Squeal after recap

[Smoke Monster]

I recapped my Vectrex and installed a debuzz audio pre-amp using console5's kits, and after installing everything seems to be up and working, except for a new high-pitched squealing sound and a faint smell of ozone. The squeal is from inside the Vectrex, not the speaker.

I'm going to let it discharge for a couple of days before opening it up for an inspection (and then manually discharging it), but my only guess is that maybe I didn't reconnect the flyback connector to the tube securely enough. Could this be a potential cause of the sound and smell?

[vtk]

hi, perhaps you can temporarily remove your new audio preamp to check that is not related to the new squeal sound you have ... to do this if i remember correctly, you can cut the input and output wires which are leading to your small preamp circuit board, and then join those two wires together, (so as to override the preamp circuit board).

 

i have a bad memory but i think when i performed the audio preamp circuit board on my vectrex some time ago, i may have had squeal also. in the end, personally i decided to remove and abandon the audio preamp circuit board, and just stick with installing the 'shielded buzz reduction (ground) cable' instead ... but also, along with that cable, i placed a resistor around 2k (not 1k) in value, in the position shown in the picture below. this gave me a much better useable sweep of the vectrex volume pot (before using that resistor, i was noticing that the upper range of the volume knob was way too loud (annoying/ear piercing), so i wanted to remove that).

 

so now with a 2k resistor in place, when the volume knob is at maximum it is 'quite loud', but no longer 'crazy annoying loud' and doesnt rattle (distort) the speaker anymore. .. also, adding that resistor has reduced some noise/buzz (because it has removed some of the gain/volume.... which is ok with me as i didnt want all of that volume i would never use anyway....)

 

regarding the suction cup, you should be able to just give it a bit of a final adjust by pushing it down onto the tube (hand squeezing around the middle-rear of the suction cup) and moving it around a little (ensuring that both legs are properly inside the hole in the tube of course)

 

Edited January 31, 2016 by vtek

[+madman]

Definitely sounds flyback related, did you try the old tried and true low tech method of fixing flyback issues by giving the Vec a couple hard whacks with your hand?

 

I was unaware of that amp kit, that's something I wanted to do about 10 years ago, but never got around to it. I was pretty happy with the cutting the ground wire mod + shielded audio cable. It's just not the Vectrex without some buzz

[+save2600]

…or a slightly wavy screen.

[Smoke Monster]

hi, perhaps you can temporarily remove your new audio preamp to check that is not related to the new squeal sound you have ... to do this if i remember correctly, you can cut the input and output wires which are leading to your small preamp circuit board, and then join those two wires together, (so as to override the preamp circuit board).

 

i have a bad memory but i think when i performed the audio preamp circuit board on my vectrex some time ago, i may have had squeal also. in the end, personally i decided to remove and abandon the audio preamp circuit board, and just stick with installing the 'shielded buzz reduction (ground) cable' instead ... but also, along with that cable, i placed a resistor around 2k (not 1k) in value, in the position shown in the picture below. this gave me a much better useable sweep of the vectrex volume pot (before using that resistor, i was noticing that the upper range of the volume knob was way too loud (annoying/ear piercing), so i wanted to remove that).

 

so now with a 2k resistor in place, when the volume knob is at maximum it is 'quite loud', but no longer 'crazy annoying loud' and doesnt rattle (distort) the speaker anymore. .. also, adding that resistor has reduced some noise/buzz (because it has removed some of the gain/volume.... which is ok with me as i didnt want all of that volume i would never use anyway....)

 

regarding the suction cup, you should be able to just give it a bit of a final adjust by pushing it down onto the tube (hand squeezing around the middle-rear of the suction cup) and moving it around a little (ensuring that both legs are properly inside the hole in the tube of course)

Thanks, I will mess around with it a bit and then try bypassing the amp if all else fails. Was your squealing coming from the speakers though? Mine is coming out of the unit itself.

 

Definitely sounds flyback related, did you try the old tried and true low tech method of fixing flyback issues by giving the Vec a couple hard whacks with your hand?

 

I was unaware of that amp kit, that's something I wanted to do about 10 years ago, but never got around to it. I was pretty happy with the cutting the ground wire mod + shielded audio cable. It's just not the Vectrex without some buzz

I haven't tried smacking it around yet, but I will the next time I have it powered on. That should remind it who's in charge too

 

Don't worry about losing the buzz, it's definitely still a prominent feature but just at a much lower volume than before.

[vtk]

.....Was your squealing coming from the speakers though? Mine is coming out of the unit itself.....

 

unfortunately i cannot remember,

if you do determine what is causing your squeal please let us know here, as i would be interested to know, thanks (and best of luck)

[Smoke Monster]

I had a chance to work on the Vectrex tonight but I've still got the squeal. I removed the audio preamp and re-seated the flyback's attachment to the monitor. I'm pretty sure that the sound is coming from inside the metal box of the flyback transformer.

