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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 21st Nov 2020, 12:10 pm   #1
DMcMahon
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Default ITT SL 530 Cassette Recorder Check Out

Just been checking out my recently acquired ITT SL 530, a portable mono cassette player/recorder that I brought through the Forum. It was sold as working but with a hum problem. Radiomuseum.org indicates 1986 vintage, I thought it looks possibly a little older.

Quite nice looking and in quite good condition for its age.

Mechanically everything works well and playback sounds OK. There is a nasty hum problem present in Play (also same when Play paused), FFD and RWD, not present when nothing selected.

The hum starts as the volume control is moved away from zero and gets worse as volume control setting is increased.

Occasionally for no obvious reason the hum goes away completely so the fault is not a hard/permanent fault. Only so far checked with mains connection, not yet tried batteries.

So need to go inside to investigate, cannot find a schematic for this particular model, have found schematics for possible similar models SL 520, SL 600 & SL 53 (Automatic)

David
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Old 21st Nov 2020, 5:54 pm   #2
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Default Re: ITT SL 530 Cassette Recorder Check Out

Hum still present when on batteries but as before intermittent in nature.

Internal microphone recording checked OK.

Opened up the unit and tried to see if could either induce hum when no hum present or make hum go away when hum was present, by disturbing various cables and connections.

With hum present could make it worse by disturbing screened cables (to heads and microphone) and/or just putting hand close to areas of the PCB (which may well be normal). With hum not present could sometimes make it come and go by tapping areas of the PCB.

Neither of the above was very conclusive. Gave the PCB a good visual under magnifying glass, but no obvious dry/bad solder joints found.

Have now got the PCB hinged back to inspect the components side, had to disconnect various wiring including the heads wiring so not really practical to do live operational checks with the board hinged back.

The PCB is marked SL 520 so hopefully the SL 520 schematic I found will match it.
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Old 21st Nov 2020, 6:17 pm   #3
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Default Re: ITT SL 530 Cassette Recorder Check Out

It's worth cleaning the record / playback switch , Mick.
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Old 21st Nov 2020, 7:12 pm   #4
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Default Re: ITT SL 530 Cassette Recorder Check Out

Thank you Mick, that is on the plan.

It is also very noisy (even with volume turned right down) when the Play, RWD and FFD buttons are initially pressed.

David
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Old 21st Nov 2020, 8:13 pm   #5
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Default Re: ITT SL 530 Cassette Recorder Check Out

Quote:
Originally Posted by DMcMahon View Post
The PCB is marked SL 520 so hopefully the SL 520 schematic I found will match it.
I see on the SL 520 PCB layout drawing it shows the board numbered as SL 520, but it is not same as the SL 520 PCB fitted to the SL 530 but probably is very similar.
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Old 22nd Nov 2020, 11:50 am   #6
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Default Re: ITT SL 530 Cassette Recorder Check Out

Cleaned the record/playback switch.

Inspected component side of the PCB, nothing untoward seen.

Hinged up the tape deck mechanism to be able to properly clean the heads, capstan shaft and pinch wheel, they were not bad. Also did this to be be able to access/remove the tape counter because the counter will not reset to 000.

Taking the counter apart it can be easily reset by manually pressing the reset bar but the little reset plunger does not make contact with the reset bar, almost as if something is missing, nothing appears broken. Could do with a tiny angled spring under the reset plunger and going over to the reset bar, will see if I can make something, although it might be a lost cause.
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Old 22nd Nov 2020, 1:32 pm   #7
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Default Re: ITT SL 530 Cassette Recorder Check Out

Early Tanashin mech.
You have eliminated the rectifier and amp power output stage. The hum is likely from cracked earthing joints in the preamps, vol control area or mode switch. I also had one case where the actual head was bad and acted as antenna!
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Old 22nd Nov 2020, 5:44 pm   #8
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Default Re: ITT SL 530 Cassette Recorder Check Out

Thank you Ben for your inputs. Have been doing some diagnostics with the component side of the board uppermost, to do this have to have the head wiring disconnected.

Getting quite strong hum now most of the time, that I cannot make go by tapping the PCB, maybe with the floating head connections this is inducing this particular hum. One strange symptom is that touching with a finger (or a meter lead) battery negative (anywhere on the PCB), which is the common ground connection, the hum level reduces a lot.

There is one small area of the PCB that is really sensitive to a finger near it, so far cannot find anything actually wrong. Probably will have to reconnect the heads and work on the artwork side of the board to see if the present hum symptom is a red herring.

It is all very awkward to work on.

David
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Old 22nd Nov 2020, 5:50 pm   #9
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Default Re: ITT SL 530 Cassette Recorder Check Out

Quote:
Originally Posted by DMcMahon View Post

So need to go inside to investigate, cannot find a schematic for this particular model, have found schematics for possible similar models SL 520, SL 600 & SL 53 (Automatic)

David
SL 600 & SL 53 (Automatic) schematics bear little resemblance, SL 520 is very similar but not quite identical.
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Old 22nd Nov 2020, 5:54 pm   #10
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Default Re: ITT SL 530 Cassette Recorder Check Out

Quote:
Originally Posted by DMcMahon View Post

There is a nasty hum problem present in Play (also same when Play paused), FFD and RWD, not present when nothing selected.

