Picked up a Sunn stage lead for next to nothing. Has power but no sound. Found an open 680K resistor which I replaced. When I powered it up it fried one of the power resistors on the board. I couldn't find a schematic for it anywhere so anyone had any luck with these?
The Sunn Stage Lead is extremely rare. Probably only made one or two years. I'm a moderator on the Unofficial Sunn site and there are no known schematics. Let me know if you find one. Probably close to the Alpha or Beta series.
http://sunn.ampage.org/
So I am finding out. I believe they only made it in 1975.
This cab has 4 - 10" speakers. One was blown. So I'm on the hunt for one of those as well. I plan on completely restoring this amp.
The info I have says the Sunn part number of the speaker is 81-0116-00-90. A 10 inch 16 ohm speaker also used on Beta 410. Speakers might be stamped 0116 on the back if there is no Sunn label. Once in a while you'll see a Sunn speaker on Ebay.
If you find none , and reconing is impossible or 3x the price of a new one, remember that Jensen makes the *very* good and relatively inexpensive MOD speakers.
Many of them are available in 16 ohms.
The MOD 1050 in 16 ohms would be a perfect replacement.
They are available in incredible 4/8/16/32 ohms.
Yep there were numbers stamped on the speakers alright. I'll check that tomorrow. I'm gonna try as best I can to keep it original. Thanks for the info!
If possible, post a few close up pictures.
A front one, a back one and some closeup of whatever inscriptions it might have.
It was probably OEM made for them by Eminence or CTS or Pyle.
I guess Jensen was out of the picture by then (they were getting heavily into car speakers) .
(http://i1092.photobucket.com/albums/i414/markorock37/003.jpg)
(http://i1092.photobucket.com/albums/i414/markorock37/005.jpg)
(http://i1092.photobucket.com/albums/i414/markorock37/006.jpg)
(http://i1092.photobucket.com/albums/i414/markorock37/001.jpg)
1) your speakers have no manufacturer´s codes (unusual) but show, as loudthud said, Sunn´s own internal code.
They have Alnico magnets (which were dropped everywhere between 1973/75 ) so in theory you shoud have it reconed (ask Ted Weber) although if too expensive or inconvenient, replace it by one of those I suggested earlier.
The amplifier board knows nothing about magnets.
Anyway keep the blown speaker frame, maybe in the future you change your mind.
2) from what´s visible in your slanted picture, the power amp has a driver transformer.
Please take a new one "from above" to see the entire board and how many power transistors it has, to compare it to better known models.
*Maybe* the power amp matches something .
Also show a closeup of the burnt area.
Just shooting in the dark.:(
*Maybe* your power amp matches or is somewhat close to this Sunn Studio Bass
Best pic I have at the moment, top right corner of the board just left of a ceramic resistor are two brown resistors. The one on the bottom of the two went up in smoke. Thanks for the schematic, looks similiar.
(http://i1092.photobucket.com/albums/i414/markorock37/002.jpg)
Definitely keeping the speaker. I'll try to find a replacement but I will get it reconed if I have to. I'm sure this will be worth more in the long run if I keep it as original as possible. I do know those speakers sound good!
The burned out guy is a 33 ohm 10%, is it safe to say a 5W will be sufficient?
No, because *someting else* is burnt.
First repair that.
I´m quite sure the schematic reasonably matches the one I posted, with maybe a couple different values to account for the higher power (which means higher rail voltages).
That 33 ohm you mention probably is a 1/2W or at most 1W resistor, no more.
Your output transistors are probably blown, check them for shorts.
Search the Forum, it has been explained lots of times.
Also build a series lamp limiter (search it too).
Beware that Resistors R142/142 , labelled 33 ohms, are really 0.33 ohm ones.
I *hate* when they drop the leading "0" , which makes it easy to miss the dot.
The better European system is to use a letter to indicate the dot, so 0.33 (or even worse: .33) ohms is written as 0R33 ; 1200 ohms is written 1K2 (instead of 1.2 K which might be misread as 12K if you miss the dot, specially in a photocopy or poor scan)
Oh well.
