching the -15V trace of its CW terminal.

short.jpg

[tele_player]

Interesting. You have built these before - what changed? Are you using a different pot?

[davebr]

Maybe this will help a builder someday.

There was a hard to find short between quiet ground and -15V on the motherboard of a 208 I'm currently building.
It was caused by a metal tab below the CO fine tuning pot touching the -15V trace of its CW terminal.

I didn't recognize this tab from the photo but it is the side profile of the tab nearest the pins. My analysis says it is also an issue for the MO fine potentiometer as well, but not for the waveshape potentiometer. Thanks for the notice.

I regularly check tab clearance and break many off on other Buchla builds and admit I've missed these. I've built plenty of 208s but haven't run into this before. Note that the tab simply connects to the shaft so the shaft has to short to the panel, which is likely, but possibly with some resistance.

I just checked both the MO and CO and by pressing on the knob and I can cause continuity between the shaft and -15V. I simply desoldered the potentiometers from the back and pulled them higher so the pins were flush with the PCB and resoldered, then dropped the knobs back down. This provides adequate clearance to the tab and the pins and two soldered tabs hold the potentiometer sufficiently.

If you cannot desolder the potentiometers I suspect a sturdy piece of insulation slid down over the shaft would do in a pinch.

I also grind one tab higher on the Alps sliders as they nearly touch a trace on many of the positions. While this wouldn't cause a short, I don't like the idea of floating metal touching a trace.

Dave

[papz]

It's surprising a short occured with the MO fine tune since on the 208p motherboard (and I think 208r but don't have one on hand to check), no top trace passes under the MO pot.
In my builds, I tighten a nut behind the panel to improve the pot's stability, which makes a good contact to 0V.
I've built many with the same pots and PCBs indeed and it's the first time this happens, also no one reported similar issue so far, it obviously depends on the pot position.
I fixed it the same way as Dave suggests, pulling the pot without unmounting the panel.
One could simply bend this tab for the CO pot preventively before installation.

[davebr]

Rats, I read the board backwards. I only have a image of a EMS board and There is a trace similarly placed under the waveshape pot. However, when I wiggled the MO pot, I might have moved the motherboard and gotten the CO to touch. I guess I'll pull it again and raise that pot just in case.

Dave

[papz]

I added to the build notes a recommendedation to preventively bend the pot's tab.
If there's another run, these traces will be rectified.

[AlanP]

My 208papz build is working now, and sounds very good (I followed Mr Brown's vactrol matching recommendations, and got parts kits from Boops (a new card 12 from him as well) and SA Modular.

The only issues I had were where I missed a pin on an IC socket.

[papz]

You deserve the P at the end of your nickname.

[jonen]

I'd rather desolder each transistor and test it to get more reliable results.
A cheap tester like this is good to have and enough to spot faulty ones : https://www.befr.ebay.be/itm/Transistor ... 3431627171

I finally got my tester and was able to correctly test all transistors, all were recognized! Any idea, what else I could check? This all seems really weird to me because it all used to work just fine. Here is my problem again:

Unfortunately, my modulation oscillator stopped working. I am using the pa726 boards and for some reason the voltage regulator on it burned through, not really sure why, maybe a shortage accidentally caused by me.

I have since replaced the voltage regulator and confirmed the pa726 to be working by testing it in the complex oscillator. I also replaced all ICs on card 6, however there is still no sound, and IC2 (RC4136) gets really hot. Now I suspect that one of the transistors needs replacing, but I am not sure which. I took some voltage measurements, assuming they are all dc, I am not sure if this is true. Would there be any chance for anyone to verify these? I would greatly appreciate it, since those transistors are quite difficult to source.

[papz]

Check the resistors and caps, maybe one fried when the shortage occured.
Check the contacts of the pA726 mounting by moving it.
Check if the MO modulates the CO in AM and FM and if signal is present at the mod CV out, if so, it's still working and the issue is elsewhere.