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Peavey Renown

Started by Hawk, June 26, 2015, 09:12:28 AM

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Hawk

Readings:
I didn't tests the new transistors but might just in case. Here are test results from 4 of the six original output transistors in amp. All transistors were removed for all readings.

TR 7 Showed Good on DCA55 Tester. Resistance Readings trans. out of circuit.  E/C-C/E-0L/0L,  B/E-E/B 75M/0L, C/B-B/C- .71M/0L

TR10 Showed Good on Tester.  Res. readings trans. out of circuit. E/C-C/E-0L/0L, B/E-E/B 5.7M/0L, CB-B/C .6M//0L

TR11 Showed Good on Tester. Res. readings trans. out of circuit E/C-C/E-0L/0L, B/E-E/B 6.8M/0L, C/B-B/C .703M/0L

TR6 Showed Good  on Tester. Res. readings trans. out of circuit E/C-C/E-14.8M/0L, B/E-E/B 7.17M/0L, C/B-B/C .7M/0L.

phatt

Just remember;;When reading low Ohms on a DMM it's wise to self test first.
Bridge the tips together and note the reading of the leads,, it won't be zero.
Now measure the resistor value and subtract the lead resistance that will give a more accurate number. :tu:
Phil.

Hawk

Thanks Phil. My DMM zeros out completely when touching the leads. I'm still wondering about the power transformer as you suggested. I'll do some resistance readings on the 42V windings. From red-to-red I get .5ohms. Red to center tap-.3ohms, other side .2 ohms.....seems low!!

Hawk

I used R.G. Keen's transformer test. Since I used a 9V battery I reduced the resistance to 10K. The bulb didn't flash. But when I removed the resistance the bulb did flash. Not sure if I've used the correct ratio of resistance to voltage (my battery reads 7V so I reduced the resistance down to 10K instead of 100K). Hmmm....I then tried using a 100K and again the bulb didn't flash but did without the resistance.

Could a small internal short cause the voltage to drop when connected to the power amp as is happening to my amp?
Can putting a load on a transformer with an internal short create an Inductive Kickback in the transformer so that current tries to flow in the transformer to compensate for the short and therefore not send current out into the power amp?
??? ??? ???

phatt

#79
Hello Hawk,

I think you will find the neon will only light up (flash) when you break the connection, if no flash then you have a problem. Unlikely the transformer has a fault but I thought it might be worth a mention.

I'm with Enzo, there is a short somewhere loading down the rails and I quote from his previous clarification;

Enzo said; "I mentioned before not to focus on the 1.4v rails.  With the bulb limiter, some serious short in the amp lights the bulb brightly meaning the amp itself sees very low voltage."

That short could be anywhere.  :grr :grr :grr

Even a breakdown in the isolation shims of the power transistors,, only takes one to fail.
A hairline crack in a trace,,, the list goes on.
Phil.


Hawk

So when the screws go through the output transistor hole they insert through the insulator but the thread makes contact with the case collector and that is cool. Right? I should test to make sure the collector isn't grounded out to the heat sink......or that the base and emitters aren't grounded out.....thanks!

g1

Quote from: Hawk on August 06, 2015, 03:36:59 PM
I should test to make sure the collector isn't grounded out to the heat sink
Yes

phatt

See pic;;
If Any of the parts marked *C* come in contact with the heatsink then it's a dead short for sure.

The little washer down the bottom has a locating detent and that keeps the bolt from coming into contact with the sides of the hole as it passes through the heat sink.

Even If the parts and holes are well made and aligned you still need to take care when bolting it all together.

You only need a bit of conductive crap to lodge in the hole and it's all for nothing. :'( :'(

Yes ALWAYS :trouble :trouble Use your continuity buzzer to check for shorts to the collector when you mount TO3 transistors. :tu:

And as you have already noted,,, the B and E pins need to be checked for the same reason,, Some gear may even have protective sleeves on those pins to stop any chance of contact of pins to the heatsink.
Hope it helps,, Phil.

phatt

Just had closer look and cross referencing some of this;;

Now I just goo'd the number on those new looking power transistors.
Best I can make out from the photo is MJ150250.

Which seems non existent but MJ15025 is legit.

