Hi

I have been having some problems with the BC9000 and have traced the fault to a "dodgy" FET - 3TR102/202.  The original manual shows that the part is PF5013 but on the PCB, the part is visibly PF5101.  In any event, all of my local contacts have no idea what to use as an equivalent replacement part.  Has anybody any ideas of a readily available FET that will do the same job and can you provide me with a supplier and part number.

Thanks for your help

Geoff

Geoff,

What's wrong with the transistor ?
I would think that any good FET should work, it's not
particular high frequencies or high power.

Martin

Hi Martin

Thanks for your contribution.  You are probably correct in saying that the FETs are probably OK and that the fault is most likely somewhere else.  It is just that when I go to all of the trouble to take a board out, I usually try and replace as much as possible, especially when the components are relatively inexpensive.  I would still appreciate a brand number or model number of an equivalent FET that works in this application and then at the very least, I will have a few spares around.

Over the last week, I have change the electrolytics on board 3 and tidied up some dodgy connections.  With the updated board, I then did a lot of work injecting the playback amplifier with a range of frequencies and measuring the outputs at the inlet to trimpots R160/260.  The problem that I am trying to rectify is a degraded volume on the left channel and a disproportionate loss of the higher frequencies.  So far, my efforts have shown that the playback amplifiers might be OK since the input and output values were fairly comparable on both channels.  There are a few funny DC readings around the FETs that I cannot explain but they don’t seem to be affecting the overall performance.  This sent me back to the tape head and I have made some alignment changes that have helped restore the higher frequencies.  Unfortunately, the left channel output still appears to have only 70% of the right channel so there might be a problem with the head itself or the connections.

Thanks for your help

Geoff

Hi Jeff:

Have you checked the obvious:

1) Cleaned tape heads meticullously

2) Swap the L+R channels on the output of the tape head to the input amplifiers. If the problem switches channels then its not the audio amps.

3) Check for proper azimuth. The BC 9000 came with an alignment tape and a tiny screw driver, which means someone could have fiddled with the tape head!

Derek

Hi Derek

Thanks for your input:

1.  Tape heads cleaned as well as I could.

2.  I made up a short bridge to swap the channels and confirmed that the problem goes with the head.  My next step is to make up a longer bridge and bring in the playback channels from a second BC9000.  That should give me a lot more information on the outputs and frequency distributions.

3.  I don't have the azimuth kit but did alter the head alignment using a two channel scope and a self produced standard tape.  That is how I was able to get an improvement in the higher frequencies so the azimuth was out.  I have also tried altering the shims supporting the head but wasn't able to get any further improvement.  Oh for one of those standard tapes that the manual refers to!!!!

Keep the ideas coming, they are all appreciated because I have so much to learn about this complex machine.

Regards

Geoff

PF5101 is discontinued and I would expect the PF5013 to be even more so since I can't find a trace of it anywhere (or just a typo in the manual, it wouldn't be the first, nor the last...).

Anyway, it's an N-channel junction FET with I_DSSmin = 4.5 mA and pinch-off voltage of 1.0V max. With these specs, my cross reference books give 2SK130, -147, -162 and -363 as similar models, but they may not be as good otherwise (most notably noise specs) and the pinouts may differ. I would advice you leave the FETs alone unless you're sure they are not OK.

Regarding the test cassette, try to find some ready made tape (i.e published by a record company, there must be a better word for this ) from a flea market or something that hasn't been destroyed by 10 years in a glove box of a car - they are usually quite good regarding the azimuth angle, then just adjust it by ear. Not as good as a real test tape and an oscilloscope, but near enough...

Swapping the two transistors will tell you if one is bad.

I agree about the test tape. You can come a long way using a good quality recording. Pre-recorded tapes are cheap
today, choose a newer one from a good (read: rich) artist/producer like ABBA or similar.

Martin

Hi Tournedos

Thanks very much for taking the trouble to find the FET specification that you supplied.  At least that gives me something useful to work with.  I will talk to our local suppliers about the equivalents that you recommended and see if they can get hold of some.

I am now tending to believe that the FETs might still be OK and I will leave them alone for a while.  What sent me down the wrong track was the fact that the source voltage on the left channel FET was very low compared to the right channel, +0.03V left compared with 0.12V right.  I have since found that this type of voltage difference is the same on my second machine and have no idea why the DC voltages should be so different for each channel.  At first, I thought that it might be a component tolerance effect but the fact that both machines have low voltage left channels leaves me confused. 

Regarding the test tapes, I do have some pre-recorded tapes that I use as a reference between my different machines.  However, I am not very good at picking up audio subtleties and will continue to use the scope and my home grown test tapes.  At the moment, I think that the azimuth and alignment settings are fairly close and will continue to hunt around to find the weakness on that left channel.  As long as the signal gap continues to decrease and there is a proper transfer of all frequencies, I start to feel happier to take out the difference on the R260 trimpot.

Thanks for the help

Geoff