Dunlap-Clarke Dreadnought 1000

This thing is a monster!

DSCF0497

Front view of the amp. This thing wieghs around 80 lbs, and is called the 1000 because it will dump 1000 watts into 2 ohm loads. What was wrong? It blew fuses at power-up. The customer stated that a repair shop had looked at it and given up. He was, however, diligent and had accumulated a lot of information from online sources. What we did not have was a complete schematic.

The output circuit is fully complementary symmetry, and is most like a servo amplifier. The input goes to an op-amp and the feedback to that op-amp comes from the speaker output. The simplified schematic I had showed that much. The speakers are DC coupled to the output stage through fuses. Those fuses were not blown.

DSCF0498

Of course, with blown fuses the power supply was checked first. The customer stated that the filter capacitors (huge blue cans) had leaked in the past and been replaced. He also mentioned that the output devices had been replaced. The power supply was disconnected from the amplifier circuit and brought up on a variac. No problems. So, the driver boards were removed, and each bank of output devices was checked for resistance to the positive speaker terminal. If output devices fail short, this test will show that short. In fact, the left side of the amp had shorts and an odd mix of output devices. Upon removal, some originals present were failed short. Some incorrect replacement devices were failed open. And they were mixed, which is bad practice.

According to our information, the original output devices (2N5631/2N6031) were selected by Dunlap Clarke for this amplifier based on a forward breakdown voltage of 180V, from devices specified at 160V. This is a vital clue, because those incorrect replacements were 150V devices.

At this point the repair moved into the realm of restoration. How to get the best results, in the least time for the least cost? Knowing a little about why prior replacements had failed, all new output devices were selected. ON Semiconductors’ MJ21193 and MJ21194. These are 250V, 250W, 16A devices. This is far beyond the originals and 10 of each came in just under $100. New mica insulators, replacements for 8 electrolytic capacitors and 4 39K 1W resistors on the driver boards, and a bag of 8A fuses were also ordered.

On each driver board were the 4 electrolytic capacitors, which looked OK but were old, and 2 burned 39K resistors. These were replaced as routine. Then the output devices were replaced. This is where I found that only the right side output devices had been successfully replaced in the past with some Motorola devices. All were replaced with the newly selected devices. Each new device was installed with a new mica insulator and fresh silicone grease. In fact, 16 devices are output and 4 devices of the same type are used as drivers, which are mounted separately directly to the chassis under the heat sinks.

DSCF0499

The above, a view from one side, shows everything in place.

The result? I only have an 8 ohm test load, so I tested each channel with that. Specification is 250 Watts into 8 ohms, and I measured approximately that on each channel. Nailed it! So I have to ask: How can an established professional repair shop fail at this? They’re the pros, right? This challenges that assumption!

16 thoughts on “Dunlap-Clarke Dreadnought 1000

      • No, that isn’t something I can casually do for fun. I do a lot of reverse engineering in my work, making my time more valuable than ordinary technicians.

    • Hi,
      Sorry for the disturbance. I did send you a mail requesting what is the adjustment required for the two pots at the driver board?
      I got one dreadnought 1000 which is not working. I am trying to trouble shoot.

      Kind regards.

      • Hi Raj,
        When I did this repair, I was given a paper with a partial schematic on it that I returned to the customer. I can tell you these:
        1- Shorted output transistors will show as shorts across the power supply and blow fuses
        2- Shorted output transistors will also show as shorts from the speaker terminals to either positive or negative rails IF the output fuses aren’t blown
        3- It is probably a good idea in general to replace all electrolytics in the unit, and if output transistors are shorted, replace the entire lot using new mica washers and thermal grease. Use ON Semiconductors’ MJ21193 and MJ21194. If you can’t get these, I can find another sub for you.

        Of course you could send the unit to me for a complete overhaul… 🙂

      • Thanks for your reply,
        I have managed to identify the function for the two pots. For dc offset and bias adjustment.
        Will look into the MJ’s.

        Regards,
        Raj

      • Hi Bil,
        I have refurbished the Dreadnought 1000 with MJ15024/15025. The amp has been working for past two months. No issues at all.
        I would say the sound is incredible. Bass is something not many amps can do like it.

        regards,
        Raj.

      • Those look great too! Mouser shows the MJ15024 as obsolete, so I should mention that the currently available devices are MJ21193G and MJ21194G, for anyone listening.

  1. Good job of troubleshooting. The devices have to have a 180 volt or greater breakdown. Manufacturers
    breakdowns were variable so we selected devices using the lower breakdown power transistors for lower powered amps. Checking between the power supply and the speaker terminal is the best way to check for shorted
    devices without removing individual devices. A variac with a current meter is essential. Once the supply\
    holds you are all set.

    The main power supply can be fused with a .25 amp fuse. The amp with function
    with AC line voltage as low as 30 to 40 volts AC. Driver boards will balance up and function at this
    level. The amp will run at plus and minus 25 volts DC up to plus and minus 90 volts.

  2. Hi Ron! Looking to contact you about posting the service manuals, etc. I have some of the old devices with the red fingernail polish dot, to confirm tested, Also need advise on common mode conduction
    Best Regards, Phil

  3. Phil,

    I personally wouldn’t risk using the original output devices on a unit like this. High temperature & high power operation near the device limits causes “electromigration”, which will degrade and fail the devices over time. Modern components like the MJ21193/21194 devices will outlast and outperform the originals. I know a lot of people would probably disagree, but the owner of the amplifier in this article has had it in continuous operation for over two years without any troubles, and raves about the sound.
    I do not know whether Ron Dunlap will see your questions here and be able to respond. I do not myself have any literature on this amp.

    -Bill

  4. I have one of these and have it plugged into a voltage regulator set to 90 volts. Lots of power for my electrostatics and no failure of the amp in 30 years now.

  5. Micromega,

    Generally you would not notice any change. The entire audio chain becomes like a single unit when inverse feedback is applied. One thing that could change is noise figure, another might be improved handling of high level inputs, because many modern op-amps can swing closer to the power rails.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: