SWR 350 Bass amp - There's a short...no there isn't...yes there is...no there isn't...

[christarak]

Greetings my learned friends. I have had my arse severely kicked by a SWR Bass 350 all day today, and I'm hoping you can help me out.

Here is a brief summary:
The amp shows as a significant short on my bulb current limiter (speaker load plugged in).
I removed V+ from the rectifier and the short was gone. Easy…bridge rectifier failed, me thinks…but no. I tested voltages. They were a little low going through my Bulb limiter, but otherwise OK.
I reconnected V+ and the short was back. No great surprise there.
I removed the speaker load to check for DC at the speaker out (there was 0.5v), and removing the speaker removed the short!! What? Better check those output transistors, me thinks. But no. I (Fluke 87V) diode tested all diodes and transistors, and all checked OK.

And then came the mysterious haunting of the SWR ghost.
The fault is dependent on the ORDER of connecting the speaker! Yes, I know it sounds weird. No, I haven’t been drinking…yet … although I am seriously considering taking it up.

• If I power up the amp with no speaker connected, there is NO short.
• If I then connect the speaker with the amp still on, there is NO short and I can hear my test signal. The volume seems reasonable (but I didn’t push it too hard)
• If I power down with the speaker still plugged in, then power up…the SHORT is back! If I then unplug the speaker, the short is gone.

The only unexpectedly hot component was the stupid hot (100C = 210F) R7 (2K7), so I replaced it with a higher watt metal film and elevated it off the board. The result is the same crazy situation of the order-dependent short. In the “non-short” mode (speaker out) R7 has 55VDC across it (V+=65V). In the “short cct” condition R7 has 1.2VDC across it with V+=15V (dragged down by the fault through the current limiter)

I hope you can help me out with some suggestions. Meanwhile, its dinner time here in Sydney, so that’s my excuse to try some Cabernet-therapy.

[Jon Snell]

Check the main smoothing capacitors, if one is high ESR or low value, when the amplifier draws current at switch on with a load, the amplifier will go unstable.
The other issue I have has is R27 open circuit causing the amplifier to oscillate at HF.

Have you unplugged J4 SCON 2 and powered up?

[Mick Bailey]

What was the original fault that caused you to run the amp on a bulb limiter?

Some amps don't like running on a limiter - Sessionette 75 with the ILP 'brick', some Trace Elliott and others I can't recall. They light up the limiter and the speaker output swings to a rail voltage making it look like the output transistors are shorted. If there's no significant DC on the output without the limiter and without any load, the amp may be fine. I use my bulb limiter quite often, but always bearing in mind it can give false results. Running the amp up on a variac and monitoring current draw is my preferred option when I get unusual bulb-limiter results like this.

[christarak]

Check the main smoothing capacitors, if one is high ESR or low value, when the amplifier draws current at switch on with a load, the amplifier will go unstable.
The other issue I have has is R27 open circuit causing the amplifier to oscillate at HF.

Have you unplugged J4 SCON 2 and powered up?

Thank you for taking the time to reply, Jon. Much appreciated. I unsoldered the input from the preamp to the power amp (sadly my version has no connectors). The same result. When powering up the "short" appears on the limiter. I unsoldered the 22ohm resistor which showed as a short, being across the inductor. It was spot on 22ohm. I checked the ESR of both the main 6800uF filter caps. Both measured at 0.05ohm.

[christarak]

What was the original fault that caused you to run the amp on a bulb limiter?

Now I understand what the owner meant when he said "sometimes it would work, and other times it made a horrible sound and wouldn't work". That is the exact same Ghost in the Machine I am seeing. Interesting suggestion about the Limiter itself causing an issue Mick. I will try you Variac idea now and report back.

[christarak]

Some amps don't like running on a limiter - Sessionette 75 with the ILP 'brick', some Trace Elliott and others I can't recall. They light up the limiter and the speaker output swings to a rail voltage making it look like the output transistors are shorted. If there's no significant DC on the output without the limiter and without any load, the amp may be fine. I use my bulb limiter quite often, but always bearing in mind it can give false results. Running the amp up on a variac and monitoring current draw is my preferred option when I get unusual bulb-limiter results like this.

I was hopeful, Mick, but the same result through the Variac. With speaker plugged in, the "short" kicked with 32VAC on the mains (we're 240VAC in Australia). Then with no speaker connected, the amp went up to 240 with no issue, although I did hear some transformer-like buzzing.

[J M Fahey]

The answer is in the brand, it´s a *guaranteed* "problem" on SWR amps and I posted this many times.
Many other amps do it too; although Peavey never does, Crate/Ampeg neither, etc.

