Re post #159 your diagram is correct. (If you reverse the polarity it will behave like a forward biased normal diode so it would have about 0.7 volts across the junction.) You just use ohms law to work out the resistor value. As it is a 9.1 volt zener you must more than 9.1 volts available. So your 12 volt power supply is your only option. So you will have 2.9 volts across the resistor. (We will call it 3 volts to make it easier to the calculations in our heads.) From the data sheet the voltage rating is given with a current of 5 mA. So R = 3/0.005 ohms = 600 ohms. As you have such a limited stock of resistors you can use three 200 ohm resistors in series. I don't know what you mean by "the common lead" in your last sentence.
Re post #160 I think that that gives a reasonable description of you problem. They will probable ask you questions and ask you to do various things to try to isolate the problem.

Les.

 

As the diode is connected in reverse for this test; the voltage test across D477, would the common lead connect to anode?

I do beg my pardon on that Les; I should have made it clearer!
As the diode is connected in reverse for this test; the voltage test across D477, I will be connecting the common lead from my meter to the anode?

 

I assume by common you mean the black meter lead. Yes, the black lead to the anode the red lead to the cathode. It would not matter if they were the other way round as the meter would just display the negative sign in front of the reading. The voltage across the diode can only be one way round as we know the polarity from the power supply.

Les

 

Les... I suggest taking D477 out of circuit and testing it. It is specified to clamp at it's rated voltage of 9.1 volts with a current of 5mA.

So; I have 12.3vdc supply
3 resistors = 0.599 k ohms
Voltage readings across D477 4.99 to 5v

The voltage is way down. It's impressive what you can do with a smartphone and connected to a good website

 

It looks like you need a new zener diode. Have you had any replies on the other forum about the rewind problem ?

Les.

 

Zener Single Diode, 9.1 V, 500 mW, DO-35, 5 %, 2 Pins, 200 °C - BZX79-C9V1.

CPC have these in stock, it will pass a max of 500 milliwatts; is this going to be a good replacement, the existing, which you kindly have identified, the new one will need to cope with a current of 5mA?
https://cpc.farnell.com/nxp/bzx79-c9v1/diode-zener-500mw-9-1v/dp/SC05702

I’m going to make up a small order to qualify for free postage; could you use a pair of these Pro’sKit Angled Side Cutters 100mm - 1PK-501A, I bought these to assist with removing those IC’s, I took them to my bench grinder and ground a small amount off, I could then get them in between the IC pins from the top face to cut each pin, with other components next to the IC you cannot get near to cut from the side, there are 3 photos on my OneDrive showing the finish grinding, I will of course give them the same treatment?
https://cpc.farnell.com/pro-s-kit/1pk-501a/4-micro-cutting-nipper/dp/TL01306
https://1drv.ms/f/s!AuQDAPiShOjc9RoD2G3xlbzfvUuQ

I would have liked a pair micro pliers with a bent nose at an affordable price as they won’t get a lot of use; to assist with fitting and removing components, I think I will have to make do with these?
https://cpc.farnell.com/pro-s-kit/1pk-501e/4-micro-long-nose-pliers/dp/TL01310

It looks like you need a new zener diode. Have you had any replies on the other forum about the rewind problem ?

No not yet; I am sure that I will get an answer later on.

 

That zener diode should be suitable. (You seem to be getting mixed up between "m" (10^-3) and "M" 10^6) It might be worth getting the zenner diode on ebay to save having to make up a larger order to use CPC, Farnell, or RS This is one source on ebay. I keep a list on the computer of things I am getting low on so when I want something quickly I can just make up the order value from things on the list. I have a very nice pair of Lindstrom angled side cutters that get into small spaces. I also have a pair of thin pointed nose pliers similar to the ones in your link. (Although these are not Lindstrom.)

Les.

 

I think it is more likely to be the diode that is faulty. I think there is another fault on the speed controller as from a quick look at the speed control circuit I would expect the way the zener seems to be faulty would cause the speed to be slow rather than fast.

The new Zener diode is fitted; the replacement that you kindly sourced.

This is the situation now; play is still fast and the tape is being feed too fast via motor 1 and as a consequence tape isn't being taken up fast inough, motor 1 needs slowing down, I think?
D457 on U405 Main Board; it is a Diode voltage regulator BZX79/C4V7, this seems to be related to motor 1?
Voltage readings across -8.79v
To Ground
Anode side 8.3mv
Cathode side 8.8v

 

It looks like replacing D477 has made things worse. Have you have checked the voltages on D477 since you have fitted the new one ?

D457 on U405 Main Board; it is a Diode voltage regulator BZX79/C4V7, this seems to be related to motor 1?
Voltage readings across -8.79v
To Ground
Anode side 8.3mv
Cathode side 8.8v

I can't find how you think the A6 rail (Which D457 regulates.) can effect the motor speed. I can only find it being used to power some analogue switch ICs on U405 I can find nothing to do with motor control on U405 which only deals with the audio path and the A6 rail does not seem to exit from U405 to be used anywhere else. Also it is shown as being a 9.1 volt zener not a 4.7 volt one. Your reading of 8.8 volts fits with it being a 9.1 volt zener. Can you tell me where on the schematic you think the A6 rail could effect the motor speed ?

