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I would expect to see readings approximately in the range of 30-40 Vac on pins1-2 and 6.5-9 Vac on pins3-4 for the transformer only. Once P1 is connected to the the power supply board, the same measurements should drop to the lower end of those ranges.

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It is not clear to me what you are actually measuring.

For the transformer with a disconnected P1, the places to measure are between P1-1 to P1-2 and between P1-3 to P1-4. This should be done with the A/C setting of your multimeter.

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Note: I’m basing my comments on the schematic for a BC6500 since I can’t find one for the BC7000. I’m assuming that they are similar enough.

It looks like to test the power supply stand-alone, you could pull P2 and P3 (with P1 connected). I think that you will only see the +5 and a pretty lumpy +22v signals at this point. The +22v will likely be somewhat higher since there is no-load on it.

Grounding P2-2 should turn-on the +14.8 and +15.5v outputs (as well as the +4.2v? output).

I believe that you should be able to debug the power supply in this configuration without the protection circuitry turning things off.

I may of misunderstood your earlier comments. Were you saying that you were seeing 5v A/C on the secondaries while seeing +22v DC on the output of the rectifier? If so, this wouldn’t make sense. Could you have been measuring the second, lower voltage secondary?

I don’t know what the exact current draw for the boards would be. My SWAG would be somewhere in the range of 50mA to a couple of hundred mA. Checking the fuse ratings and scaling the currents can give a rough idea of what to expect as a maximum.

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For reference, the crack at pin 7 is the kind of thing to look for. The “stress rings” around the other pins look like they are getting close to failing.

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The transistors and load resistors “look” similar to my working Penta2’s (i.e. no obvious overheating). I wouldn’t mess with them right now.

Probably not the problem, but are you sure you have the trim 100 and 220 trim pots in the correct location?

While you are double checking… check the polarity of the power supply capacitors. I recall that the markings on the original capacitors could be misinterpreted.

The soldering on some of the transistors doesn’t look like I would expect coming from the factory. I would clean-up the flux residue and do a closer inspection. It might be just the camera angle, but the solder from TR32 seems close to the trace.

I would carefully inspect the board connections even if you didn’t touch the connectors. I use either a 7X loupe and a bright light or take a picture with a macro lens and zoom in on the pictures on the computer screen. Even if I don’t find any obvious issues, I usually reflow solder on the board connectors as a precautionary measure.

I had trouble setting the bias on one of my other B&O amps. It turned out to be a bad ground connection to the preamp. The symptoms were similar to what you described where the full range of adjustment on the trim pot wouldn’t get the bias to a proper value.

I am also interested in hearing from others the possible causes for the amp running hot.

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Apologies in advance if you already know all of this…

There is actually a lot going on inside a transformer from an electromagnetic point of view. A detailed explanation of exactly what happens is probably beyond the scope of this forum and would put most people to sleep. A simplified explanation is that the secondary windings have resistance, as the current output increases, the output voltage of the transformer decreases proportionally (i.e. V=I*R or more specifically V_out = V_noload – I_output*R_windings).

If you did a direct short of the transformer output, you would expect the voltage output to go to zero, right? Your circuit board is simply a load somewhere between a no-load and a direct short.

The rectifier doesn’t really change any of the above. All the rectifier does is switch what phase of the transformer’s A/C output is connected to the rectifier output at a given time. The current load on the transformer is still there.

There “could” be a problem with your transformer, but I believe that these kind of issues are rare. Transformers are generally robust and reliable devices that fail pretty decisively.

Have you tried to measure the current draw of your board? This can be done with a multimeter (be prepared to replace the fuse in the multimeter if the current draw is above the rating). A better option is to use a bench power supply with a current limit. Set the current limit to less than you expect the board to draw and increase slowly until you get a better idea of your situation. The bench power supply approach has the benefits of not having to deal with the protection circuitry kicking in and making taking readings difficult, as well as limiting the possibility of further damage to the board.

After you use the bench setup to find/fix any possible power related issues, it will be easier to debug the rest of the functionality.

Note:  I would keep following Martin’s advice as it will likely get you to a solution faster. My suggestions are in the spirit of trying to get more information so that it is easier for people to help.

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alf,

I wouldn’t expect that the transformer is bad. It is normal for an unregulated power supply that the voltages be pulled down under load. Voltages lower than what is indicated on the schematic would imply that you have a short or some other similar fault on the board.

Do you have a bench power supply? If so, connecting the board to a bench supply is helpful for debugging.

A voltage sag from 40v to 5v seems to be more than a “sensitivity issue”. It sounds like a genuine fault and the protection circuitry is doing its job.

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I’m certainly not an “expert” on these amps, but I’ll try to help by asking some general questions. At the very least, there will be a bit more information here in case someone with in-depth knowledge of the common Penta amps failure modes chimes in.

Are any of the main transistors or load resistors discolored?

What are the actual voltages on the +68v and -68v power supplies?

Did you replace the power supply capacitors?

Have you reflowed the solder on the board connectors? Or at least carefully checked for cold solder joints?

It also might be helpful if you could upload some pictures of the component and trace sides of the circuit boards.

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Did you replace the trim pots?