All SS amps "wake up stupid" since lots of electrolytic capacitors (read that as high capacitance=long time constants) need to be charged to working voltage, and until then circuit is unbalanced ....sometimes big time.

During that startup period you have high voltage, even rail voltage transients at the output, and result is typical turn on thump.

High voltage on a speaker = high (maximum) current being pulled by a rail = voltage drop on rail.

Many amps recover in a second (anyway showing thump) but many latch against that rail and *keep* it down, can not self recover, think a belly up turtle.

Of course: no speaker or any load=minimal current peak.

With load but without a bulb limiter, mains can provide the brief current peak, so amp starts (almost) normal.

That said, the problem your customer complained about is something else, I doubt he turns amp on through a bulb limiter on stage.

[Jon Snell]

Q13 is the slow start control. Check the timing is working as it should stop any thump in the speaker and if not working could be your cause.
If you stop the AC feed to D7 it will have a very low quiescent current because Q14 the current source is effectively switched off. If it still works without the AC feed, there is your fault. If C16 is dry then there will be no time lag.

[glebert]

SWR combo amps have a common problem of broken legs on the power transistors, and they will at times make contact and work and sometimes not. Haven't heard of it on a head, but I would check it out.

[christarak]

Q13 is the slow start control. Check the timing is working as it should stop any thump in the speaker and if not working could be your cause.

Thank you Jon. I could understand how that circuit worked, but I missed the important point of why it was even there. The clouds parted. Yes that part of the circuit did NOT work correctly with the Bulb Limiter in circuit and the speaker connected. But I took a deep breath, bypassed the limiter, and bingo...the slow start circuit worked, and all the levels around Q13 and Q14 were correct.

[christarak]

The answer is in the brand, it´s a *guaranteed* "problem" on SWR amps and I posted this many times.
Many other amps do it too; although Peavey never does, Crate/Ampeg neither, etc.

All SS amps "wake up stupid" since lots of electrolytic capacitors (read that as high capacitance=long time constants) need to be charged to working voltage, and until then circuit is unbalanced ....sometimes big time.

With load but without a bulb limiter, mains can provide the brief current peak, so amp starts (almost) normal.

That said, the problem your customer complained about is something else, I doubt he turns amp on through a bulb limiter on stage.

Well Jon. What can I say? You make me feel dumb sometimes. You were right, and I would NEVER have thought about my limiter contributing to the problem in a million years. Of course, I didn't believe you, so I put in a series 1A fast blow in the 240v supply. No problemo. The amp started up normally. I guess this latching effect is the same reason the problem appeared with no Limiter but on the Variac.
It makes sense when you say it, JM. A transistor starts working straight away, while the caps are still charging, but a valve has time for a "coffee" while the cathode heats up and the caps charge.

Its quite early here, but when its after 9am, I will call the customer to get a clearer description of the problem.

[christarak]

SWR combo amps have a common problem of broken legs on the power transistors, and they will at times make contact and work and sometimes not. Haven't heard of it on a head, but I would check it out.

Hi Glebert.
Even though the amp is now working (after following JM's advice to remove my Limiter), I am still not sure that I have resolved the customers original fault after spending a whole day chasing my own self-induced fault.
Yes, this is a SWR head, but I will still check those pins. I need to talk to the customer again, and I'll be back later in he day. Thanks again.

I am envious of your experience that you can offer up advice like that. That knowledge doesn't come in a book. Its the years of experience that you and the other guys here have earned. At my age, I doubt I will ever catch up. Still, I do a good job for my customers, and I don't charge for my "stupid time". But with a high "stupid to smart ratio" I will never own that Ferrari...or Kia.
I hope you were well clear of the fires. They were all across our TV stations. Australians are very empathetic to other countries that get hit by bushfires, as its an annual cause of devastation here.

[christarak]

That said, the problem your customer complained about is something else, I doubt he turns amp on through a bulb limiter on stage.

HI Jon. I just got off the phone with the customer. He said the fault that he experienced was (i) the sound getting farty, then soon after, (ii) a loud hum (which is what I heard with the Limiter). He immediately turned the amp off. (iii) Some hours later, he turned on the amp with no hum, but wants it to be a reliable amp. In light of the speaker load situation we found here, I asked him to check his cabinet and lead with another head.

[Jon Snell]

Clean J2 for the farty sound, send/return jack. Contacts are dirty as it is never used.
I would replace C16 anyway.
Pleased to help.

[christarak]

Clean J2 for the farty sound, send/return jack. Contacts are dirty as it is never used.
I would replace C16 anyway.
Pleased to help.

Will do! Thank you Jon.