Les.

 

It looks like replacing D477 has made things worse. Have you have checked the voltages on D477 since you have fitted the new one ?

Board U302 Motor Control; Diode voltage regulator, voltage readings with cassette in. First reading across, second readings to ground, anode (A) side first followed with cathode.
D467-C4v7. -8.83v
A -11.89v
C -3.0v
D477-C9v1. -9.0v
A -11.89v
C -2.8v
D476-C7v5. -7.63v
A -11.0v
C -3.39v

Please accept my apologies; I certainly did make a wrong move with the latter part of my post yesterday, my mind isn't as it should be, I need to try harder!

 

Hi Michael,
I assume you mean the cathode of D477 is at -2.8 volts with respect to ground (NOT +2.8 volts.)
D467 looks to be open circuit so will need replacing. This is probably the cause of M1 running far too fast. With its cathode being at -3.39 volts it is biasing IC435 outside it's common mode voltage range (0 to V+ -1.5 volts) This will be preventing tachometer pulses being detected so the speed control loop will think the motor is stopped.

Les.

 

I assume you mean the cathode of D477 is at -2.8 volts with respect to ground (NOT +2.8 volts.)

Thanks; I have amended my last post.

D467 looks to be open circuit so will need replacing. This is probably the cause of M1 running far too fast. With its cathode being at -3.39 volts it is biasing IC435 outside it's common mode voltage range (0 to V+ -1.5 volts) This will be preventing tachometer pulses being detected so the speed control loop will think the motor is stopped.

With my observations on my last post; regarding the voltage readings taken across these types of regulators, are we looking for a close to the same reading that matches the governed/regulated voltage, otherwise how would we know that it is 9.1v, or whether it is good or bad, on a D477?

I have found a match for D457 BZX79C4V7 I think; based on the previous one that you kindly found, with a maximum current of 1.3W and it is from the same supplier https://www.ebay.co.uk/itm/10-4v7-Zener-diode-4-7-volt-1-3W-DO41-ROHS-BZX85C4v7/262665831293?_trkparms=aid=222007&algo=SIM.MBE&ao=1&asc=20170920101022&meid=e23bb4d236f746769c01f25649ce6f7e&pid=100010&rk=2&rkt=12&sd=302101187936&itm=262665831293&_trksid=p2047675.c100010.m2109 would you kindly give me your approval, thank you Les.

 

With my observations on my last post; regarding the voltage readings taken across these types of regulators, are we looking for a close to the same reading that matches the governed/regulated voltage, otherwise how would you know that it is 9.1v on D477?

The voltage across a zener diode is not totally independent of the current through it. It has a dynamic resistance so once the voltage reaches the zener voltage it increases slightly as the current is increased. The voltage is specified at a particular current through the diode. There will also be a tolerance on the voltage rating. You need to read the data sheet on the particular diode that you have. So a 9 volt reading for a 9.1 volt zener is close enough but a voltage of 8.8 volts for a 4.7 volt rated zener is too high.
The zener in your link should work but it will be a bit larger physically as it's power rating is 1.3 watts. The original one was 500mW (Half a watt.) You will have to make sure it will fit. By the way Watts is not a unit of current it is a unit of power. So a 4.7 volt zener will handle a current up to 1.3 Watts/4.7 volts = 0.27 amps (270 mA) This is a link to the data sheet on the BZX85 series zeners.
The dimensions are near the end of the data sheet.

Les.

 

The original one was 500mW (Half a watt.) You will have to make sure it will fit. By the way Watts is not a unit of current it is a unit of power.

Thank you for that; I have chosen the 500mW (milliwatt) to match the existing https://www.ebay.co.uk/itm/10-4v7-Zener-diode-BZX55C4V7-4-7-volt-500mW-DO35-ROHS/280404639785?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
I found some further education on the Zener Website; which explains more https://www.electronics-tutorials.ws/diode/diode_7.html
The originals were 79 series; the previous Zener was a 85 series and the one just ordered is a Zener 55 series, the only difference I can see is the power ratings, I assume and hope there is nothing more to be concerned about?

The data sheet below is for the BZX79 series Voltage Regulator Diodes

 

Hi Michael,
The BZX55 series of zeners are 500 mW so they will be about the same physical size as the original BZX79 series so thy should be a good replacement.

Les.

 

D467 looks to be open circuit so will need replacing. This is probably the cause of M1 running far too fast. With its cathode being at -3.39 volts it is biasing IC435 outside it's common mode voltage range (0 to V+ -1.5 volts) This will be preventing tachometer pulses being detected so the speed control loop will think the motor is stopped.

