Broken valvestate 8080

[Enzo]

Don't even go to the IC yet.

Your power supply is missing the -15v rail.

R102 has -47v on one end and next to zero on the other (where -15v belongs).   Now either R102 is open, so the voltage can;t get to the othger end, or the -15v is shorted to ground.   This is easy to determine.   

The other resistor R101 will naturally get pretty warm during operation.  COmpare R102 to it in terms of heat.  If R102 is open, it will be cold.  But if the reesistor is intact, but the zener or other item on the -15v end is shorted, then R102 will be VERY hot.

And it is also possible the zener is shorted and that caused the resistor to open.  SO if you find the resistor open, you still have to check for shorted -15v.


About the op amp ICs.  They are powered by +15 and -15, but they have no ground connection.   They sometimes fail and short the two power supplies together.   This makes them hot, and the two supplies will BOTH be very low or at zero.   But a failed op amp IC really doesn;t have a way to short just one power rail to ground.

SO my main suspect still remains the zener.

I forget which one is -15, but it is the one connected to R102.

[JamesAlexander93]

Sorry I got R101 and R102 the wrong way round when I gave you my measurements, it's actually R101 that's giving me the -0.08V reading not R102. And yes R101 seems to have shorted, it's VERY hot. So I'm guessing it's ZD1 that needs replacing. And that's a 15V 1.3W zener? Guess I'll order some. How exactly does a zener fail? Is it something that can wear out gradually? I'm just wondering if it would be worth replacing both of them as if one has failed the other could have have problems too. yes?, no?   

[Roly]

It may be a coincidence, but if one of your 15v rails is missing, your op amp will try to center on what is left, and that would be just about 8 volts.

Very well spotted.   :dbtu:

How exactly does a zener fail?

That is a very good question.  There are a number of failure mechanisms in solid state devices, but the best bet here would be heat, and thermal cycling.  Semicons generally don't like getting too hot since it tends to reactivate physical/chemical processes used during their manufacture.  Thermal cycling is a bit easier to understand since it results in physical stresses that change from cold to hot to cold and physically "work" the various parts of the semicon, the dice itself, its interconnects, the encapsulation, and in particular the seals between the lead-ins and the encapsulation.

Should you replace both?  I have to say that Zener diodes in general are not my favorite devices.  This may have more to do with designers pushing them to the max than anything inherent to the devices themselves, but generally they do seem rather failure-prone, so I'm inclined to say "yes", renew with fresh Zeners in both positions.

[JamesAlexander93]

Thanks roly, I'll replace both. I'm a little unsure as to which diodes to buy though as I have two schematics for the 8080 power amp and one tells me the zeners are 15v at 3W and another tells me they're 15v at 1.3W. Do you know which one is correct?

[J M Fahey]

Probably the original ones were the 1.5W type and after a certain history of problems (like yours   ) they upgraded to 3W .
Higher power capability never hurts. 

[JamesAlexander93]

Okay, 3Watt ones seem to be a little scarce would it hurt to go up as far as 5Watts? Or is that too much?

[Enzo]

Scarce?   First place I always look is Mouser, and they have:

15v 3W   1N5929 for 37 cents each, and 1500 of them in stock

15v 5W   1N5352 for 36 cents each, and almost 15,000 of them in stock. 

Take your pick.  For 36 cents, replace them both.   Watch which way they go in.


Can you tell what parts are in yours?  Your amp may have 1.5W zeners, and maybe the 3W on more recent drawings is the step up you need.


Are your existing ones mounted right down against the circuit board?   When you install new ones, leave teh wires long enough for thgem to sit up in the air a little - half an inch maybe.  That allows more air around them for cooling, PLUS the extra wire length will act as a small heat sink - honest.

How do they fail?   All manner of things can go wrong with them, including random component failure.  Heat is not their friend, but they run hot by their nature.  SOme other component failure COULD have exposed them to some unwanted voltage or current, and stressed them.   Those are matters of why.  As to what goes wrong inside them, they can suffer any failure a regular diode can, plus their zener voltage can wander way off.   They can short, they can open, they can get intermittant, they can even get noisy.