David
When nothing is selected on the control keys the recorder is not powered, so this explains why there is no hum then.
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Old 23rd Nov 2020, 9:40 am   #11
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Default Re: ITT SL 530 Cassette Recorder Check Out

Spent quite a bit of time trying to locate source of the hum (with heads still disconnected) without any success.

Photo shows the hum at max volume at the output of the TBA 820M audio amp IC (IC 601), around 600mV peak to peak. Input to amp was around 30mV, hum signal was basically the same on mains and batteries.

Mains ripple on the DC supply seems fairly high at 160mV

As far as I could, checked all ground connections at components and cables etc both my continuity (to battery -ve) and voltage measurements (to battery +ve) all checked out good. Once again carefully checked all soldered connections and tracks for breaks/cracks using strong magnifying glass but all good, I did resolder a few slightly suspect looking soldered joints.

Very sensitive to pickup from hand being close by, were the first 2 pre-amp transistors area (Q1501 & Q1502) particularly the electrolytic capacitors (CE1501, CE1502 & CE1504).

Will refit the PCB and reconnect the head wiring and recheck.
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Old 23rd Nov 2020, 11:58 am   #12
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Default Re: ITT SL 530 Cassette Recorder Check Out

Have you gone to town on the slide rec-pb switch yet?!
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Old 23rd Nov 2020, 12:35 pm   #13
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Default Re: ITT SL 530 Cassette Recorder Check Out

Yes has been cleaned twice with no change. Also get loud noise clicks when Play/RWD/FFD selected (when power switch is enabled).

David
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Old 24th Nov 2020, 9:23 am   #14
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Default Re: ITT SL 530 Cassette Recorder Check Out

While checking out the hum issue discovered that the tape deck metal chassis is floating/not grounded and the metal work is very sensitive to be touched. Could see no way that the chassis would connect to ground. Tried adding a temporary ground connection which only reduced the hum level by a small amount, so removed the ground connection.

Refitted the chassis and reconnected the heads wiring. Hum level now probably around 50% lower (chassis still not grounded).

Resoldered quite a lot more of the various component connections particularly ground connections (none looked bad). Hum now appears fixed (or at least has gone away for the time being). While doing this noticed that the main drive belt did not look like it was position correctly and found that during the refitting of the chassis had got the belt hooked behind an electrolytic capacitor.

Unhooked the belt but the belt the fell off the driving gear and I could not get it back around the gear, so had to remove the chassis (which is not as easy as it sounds) to be able to relocate the belt correctly. Refitted the chassis and then found that the tape would not play and found that there was quite a bit of tape tightly wrapped around the pinch roller.

Had to cut the tape to get the cassette out but could not remove the tape from the pinch roller in-situ so had to remove the chassis again ! getting good at this now

Not sure how the tape got wrapped around the pinch roller, it must have occurred during the hum tests (I had forgotten the cassette was even fitted) due to the belt being around the capacitor.

All back together now and working well with no hum. Next to check line recording from the DIN socket and try and fix the tape counter reset mechanism.

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Old 24th Nov 2020, 10:14 am   #15
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Default Re: ITT SL 530 Cassette Recorder Check Out

Well done!

Ha ha! Your tale of woe reminds me why I don't often play around with cassette machines any more!

N.
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Old 25th Nov 2020, 12:26 am   #16
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Default Re: ITT SL 530 Cassette Recorder Check Out

Glad you got a result. It can be a chore! Re: belt fitting. As Nick would probably agree (!), a set of dental tools and some types of tweezers from a pound shop pays dividends when it comes to threading belts around where your fingers cannot get. I have saved lots of needless disassembly this way.
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Old 25th Nov 2020, 11:57 am   #17
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Default Re: ITT SL 530 Cassette Recorder Check Out

Checked Line in recording, I think it probably is OK but it is not erasing the old recording (at all) so difficult to be sure.

It was erasing fully when I previously tested internal microphone recording.

So need to check out the erase circuit and head.
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Old 26th Nov 2020, 9:32 am   #18
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Default Re: ITT SL 530 Cassette Recorder Check Out

Checked out the Bias Oscillator. Zero output for the Erase and a small signal for the Record bias, very surprised it was able to record at all in above post ?

Traced problem to 2 solder tracks/pads shorted together by solder bridge, must have happened when I was resoldering some of the solder joints during the hum investigation. The short was across capacitors C2801 & C2802, these being connected around the Bias Oscillator transistor Q2801.

After removing the short now get very healthy 57kHz (17.5uS) signals, 40V p-p for the Record bias and 75V p-p for the erase, surprising so large from a 6Vdc collector supply to the transistor.

Now erase fully works and record and playback are working very well. Also checked the DIN line output signal which works well, also used it to do an Audacity recording.

David
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Old 26th Nov 2020, 7:25 pm   #19
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Default Re: ITT SL 530 Cassette Recorder Check Out

Forgot to mention that this must have automatic recording level control because the volume and tone level controls have no effect on the recorded level as judged by playback and there is no record level monitoring.

Attached is a short extract of a MP3 test recording, it is German and the track is titled "Symbolism" and I am pretty sure no body would be able to identify the unknown artist.

Have had to attach it as a ZIP file to be able to attach the file.

David
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Old 26th Nov 2020, 10:39 pm   #20
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Default Re: ITT SL 530 Cassette Recorder Check Out

So, in a nutshell, was the hum being caused by dry earth joints?
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