• Please review our updated Terms and Rules here

Commodore PET - buzzing sound and data issue

R

rglenn

Experienced Member
Joined
Feb 16, 2014
Messages
104
Location
PA
I recently won an auction for a Commodore PET 4008 from eBay. The system runs quietly for about 5 minutes and then begins to buzz. I think the sound is coming from the back left side of system. The noise does not seem to affect the system's operation. Any suggestions on what may be causing the noise, does it matter and how to correct?

Also, if I copy a pet program from a Commodore 64 to cassette and load it on the PET, it finds the program, loads it, says it is ready but when I type LIST, nothing is there. I am able to type in a program directly on the pet, save it to cassette, type NEW, reload the program and it works fine. I can also take the cassette unit over to the C64 and load, run it just fine.

Thanks for any ideas.
 
Re: the save/load problem.

BASIC programs saved on the C-64 will not load in the right location on the PET to be seen by the BASIC interpreter. PET BASIC starts at location 1025 ($0401 in hex) and the C-64 starts at 2049 ($0801H). In essence then you have to load the program into the PET, move it to the right location, then save it back out to tape to get your permanent copy.
Here's what I did on my PET to achieve this:

  • Type NEW to clear any old programs
  • Enter 0 REM
  • LOAD in the program you want to convert
  • When loaded, enter the machine language monitor by entering "SYS 1024"
  • Display the first part of the BASIC by typing: "M 0401 0408"
  • Now change the line-link to the memory location of the program to be moved; use cursor keys to move up and change the first two two-digit numbers to read: :0401 01 08 00 00 8F 00 00 00
  • Press return to change the bytes then enter "X" to exit the monitor.
  • Type LIST, you should see the 0 REM followed by the program.
  • Enter 0 to delete line 0 which will move the BASIC to its proper place.
  • Save the program.


I got this method from a FAQ here:
http://www.zimmers.net/cbmpics/cbm/PETx/petfaq.html

Tez
 
Wow - that was fast - thanks, I'll give it a try!

And it worked perfectly.
 
Last edited:
You're welcome.

I'm not sure what's causing the buzzing though. Obviously something that starts doing it when it warms up. The PSU transformer maybe? I wouldn't know how to fix such a thing.
 
About buzzing: try to exclude the line filter; it's a cylinder, similar to a capacitor, connected to the AC line. The computer will work fine even without this component.

BEWARE: Line filters can explode (literally) when the computer is working (it happened to me and wasn't funny!), so keep your face and your hands far from it. In other words, don't try to put your ear close to it in order to discover if it's buzzing or not. Instead, unplug it and see if the buzz has gone.

--Giovi
 
giobbi said:
About buzzing: try to exclude the line filter; it's a cylinder, similar to a capacitor, connected to the AC line. The computer will work fine even without this component.

BEWARE: Line filters can explode (literally) when the computer is working (it happened to me and wasn't funny!), so keep your face and your hands far from it. In other words, don't try to put your ear close to it in order to discover if it's buzzing or not. Instead, unplug it and see if the buzz has gone.

--Giovi
Click to expand...

Giovi,

Good idea about checking the power filter. I'm not sure about how easy it is to get to. For safety it is enclosed in a metal box at the power input at the back of the PET in the 8032.
 
Thanks for the suggestion. I was able to get to the metal box at the power input but there does not appear to be a line filter in the box. After putting it back together, now the buzzing starts right away without warm up. I am thinking it is the transformer. Perhaps some of the hum/buzz is normal?
 
Last edited:
rglenn said:
Thanks for the suggestion. I was able to get to the metal box at the power input but there does not appear to be a line filter in the box. After putting it back together, now the buzzing starts right away without warm up. I am thinking it is the transformer. Perhaps some of the hum/buzz is normal?
Click to expand...

A lot of buzzing is not normal and sometimes mean the transformer should be replaced. However if the lacquer protecting the plates look OK, it may mean the screws that hold the transformer to the chassis need to be tweaked for proper tension. With power off, tighten the screws and see if things get better or worse. Shock mounting to the chassis with fiber washers may help also.

Perhaps you should look into repairing your line filter with something like this (these voltage and amp ratings look OK). Find one with dimensions that fit in the little box.

http://www.jameco.com/Jameco/catalogs/c141/P104.pdf
 
I'd certainly suspect the transformer; I'm not sure that they all even had a line filter (especially the versions with a non-removable line cord) and the only remote reason it might buzz would be a cap about to blow.