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notwist:  You asked about more detail… This video does a good job demonstrating some of the things mentioned above.  https://www.youtube.com/watch?v=aBnIB4lDPzk

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You probably created, or already had and didn’t notice, a ground loop. It would be helpful to anyone trying to help you if you could provide a diagram of your system that includes all audio/data connections between components as well as power connections.

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You might be able to figure out some compatibility information by looking at the part numbers listed in the service manuals. I haven’t done this for the Beomasters that you have listed, but have done it for some other ones.

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A replacement spring could also be made from spring steel sheet stock. It is a simple shape that could be cut out with a Dremel or nibbler tool. The stock springs are 0.5mm thick.

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Martin,

Yes. Yes it has!

Martin, you’re awesome! I don’t know how long that would have taken me to find, considering that it took a while to find even when I was explicitly looking for it.

Here is the PPM is working as expected and I have clean, clear music coming from the headphone jack.

Thank you again, Martin and Mark, for your suggestions and advice,

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For IC101 and IC201 the pin voltages are the same

Pin28 – 13.51v

Pin5 – 0.040v (play)      0.027v (stop)

To check for the signals, I’m using the tape with the recorded calibration tones. This makes it easy to distinguish a signal from the background noise.

The plot below is the Pin4 input for the left (top/yellow) and right (bottom/magenta) channels

Here is a close-up of the Pin4 left channel with some measurements.

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Martin,

I don’t have access to the machine right now, but it is a later model with the two Hitachi HA12038 chips on the Dolby board. The tapedeck is consistent with the schematics in the Beocord 9000 Supplement manual.

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Part of my reasoning for running the calibration process is that it is a quick way to test that the BC9000 is recording. It provides a test recording that is tolerant to any head alignment problems since recording and playback are done at the exact same head azimuth angle. This test also excludes some of the line-level circuitry that I’m not trying to test yet.

The newly recorded test tones play reasonably load and clear on other tape decks. I don’t know exactly what they should sound like on other equipment or have any quantitative measure of the record levels since my other playback devices don’t have any meters. This might all be a moot point since I’m not trying to address any record issues right now. Everything that I’ve tried so far indicate that recording is generally working OK.

During the calibration test I expected to see record levels around 0dB +- 1 or 2dB. The BC9000’s PPM was just barely hitting -8dB. I generally believe what I am seeing on the PPM since it is consistent with what I’m hearing at the headphone output. I’m not having much luck probing the signals that I have since the signals are so small and connecting test equipment pulls them down even further. This is one area where hearing about any tricks for testing or expected behavior would be appreciated.

The circuit path that I’m concentrating on is from the tapehead to the headphone output. This includes the fewest number of stages (i.e. the simplest test scenario).

Tapehead -> JFET stage -> IC8 opamp stage -> BJT transistor stage on board 4 -> BJT transistor stage on board 2 -> (part of) Dolby chip stage -> headphone amp or PPM

The only stage that I’ve ruled out is the headphone/PPM stages, mostly because they corroborate each other both on tape playback and with the “injected noise” when I touch the board 2 signal inputs.

I am trying to be cautious with any experiments that I try to minimize losing or messing up any of the factory calibration settings. My access to the “test tapes” mentioned in the service manual is limited to the “azimuth tape” that was included with the BC9000. I’m also assuming that the AF meter mentioned in the service manual is an analog meter. Advice on testing with more modern digital instruments would also be appreciated. I really don’t know what is truly important to get perfect versus where “close enough” will work.

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Here is a picture of the tape head

And the meter during playback. The left channel is “stronger” than the right, but neither gets significantly above -20dB.

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Mark-sf:  The tape heads are clean and have very little wear.  I haven’t tried to do any tapehead adjustment/alignment since the signal level is so low that I wouldn’t trust the readings. I don’t have any reason to believe that the tape head is out of alignment.

I did take some measurements of the playback head itself.

Channel1     212 mH @1kHz     487 ohm @1kHz     430 ohm @DC

Channel2     215 mH @1kHz     521 ohm @1kHz     433 ohm @DC

I don’t know what the exact readings should be, but the values seem reasonable.

Martin:  I see the same symptoms (levels not greater than -20dB) with tapes recorded (years ago) on this BC9000 and prerecorded tapes. The same tapes play normally in other tape players.

I’ve also tried to go through a record calibration process. The calibration does not succeed, but when I play back the section of tape where the calibration tones were recorded I get the same -20dB signals.

I tried swapping out the IC8 opamp with a known working one that came out of my upgraded Pentas. This didn’t make any difference.

Also, when I touch the input connections to the Dolby board, I see a jump in the meters up to about -3dB. I assume that this indicates that the Dolby chips are alive.

My current thoughts are that the problem is in the JFET and trim-pot R148/R248 stage. Does this make sense?

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I found a short to the metal chassis of the tape transport. It looks like when I plugged-in B6-P19 it made a connection to the chassis via the tape head mount, to the tape head angle adjusting spring, to the tape head sliding bracket, to the transport chassis. This unlikely, and easily opened, connection likely explains why I didn’t catch the short when I checked continuity between all the pins on the 14 and 16 pin connectors and the ground pin on B6-P19.

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