1967 Ampeg BT-15

[joshdfrazier]

So, a little backstory on this amp before I discuss the issue(s).

This amp has been played alot since we bought it. It's been on month long tours, played every night at (almost) max volume. It's always been great, and never needed any work besides a cap job and some new pots. One night at practice, something failed, and the amp started humming, and no sound came out.

I took it home, tested the output transistors, they were bad. replaced them and the 33ohm resistor that blew, and the 10ohm that looked a little crispy. I turned it on, and the same thing happened, taking out the output transistors. Keep in mind I did this awhile back, before I had any experience with solid state gear (stupid). Anyway, it's been sitting in a corner ever since.

I started tinkering around with it last night. I tested every transistor on the board, as well as the TO66 transistors, and every one of them tested okay. The 300ohm resistors are okay, as well as the .47ohm. The power section has all new caps. Transformer is fine, no shorted diodes anywhere.

Is it possible that a transistor can test okay, but fail when under a working load? That's the only conclusion I can come to at this point.

[Enzo]

Of course it can.   Ever get injured, like a sprained ankle, and you feel OK sitting there, but as soon as you try to walk on it, YEOW!!.   Your meter tests things with a tiny battery voltage, and under those conditions the transistor may check OK.  But under real world conditions, now your parts have 30-40 volts on them or something and we try to draw entire amperes of current through them.  We find out the parts are not up to it.


basically a meter can tell you something is definitely bad, but it can only tell you it MIGHT be good.


Resistors do not burn up on their own.  ANy time a resistor goes, you can pretty much count on a semiconductor connected to it failed.

[joshdfrazier]

I guess it's worth mentioning that most transistors measure about .6 or .7 v from collector to base with the diode test function, which seemed a little high. These are very old transistors, though.

I get an earful about shotgun repairs from you guys. What are my options, besides replacing all semicons on the board? The little TO66 guys (2n3767, 2n3741) ain't cheap... about 8 bucks a pop.

[DrGonz78]

So do we have DC offset on the output? What do the B+ rails measure? Of course measuring everything with a current limiter in place. I think we should start at square one to get the full perspective of what we are repairing. Is there a short in the amp that is causing excessive current draw? Just need more info about where we are at today on this amp.  :tu:

[Enzo]

What are my options, besides replacing all semicons on the board?

What if your problem is not a semicon?  An open resistor or a broken solder joint or cracked copper trace are just as likely.

To test a transistor it must not only act like a diode from base to collector, but also from base to emitter, and importantly, collector to emitter should check as open.

DO NOT connect a speaker or load to the amp until we know it is stable.

Look up "light bulb limiter" then make one and use it.

[Roly]

Can you nominate (by Rnnn) which resistors fried?

This has a slightly horrific bias setting arrangement.  If the wiper of R824 should happen to lift off the track it could smoke the output stage.  Smart designers don't do it like this any more.

Compared to your Jordan this is "big kids sandpit" and will be very unforgiving of mistakes.  You must only power this up via a limiting lamp, say 40W, until we are quite certain all faults have been cleared.

[g1]

the same thing happened, taking out the output transistors. Keep in mind I did this awhile back, before I had any experience with solid state gear

If you didn't have any experience at that point, can you be sure the output transistors were bad?  I think you should drop that assumption if they are currently measuring ok.

[gbono]

So back to your original post - does the fuse blow? I would suggest that you start at the power supply and check that the amp has a functioning   bridge and filter capacitors. If you have a PS then you might be able to pull the output devices and check and see if there still is an over current issue. I have one of these amps that was brought in disassembled in a box - long term project.

The junctions of bipolar transistors should read low resistance (.7V for Si) in the forward direction and high resistance in the reverse. That's CB and EB the CE should not read low resistance in either direction. This is a very crude test at best. Similar to using a tube tester to test a tube - if it reads bad it's bad if it reads good well maybe.

Does you amp have an electroluminesent front panel? Be careful with the wiring here since there is high voltage present.

[joshdfrazier]

Thanks for all of the replies!

