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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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15th Nov 2011, 8:00 pm | #1 |
Rest in Peace
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
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Distorted sound on both decks of a Coomber 844
As some members will know, Coomber make things for school & college use.
They are usually very helpful, but no longer have circuit diagrams for the 844. This model has two identical cassette decks, both are recorders,and, since I removed the circuit boards to replace drive belts. etc. the sound from both is very distorted. I've checked various things such as supply voltages, record/playback switching, etc., and even fitted new heads to no avail, the latter after establishing that, with the heads disconnected, a sinewave injected into the point where the playback head connects is reproduced without distortion. What's niggling me is why should the same fault occur on both decks, when all I've done is unsolder & re-solder connecting wires I even tried de-magnetising the original heads before changing them. This made no difference. This is turning out to be a job with much Can someone PLEASE shed some light on it?? |
15th Nov 2011, 8:39 pm | #2 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
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Re: Distorted sound on both decks of a Coomber 844
Hi I am not familiar with this item but does it have an aux input you could try feeding a signal into to see if the output stage is OK.
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15th Nov 2011, 8:45 pm | #3 |
Heptode
Join Date: Oct 2008
Location: Aalborg, Denmark
Posts: 903
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Re: Distorted sound on both decks of a Coomber 844
Hi,
This is what I have. Hope it works out for you. rgds, /tri-comp |
15th Nov 2011, 10:12 pm | #4 |
Nonode
Join Date: Dec 2002
Location: Coulsdon, Surrey, UK.
Posts: 2,109
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Re: Distorted sound on both decks of a Coomber 844
Mmmm
Put logically, with these symptoms, this fault seems impossible. I'm tempted to wonder if the problem lies with the recording process, rather than playback ? Especially given that injecting a test signal into the playback chain produces a clean ouput. Do you have a known good tape with which you can verify the playback ? At first pass, I'm tempted to lay the blame with something like a faulty bias supply which is common to the record head on both decks ? Andy |
15th Nov 2011, 11:16 pm | #5 |
Octode
Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
Posts: 1,786
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Re: Distorted sound on both decks of a Coomber 844
Just a guess but is it possible that you have transposed any of the connectors/connections when re-fitting the circuit boards? Many units of similar design to your Coomber model used connectors where this could easily be done by mistake.
Regards Andrew |
16th Nov 2011, 12:05 am | #6 |
Moderator
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
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Re: Distorted sound on both decks of a Coomber 844
What happens if you play a known good tape? Is it distorting? If playback of a known good tape is distorted, then forget the recording chain and concentrate on the playback side of things
If its only on its own recordings then the bias level might be a good place to start. |
16th Nov 2011, 10:58 am | #7 |
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Location: N.W. Oxfordshire(Chipping Norton)
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Re: Distorted sound on both decks of a Coomber 844
Thanks for the circuit diagram, etc., tri-comp. It should(I Hope)help me to trace the fault.
Just to clear up a few points that maybe weren't clear in my OP. The fault is present when playing tapes either recorded on other, known good machines, or commercially prerecorded tapes, all of which play correctly in other decks. In reply to Andrew's comment, which is a fair point-I will double check, but, AFAIK, all my rewiring is correct, since I made notes before I unsoldered any wires. The two circuit boards are identical, and are each connected to the main PCB with two ribbon cables, one 12 way, the other 6way. I've checked, and swapped over these ribbons to no avail,etc. What I haven't yet done is to make a test recording on this machine. Maybe that's the next job! |
16th Nov 2011, 2:45 pm | #8 |
Retired Dormant Member
Join Date: Apr 2008
Location: Banffshire, Scotland, UK.
Posts: 191
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Re: Distorted sound on both decks of a Coomber 844
Thinking logically about this
If I have read the postings correctly it replays recordings made by other recorders with distortion so the fault must be present with the machine on the replay side ( maybe recording too ) It was ok before you serviced it it reproduces a sine wave ok injected at the replay head connection on the pcb When you say the sound is distorted ? what do you mean by distorted, ie could it be hum mixed with audio ? The thoughts that spring to mind are an earth connection disturbed from the deck to the amp or a poor connection to the head itself maybe by the connectors being removed and replaced circuits carrying very small signals are very prone to connector trouble as they dont burn themselves clean in operation regards val33vo |
16th Nov 2011, 11:37 pm | #9 |
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Re: Distorted sound on both decks of a Coomber 844
To me, it sounds like the noise caused by clipping(i.e. crossover distortion)but I know the
inbuilt audio amp. & speakers are O.K. As to the connections, yes I did(of necessity)unsolder those to the rec/pb & erase heads, switches & motor on the deck PCBs in order to gain access to the mechanisms of both decks, and, as noted in a previous post changed the heads. Two thoughts occur as to the cause, though I would have thought both unlikely. (1) Initially I de-soldered & re-soldered with a Temperature controlled iron, the electronics of which are in the handle, (2) I use 'proper' i.e. leaded solder, whilst the makers may have used unleaded, although as the machine is quite old, I wouldn't have thought so. Could either of these be the cause of the fault? |
18th Nov 2011, 10:13 pm | #10 |
Retired Dormant Member
Join Date: Apr 2008
Location: Banffshire, Scotland, UK.
Posts: 191
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Re: Distorted sound on both decks of a Coomber 844
Livewire
It could well be crossover distortion if so it will be much worse at low volume levels ( due to the signal to distortion ratio being much greater ) If it is crossover distortion you are looking at the bias circuit on the output transistors, be carefull here whilst fault finding as too much bias will cause the output trannies to overheat and self destruct the usuall set up is an amplified diode ( just a transistor connected between the output pair bases ) or alternatively 2 diodes, they are usually in thermal contact with the output trannies heatsink so as to conduct more when the heatsink warms up and thus reduce the bias regards val33vo |
19th Nov 2011, 10:50 am | #11 |
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Re: Distorted sound on both decks of a Coomber 844
I hadn't looked at it that way, val33vo, though why it didn't happen before I disconnected the cassette mechanisms I don't know. Maybe just coincidence. The amplifier in these machines is an integrated circuit(TDA2122, I think)of about 2 x 5w power, easily changed if
necessary. Maybe the fact that there is no detectable distortion right from the i/p on the tape recorder PCBs, with the heads disconnected) that's been fooling me. Logically, the fault has to be in something common to both decks, which is basically the power supply and amplifier. |