Vintage Ampeg BT15 Help

[erock0138]

Also, I forgot to mention that the output transistors on the heat sink were removed when I did the light bulb limiter test.

[J M Fahey]

Well, now you'll have to check all those voltages kindly provided by Ampeg.

[gbono]

If the electroluminescent "panel" doesn't light - check the imbedded .25A (?) fuse on the PCB for the panel.

I have one of these amps that came with the "flip top" speaker enclosure (Altec 418 loudspeaker) and dolly. Timely thread since I'm going to get the amp back together now.

The large aluminum electrolytics (2500uF/80V) were damaged/marginal and I have yet to find a nice "radial" replacement - what are you going to use?

[soundguruman]

If the electroluminescent "panel" doesn't light - check the imbedded .25A (?) fuse on the PCB for the panel.

I have one of these amps that came with the "flip top" speaker enclosure (Altec 418 loudspeaker) and dolly. Timely thread since I'm going to get the amp back together now.

The large aluminum electrolytics (2500uF/80V) were damaged/marginal and I have yet to find a nice "radial" replacement - what are you going to use?

2200, 3300, or 4700 are the standard values and any would work just fine.
Aluminum Electrolytic Capacitors - Snap In | Mouser
There is no need to use 2500. That is a non-standard value.
check for the proper lead spacing, or drill holes to accommodate the new caps.

[erock0138]

If the electroluminescent "panel" doesn't light - check the imbedded .25A (?) fuse on the PCB for the panel.

I checked it, it was good. Which pointed me back to the transformer only putting out 200 v.

The large aluminum electrolytics (2500uF/80V) were damaged/marginal and I have yet to find a nice "radial" replacement - what are you going to use?

There were some at digi key, that were 2500 100V, but I haven't looked at the size yet to see if they're comparable.
I'll probably just take soundguruman's advice and use something that I know will fit size wise. I can let you know if i get something that works when I get around to it.

[erock0138]

I have one of these amps that came with the "flip top" speaker enclosure (Altec 418 loudspeaker) and dolly. Timely thread since I'm going to get the amp back together now.

I wish I had the cabinet! That would be ideal, but unfortunately mine didn't come with one. I have a 1 15'' cab I'll end up using with it though.

[soundguruman]

I checked it, it was good. Which pointed me back to the transformer only putting out 200 v.



There were some at digi key, that were 2500 100V, but I haven't looked at the size yet to see if they're comparable.
I'll probably just take soundguruman's advice and use something that I know will fit size wise. I can let you know if i get something that works when I get around to it.

It's not bad to use 4700, 100V
Caps have improved a lot, size and cost wise.

[erock0138]

Update:

Alas! I floated the supply. I found a bad solder joint on one of the big capacitors. What was making it short immediately. So, powered it up with the light bulb limiter and the bulb dimmed after a second (Good sign!)

I left it on for a few minutes, then started smelling a dry burning smell, so I quickly powered it off and noticed that the 300 ohm 20 Watt resistors from the power supply were super hot, about 110 degrees F according to my digital thermometer.

So, I've narrowed down the trouble area quite a bit (Hopefully)

[erock0138]

Also, forgot to mention I checked dc at the output, and was getting 20 V DC.
Also, Checked the big filter caps and was getting correct voltages. Haven't sent an audio signal through until I can determine other issues (Obviously)

[soundguruman]

Also, forgot to mention I checked dc at the output, and was getting 20 V DC.
Also, Checked the big filter caps and was getting correct voltages. Haven't sent an audio signal through until I can determine other issues (Obviously)

Another place to check is the AC power input connector.
The solder joints burn out on those, too.

[J M Fahey]

Update:

Alas! I floated the supply. I found a bad solder joint on one of the big capacitors. What was making it short immediately. So, powered it up with the light bulb limiter and the bulb dimmed after a second (Good sign!)

I left it on for a few minutes, then started smelling a dry burning smell, so I quickly powered it off and noticed that the 300 ohm 20 Watt resistors from the power supply were super hot, about 110 degrees F according to my digital thermometer.

So, I've narrowed down the trouble area quite a bit (Hopefully)

Not that bad. Those resistors in *normal* use dissipate around 5W continuously, so they *are* hot.
And 110ºF is nothing (for them).

Do you get around 38/40V on the positive of C34?
And on its negative?
With or without speaker connected?
Do you get 75/80V on C30/31 positive?
All these relative to chassis/ground.

[erock0138]

Sorry for the delay, have been super busy as of late.
The voltages I previously gave were with all of the output transistors on the heat sink out.
So, I reinstalled them and powered the unit on to get capacitor voltages. The fuse didn't blow, but it immediately burned up a resistor (Looks to be 10 ohms) I'll have to check with the schematic to make sure. I'm guess one transistor is shorted? Even though the fuse didn't blow, it just went up in smoke.

So I'll be looking for some of the transistors after testing them, But cannot find many available, anyone have any idea of where to maybe find some?
2n3741 PNP
2n3767 NPN

Thanks again!

[Jazz P Bass]

Find 2N3741 Stock and Compare Prices Across the Most Reputable Distributors in the Industry.
Find 2N3767 Stock and Compare Prices Across the Most Reputable Distributors in the Industry.
Them puppies ain't cheap.
Quest has them both.

[erock0138]

Find 2N3741 Stock and Compare Prices Across the Most Reputable Distributors in the Industry.
Find 2N3767 Stock and Compare Prices Across the Most Reputable Distributors in the Industry.
Them puppies ain't cheap.
Quest has them both.

Thanks! that's the exact type of site I was looking for. You're right, a little on the steep side. But, at least available somewhere!

[soundguruman]

Sorry for the delay, have been super busy as of late.
The voltages I previously gave were with all of the output transistors on the heat sink out.
So, I reinstalled them and powered the unit on to get capacitor voltages. The fuse didn't blow, but it immediately burned up a resistor (Looks to be 10 ohms) I'll have to check with the schematic to make sure. I'm guess one transistor is shorted? Even though the fuse didn't blow, it just went up in smoke.

So I'll be looking for some of the transistors after testing them, But cannot find many available, anyone have any idea of where to maybe find some?
2n3741 PNP
2n3767 NPN

Thanks again!

Oh crap, you let the smoke out.

2N3741 = NTE218 $5.25 Mouser
2N3767= NTE 384 $6.52 Mouser