I did test all of the transistors from C to B, C to E (all tested open) and E to B, and also doing these tests with the leads reversed. All tested as "probably maybe okay".  Also, there are no bad solder joints or broken copper traces. I spent about 2 hours one evening giving the board a good long look, and removed any residue, and making sure no solder had melted, or shifted somewhere it shouldn't be. This amp gets very hot. Actually, I've read alot of literature on this amp, and someone working for Ampeg described this amp as getting "hot enough to fry and egg on". I'm actually surprised it's lasted this long.

Anyway...

I built the limiter. the 200vac winding is fine, the electroluminescent panel lights up as it should. When the 56vac leads are connected to the board, there is almost a complete short and the bulb glows as bright as it can (60w, sorry Roly). This is with all output transistors in place.

When the output transistors are removed, the limiter bulb glows, but much less bright. Also, R830 begins to cook itself. When the amp died originally, R645 (33ohm) was toast. This amp has no fuse, actually. I'm not sure why the fuse holder was removed, but if I get the amp functioning, it will most definitely be added.

Edit: V+ rails are -50.0vdc and 50.1vdc with limiter in place.

Also, the schematic calls for 2500uF 80v caps. Whoever recapped it used 2200uF 200v caps. I've heard of going higher in capacitance, but not lower. Besides storing way too much voltage, should I be concerned?

[Enzo]

Using higher voltage caps will not increase any voltages.   All it means is the cap could withstand higher voltages.

2200 and 2500uf are too close to worry about.  Not an issue.  If I had to replace 2500uf caps and only had 2200 ones, I;d use them.   if I had to order caps, then I might look into 3300uf as an alternative.  2500 is not a standard value these days.

Or since they were doubled to make 5000uf, today I might just install a single 4700uf, or maybe a step larger if I feel generous.

33 ohm R645 is part of your zobel stability network.  It must be there to prevent oscillation.  Make sure the cap in series with it is OK.

Schematic shows a 4A mains fuse.

R830 is in the emitter leg of the low side driver T831 (it looks like, hard to read)  Current to heat that resistor comes through that transistor or the low side outputs.

T822 appears to be the bias transistor, it says there should be 1v across it E-C.   I would turn it to the coldest setting, the wiper up against the 470 ohm end.

That your 33 ohm burnt tells me the amp was probably oscillating, and would explain both hi9gh heat and high current draw.  Scope the output for RF.   And NO LOAD.

[J M Fahey]

Agree and add:

1) this amp does NOT have a split supply, so it's ground and +78V .... which will be less thanks to the lamp limiter.

2) output is capacitor coupled so dead amp or not, you will NOT have high DC volts at the speaker output.

Although at turn on without a load, you will have some +40V there which will drop to 0 after a few minutes, until C841 (2500uF) discharges. (Time constant 2500uF and 10K)

On the contrary, on a good amp, you must have around 1/2 +V (around 40V) at the speaker out rail, meaning T843/44 collectors or T841/42 emitters, BEFORE the output coupling cap.

So in a nutshell:

3) measure the following transistors for CE shorts in the Multimeter diode check scale.

To avoid messing with them too much, check first with them in circuit.

As Enzo says : "nothing can hide a short"

If they don't show a short, for now we pronounce them good.
If they measure shorted in circuit, we pull them to re-measure outside.

Check:

Power: T841/42/43/44 .

Drivers: T 832/31

"Class A"/VAS: T821

pre-VAS: T811

If any bad, replace them.
Outputs: any modern good TO3 NPN power.
If I ain't wrong Enzo usually suggests some MJ1502x , let him provide an exact number.

NPN driver 2N3767: forget obsolete expensive TO66 cans, apply some ingenuity and replace it with plastic TO220 cased TIP31C

Post closeup pictures of the original one, both sides of the PCB and I'll explain you how.
TO220 case WAS designed to replace TO66 .

PNP driver 2N3741: same here, use TIP32C.

VAS 2N3440: use MJE340  . Check pinout and bend leads as needed to match each leg with the propper hole

Pre-VAS 2N3391:  well, this one is still widely available, so you can splurge and use the original one

Just in case, order 5 or 10 , they are cheap and used elsewhere.

T822  2N4315 is somewhat dated but still available, it's just an NPN TO92 case small transistor, probably Enzo can suggest a modern replacement, or you can use one of your spare 2N3391 .
They are pin compatible, but anyway check and compare datasheets yourself.