Les.

I've removed D467 and connected to make a diode test, see my image, without explanation.

I have added a image to post #168 and edited the latter part of post #174. I am in progress of fitting the new Zener diode

 

D467 Zener BZX55C4V7 voltage regulator diode fitted; voltage readings across followed Anode side to ground and Cathode side to ground.
-4.73v
A. -11.89v
C. -7.16v
D477 and D476 voltage readings are still the same as in post #170

Main Board U405 there are 2 regulators; I have taken these voltage readings in the same order as usual.
D457 BZX79C9V1
Across -8.81v
A. +8.3mv
C. +8.8v

D458 BZX79C5V1
Across -4.7v
A. -4.69v
C. +5.3mv
Both diodes voltages readings across are under the expected value and I think the polarities aren't correct, this is a concern ?

Play now works very well; a big thank you Les.

I am getting interference noise; whilst playing a cassette it sounds something like crackle/popping in the background, like when you turn/pull out the headphones from the socket, this isn’t a continuous noise, it just happens once every few seconds, is this a suppressor problem?

 

The rewinding issue is resolved; http://forum.mfbfreaks.com/viewtopic.php?f=5&t=8426&p=244255#p244255

 

Hi Michael,
Sorry for the delay in replying to to posts after February 27th. I have been on holiday. Re Post #177. Although the voltage reading on D457 & D458 on U405 are a bit lower than expected I don't think this will be the cause of the crackle/popping noise. I can't think of a way to trace the noise problem without an oscilloscope. Does the noise come on both left and right channels ? If it is only one channel then it is more likely to be in the audio path. If it is on both channels it is likely to be on one of the power supply rails. Does your recorder have the option of monitoring the input signal on the headphone output when recording ? You could try playing back with the playback pre-amplifier board U409 removed. You will not hear the recording but if the noise still occurs it must be from latter in the audio path.

Les.

 

Re Post #177. Although the voltage reading on D457 & D458 on U405 are a bit lower than expected I don't think this will be the cause of the crackle/popping noise.
Les.

Zener Single Diodes voltage regulators

Have now replaced all Zener voltage regulators on this unit the readings are set out in the usual way below, first reading across, second readings to ground, anode (A) side first followed with cathode.The readings before can be seen by looking back at post #170 and 177.

Main Board U405:

D457 BZX79C9V1
Across -9.01v
A. +8.2mv
C. +9.02v

D458 BZX79C5V1
Across -4.84v (Note this reading; it is under?)
A. -4.83v
C. +5.4mv

Motor Control Board U302:

D467-C4v7
Across -4.73v
A -11.89v
C -7.16v

D477-C9v1
Across -9.0v
A -11.89v
C -2.91v


D476-C7v5
Across -7.52v
A -11.0v
C -3.54v

Philips Service Manual; gives reference points for voltage readings shown on the Main board U405 drawing and the schematic diagram (bottom right). Note some of these references are quite precise to the point of a volt, giving the impression that they are important!

Some of my voltage reading are shown in the attached file; Philips N 5748 Transistors 24April19, the rest are shown below, readings in red are the reference voltage followed with the actual voltage in black.

B -12v -11.98v (Under)

B2 -5v -4.83v (Under)

A1 +15v +15.15v

A2 +12v +12.04v

A3 +12v +12.15v

A4 +14.5v +14.63v (Over)

A5 +11.5v +12.06v (Over)

A6 +9v +9.03v via D457/R596 diagram (via R589 +8.42 shown on the drawing. Under)

Cr -4v (+15v) +15.13v (via C796/R600)

Recording Amplifier Board U406 Power Supply

Pin 2 +15v +15.13v
Pin 7 -12v -11.96v

Does the noise come on both left and right channels ? If it is only one channel then it is more likely to be in the audio path. If it is on both channels it is likely to be on one of the power supply rails. Does your recorder have the option of monitoring the input signal on the headphone output when recording ? You could try playing back with the playback pre-amplifier board U409 removed. You will not hear the recording but if the noise still occurs it must be from latter in the audio path.
Les.

Interference Noise I removed the playback pre-amplifier board U409 and the noise had stopped, although you said “You will not hear the recording but if the noise still occurs it must be from latter in the audio path” I think you meant this board?

As I am intending to renew all the transistors in this unit I replaced the ones in this board next, yes, the noise has almost gone or has gone; I will be able to listen better, when it is playing through the speakers and with different cassettes, which will be when we are satisfied with the voltages.

I have been keeping a record of the reference voltage and hFE (current gain or amplification factor of the transistor) readings, look at the low readings on U409) they are shown in the file called Philips N 5748 Transistors 24April19 these have improved after renewing each board of transistors, all the transistors have been replaced now. The two photos show transistor BC337-16 which should give 100 hFE min.

I am still concerned that some of these voltages need to be corrected and I am wondering if I have damages a transistor on a board that I had already renewed before I caused these faults?