Hitachi SDT-400 music centre - help

[steveluigi]

Hello again.
As you were all so helpful with the above equipment the last time I asked, I thought I would chance my arm and try again.
Thanks to your advice and help I got the machine in full working order and it still is, BUT! I have but 1 small problem.

It is with the record deck. I can't get the deck to the right speed. What I mean is that I am trying to get the stroboscope to stand still on the correct dots for the speed on the outer edge of the turntable, but the the dials that adjust the speed 33 & 45, will not adjust enough to get the dots static, and the deck is running slower then it should. The speed is steady, but slower than it should be due to the lack of being able to adjust any further.
Also when trying to adjust the speed with the knobs, the speed adjustment is quite erratic speeding up and slowing down jerkily.
I have just had the turntable off and cleaned some black goo off the belt runner and off the belt itself, but this hasn't helped.

Has any kind person any idea what I need to do please?

Thank you so much
Steve

[GP49000]

Not having specific experience with this model, but noting that it is a belt-drive:

I note that you mention cleaning up the drive surfaces of belt goo, but have you also replaced the belt? Or at least done some research to see that you have the correct belt? The references I can find on the net say the SDT-400 takes a 25.0 inch belt (total length). If someone substituted the more common 23.6 inch belt, the too-tight belt could be dragging down the speed.

If the belt is correct: there should be secondary speed adjustments normally accessed by a technician. Usually on turntables of this type there will be two small, trimmer-type controls, one for 33rpm and the other for 45rpm, on the speed governor circuit board. They are there to bring the adjustment range of the main, user-accessible speed control into range. The procedure for adjusting the secondary controls is to put the main pitch control at its center position and LEAVE IT THERE, then adjust each of the secondary speed adjust controls (use a PLASTIC screwdriver if the controls are made of metal, because they will be LIVE) so the strobe lines are stationary at the applicable speed. The goal is to put "correct" speed at the center of rotation of the main pitch control, with an equal amount of adjustment in each direction.

However, DON'T just adjust if the belt is too tight, because that will overstress the motor and the governor circuit. Make sure the belt is correct, first.

[Nickthedentist]

Hello,

I had one of these, bought secondhand about 20 years ago. I had exactly the same problem as you describe.

The record deck is very nicely made, and is a delight to use when it's been serviced. One of the nicest I've seen on an all-in-one music centre.

You need to:

(1) Ensure the motor pulley and turntable rim are clean and free from residue from the old belt,

(2) Replace the belt,

(3) Clean the 33 and 45 fine speed adjustment potentiometers with some Servisol 10 switch cleaner squirted into them,

(4) Replace the 33/45 select microswitch. It is possible to take it apart and polish the contacts with Brasso, but it wasn't designed to be disassemnbled and ISTR that you may have to drill out a rivet or two then use a dab of Superflue to keep things together again. It was this microswitch that was the worst offender in my machine.

(5) Finally, there are two preset speed adjust pots on the PCB, which ISTR are accessible through holes in the platter. These too need to be cleaned with a tiny drop of Servisol and twiddled back and forth a few times, noting their original positions before you do so.

Ensure no Servisol gets on the drive path, or else the belt will slip.

Nick.

You may also

[Tractorfan]

Hi,
I don't know this deck, but I'd be very surprised if the trimpots are live. These decks usually run on about 12volts via a transformer, usually the one that supplies the rest of the music centre. I've never ever seen a belt drive deck that had speed pots with a mains powered motor. The two I've got both have isolating transformers.
But don't take my word for it.
Cheers, Pete

[steveluigi]

Thank you for your feedback, I will try the suggested fixes and see if it helps. When I took the belt off I measured it to see if I could find another one and the measurement was approx 12" folded, so I would say approx 24"-25" in total length taking into account the ends when folded that I couldn't measure, so according to GP49000 account that is about right. The belt doesn't seem stretched or tight when fitting it, but will try a replacement to see if it helps.

I will also check to see if I can see the adjustment screws and try adjusting them to see if that helps also.

Again thank you for your help

Steve

[steveluigi]

Hi again.
I have attached a couple of photos with the turntable and belt detached.
I am looking for the adjustment holes but can only find 2 close together that may be them. They are the one in the close up shot that also show the adjustment knobs on the other side of the tonearm, are these the ones please?
They are very small if so and on shining a bright light inside them I can not see any screws to adjust, not saying they aren't there, just sating I can't see them.
The other thing is how can I adjust the screws when the platter is in place as it covers the holes?

Thank you again
Steve

[steveluigi]

Just a bit of added info, the turntable is model number SAR-610H1.
I have further inspected the 'holes' mentioned above and there are no screws in there.

[Nickthedentist]

Is one of the preset pots here (circled in red)?

You may need to glance under the deck to find them unambiguously.

And no, they're not live. The only bit you need to take care with is the neon strobe.

But the microswitch(es?) will almost certainly be the main culprit!

Nick.

[steveluigi]

Hi again thanks and yes, there is a pot there, It looks to have an oblong cutout at the top, is that it?

Update - Yes there are 2!

[steveluigi]

Do the pots turn?

[Nickthedentist]

Yes. Basically, they're just like the ones with knobs on, but are only designed to be adjusted during manufacture and servicing, not on a day-to-day basis. You need a small screwdriver.

Don't force them beyond their end stops, and note their position before twiddling so that you can reset them to their original position if necessary.

Nick.

[steveluigi]

Thanks Nick, just ordered some 'super servisol' so will have a go when it arrives and let you know the outcome.
Steve