I planned on replacing the output transistor at the same time, I am aware sopmething else is wrong down the line, why else would resistors blow, I figured that may be the cause as it usually is. I have bands of orange, orange, black, silver on the resistor, that is a 33 ohm and it looks a lot bigger than a 1 watt.
What is it connected to?
Try to find it in the schematic.
Even better, "correct" the schematic by going over it part by part and labelling them with the actual values used in the board.
Voltages too.
I was referring to the emitter resistors, don´t know what on the schematic is that "33 ohm" one.
If I use the Studio Bass schem its R140 or R142 33ohm 10W. Glad I found that 10W figure. I'll get my parts today but may be after xmas before I get any more time in it.
Thanks for the info JM.
That amp (Stage Lead) is the same as the Studio Lead. The Studio Lead and Studio Bass use the same PCB, they just leave off the parts for the reverb. R140 and R142 should be 0.33 ohm 10W. If one or both are blown, you have a shorted output transistor, Q1 and/or Q2. Remove Q1 and Q2, power up the amp and check that the collector of Q1 (terminal 5 on the PCB) goes to about 25V. This will verify that the biasing resistors are ok and there is not a short in the driver transformer.
Loudthud´s suggestion is great, because the driver transformer separates the power transistors from the rest of the circuit, (speaking of DC), so you can safely test for the voltage he indicated (which is about 1/2 the Rail voltage, what you have on the top transistor´s collector)
I add that you should check the Base to Emitter voltage on each of the output transistors (without transistors, just at the PCB connection points)
They should be both the same and around 500/550mV.
Post results before re-mounting said transistors.
Sounds like a great idea, I will try that and post what I find.
This should help. The schematic and layout from the Studio Lead which should be the same as what you have. What are the numbers on the power transformer? The Studio Lead used 28-1650.
Thanks loudthud! This is the real schematic, mine uses PT 28-1650 as well. According to the new schem, its R137 that burnt, 22 ohm, 2W resistor. Apparently what I thought was orange bands on that are actually just really faded red ones! Again thanks for the schematic, I'll get the parts today and do testing Q1 and Q2 out of the circuit. I have some extras laying around is there a way to test these?
Pulled Motorola 22-3055 transistors out. I have a bunch of MJ15003 and MJ15004 - are they interchangeable with the 3055's?
I picked up some 3055's at the local electronic supply. Replaced the burnt resistor with same spec, fired it up w/o the transistors in place and looks like its running now. Q1 on pin 5 was 24.5v and Q2 pni 8 52.1v, Q1 base to emitter 654mv, Q2 base to emitter 632 mv. Fire away if anything sounds bad or good before I put the rest back together.
Sounds reasonably good.
Put the transistors and turn the amp on; still with the lamp limiter in series and without speaker connected.
Re measure and post voltages.
Should read fine.
Plugged it in with the transisitors in with the lamp limiter, bulb barely glows. I was going to plug it in to a speaker but the cab wasn't wired up when I got it, anyone have any ideas how the speakers were wired and what ohms is the output?
Given that you have 4 x 16 ohm speakers and the low (47V) power rail voltage, I think they probably wired all in parallel, as to give a total 4 ohm load and an approximate 40/50W RMS which sounds reasonable.
I very much doubt they built a 4x10" amp to get just 12W into 16 ohms, which would be the other option.
You can wire the 3 alive speakers in parallel, pull the bad one for future reconing, and cover its hole with a plywood square bolted there.
You should end up with a fine practice/Club clean guitar amp.
Even overdriving it won´t sound bad at all, because of the transformer driven stage.
Try it.
Any MP3 will be welcome ;)
Sounds good. I found the speaker plug with 2 pairs of leads I figured it must have been wired series parallel. Couldn't find a speaker on ebay at the moment so sounds like I'll be sending it out to get reconed. I'll put a mp3 up soon.
Got everything hooked up and had some sound for about 15 seconds before R137 started smoking again. Pulled transistors back out, one tested bad. Put 2 new ones back in that test good (I didn't test the new ones the first time) with an ohmeter. Replaced R137 although it tested ok, it did get red hot. With it back together I get sound but its really weak like its not the full signal or power. I tried it with another cabinet too, since I only tested the original speakers with a 9 volt, same thing.