BUT  Ouch!! it's a PNP power transistor  :o :o and if that be the case then no wonder the amp does not work as the circuit uses all NPN power devices.
Phil.

Hawk

QuoteBUT  Ouch!! it's a PNP power transistor  :o :o and if that be the case then no wonder the amp does not work as the circuit uses all NPN power devices
Ahhhh!!!!!!!!!  I ordered these from Peavey and they sent me the wrong ones. I know I quoted them the original part and I even told them the cross-reference part as well. I didn't think to check before inserting. Okay, there's hope!! Thanks Phil for noticing! Awesome!! I'm going to order from a local supplier and get the right ones.

Also, Phil, thanks for the post on continuity buzzer. I never use it and now will always use it when looking for shorts. So, yeah, I'm finding shorts on the output transistors  from collector to heat sink, and it comes down to the screw making contact. However, Peavey did not supply any kind of isolating washer. The screws go right down through the holes in the heatsink and the holes are tight to begin with. Is there something I can use/purchase to stop the screw from making contact with the hole?

phatt

Re the buzzer which is likely the diode tester,  be aware they are not fool proof and can give false reading. I'm not sure of the threshold resistance but others may know more.

I like the buzzer because it allows you to focus on the circuit without having the distraction of keep looking at the meter.

I recall *Enzo* mentioning parallel components causing misreadings and this is the case here as power transistors are often paralleled by very low resistors.
I guess ideally you mount all the power transistors on the heat sink and test for shorts *Before* connecting to the rest of the circuit but (by what I can make out) that is not easy to do in this case.
Hopefully Enzo might be able to shed more light on this. 8|
Hang in there all is not lost. :tu:
Phil.

Hawk

#86
Thanks Phil!  So some good news. The light bulb is not lighting up!!!!!!!!!!!!!!!!!

70 volts variac, 20 Volts collectors of Transistors 6 and 7 (12 removed, -20 volts on emitter resistors of 10 and 11, 13 removed. -.57 on collectors of 10 and 11.

So, here's my question: Yes, I understand that the collectors can't short out to the heat sink but the heat sink is part of ground, and 6 and 7 aren't shorting and 11 and 10 collectors even if I remove the screws they short out (beep test, so I get a beeping sound). I removed the screws so that I wouldn't get the beeping sound and I didn't so I figured no shorts and decided to power up the amp for a short while to see what I could find. Are 11 and 10 collectors supposed to be shorted  ground??? Do my collector voltage readings on 10 and 11 make sense? they are connected to ground so it makes sense to me:tu:




g1

Can you post a pic of the underside of the board in the power transistor area?

g1

  Also, there may be a short to ground from where ever those screws connect to, that's why the picture will help.

Enzo

Peavey will provide insulating stuff if you ask for it, but otherwise the transistors do not "come with" them.  And really, all insulating materials were, or should have been, already in the amp with the bad parts.  The screws need not to touch the heat sink, but also, under each transistor is an insulating piece, and those can get damaged.  A chip out of it can let the transistor body touch the heat sink.  SO make sure all those insulators are in place and intact.

If peavey sent you the wrong parts, first look on your packing slip or invoice to see exactly what part number they SAY they sent.  is it the same number as on your new parts?  We want to figure out if they placed an order for one thing but shipped a different thing, OR did they enter the wrong part in the first place and then shipped it.  Then CALL them and tell them you received the wrong parts.  Chances are they will just send the right ones to you and won't want the others back.  Give them a chance to correct any mistakes.  WHy pay twice?

Collectors of Q10,Q11 grounded?   Look at the schematic, any ground connections there?  I see none.  However, if you have that output transformer connected, it will look like a short to ground.  We were supposed to have that disconnected, is it?  Also, if you have a load on the output, that also will make those collectors look grounded.  We were supposed to be working unloaded, are we?

Beep tests are fine within their limits, but use the ohm meter.  There is a HUGE difference between zero ohms, as in a dead short, and something like 22 ohms or 50 ohms.  Those resistances will still beep the continuity test, but they indicate resistance instead of a short.  For that matter, most meter beep functions are diode tests which should present a voltage on the scale, so it may beep and show half a volt, whch sounds like a diode junction or transistor junction rather than a short.  Don't assume your test gear is telling you more than it is.

If the amp has MJ15025 installed instead of MJ15024, good catch.