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ZX Spectrum 48K Repair. I could really use some help from the other side of the pond repairing this...

It was a little hopeless at first but I must say most of that is my fault as I am completely unfamiliar with this computer. I have learned alot thanks to you all so its been fun. Next time (who knows with the state of things) we make it to England to visit my Sister in Law maybe I can find another Sinclair machine or BBC micro.

And thank you sir!
 
Just got an email today saying its getting ready to ship. I suppose it was the Chinese New year that delayed it.
Disregard that. The tracking for this has not worked since the day they issues me the tracking number. Until today that is, it finally shows it was "Item Accepted". I have that happen on certain things I buy from china on eBay. They are marked as shipped right away but may take as long as 3 weeks to actually get shipped. I hate that. Consindering I have already been waiting several weeks. But what can you do, you cant beat the prices..
 
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Hopefully it will come soon and I know your pain. I have three 48k membranes here in Australia, and if you were local, I could have helped you with a replacement while you waited and swapped later, but you are right. Buying the same item online from ebay costs 2 to 3 times the cost :(

I also have to order two new membranes from two new spectrums I bought that just arrived this week. Both are the + variety with the hard keyboard. So I too will venture to their site. Hopefully the post is back to working normally now.

In the mean time, have you had a look at the ZX Spectrum TOSEC archive on archive.org? It's a great place to find all those memories from 30 years ago, and will help your wife relive those years.

I'm really looking forward to you posting the reaction when she finally sees it :)

David.
 
So the Keyboard membrane came in finally. Works just fine. I cleaned up the case really well and washed it thoroughly. I removed the deteriorated foam feet (one was missing anyway) and replaced with some small rubber sticker feet I had. I cleaned off the old dried adhesive and washed the membrane. Put it all back together and it loooks better and the KB works.

So next step try and diagnose bad ram. I am trying to load the test diags from a TAP file from this site: https://github.com/brendanalford/zx-diagnostics
At first I tried my cell phone with a mono audio cable. Nothing. Then my old Ipad.. Nothing. I cant even hear the audio coming out the ZX spectrum speaker which I thought was supposed to happen.

It occured to me the cassette jacks might be dirty so I used some bristle brushes dipped in deoxit and cleaned out the audio jacks. I dont remember what triggered it but the thought occured to me the audio may not be loud enough or the ZX was not sensitive enough to "hear" it. I decided to get some amplified speakers, connected both my phone or Ipad into the speakers and use the mono audio cable from the zx spectrum to the headphone jack on the speaker. That way I can modulate the volume and the tone knob.

This finnally gave results. I could see the red and blue bars for the first time as well as hear the sounds come out the spectrum when the audio knob on the speaker was sufficiently high enough.
This seems strange to me as the volume on the phone and ipad were MAX and it still didnt trigger anything, why is that? Could something on the ZX spectrum that picks up the audio signal be faulty?

Anyway with lots and lots and lots of playing around (hours.. maybe 3) I was able to get the actual file name to come up on the zx spectrum screen. If the tone or volume goes up or down a hair, some letters in the tile might not be right.
Either way I never got it to load anything. I would get BASIC errors, or Tape errors, or RAM errors. I stopped after sufficient frustration. In the end I used my laptop, with the stereo speakers and a program called OTLA https://code.google.com/archive/p/otla/downloads got the idea watching a video of Mark fixes stuff. Seemed to give the best results so far anyway.

On another note. The small 4:3 LCD TV I thought was working only seems to work in monochrome. Any adjustment on the ZX which was color shows a complete mess. I decided to hook it up to our Primary Television downstairs and it works fine on with proper color (although I will need to tweak it when its all said and done for best picture.

So I am at a loss. I cannot load a single program even the tiny 8KB diagnostic program. Should I build the diagnostic ROM at this point? Any Ideas?
Really frustrated.
 
Glad the keyboard membrane came in the end.

>>> Really frustrated.

This was all part of the 'fun' with anything from Sinclair...

>>> Any Ideas?

Fortunately, I hope I have!

There is only one (1) pin on the ULA that handles the cassette interface and speaker. It 'doubles up' as both an input and output.

You are correct - any cassette input or output should be audible on the speaker. If not, something is wrong somewhere. There is a simple diode and transistor driving the speaker - so this should be a relatively simple issue to resolve.

Have you heard anything at all from the speaker? If you can use the Spectrum now via the keyboard, have you tried the 'BEEP' command to see if you can get any sound at all from the machine?

The cassette input/output from the Spectrum contains Resistor/Capacitor filtering. In my experience, a modern 'digital' device doesn't work too well with old fashioned computer cassette interfaces. For such occasions I use a really old analogue cassette recorder/player.

