Then the hard task begins.
Check the voltage on each channel at each stage, that will give you some idea.
And if you have an analog multimeter, remove the head wires from PCB, put the machine on, and play music, check the needle moves in AC mV range, or the lowest AC range available on the meter.
If the readings are similar, swap the channels to confirm output from head.
Then switch off, power off from mains, remove mains cord from wall.
Then check each and every wire and switch for continuity, and all the caps for polarity.
Then check for resistors which may have shorted to ground or are open.
Then the diodes, transistors etc.
Make notes, take photographs.
Most likely it is a stupid fault caused by a hasty repair attempt, by somebody who was over confident.
Check the voltage on each channel at each stage, that will give you some idea.
And if you have an analog multimeter, remove the head wires from PCB, put the machine on, and play music, check the needle moves in AC mV range, or the lowest AC range available on the meter.
If the readings are similar, swap the channels to confirm output from head.
Then switch off, power off from mains, remove mains cord from wall.
Then check each and every wire and switch for continuity, and all the caps for polarity.
Then check for resistors which may have shorted to ground or are open.
Then the diodes, transistors etc.
Make notes, take photographs.
Most likely it is a stupid fault caused by a hasty repair attempt, by somebody who was over confident.
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Hmm, that is very unfortunate 🙁
Although you say nothing has changed (and in a sense that is good because it means you have replaced the parts correctly) I would still recheck those same voltages as you did before on the transistors. Just check they are similar to how they were.
Without a circuit this gets very very difficult indeed. The REC/Play switch is a complication because typically that will switch the circuit configuration around.
Don't lose sight of the fact that the voltages show a problem in one channel. The two NPN collectors are wildly out and the PNP ones are not. That suggests the first stage uses just the two NPN's in what will be a standard type configuration.
I'm going to post the circuit of such a layout here. Although yours will differ greatly in details I think the basic setup will still be similar. The emitter volts of that 2SC device is extremely low. It might be worth a swap just to eliminate it.
This is a typical two stage amp for a head. The R/P switch interacts with everything...
Although you say nothing has changed (and in a sense that is good because it means you have replaced the parts correctly) I would still recheck those same voltages as you did before on the transistors. Just check they are similar to how they were.
Without a circuit this gets very very difficult indeed. The REC/Play switch is a complication because typically that will switch the circuit configuration around.
Don't lose sight of the fact that the voltages show a problem in one channel. The two NPN collectors are wildly out and the PNP ones are not. That suggests the first stage uses just the two NPN's in what will be a standard type configuration.
I'm going to post the circuit of such a layout here. Although yours will differ greatly in details I think the basic setup will still be similar. The emitter volts of that 2SC device is extremely low. It might be worth a swap just to eliminate it.
This is a typical two stage amp for a head. The R/P switch interacts with everything...
One good thing is that you have a working channel to compare, that is handy, as there seems to be no schematic available.
See if the supply to the transistors is through a switch, that may have a bad contact, if you can find the right contacts, just put a jumper as a temporary check.
See if the supply to the transistors is through a switch, that may have a bad contact, if you can find the right contacts, just put a jumper as a temporary check.
@Mooly and @NareshBrd . Big thanks for your continued support. I'll do all the tests tomorrow and I'll let you know. Thank you again.
In a cassette recorder like this, the record and playback function is done by the same amplifier stages, switched over by a rec/play switch. The playback usually has a preamplifier in front of the universal gain stage and volume control. This preamplifier is one source of fault, because it is not common in rec/play.
Have you tried recording and playing back that recording on a different cassette player? If both channels are OK, then
a) the head is OK
b) the above mentioned combined rec/play amplifier is OK
c) the playback preamplifier is suspect
d) there is a chance that the rec/play switch has some fault in the bad channel on play.
Have you tried recording and playing back that recording on a different cassette player? If both channels are OK, then
a) the head is OK
b) the above mentioned combined rec/play amplifier is OK
c) the playback preamplifier is suspect
d) there is a chance that the rec/play switch has some fault in the bad channel on play.
Thank you all for your generous help, much appreciated.
I’d like to do a quick recap of what’s been done just to have a starting point for everybody.
1. All the components on the Left ch. have been tested: transistors and diodes off board, resistors on board, matching values. Four out of five transistors have been replaced, the Dolby IC has been replaced as well. Capacitors have been just charge/discharge tested, on board. Not very accurate, but at least we know they’re not short, nor dead.
2. Tensions on the transistors terminals have been tested and reported in a previously shared table. First two NPNs (the closest to the heads) show weird values compared to the respective ones on the working ch.
3. The left ch. is working in Rec mode. I fed some audio into the RCA input jacks and I could hear both channels. The input faders work fine too.
But the VU Meter doesn’t move and I get no recording on the infamous ch. What @lcsaszar suggested has been tried earlier, and the result is no audio on the Left ch.
4. It looks the main suspect is the Rec/Play multi-switch, but it’s been thoroughly cleaned by disassembling it. Contacts have been degreased with isopropyl alcohol, deoxidized and treated with CRC before to be put back in place.
5. The only thing I didn’t check is the small board connected to the back panel (pic attached). Since it holds a kind of DIN socket for remote Rec/Play, I think maybe somebody in the past could have plugged a wrong device in, or the remote itself was shorted or damaged in any way. I will check it and maybe I will try hard to reverse engineer the little PCB, it’s something I never did before, but let’s see.
PS I don’t have the hypothetical remote control.
Thanks for your patience
I’d like to do a quick recap of what’s been done just to have a starting point for everybody.
1. All the components on the Left ch. have been tested: transistors and diodes off board, resistors on board, matching values. Four out of five transistors have been replaced, the Dolby IC has been replaced as well. Capacitors have been just charge/discharge tested, on board. Not very accurate, but at least we know they’re not short, nor dead.
2. Tensions on the transistors terminals have been tested and reported in a previously shared table. First two NPNs (the closest to the heads) show weird values compared to the respective ones on the working ch.
3. The left ch. is working in Rec mode. I fed some audio into the RCA input jacks and I could hear both channels. The input faders work fine too.
But the VU Meter doesn’t move and I get no recording on the infamous ch. What @lcsaszar suggested has been tried earlier, and the result is no audio on the Left ch.
4. It looks the main suspect is the Rec/Play multi-switch, but it’s been thoroughly cleaned by disassembling it. Contacts have been degreased with isopropyl alcohol, deoxidized and treated with CRC before to be put back in place.
5. The only thing I didn’t check is the small board connected to the back panel (pic attached). Since it holds a kind of DIN socket for remote Rec/Play, I think maybe somebody in the past could have plugged a wrong device in, or the remote itself was shorted or damaged in any way. I will check it and maybe I will try hard to reverse engineer the little PCB, it’s something I never did before, but let’s see.
PS I don’t have the hypothetical remote control.
Thanks for your patience
Attachments
Update: I removed the only two transistors from the small board (another couple of C458).
Legs are totally black, but they look ok according to the China tester.
To be continued...
Legs are totally black, but they look ok according to the China tester.
To be continued...
Just a thought.
If you have a 5-pin din plug between the input and output RCA´s, it is NOT a remote connection.
It´s a DIN-connection for rec./play to use INSTEAD of the RCA´s. A signal connector..... not a connector for remote control.
A lot of radio´s/amp`s (european) like Grundig/ITT/B&O etc. used only DIN.
It´s coupled reversed from normal DIN, so your deck will have Audio In on pin1 & 4, Audio out on pin 3 & 5 and ground on the middle pin (2).
If you have a 5-pin din plug between the input and output RCA´s, it is NOT a remote connection.
It´s a DIN-connection for rec./play to use INSTEAD of the RCA´s. A signal connector..... not a connector for remote control.
A lot of radio´s/amp`s (european) like Grundig/ITT/B&O etc. used only DIN.
It´s coupled reversed from normal DIN, so your deck will have Audio In on pin1 & 4, Audio out on pin 3 & 5 and ground on the middle pin (2).