Checked R136, 138, 139 all are in spec. I'm not wanting to trust Q106 if thats playing a part in this.
A bit more testing I found about 10V at pins 11 and 14 from driver transformer to Q2. Pins 12 and 13 from driver transformer to Q1 is less than a volt. Sounds like the driver transformer is bad to me. Its a model 28-4300
Quote from: markorock37 on December 27, 2011, 06:45:21 PM
A bit more testing I found about 10V at pins 11 and 14 from driver transformer to Q2. Pins 12 and 13 from driver transformer to Q1 is less than a volt. Sounds like the driver transformer is bad to me. Its a model 28-4300
10V is a little low. Pins 11 and 14 should be at half the supply voltage (+47). They should go to that voltage when Q1 and Q2 are removed and should be about the same when a good pair of transistors are installed. You can see that R128, R137, R136 and R139 form a voltage divider. Some current goes through R132 but not much. C1 could be bad, but it wouldn't affect the voltage when no speaker is connected.
These transformers develop shorts between windings. Disconnect 11, 12, 13 and 14 and then check that those wires measure infinite ohms to 15 and 16 and to ground. Mercury Magnetic$ makes the only replacement transformer available. If you find a short, there are two more windings on the transformer that were not used in this amp. They probably just cut the wires off. Any chance you can see them and maybe solder a wire to them? One pair is Red/Orange, the other is Yellow/Green.
Do you know how to check transistors out of circuit with an ohm meter? Basically you should see a diode between base and collector and another diode between base and emitter. Collector to emitter should show a high resistance or open.
Oh well.
It´s discouraging when it happens, but we must go on.
Hope it´s not the driver transformer.
*If* it is, considering what MM usually asks for their iron, I´d ditch the Power amp and put a small chipamp there, fed by that PSU and driven from that Preamp.
Sound would be very good.
Anyway, let´s try it once more:
I have 2 suspects for what happened:
1) fake 2N3055, specially if from a small supplier (the 2 or 3 "big" ones can still be trusted, sort of).
The situation is so bad that I had to stop using them after 35 years of faithful service, over 8000 amps built around them.
2) Overbiasing and thermal destruction.
I suggested earlier you would find "500/550mV" across BE junctions (across R137/139) , you found around 650mV.
Too high in my book.
Yet, "since Sunn made it that way" I said nothing.
Please solder a 100 ohm resistor (1/4W will do) in parallel with each 22 ohm one.
Measuring without the output transistors you should have around 500mV across each of them, and 1/2 47V (1/2 +V rail) on point 5.
Please confirm.
You might also have R142 open (so all current must pass through R137 = "red hot"); check R140 too.
Do not replace transistors (which should be checked "outside" the amp) until posting found voltages.
Good luck.
EDIT:just checked MM´s ordering page:
QuoteSUNN-CONL-INT Concert Lead -- Drop-in Upgrade! 170.00
ARRRGHHH !!!!!!!!!!!!!!!!!!!!!!! 170 bucks !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Well, looks like the driver trans is good, it measured infinite on all secondary leads. I did remove R140 and R142 and both measure around 2.5 ohms = should be .33 ohms. For whatever reason, pins 11 and 14 are at 24V now with the transistors removed. One of the new transistors is bad, and one of the originals was bad too. I have 3 good ones left. I'll get a couple new .33 resistors installed and then take a reading with those 100 ohm resistors installed in parallel with the 22 ohm ones.
Quote from: markorock37 on December 30, 2011, 01:56:41 PM
For whatever reason, pins 11 and 14 are at 24V now with the transistors removed.
That's what you should expect.
Quote from: markorock37 on December 30, 2011, 01:56:41 PMOne of the new transistors is bad, and one of the originals was bad too.
There is the possibility that something in the circuit killed one of the new ones when you installed it. Did you find a short when you checked the transistors with an ohm meter? Check for shorts from the case of those output transistors to the chassis before you power up the amp. This is a good time to have one of those light bulb limiters, it will usually save the transistors if there is still a problem.