 

Other symptoms include: slight zooming in and out of the picture and unstable video. The squeal is not there when the machine is first turned on, but starts about 10-20 seconds after powered on.

[Smoke Monster]

Measuring the voltage at J204, I get:
-13.01V

+4.90V

GND

-4.95V

 

Is +4.90V close enough, or can this be tweaked up to 5V using a pot somewhere?

[Rolo]

The voltages are completely good (we don't know the accuracy of your voltmeter anyway). The LM340 takes good care of the +5VDC.

 

The sound of your flyback transformer might origin in some parts (windings, magnetic core) vibrating due to periodically building up and breaking down of the magnetic field, with the 555's frequency. In this case, this is just annoying, but not a real problem.

Either it is just something shaken a tiny, little bit loose, then let it sing, or the transformer is at the end of its lifetime (which happens to old CRT-tv-sets all the time), then you can't repair it anyway, but just replace it.

The solder points are all good? No cracks? No sparks?

 

If you dare and if you have the equipment, you could try the power board adjustments, described in the service manual, page 15. Maybe you can observe something useful.

[Smoke Monster]

The voltages are completely good (we don't know the accuracy of your voltmeter anyway). The LM340 takes good care of the +5VDC.

 

The sound of your flyback transformer might origin in some parts (windings, magnetic core) vibrating due to periodically building up and breaking down of the magnetic field, with the 555's frequency. In this case, this is just annoying, but not a real problem.

Either it is just something shaken a tiny, little bit loose, then let it sing, or the transformer is at the end of its lifetime (which happens to old CRT-tv-sets all the time), then you can't repair it anyway, but just replace it.

The solder points are all good? No cracks? No sparks?

 

If you dare and if you have the equipment, you could try the power board adjustments, described in the service manual, page 15. Maybe you can observe something useful.

Thank you for the excellent explanation Rolo.

 

I think that you're right about the sound being caused by vibrations. The squeal changes pitch and amplitude depending on the game that's being played, for example the Cantina Band demo doesn't cause any squeal but Spike makes it sing so loud that it causes the screen to freak out (zooms in/out and dims).

 

If there is nothing that I can do to repair the transformer, then I must look in to replacing it.

[Smoke Monster]

I may have found the source of the problem. When I was testing yesterday the fuse blew out of nowhere. Looking for shorts, I found that all three legs of the BU407D (Q502) transistor have continuity, and it looks discolored in one corner as you can see in the picture below. I've already ordered a replacement.

 

The BU704D drives the flyback transformer, so is it possible that this is the root of the Vectrex's problems?

 

[Rolo]

Will be interesting to see. Since your system was working, that transistor was working. Why did it die? Just like that or because of a final malfunction of the flyback transformer. Let's see how long the replacement part will survive...

Good luck!

[Smoke Monster]

I'm afraid that your hunch is probably right. I removed the transistor last night and those three points on the power board all still have continuity, which I believe means that something else must have blown. Once removed, the transistor doesn't bridge its three pins, which probably means that the flyback transformer is the cause I imagine.

Is the flyback a repairable part? It seems like that would pretty much put me at a dead end otherwise. At this point, I'll either have to give up or send it along to someone who can repair it. If you have a good recommendation, please PM me.

Edited February 11, 2016 by Smoke Monster

[Rolo]

Hmmmm,

 

 

transformer is at the end of its lifetime (which happens to old CRT-tv-sets all the time), then you can't repair it anyway

You probably can't repair it, if your not an expert in making transformers. I tried that once, when those small solar driven neon lights came out. My newly wound transformer worked for 60 minutes - and sparked again .

 

Have you ever looked at the damper diode D503 (1N4005), which probably protects "your" transistor from unwanted oscillations (ringing) when the inductive load is switched off? Is that still ok? This would explain the squealing noise.

And maybe C505 and C506 nearby, also not shortened?

[Smoke Monster]

Thanks for the help. It's hard to test anything right now, since most all of the positive and negative points on the board show continuity due to the (probably) blown transformer.

 

I've looked over the board a hundred times now, and will next remove the flyback transformer to make sure it's the cause. Hopefully I can then find someone to repair or rewind it.

[Rolo]

Check the diode first! And the safety-cap. There are just one, two or maybe a few windings on the primary side of the flyback transformer. If you're doing a DC-resistance measurement, that looks like a short. Transformers need alternating current to work, as we all know. Don't be fooled.

 

If that leads to nothing, let the board like that and go visit an old fashioned crt-tv-repair-shop. They do know all of the usual flyback driving circuits. That is a tricky analog part and surly requires some experience!

 

PS: Where you're from? (Shipping!)

Edited February 12, 2016 by Rolo

[Smoke Monster]

Thanks again for your help, it's giving me the optimism to get this thing running. All of this is beyond my experience and the guidance is much appreciated.

 

I just went to test D503 and it's not populated on my PCB. I've attached a picture below. I could order an 1N4005 and install it there if it will help.