As to figuring out what was wrong with the amp, despite what my wife says, sometimes I get the right answer.

[phatt]

As to figuring out what was wrong with the amp, despite what my wife says, sometimes I get the right answer.

Clever Man if you can use your Wife as a zener diode tester!!! :lmao:
Mine is Obviously not working well,, as it gives false results all the time. :loco

ps; I had my SS Laney work horse go dead a few months back while playing and sure enough dead shorted Zener diode for opamps. :grr

The drop resistor was hot as hell,,,which was my clue and sure enough two new Zeners and everything was fine. :tu:
Replaced a few suspect caps while inside it.

Phil.

[Roly]

Wife as a zener diode tester!!! :lmao:

{Ooooh no, I'm not going anywhere near that.    }


The power rating of a zener, like a resistor, is how much power dissipation the device can handle without being damaged.  The only problem you are likely to strike is that as the power rating goes up so the devices tend to get physically large (and in this case lead thickness).  So apart from that there is no reason why you couldn't use 5 watt zeners if that is all you can get.    Going lower simply means it will overheat and burn out in a hurry.

[J M Fahey]

false results all the time

Well, that's *good*.
You can ask and then do the exact opposite, with excellent results. :tu:
The problem is when the tester (electronic, human or female   ) gives random answers, which average 50/50% or thereabouts.

[Enzo]

Hmmmm...

Yes, my wife is like a zener diode.  If things go her way, they flow just fine, but when things go against her way, she resists them, until it gets to the point she gives in.  Things just avalanche from there.

[Roly]

human or female

female =/= human ... I often wonder.   :loco

My wife's Subaru Liberty has developed an occasional and total loss of power for a second or five that is driving her, her mechanic, and therefore me, stark raving nuts.


Ha, techo-pun by Enzo  :lmao:  (just to let you know Zener/avalanche wasn't wasted).

SWMBO is more your 1N4007 - gets a bit hot and bothered if her PIV is exceeded.

[chezqui]

First thing, is the speaker cone moving hard forwards or backwards when the amp is switched on, or staying roughly in the rest position?  If it is moving to one end of its travel and staying there it must be disconnected to avoid damaging it while other tests are carried out.

Second thing; take a known good lead and bridge the Effects Loop Send and Return - any change?

Try feeding a signal into the Effects Return, say from an MP3 player or the like - any response?

Try feeding the Effects Send into another amp - any response?

These test will help narrow down which section of the amp is playing up.

Hi Guys

Newby to this forum. I found it because I have the same amp problem.

I tried "take a known good lead and bridge the Effects Loop Send and Return" and it made huge difference so can you help me now. What is my next step please?

[phatt]

Hi chezqui and Welcome, :tu:

So you now are faced with 2 options;
1/ Leave the short lead in place which has fixed the problem and likely cost close to nicks. :dbtu: :dbtu: :dbtu:

OR

2/ Spend a lot more money, time and effort pulling it all apart and replacing with a new switching socket which will more than likely have the very same design flaw as the original part,, which means it will happen again down the track.

A majority of the socket designs that are used in most guitar amps nowadays are just crap and are a very common fail point. The cost cutting is now so extreme that you are lucky to get more than a few years of service before a socket fails. The plastic itself is no longer plastic the molded plastic sockets have filler mixed into the plastic to make it go further hence they become very brittle and crack. the metal tab contacts are only half the thickness of the older types. And the list goes on,,,,All in the name of $$Profit$$

So Brand Name now is just meaningless marketing BS. :trouble :grr :trouble :grr :loco :duh

So obviously my advice would be leave it alone; The short patch lead is the most reliable fix. :dbtu:

You can buy electrical cleaners which may help,, spray and insert the plug in and out many times to help clean and dirt and oxidation from the contacts but I've found the patch cord to be the best fix. :tu:
Phil.

[chezqui]

Great answer, thank you. Spookily I have taken the leads out and it seems to be OK at the mo.

Still some frying bacon going on but we can maybe discuss that later.  :dbtu: