Repairing Sunn Beta Lead

[BudJillett]

Hi All,

I'm in the midst of repairing my Sunn Beta Lead amp head after it blew out some final stage transistors and supporting components. The meltdown also took out the infamous 14506 on the pre-amp board, which I've completely removed.

I have the power amp board back together after replacing the transistors and other compromised components. I can play through the amp if I plug into the pre-amp "from" jack. It sounds great and I can crank some power, but the R25 power resistor gets VERY hot and the voltage at the TIP30 collector reads significantly lower than it should - 23vdc instead of 30.

All the voltages on the TIP29 side of the power amp are within a volt or so of spec and the TIP29's associated power resistor runs (relatively) cooler.

My question is this: With the 14506 still missing from the pre-amp board, would this throw the voltages off such as to cause the symptoms I'm seeing above? (i.e. hot R25 and somewhat suppressed voltage across the TIP30).

Or should I suspect some other compromised component on the power amp board?

Any help/info/ideas/thoughts appreciated.

I did find a source for the 14506 on eBay, but I've yet to get it in.

Thoughts?

Peace,
Bud

[loudthud]

Check or replace any capacitors on the -15V supply.

If the 14506 died, there was probably an over voltage condition on the -15V supply. That condition may have damaged other chips on the preamp that are now drawing too much current. With the 14506 removed, both the Red and Green LEDs should be off and thus no signal can get through the preamp. Ground pin 15 or pin 7 (or both) to enable one or both channels.

IC103 is a prime suspect because it's current would go from -15V to ground, but check that none of the inputs are floating. This can be tricky because a DVM can pull a floating input to ground, some of the inputs have a 4.7 Meg resistor that pulls the input to -15V when no guitar is plugged in. Outputs should also be pretty close to a rail. Note that current on the -15V supply changes substantially when a footswitch is plugged in.

I suggest that you install sockets for IC103 and IC106.

[BudJillett]

Thank you so much, LoudThud!

The 14506 definitely died. I'm not sure what died first though. First the amp was making a crackling noise for awhile, almost like a loose speaker wire. Then the finals blew. I also had to replace the 7404, just FYI. I'm afraid the initial cause hasn't changed and this is why R25 is getting so hot.

I did notice that when I changed channels on the foot switch, the voltage at the TIP30 would go from about -23 to -18. It would then stay that way, even after I changed back to the other channel. It would stay that way till I reset, by powering off and then back on.

I will definitely try the checks you recommend, but I might not be able to get to it till this weekend. I will definitely post back and to let you and everyone know the results.

I have a friend in the semiconductor sales business and he's getting the 4011 ICs for me as a work-around for the 14506 -- found that work-around in another great post on this forum. So glad to have found the SUNN forum.

Thank you for all of your help.

Peace,
Bud

[loudthud]

IC103 should be a 74C04 or 4069. A 74HC04 or 74HCT04 won't work.

[BudJillett]

>>IC103 should be a 74C04 or 4069. A 74HC04 or 74HCT04 won't work.

Okay, now I know why it says "Hero Member" next to your user name -- I had installed an HC04 in place of the 74c04. So now I have a 74c04 on order from my buddy as well. Thank you!

There's 1000 other things I should be doing but curiosity got the better of me. So here are my findings after trying some of your suggestions:

With the dubious 74HC04 still in place and the 14506 removed, I brought pin 15 to ground via a 470-ohm resistor and the RED A Channel LED came on.

With my guitar plugged into the "both" input, I could not get a signal to pass through the pre-amp and the channel emitted a strange hum. Not loud, but a hum.

So, then I brought pin 7 to ground, returning pin 15 to it's floating state and the GREEN B Channel LED came on. I heard a normal amp hiss/hum (quiet background type) and I was able to get a signal to pass through the pre-amp, which is an understatement. With the drive on about 2 and the "level" on 1 and the master volume on about 1, it was already pretty loud connected to two 12" (4-ohm) speakers.

I wonder if it was almost too loud for the given settings? I can't really remember because this "repair" started about 10 years ago (long story) so I can't remember how loud the amp was. But it seems too loud for the given settings and I'm thinking this is also a symptom of whatever the problem is.

So it looks like I have to pull out the pre-amp board again and start replacing caps along the -15 route, per your other suggestion, which will likely have to wait till the weekend despite my curiosity

Another symptom that occurs whether I'm bypassing the pre-amp as I did last night, or passing the signal through it as you helped me do tonight, is there is a LOUD speaker pop on power-down. It's so bad that I had to disconnect the speakers before powering down in order to protect them. However, I did try it once tonight just to see if that situation improved by passing the signal through the pre-amp but it did not improve. Was the same -- very loud pop on power down.

When it was happening last night, it did make me wonder about a bad cap somewhere.

Thankfully, I did install some nice sockets for IC's 103 and 106 way back when I first began working on this. So when the parts come in, it won't be so painful -- assuming I get the root problem fixed by then!

Okay, that's everything I know

Thank you for all of your help.

Peace,
Bud

[loudthud]

Plug a guitar into the Master From Accessory jack on the back of the amp. That will disconnect the preamp from the power amp. If the amp still makes the loud thump at turn-off, the problem is most likely in the power amp with the possible exceptioin that the -15V supply collapses too fast (because of the abnormal current draw) causing the thump.

The preamp uses a dozen or so 2.2uF 25V Non-polar electrolytic caps in the signal path. These should all be replaced, they become leaky over time. If you can find a polyester or mylar cap that is small enough, you could use those, they will last much longer. In fact it's not a bad idea to replace all the electrolytic caps in the amp.

The 4000 and 74C series chips can operate on 15V no problem. The 74HC and 74HCT parts IIRC are only rated to 7V. Each channel has a JFET switch to turn the signal on and off that is controlled by the CMOS logic, IC103 and IC106. IC103 has to work for the switch to operate properly.

[BudJillett]

Thank you, LoudThud,

I plugged my guitar into the Master Accessory "from" jack and the loud-clack-on-power-down problem disappeared.

That finding, combined with the other info you gave on the 74HC04's voltage spec told me I better pop that IC back out of the socket.

Once I did that, I can now plug the guitar into either of the A or B accessory "from" jacks and play through the power amp with no excessive click on power-down. I'm still able to select a channel by grounding pins 7 or 15 and the only real difference is the A channel has more high end hiss.

Other than that, I can now play through the power amp as long as I want without R25 getting excessively hot. I just got done playing and it felt good to play through the amp for a long stretch. Hitherto, I had been only powering up long enough to take measurements or play for a couple of minutes to see if R25 was still heating up too much.

Can't pass a signal through the main inputs with the 7404 out, so I guess I have to wait till my parts come in to finish testing on the pre-amp board.

If I'm lucky, I'll pop the parts in and all will be well. Otherwise, it's back to the R25 heat-up problem and I start replacing caps. Either way, it's good to have a plan and good to have isolated the remaining problems to the pre-amp board.

I feel I'm getting close, which is a blessing because when this thing fried it fried good. It had taken out the TIP30, R25, the 15vdc Zener on that circuit, the 14506, the 74c04, a couple of the power transistors . . . probably other stuff, too, that I can't remember after 10 years. Don't recall if I had to replace the TIP29 or just did so for good measure. Either way, I've got heat sinks on both TIPs now. Oh, and I had to re-run some traces--more than a few. It was a pretty nasty meltdown.

Probably won't have any further news till the parts come in.

Thank you for all of your help and insights. You've definitely saved me from barking up wrong trees!

Peace,
Bud

[BudJillett]

Well, I had some time for a little Christmas vacation project. Building the replacement for the 14506 was more challenging than I thought, but I got it built and popped that and the 74c04 in their respective sockets.

The good news is that the foot switch logic is back and I can pass a signal through Channel B of the pre-amp. So, things are progressing.

It was good to hear overdrive once again from this amp.

Channel A switches over just fine, but can't get a signal through the pre-amp. Channel B has a semi-loud click on power down, but nothing like before. Channel A makes all the right sounds when I turn the drive & EQ knobs but no guitar amplification. Interestingly, Channel A has no click on power-down, even though it seems to be the "bad" channel.

Channel B totally CRANKS amplification-wise as my entire condo association found out when I accidentally switched over to it before turning down the volume knob.

So, I think I need to check my inventory for caps and start replacing them per your advice. I'll post another update after that step. Might have to wait on more parts.

Progress every time I work on it, so I'm happy with that.

Thank you again for all of your help.

Happy Holidays to All,
Bud