Just in case, order a new 100 ohms trimmer.
Rusty/corroded/weak contacts there can kill your amp.

Also remember to order micas, grease and nipples fror your TO220 TIP , if needed.

I repeat: first check all for shorts, maybe you only need to replace a couple.

Also check emitter ballast resistors R841/42/43/44  .
Replace if open.
They should look "almost like a short" to your multimeter on the 200 ohms scale.

4) turn amp on, without load and with lamp limiter.
Just to play it doubly safe, short T822 CE with a piece of wire (just tack solder it on the bottom side, pad to pad)
We'll worry about crossover and biasing later.

Lamp should blink, go down to dull red or orange, you should have reasonable +V (around 50 to 60V) and speaker out rail, *before the output capacitor C841" should be about half that.

Please post a few gut pictures, PCB from 2 sides if possible , and the heatsinks.

[Roly]

was brought in disassembled in a box

Doncha just love that?  Last job I can remember like that was an electric drill that was brought in in a bucket.
"Do I get a discount 'cause I've started the job for you?"
"No, you get charged double for sorting out your mess."    :trouble


A 60W lamp is okay, I was just being conservative.   :tu:

Since Enzo and JM have covered the ground well I'll just reinforce the point JM made about putting a link across T822, that is between the bases of the drivers T831 and T832.  If there is a problem with the bias circuit this will prevent it doing any more harm and we can sort it later after you get the amp basically functional again.

Also, the schematic calls for 2500uF 80v caps. Whoever recapped it used 2200uF 200v caps. I've heard of going higher in capacitance, but not lower. Besides storing way too much voltage, should I be concerned?

No.  The voltage rating of a cap is the voltage it can withstand without being damaged.  Be they 80V or 200V rated they can only store the amount of voltage that is applied to them from the power supply - they can't generate extra voltage.  All this change in voltage rating means is that they have a much larger safety margin.

Electrolytic caps have a fairly high tolerance on their values, so it is quite possible for a nominal 2200uF to be above 2500uF and a 2500uF to be below 2200uF.  Designers take this into account and specify values that are somewhat larger than required anyway, so there is a fair leeway in the nominal value.


{Not related to your problem, but I have to remark on the notation on the mystery component in series with the EL panel "0.22uF/1000V OR 1/4A fuse OR 20k 25W".  I'd love to know what is behind what may be a unique marking in the history of circuit diagrammes and can only assume that they used different EL panels at different times during the manufacturing run.  Curious.}

[J M Fahey]

{Not related to your problem, but I have to remark on the notation on the mystery component in series with the EL panel "0.22uF/1000V OR 1/4A fuse OR 20k 25W".  I'd love to know what is behind what may be a unique marking in the history of circuit diagrammes and can only assume that they used different EL panels at different times during the manufacturing run.  Curious.}

I can answer *that*:

The electroluminiscent panel looks like a capacitor to the driving voltage ....because what else can you make with 2 parallel metal plates separated by some plastic?
The fact that such plastic emits light when 200 AC volts are applied, is just a bonus.

Problem is that clumsy users can short it, no need to drive a screw through it but simply to use a non original  metallic jack or an uninsulated pot bushing shorting across it or simply overtightening some screws, so some short protection must be applied to avoid blowing that transformer.
You can either add a proper fuse (1/4 A)  or limit current with "something" in series. (.22x1000V or 20K 25W).

Did I win my Vegemite can Prize? 

[Roly]

Thank you JM.

One kilo of Vegemite and a slightly used Prime Minister are in the e-mail.  {I just wish I could send you our current one.}



Enjoy.

[joshdfrazier]

I guess I should come clean about the EL panel. The fuse to the panel was blown so I shorted it with a wire. I actually had no idea the panel was supposed to light up until about two years after owning it. The panel and amp worked fine, but the second time I used it "loud" with the panel functioning, the amp "died". I don't know if these things could be related... 

Anyway, here are some pictures. The TO220's that I ordered to fix the peavey (I had leftovers) might be a useable replacement for the to66 drivers, I haven't checked...

JM, thanks for all of the advice... I will have a chance to work on this again later this evening and will post my findings.