Author Topic: PT-80 Oscillation Problem  (Read 2842 times)

The Surgeon

PT-80 Oscillation Problem
« on: August 23, 2006, 01:02:06 PM »
Built PT-80 delay unit few days ago.
I chose 9 V power supply and electrolyth coupling caps to avoid some brightness.
Also BC549B transistor instead of 2N5088 (noticed different pinouts,couldnīt find 2N5088
but characteristics are pretty much same) and 25 K time pot because I donīt need longer
delay times. Otherwise stock.

Sound quality is superb; Iīve got Ibanez ADL and DOD FX90 and this thing beats out Ibanez.

One big problem: although the unit is completely usable, it starts oscillate when repeat pot is turned
about half the way, so I chanced pot to 25 K. No influence here so I put 22 K resistor over the pot with
very little results.

It seems that amount of repeats got some level when the oscillation starts and pot value has little or
nothing to do with it.

So can anyone help?

Soldering  and measuring components is not hard for me and component values has checked before soldering.

The unit sounds so good that I want to solve this problem. I donīt want "space craft sounds" so I donīt want
it oscillates at all.



geertjacobs

Re: PT-80 Oscillation Problem
« Reply #1 on: August 23, 2006, 03:26:41 PM »
If you use the search function with the search terms "PT-80 oscillation" you get the solution in the second topic. (the first topic is yours  ;D)

Quote
Probably the simplest way to do it in this case is to take the 22k resistor that feeds pin 12 of the compander chip, and replace it with a 10k and 12k resistor in series to achieve the same series resistance.  Now, from the junction of the two resistors, run a small nonpolarized cap to ground, and voila!  I would suggest a value, but unfortunately the computer I'm working on has the Windows applets like the calculator locked out.  I am guessing, though, that with a 10k resistor, something like a .015uf to .022uf cap might be sufficient to do the deed.

The Surgeon

Re: PT-80 Oscillation Problem
« Reply #2 on: August 24, 2006, 12:31:59 PM »
Thanks a lot; my problem`s gone ; ;D and thatīs really fine! With 10 and 12 K resistor and 0,015uF cap to
ground it works fine, "normal" oscillation happens when pot is allmost maxed. Repeats sound a bit warmer
too. I honest canīt tell which sounds better, FX90 or PT-80 (with 25 K delaypot).

zjokka

Re: PT-80 Oscillation Problem
« Reply #3 on: December 04, 2006, 03:00:45 PM »
w :icon_eek: w,

good I came across this one...
started populating my PT-80 board half an hour ago, only put in the 1k and 10ks in yet.
i'd better anticipate trouble and put the series resistors and cap to ground in from the start?
cannot hurt, can it?

where would you be without this forum?
zj

The Surgeon

Re: PT-80 Oscillation Problem
« Reply #4 on: December 05, 2006, 01:53:48 PM »
w :icon_eek: w,

good I came across this one...
started populating my PT-80 board half an hour ago, only put in the 1k and 10ks in yet.
i'd better anticipate trouble and put the series resistors and cap to ground in from the start?
cannot hurt, can it?

where would you be without this forum?
zj

Yep, just do "mod" with those resistors and 0,022 uF cap and you will get some oscillation at the latest quarter of the pot turn. This is normal (and often wanted) with good ANALOG delays! :icon_biggrin:
If youīre using the ggg layout there are empty holes for the cap. This really isnīt mod; it should be in the
layout to make it work like it should I think.

Oh yes, without this forum I`d definitely be in great trouble... ;)

RDV

Re: PT-80 Oscillation Problem
« Reply #5 on: December 05, 2006, 02:09:46 PM »
My Ibanez ADL is the warmest delay ever. The PT80 is nice, but it doesn't sound analog to me. It actually sounds a bit like my old Digitech PDS1002. Digital, but with less fidelity in the repeats.

Just saying.

RDV

The Surgeon

Re: PT-80 Oscillation Problem
« Reply #6 on: December 05, 2006, 02:34:28 PM »
My Ibanez ADL is the warmest delay ever. The PT80 is nice, but it doesn't sound analog to me. It actually sounds a bit like my old Digitech PDS1002. Digital, but with less fidelity in the repeats.

Just saying.

RDV

I first built Rebote 2,5 and after that I sold my Ibanez ADL, sound was pretty much the same. After got my PT-80 working the way I wanted Iīm now selling my DOD FX90 which really is warmer than ADL (I got four analog delays at the same time). So itīs a matter of taste.
I used electrolyth coupling caps in PT, maybe made repeats a bit softer. I also took out about 330 ms of delaytime because longer delays loose their quality. Iīm not a builder; Iīm after a good tone (perhaps that made me first build my own tube amps). So I found out that itīs possible to make new, good sounding fx rather than fix those already stomped out. There is allways possibility to make some "tuning" during the process. After the DOD is gone I donīt have commercial pedals any more.

zjokka

Re: PT-80 Oscillation Problem
« Reply #7 on: December 05, 2006, 07:55:36 PM »
w :icon_eek: w,

good I came across this one...
started populating my PT-80 board half an hour ago, only put in the 1k and 10ks in yet.
i'd better anticipate trouble and put the series resistors and cap to ground in from the start?
cannot hurt, can it?

where would you be without this forum?
zj

Yep, just do "mod" with those resistors and 0,022 uF cap and you will get some oscillation at the latest quarter of the pot turn. This is normal (and often wanted) with good ANALOG delays! :icon_biggrin:
If youīre using the ggg layout there are empty holes for the cap. This really isnīt mod; it should be in the
layout to make it work like it should I think.

Oh yes, without this forum I`d definitely be in great trouble... ;)

Just took the two resistors and a 15n cap and let the latter float until further notice. Was really interested whether I could get this one to work and wasn't expecting much. So I was very surprised that I got play through... but no delay. I learned that if that it the case, the middle lug of the level pot is disconnected. Now it works just great, though didn't have 50k pots, used 100k.

And ..oscillation at high levels without the cap...

And you are so right, there should be a channel for updates, improvements, essential tricks like these. I'll send Mark a pm to thank him, really.



the Hammer cap is in the front, with a free leg still pointing right.

and here it little PT: really discovering the close up setting of my digital camera lately


bdevlin

Re: PT-80 Oscillation Problem
« Reply #8 on: January 24, 2007, 01:31:08 PM »
I was able to tame my oscillation problem by replacing the 22k with the following values:

13.5k (to Pin 12)
12k  (to .001 cap)
.02 uF cap to ground from the junction of the two resistors.

I tried a 10k, 10k, .01 uF combo but that did not do too much good.

Brings me to a question.  What impact does it have with one of these resistors being larger than the other?  And, which one should be larger?