 

C505 and C506 aren't shorted and look good.

 

On a properly working Vectrex, should the three points of Q502 have be shorted, or does that indicate a problem as I was assuming? I haven't plugged it in to test again since the fuse blew on Monday.

 

[Rolo]

Hi,

 

that is funny and strange, too! Now, I have to open up my Vectrex and have a look. Until now, I was just looking onto the schematics. Do they really just send the reverse voltage right to the transistor?

At the moment I don't have an idea. Give me some time, please. You can search the yellow pages in the meantime ("T" like TV) .

[Rolo]

Okay, I opened it up. Know what? I don't have D503 either! Obviously they found out, that the transistor survives this and that the diode is not "necessary". The schematics and all books I saw tell the opposite.

 

Quick measurement (with Q502 soldered in):

-No shorts at the pads, but megaohms.

- 5 ohms between base and emitter (1 and 3) since there are coils in parallel, which a DC measurement does not see (not the complex part at least).

 

Did you do any measurements, when your fuse blew? Maybe your probe choked the oscillation somewhere?

Or maybe the windings isolation inside the transformer epoxy block got too hot and failed somewhere.

[Smoke Monster]

Thank you for providing me with those readings. They gave me some momentary hope, since mine matched yours.

 

So today I put a new fuse in and powered it on. The Vectrex booted to the startup screen for about a second, but then the screen went black and the fuse blew again.

[tep392]

Did you check the entire board to make sure you didn't create a solder bridge somewhere? Look for drips of solder that you could have created while desoldering and soldering components.

[Rolo]

Yes, what can we check next?

The fuse blows, so something is drawing much more current, than nominal.

 

Your transistor survived or is it dead?

Maybe you can desolder and isolate T502 Pin 6 and/or Pin 7, which means disconnecting the flyback transformers primary winding. You'd steal all crt voltages, that way.

The logic board's voltages stay unaffected. MINESTORM should play in the speaker. The fuse should not blow. You could look at the flyback driving voltage (oscilloscope).

 

Maybe we can go on from there.

 

the flyback transformer to make sure it's the cause. Hopefully I can then find someone to repair or rewind it.

 

Usually it's moulded in epoxy resin. If so, it will be nearly impossible to repair it.

 

 

[Smoke Monster]

Did you check the entire board to make sure you didn't create a solder bridge somewhere? Look for drips of solder that you could have created while desoldering and soldering components.

I suspect that this is caused by something that I've done wrong in the recap, but I've gone over the boards a hundred times looking and probing for problems and so far I've come up short.

 

Yes, what can we check next?

The fuse blows, so something is drawing much more current, than nominal.

 

Your transistor survived or is it dead?

Maybe you can desolder and isolate T502 Pin 6 and/or Pin 7, which means disconnecting the flyback transformers primary winding. You'd steal all crt voltages, that way.

The logic board's voltages stay unaffected. MINESTORM should play in the speaker. The fuse should not blow. You could look at the flyback driving voltage (oscilloscope).

 

Maybe we can go on from there.

 

Usually it's moulded in epoxy resin. If so, it will be nearly impossible to repair it.

Yes, the transistor was good after all. I will try your suggestion to isolate T502 6 and/or 7 next and report back.

[Smoke Monster]

Sorry to double-post, but I can't edit my last post.

 

Rolo, I forgot to ask you if there is any benefit to installing an ln4005 diode at D503? I have a pack of them on hand since I ordered them before we figured out that it's not actually there. If so, do I need to remove one or both of those J1 & J2 jumper wires? I know that I have bigger problems to solve right now, but I am curious about this too

Edited February 21, 2016 by Smoke Monster

[Rolo]

Interesting: Looking at the pcb layout in the service manual, I notice D503 is printed with a dotted line-style, not solid like the other parts. Obviously they were not sure themselves somehow? (This would be a question for Jay Smith during some convention.)

Another detail: the service manual is contradictory concerning the diode type: schematic says MR852, part list says 1N4005 on page 23 and MR852 on page 24 .

Let's have a look at the diodes' datasheets:

1N4005: 600V, 1 Amps, fast switching Schottky-type

MR852: 200V, 3 Amps

Without knowing the real voltages and currents in the circuit, I would guess, both might be a reasonable choice.

 

The jumpers: Aren't they not just connections, where traces would be crossing on the board? I have no good photo at the moment. But guessing from what I have, I'd say they are connecting transformer pin 7 to the caps C505 and C506. Maybe you can confirm this on your disassembled Vectrex. They are not parallel to the damper diode, so leave them in.

But indeed, they are an easy way to disconnect transformer pin7 from the board!

 

I'd probably try to get the unit running first. Then you could do the diode experiment. In the end, thousands of Vectrexes seem to run with out the damper diode. Obviously the small flyback transformer for that small tube does not produce such a mighty reverse peak, that the transistor is getting some damage. You simply could reduce stress on the component, but will probably not notice anything yourself. Just guessing.