Nobody gave you an answer before, but the MJ15003 is a good substitute for 2N3055.
Light bulb limiter has been on it since I lost a transistor. Good to know MJI5003 will work..thANKS. Did check them with an ohmeter. I didn't get any time to really get at it today, but should get somewhere this weekend.
I put in new R140 and R142, also soldered in the 100 ohm resistors in parallel with the 22 ohm ones. Reading a more stable 501mv at Q1 and 489mv at Q2. I haven't tried it yet but will later tonight. Problems only occur once the speaker is plugged in so hopefully its a go this time which makes me wonder about that cap in C1. The local parts guy didn't have that in stick so may have to order one if its the issue.
Yes, C1 might be bad.
Easy to test: connect a 100 ohm resistor from its negative to ground (as if it were a speaker) and read DC voltage across said resistor. You should have nothing.
If the resistor goes up in smoke .... there it is.
Another suspect: replace both C123 and R141, or simply measure that R141 still is 22 ohms, easy, and replace the capacitor; you can use 2 x .1uF caps in parallel if .22 not available.
This is the "Zobel" stability network, and if open, amp looks good but starts oscillating as soon as you plug a real world speaker there.
Might be what´s happening to you.
We will leave no stone unturned.
*When* you plugthose speakers back, still leave the lamp limiter connected, the amp should still be able to supply 10/15W , enough to test it.
Only when everything fine can te limiter be bypassed.
Good luck.
Replaced c123 and r141 was a little out of spec so replaced that too. C1 tested fine. Fired it up on the limiter and had rough sound so I Shut it down and cleaned all the jacks And pots with Deoxit. Sound is still intermittent.
Got it figured out, its making great sound now on the limiter, the light bulb gets brighter the louder the amp volume is turned or how hard you play. Haven't plugged it in straight to the outlet yet.
Looks fine to me.
If in doubt, put a bigger lamp in the limiter socket, I have a 150W and a 200W ones for that purpose, you can reach rehearsal levels with them.
If all looks fine, go ahead.
You might start by plugging the amp straight into the power line, no sound, no speaker attached, turn it on and measure DC voltages, such as +B (those nominal 47V), having 1/2 that at the centerline , proper around 500mV base to emitter.
If fine, connect the speaker, still no signal, cheak thing still stay the same, that transistors do not overheat, etc.
If fine, inject some music and play it at low power for, say, 5 minutes.
If fine, play at will.
A *VERY* cautious, step by step approach.
The *only* unknown might be fake transistors, which might pass all low power rtests, but overheat and burn at high power.
But then you have already shown your proper troubleshooting skills, simply you need to find a trusty supplier (which is not easy).
So far I´ve found all Toshibas fakes, and most the ON/Motorolas , both because of perceived prestige; while ST have been good, being relatively unglamorous.
Most "strange" unknown brands have been a fiasco.
Good luck.
Sounds good I will give it a try tonight. ST is what I put in there , I have a local electronics shop that is pretty well stocked. Thanks for all of your help, I will let you know how it goes.
Took it cautious like you said and now I'm rockin! It started to sound better the more I played it probably the caps getting fully charged. It has really good sound I have to admit I am surprised. Sounded even better with an overdrive pedal. I gave it the run through for about an hour, put the knobs back on and the amp is back in the cabinet. Thanks for all the help in locating the issues Mr Fahey and Loudthud, I learned a lot on this one and gained a ton of knowledge in the process. I'll get a mp3 up soon so you can hear how it sounds!
Quote from: markorock37 on January 07, 2012, 02:53:17 AM
I'll get a mp3 up soon so you can hear how it sounds!
I look forward to it. My brother rocks out an old Sunn bass amp and loves it.
**edit**
He is a bass player though, not playing guitar through a bass amp.
Here it is finally had time to record an mp3 of the Sunn Stage Lead. It was early morning so my chops weren't up to speed yet..lol.
http://www.isound.com/mark_hawkins (http://www.isound.com/mark_hawkins)