I am not sure of the version you have, but here is the relevant part of the schematic from an Issue 6A board:

1647527268738.png

The key pin of the ULA is pin 28 (CASS?SPKR) to monitor with an oscilloscope.

Dave
 
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Hi Dave. My board is an issue 2. Yes with the amplified speakers I mentioned turned up past 70% volume I start getting the sounds from the speaker in the zx spectrum and beeps.
 
Just out of interest, how did you physically connect your iPad to the Spectrum?

These devices are normally stereo - but the method of connecting a stereo output to a mono signal can vary depending upon what cable/connector you have. If you just decide to use the left or right channel - is the audio on the channel you are using? If the left and right outputs are combined together - this can cause problems.

It is also possible that the signal level from your iPad is not high enough at all (voltage wise) for the spectrum. I normally connect an iPad / iPhone to our mixer desk at Church for background music - but we connect it to a low-level microphone input and adjust the level appropriately. I am not sure what the specifications are for the iPad and various phones these days - but they may not provide enough voltage - period. Amplifying the signal would overcome this (as you have done).

Dave
 
Yes Dave at first I tried a direct connection with my phone and my ipad. At full volume on both devices it would not register the bars or the program name. That is why I put amplified speakers in between. Once I did that I was able to get at lease the bars and the program name, as well as start to hear sounds come from the ZX's internal speaker.

I am using a brand new Mono 3.5mm audio cable https://www.amazon.com/dp/B001O5F280?psc=1&ref=ppx_yo2_dt_b_product_details

"It is also possible that the signal level from your iPad is not high enough at all (voltage wise) for the spectrum." That was my thinking.. for the phone as well. That is why I added the amplified speakers in the mix.

In all honesty, I would prefer using my laptop as I am one of the few people who honestly abhor using these touch screen devices..
 
That cable probably shorts out the right-hand channel.

The tip is the left channel, the ring is the right channel and the sleeve is ground (for a stereo connector).

By plugging in a mono connector, this shorts the ring to the sleeve.

Sometimes, there is a fourth 'band' for a remote microphone. This would also be shorted to ground when using a T/S (tip/sleeve) plug.

Dave
 
Dave the only reason I bought the cable was because I thought the connectors on the ZX spectrum were mono. This video says they are mono:

qFIiFfIqIxQJg4MTouvjkd1dc56VEPqlHuGwi6vPBmM.jpg


It must be working as its showing the bars as well as the name of the program I am trying to load on the screen.
 
Yes, what I am saying is that the Spectrum is MONO, but what you are trying to drive it with is STEREO.

Whilst this may not (ultimately) be your problem - it should always be taken into account.

If the source (your iPad for example) is stereo - you should use a TRS connector at that end and then either wire the LEFT or RIGHT channel up to your TS connector at the Spectrum. In this case you will also have to check that the sound recording is made on both the LEFT and RIGHT channel of the stereo image equally (at the source).

I do a lot of professional live audio mixing gigs and the confusion with cables abound plenty. I have seen people convert balanced XLR into unbalanced MONO and get nothing out at all. I have also seen people convert from stereo to mono and also get virtually nothing out as well! I don't (however) think this is the issue with your setup though.

Dave
 
Agree, you MUST NOT ever use a cable which shorts one of the stereo outputs of a source to 0V, you risk damaging the output amplifier in the source and if not, the strain it puts on the audio PA device in the source may cause the output from the other channel to be distorted. When I do this I use a hand made cable with a stereo plug at the source end and a mono plug at the ZX end. At the source end only GND and ONE of the two output channels is connected, with the other output channel left disconnected.

I had a similar problem a while ago trying to load ZX81 programs from a small MP3 player, the problem was that the player did not produce a large enough output voltage swing for the ZX to be able to 'hear' it. I really needed it to work so I used Audacity to make a special stereo version of the audio file with one channel exactly in antiphase with the other. This made the output voltage swing BETWEEN the two audio output channels double what the difference between one output channel and GND would be.

I then connected L-out on the source to GND on the ZX and R-out on the source to Audio-In on the ZX, leaving GND on the source unconnected. The only thing about this method (apart from the complexity involved in preparing the audio file) is that the source must be powered from batteries or from an isolated PSU with no connection to GND.
 
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Ill be honest that
The only thing about this method (apart from the complexity involved in preparing the audio file) is that the source must be powered from batteries or from an isolated PSU with no connection to GND.
This really seems over cautious as there are plenty of videos showing people loading ZX spectrums from laptops, phones, and Ipads.
 
Thats how I load mine, using the original Sinclair cable plugged into one of the speaker ports of a cheap USB dongle. Works fine

Volume has to be turned up very high though.
 
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