Yamaha K300 cassette deck intermittent very low sound.

[Maclaren547]

I purchased this single logic controlled cassette deck needing at least new belts and would be very grateful for some help. It was not expensive ( about the price of the belt kit) so am looking at it as a bit of learning experience. It worked fine with great sound from both channels after the belts were fitted. I tested it both playback mode and record mode and it soon developed the sound dying away completely. Actually on further investigation, there remains sound but very very low output just about audible with headphones ( from both channels) from the built in socket and also with the line out put with gain turned right up. Sometimes after a few days, the sound has come back to normal for a period of minutes before defaulting to the very low output. The transport appears to work fine, and with record / playback switch solenoid working as it should.

I have done the usual checks looking through the threads
1) Cleaned the heads
2) Checked the head engages correctly
3) Dosed the record / playback switch with contact cleaner lots and lots of times.
4) Checked DC voltages from power supply board which appear to be all ok.
5) I have not been able to find a place to purchase the service manual but have looked at some pages from the radio museum web site.

Previous issues in the threads relate to the record / playback switch and I would like to check the continuity but without knowing what the internal layout of the switch would be, I’m not sure which pins to check.

The deck also includes a line / record mute switch. Could this be a issue ?

Appreciate anyone’s thoughts on this. I am engineer but not in electronics and in terms of equipment I have only a DVM, and a soldering iron currently.

Appended a few photos.

James

[ben]

I can't see the long rec/pc slide switch, does it have one? Decks with full logic often didn't, using ICs (or relays if early enough).

I would be looking at cracked solder joints.

What happens when you feed a signal to the line In, does it register on the meters? Does it pass the signal through to the line out in record/pause? If so, make a recording and replay it on a known good deck. Anything?

[Maclaren547]

Thanks Ben,

Yes there is a slide switch top right of picture 2 run by a solenoid. I’ve taken the cover off the solenoid to try and get better access for the contact cleaner so you can see the green coils. I guess this was a transition between a mechanical operated slide switch and what came later. I will try and carry out what you suggested and report back.

James

[Maclaren547]

Hi Ben,
Looked for obvious cracked solder joints without noticing anything. I did as you suggest feed a line in signal to the tape ‘in’ and recorded. There was no visual indication from the LED meters, but the track was recorded ok on the the blank tape and played at normal volume on my other cassette deck.

Do you think this points to the dreaded record / playback switch ? Seems weird that the signal is getting through to the tape heads ok but not on the LED level meters.

Previously just to cover all bases, I tried a type 2 pre recorded tape ( there are auto detect micro switches on the deck ) to see if maybe that might be an issue, this briefly played for about 10 seconds at normal volume with the LEDs lighting up before dying away suddenly.

I’ve appended a more detailed photo of the switch ( with solenoid switch cover removed) and the other side of the PCB

[ben]

Very odd. That absence of VU action suggests a missing voltage, perhaps the symptoms are related.Time to measure the outputs of the PSU and regulator. I had a Technics deck with a motor that was drawing excess current, that loaded down the PSU and caused the same symptoms you have.Replacing the motor fixed it.

One thing you can do to eliminate the rec/pb switch& solenoid is jiggle the switch while playing back a sacrificial tape, see if there is any interference or the audio returns.

[Maclaren547]

Thanks will give that a go

[Maclaren547]

Hi Ben,

PSU voltages look good. Cold night in my shed and played a cassette normally for about 40 mins before dying. Because of its intermittent nature which appears to somehow be related to temperature, do you think that using a freeze type spray might high light the offending component ?

[Analogue man]

It probably will, certainly worth a try.

[ben]

There could be a hairline crack somewhere on the PCB which responds to thermal expansion/contraction. Have you tried poking with something insulated like a toothbrush or small paintbrush handle with it powered and playing?

[Maclaren547]

Hi Ben,

I tried the freeze spray without success. I had previously tried gentle poking with insulated stick. Sort of getting disheartened now. Shame as it worked well after its new belts but I guess like any old electronics, it’s not unusual for an issue to show up after being out of use for so long. Thanks for all your help so far.

[SiriusHardware]

There is a KX-300 service manual available over at Elektrotanya, is that your actual model, or close enough?

https://elektrotanya.com/yamaha_kx-3.../download.html

On that page, scroll down to the 'I'm not a robot' checkbox,
Tick it,
Click the red 'Go to download' box

On the next page scroll down to where you see a 'preparing download' progress meter. Wait until that changes to a beige box with a green rectangular 'download' button and a green rectangular 'Download related manuals' box. Click the 'Download' button and use the file dialogue to decide where you want to save the manual PDF.

During the above process try not to get distracted by any other 'download' type buttons - just stay on the straight and narrow, as above.

With the manual to hand I'm sure we will be able to offer more targeted advice. Might you be able to borrow an oscilloscope from somewhere?

Edit: I see there is actually a K (not X) 300 - had no luck finding a specific service manual for that unfortunately.

[Nickthedentist]

Don't give up! But I would suggest you need to do some methodical signal-tracing now. A circuit diagram is almost a must-have for this.

[Nickthedentist]

K-200 available here:
https://archive.org/details/yamahak200servicemanual

May or may not be similar enough.

[Maclaren547]

Thanks for the suggestions. I did find the K200 and the KX300 but these are different. The K200 is most similar. The radio museum does have the K300 service manual but no easy way to buy a copy.

Nick you are right. The best way is now to go through the step by step. I particularly admire the way Manuel Caldeira on his Electronics old and new YouTube channel goes about marking what he has checked by highlighting in Green using a copy of the schematic on a graphics app.

[SiriusHardware]

The K200 is most similar.

How similar? Does the PCB inside your machine look the same as or very similar to the one in the K200 manual? Differences on the outside aren't so important, like more or fewer controls. It can happen that several models in a range sold alongside each other will use the same PCB with more features / more controls added as you go up through the range.

I have found an original (paper) service manual for the K300 for sale, but sadly it is in the USA and the cost plus the cost of getting it to the UK could probably buy you another K300.

[Maclaren547]

Thank you Siriushardware for looking on my behalf I will have a look at the w/e. The major difference externality appears to be the use of LEDs rather than VU meters.

The deck was very cheap as it needed belts which cost about the same as the purchase price So nothing to loose really. I recently bought a Hitachi SDT400 music centre, the tape deck of which was very simple to sort out with belts only ( unlike the DC servo speed control on the record deck which was a pain but all working now ) perhaps should have followed this with a simple piano key Deck but ironically all the transport works fine on the Yamaha.

[knobtwiddler]

There is a good chance that someone at tapeheads.net will have a manual. There are quite a few obsessive collectors there with tens of decks!

[Maclaren547]

Thank you for that idea I did join that forum the other day so will give it a try