Hitachi SDT-300 won't spin

[roberohn]

Hi all ?

I'm hoping some clever folks here can help me. I've recently acquired an Hitachi SDT-300 HiFi with the matching speakers that has been in a loft since the 80s.

The radio works great, I've no idea if the cassette player works as I don't have one to test that with. I do have vinyl though, however, the platter won't spin when the tonearm is raised and moved over to the platter. The orange LED does come on when the tonearm is moved so there's some activity there at least.

The original belt has melted away and hardened itself underneath the platter so I'll need to clean that up and get it replaced.

The small brass pin that the belt goes around isn't the easiest to turn manually, I'm guessing the motor is attached to it directly underneath it though there's no sound of a motor working at all.

I'd like to check the circuit board within but there is very little information online on how to get access to it, on the bottom of the HiFi are two small plates that can be unscrewed but they don't reveal much. Does this instead come apart from the top perhaps?

Here's an image of the unit from the top. I'm guessing this unit is similar to other Hitachi units of the time.

Thanks in advance for any advice ?

[Sideband]

The 'small brass pin' is actually the motor pulley, part of the motor shaft and should be completely free to turn....if that doesn't turn, there is no chance of the turntable working even with a new belt. The motor shaft/pulley could be jammed with rotted belt....not uncommon. Unfortunately I can't help with actually getting into the unit so someone else may be able to advise with that. Be prepared for a major clean-up of the motor though. If the pulley is jammed due to melted belt, you'll probably need to use acetone or surgical spirit to remove the gunk. I notice it says DC servo so the motor is likely a DC motor with a control PCB. At this stage, I wouldn't worry about any electronics driving the motor. You'll have to free the motor first and hopefully if that is successful, it may just work OK with no servo faults.

[roberohn]

Update!

Managed to get into the gubbinz of the unit and access the motor. With some cleaning and a little oil the drive shaft would spin a little with some help spinning it, under power you can feel it trying to spin and it would for a couple of seconds before stopping again. There was a little squeaking so it’s definitely dry. The annoying part is when we got the motor out of its housing to apply some oil there was a click or pop to release it. It’s not going back in flush.

I’m not sure if I’ll be able to get the brass collar over the copper gasket again. I’ll try to tinker with it a bit more but I’m thinking I’ll need to research replacement motors.

Sideband, you are right in that it’s a DC motor with a PCB attached at the bottom. Are replacements available?

[Maclaren547]

Just was wondering whether you managed to free the motor up. I have got a SDT-400 which has the same deck ( turntable) as yours. I was helped greatly by the people on this forum. I found the deck was just held down by a few screws which will give access to underside and the motor to find out why it’s stiff / seized. I did find a service manual for mine which greatly assisted access to the rest of the components.

[Maclaren547]

Great you got the motor turning again at least. I too not expecting what I found I.e the motor was in outer casing with the control electronics in between. I took it apart I was advised forum members that poor contact here caused speed instability. I thought I had bitten off more than I could chew but I took my time and managed to get it put together right eventually. It did make all the difference to the speed control. Anything in that servo circuit like the 45/33 lever and the speed adjust also needed contact cleaning as well. Just put a record on mine now. I’m always surprised how good it sounds when all set up. Good luck.

[roberohn]

Yeah I worked out how to get the deck off, two screws on the top which was easy enough. I’m struggling reassembling the motor trying to get the brass collar through the copper or nylon washer, it won’t sit back flush through it. Need to continue tinkering.

[G6ONEDave]

I have copy of the SDT300 service manual that I can scan to pdf, please send me a PM if it would be of help.

Dave

[roberohn]

Originally Posted by G6ONEDave

I have copy of the SDT300 service manual that I can scan to pdf, please send me a PM if it would be of help.

Dave

That would be incredible, thank you.

[roberohn]

Latest update the motor pulley is now spinning consistently and not stopping at any point. Next issue is that it’s struggling to spin the platter. It needs help to get started but then slowly begins to slow the platter down.

It looks like it’s not able to grip the belt. I’ll check the forum for any suggestions on how to make it grip the belt better. I feel like this is good progress though.

[Analogue man]

When you got the motor spinning did you use oil? If you did you may have got it on the motor pulley which subsequently will have migrated to the belt and the platter. what you need to do is clean the pulley, the belt and the part of the platter that the belt runs on with mentholated spirit thoroughly then put it all back together.

[roberohn]

I've recently cleaned the belt with a bit of washing up liquid and scrubbing any waxy like particles from it. I've cleaned up the pulley too with some isopropyl alcohol. After a few goes it's spinning much better now.

My next problem was the speed, at both 45RPM and 33RPM it wasn't not nearly fast enough, or it started fast before slowing well below intended speeds. I found another thread (here) which has been valuable regarding the potentiometer pots. I gave them a spray of contact cleaner and adjusted them slightly to find now the platter is spinning at a much more appropriate speed.

Still needs a little more fine tuning but I think this has overall been a success. I want to thank everyone here who's provided assistance in getting this old bit of kit going again.

[Uncle Bulgaria]

Originally Posted by Analogue man

mentholated spirit

That's 'methylated' for anyone tempted to get out the crème de menthe. I don't think any of us would advise wasting that kind of alcohol on cleaning a belt outside a typo!

Quite a lot of problems in old audio equipment ends up being to do with old lubricants and the general dust of decades. A good clean does wonders as you've discovered.

'Exercising' switches and potentiometers (rotary and slider), often with some switch cleaner is very worthwhile - though be careful as some types require careful cleaning lest the wrong chemical damages the resistive track.

Turning preset pots back and forth through their whole rotation can break up crud that has accumulated around the wiper. It's a good idea to mark the original location first, or measure the resistance in case you haven't got clear instructions as to where the thing needs to be set.

[Analogue man]

Ops reminder to self 'check spelling before posting'.