Does it sound the same when you unplug the main board?
 
Well, the buzzing is certainly coming from the transformer. Tightening or loosening the screws made a significant difference in the pitch and volume of the buzzing. I'm going to get some washers and see what happens. Thanks!
 
Let us know. My friend has an Educator 64 that's doing the same thing. Like you said, it works fine, but the buzzing is somewhat ominous.
 
I've heard of transformers that develop a buzz without a problem. If it's not drawing too much current, too much current isn't being drawn from it, and it's not getting warm, it's probably not a problem other than the annoying noise, which may be able to be dampened mechanically.
 
Ok, I added from fiber washers on the top and bottom of the 4 screw holes in the transformer. Playing with the screw tension a bit I seem to have been able to reduce the annoying buzz to a much more mild hum/buzzing sound. I'm going to run it for an hour or so to see if it stays as relatively quiet as it is right now.
 
KC9UDX said:
I've heard of transformers that develop a buzz without a problem. If it's not drawing too much current, too much current isn't being drawn from it, and it's not getting warm, it's probably not a problem other than the annoying noise, which may be able to be dampened mechanically.
Click to expand...
That's why I asked here and in the SP thread with the same issue whether it's the same when unplugged from the board (i.e. no current being drawn) but as usual my question was rudely ignored in both threads. Oh well...

FWIW loosening the bolts, liberally applying lacquer or paint and retightening the bolts often cures the buzz.
 
MikeS said:
That's why I asked here and in the SP thread with the same issue whether it's the same when unplugged from the board (i.e. no current being drawn)
Click to expand...
Yep, I thought that was kind of important. I even considered quoting you to get the point across. A leaky filter consenser will surely make the transformer buzz, like you said.

Incidentally, I wonder how many of us check to see if those big filter capacitors are working. The first thing I did when I got my 2001-N was check out the power supply with an oscilloscope. But, I don't recall ever doing that with other computers. It's kind of important, and more so as the years go by.
 
MikeS said:
That's why I asked here and in the SP thread with the same issue whether it's the same when unplugged from the board (i.e. no current being drawn) but as usual my question was rudely ignored in both threads. Oh well...

FWIW loosening the bolts, liberally applying lacquer or paint and retightening the bolts often cures the buzz.
Click to expand...


MikeS, I had no intention to ignore your question, I was simply implementing the suggestion just prior to yours. I have not disconnected the transformer from the board yet.
Regarding your last comment, are you saying to apply lacquer or paint to the bolts?
 
rglenn said:
MikeS, I had no intention to ignore your question, I was simply implementing the suggestion just prior to yours. I have not disconnected the transformer from the board yet.
Click to expand...
No sweatl From time to time I need to confirm my reputation as a cranky old curmudgeon ;-)

Just pull the connector off the board (with power off) and see if it sounds different when you turn power back on.

Regarding your last comment, are you saying to apply lacquer or paint to the bolts?
Click to expand...
No.

Power transformers are usually built on stacks of thin metal plates and if any of those plates or the coil are loose and free to vibrate then you've effectively got a 50 or 60 Hz buzzer; they're normally soaked in clear varnish or some similar coating to 'glue' the plates together.

Might be worth a try to loosen the bolts, paint all around outside and inside the metal core and around the coil with thin clear lacquer or varnish, and then tighten the bolts again and maybe even add a clamp or two between the bolts while the lacquer dries.

Sometimes just playing with the bolts, alternately loosening and tightening, can make a difference; also make sure that there is no loose (or cracked) metal anywhere near the transformer.

A common problem:
http://www.gearslutz.com/board/geekslutz-forum/402221-stopping-buzzing-transformers.html
 
Last edited:
MikeS said:
That's why I asked here and in the SP thread with the same issue whether it's the same when unplugged from the board (i.e. no current being drawn) but as usual my question was rudely ignored in both threads. Oh well...

FWIW loosening the bolts, liberally applying lacquer or paint and retightening the bolts often cures the buzz.
Click to expand...

And the answer is ....... the transformer has the same buzz when unplugged from the board
 
rglenn said:
And the answer is ....... the transformer has the same buzz when unplugged from the board
Click to expand...
Good; that means that the system board is OK and it's most likely loose laminations in the transformer (or, much less likely, a problem in the monitor or an internal short in the transformer).

As long as it's not getting excessively hot I'd either live with it or try the varnish treatment.
 
You must log in or register to reply here.
Back
Top