I think this has to be the starting point. If these transistors are the head preamp (and it sounds like they are from their location) then you have to try and find why the voltages are different. The circuit will be AC coupled stage to stage and so anything past this point is most likely not at fault as problem later on would not affect the DC conditions in the front end.
Although the record play switch looks complex they are really just a load of simple changeover contacts typically in groups of 3 pins like SPST switch.
If you look at the switch from one end you can say pins 1,2, and 3 are the first 'switch'. Pin 2 is the wiper and that will connect to either pin 1 or pin 3 depending on switch position. Now you look at pins 4,5 and 6 and now pin 5 is the wiper and connects to either pin 4 or pin 6 and so on.
It is possible the first pin may be unused and so the numbers all move up one but you get the idea. With it OFF measure on ohms and confirm you understand how it works as you move it from record to play. Having done that and seen what grouping they have you can check each switch in the package.
If we eliminate the transistors then the next suspects could be something like a small compressed disc ceramic cap (these often go leaky). Like these. Isolating one end of them would restore DC conditions if it were leaky:
@Boydk - Yeah, you’re definitely right. I’m old enough to remember that kind of European connections, my uncle used to have a Grundig like that in the 70s. I don’t know why I thought about a remote, maybe cause I have the same kind of DIN socket on a slides projector’s remote control.
I’m going to replace the transistors anyway (C458 with BC549) cause they have black legs, no matter what the tester says.
@Mooly - I agree: I have to start from there. I didn’t check the little disc capacitors, so I will, even if I just have the cheap China component tester, so I’m not sure about its reliability (still waiting for my OWON HDS242s, it will arrive towards the half of June).
As far as the multi-switch, I’m sure it works, but I’ll check it the way you suggested. I confirm it’s actually a series of two-way switches.
Thanks for your help.
I’m going to replace the transistors anyway (C458 with BC549) cause they have black legs, no matter what the tester says.
@Mooly - I agree: I have to start from there. I didn’t check the little disc capacitors, so I will, even if I just have the cheap China component tester, so I’m not sure about its reliability (still waiting for my OWON HDS242s, it will arrive towards the half of June).
As far as the multi-switch, I’m sure it works, but I’ll check it the way you suggested. I confirm it’s actually a series of two-way switches.
Thanks for your help.
For the caps just non invasively desolder one lead and measure the resistance on high ohms range. They should be totally open circuit. You can also then power up and check the voltages on those transistors with the cap isolated.
Just tried the all above, no luck. The multi-switch seems to be ok, I left out the first pin and checked the others in series of three (1 and 2 beep, 2 and 3 open; 4 and 5 beep, 5 and 6 open and so on). The same result I got testing the working side. Note that the "Play" position is the rest position of the switch, so I did the testing in that position.
There's only one small disc capacitor per channel, and the suspect one is ok, tested with one leg out and max resistance range, it reads as totally open.
I will check better the electrolytics, but I'm afraid it will take me nowhere.
The more I go on checking, the more I'm convinced that the fault is a stupid, overlooked thing.
There's only one small disc capacitor per channel, and the suspect one is ok, tested with one leg out and max resistance range, it reads as totally open.
I will check better the electrolytics, but I'm afraid it will take me nowhere.
The more I go on checking, the more I'm convinced that the fault is a stupid, overlooked thing.
I would doubt an electrolytic too, they tend to dry out and that affects AC performance as the reactance (Xc) rises. Resistors can and do fail, typically ones that have highish voltage across them... but nothing in a cassette deck is high voltage... but still it does happen if only rarely.
Going back to your voltage readings I see you have 1.29v on both collector and emitter which is a bit suspect in transistor land. Turn the power off and just see what sort of ohm reading you get in circuit from C to E. Compare with the good channel.
Going back to your voltage readings I see you have 1.29v on both collector and emitter which is a bit suspect in transistor land. Turn the power off and just see what sort of ohm reading you get in circuit from C to E. Compare with the good channel.
I had a TV repairman tell me that he spent four hours trying to find a fault...somebody had put NPN in place of PNP.
Check both channels with an open mind, do not assume, do not be tense, be extremely calm and methodical.
Check both channels with an open mind, do not assume, do not be tense, be extremely calm and methodical.