Philips reel to reel

[Copey84]

Hi sue, took the measurements again this time from the chassis instead of the negative battery terminal.
Tr1 B= -0.83 C= -4.59 E= -0.95
2 -0.99 -4.28 -1.09
3 -0.83 -5.17 -0.34
4 -3.19 -7.46 -3.32
5 -0.23 -8.64 -0.11
6 -0.23 -8.63 -0.11
From the wiring diagram the collector currents on T5+6 are 8.9v, so very close to my measured values.
Does this mean the fault is somewhere after T5+6?

[suebutcher]

T1's emitter voltage is slightly high, but that's probably OK. T3's emitter voltage is much too low. Check the resistors and capacitor around this transistor. Possibly the emitter capacitor is leaky.

If this doesn't fix it, I'd guess the fault is in one of the switches in the amplification chain. Just a guess, though. I've had trouble with complicated tape recorder switches before. Sometimes it's just the levers that move the switch not being engaged. This can happen when a machine is taken apart and put back together.

It sounds like the speaker is OK. If you needed to test it, you'd disconnect it from the output transformer, and momentarily touch a 1.5V battery across the speaker terminals. You should hear a loud click.

If you had a signal generator (basic ones are cheap and simple to make), you could confirm that the output stage (T5, T6) is working and move backwards from there to find where the break in the signal chain is.

[TonyDuell]

Maplin sell the Velleman 'signal generator' kit as order code VX94C. It's fixed 1kHz, and the waveforms are not exactly pure (it all starts from a square wave and tries to shape it), but it will do for tests like this (I use one a lot rather than getting out a better signal generator) and is easy to put together.

Yes, you could do something cheaper. Or better. But this works and is not difficult to get.

[Copey84]

Hi sue, should I check resistors 19,20 and 22 from emitter on transistor 3, along with caps c14 and c30.
Since i think the two germanium transistors 5+6 are outputting the correct voltage on collector should I be able to measure a voltage on the secondary side of transformer with multimeter?
Will have a look for signal generator, even if caps and resistors solve problem be useful to have. Seen on eBay before in kit form and will check maplin to.

[suebutcher]

Those are the parts I'd check first, plus the collector resistor.

No, you won't get a DC voltage through a transformer, they don't work that way, and strictly speaking the collector DC voltages on T5 and T6 aren't an output signal, just the average "at rest" voltage at that part of the amplifier. You could measure an AC voltage at the transformer output, but only if you had an AC test signal applied somewhere back along the signal chain.

[Copey84]

Hi all, just got back to working on recorder last night.
Have found a mistake I made identifying the the legs of transistors had effected the readings i took last week, apologies. Here are the correct results
T1 C=4.64 E=0.90
T2 C=4.48 E=1.00
T3 C=3.41 E=1.25
T4 C=7.48 E=3.22
Are these values acceptable? Seem ok to me, not much difference between them and values on wiring diagram.
When I had the recorder in play mode last night it sounded very close to playing music then it faded away and was replaced by noise that sounded more like a ww2 bomber plane.
Ive checked all the switches in play record part of circuit and they seem fine, so I'm now back to thinking the problem is with the germanium transistors, or leaky caps
Tested T5+6 E to C=949 and C to E=143. Although they are of same value shouldn't they be higher?
As always appreciate replys.

[Copey84]

Decided to disconnect one of the germanium transistors to see if it is working correctly.
Going by results it seems ok
B to E=213ohms, open circuit reversed
B to C=212ohms, open circuit reversed
C to E=open circuit, same reversed
The meter was set to diode test mode.
Back to square one and out of ideas, apart from testing caps which i think can only be tested out of circuit
Don't want to desolder components in less I really have to, traces come away from board easily, could end up with more repairs.
If I get a signal generator how would I use it to test circuit , would it definitely find fault?

[suebutcher]

The voltages are near enough, I'd say. You might be able to test some of the capacitors in place. If not, just unsolder one end and lift the cap up slightly for testing.

The WW2 bomber sound ("motorboating") is often caused by problems in the power supply and the decoupling capacitors. It suggests that the amplifier chain is working, but there's feedback from the output stage to the input stage via the power supply. Test R21, 29, and C3,19, and 26.

[TonyDuell]

With reference to the last line of post 47 :

No piece of test equipment will 'definitely find the fault'. I often have to say this when I am asked about repairing a classic computer (my main field).

What fault finding involves, or at least what it should involve (all too often people don't do this and it causes a lot of problems later) is comparing what the item should be doing with what it actually is doing, and then working out what has failed to cause the difference.

The circuit diagram and service manual, read with an experienced eye, will tell you what it should be doing. It is, for example, possible to work out what voltages to expect at any point in the circuit (if they are not given), it's possible to work out what signals to expect if a given input signal is provided, and so on.

Test gear lets you measure said voltages (DMM), view the signals (oscilloscope) and provide a known input signal (signal generator).

And then when you find something where theory (service manual) and practice (test gear) do not agree, you have to work out what could cause the difference.

[ricard]

Originally Posted by Copey84

B to E=213ohms, open circuit reversed
B to C=212ohms, open circuit reversed
C to E=open circuit, same reversed
The meter was set to diode test mode.

If the meter was set to diode test then the readings are likely in millivolts, not ohms, not that it makes much of a difference in this particular case. Still, 0.2V = 200 mV is a likely value for the forward voltage drop of a germanium diode, so the 212/213 mV reading indicates that the integral diodes (and thus probably the whole transistor) are functioning properly.

[Copey84]

Thanks for replys.
Carried out tests on components that sue had suggested.
R21 =654
R29 =138
C3 = off scale
C19=198micro farad's
C26=off scale
All components where in circuit while testing.
Resistors are lower than stated values, think they are ok though.
Dmm only good for measuring up to 200micro farad, C3 and C19 seem ok. Measured the resistance of larger cap, did seem low at 560ohms, could it be down to parallel paths?

Powered up recorder and found that distortion happens when wire from tape head is moved.
Have checked connections and wire back to PCB and all ok. Signals all seem fine at transistors. I have the head connected to wire going to PCB in screw connector temporarily since I made repair on head, didn't solder incase I needed to disconnect. It's a sound connection but could it have an effect on signal? probably not but thought I'd mention.

Also does anyone know what the load current of recorder is. When in play I measured the current at 185ma, lot of distortion though. When quiet and in play it was pulling 45ma

[Welsh Anorak]

Hi
I'm a bit unhappy about the state of a couple of those electrolytics. The only real test on these is out of circuit with an ESR meter. Howvere for the cost I'd replace the lot with decent new parts - axial parts can be found if you're keen on things looking original. The current drain sounds OK - it's a bit chicken and egg with this sort of fault. Perhaps you could try injecting an audio signal at the volume control to see if the output stage can definitely be ruled out.
Glyn

[Copey84]

Hi Glyn, disconnected the negative lead of 400 micro farad cap, and using dmm in capacitance got a value of 198 micro farad's. Although meter only goes up to 200micro farad's I thought it should have read of scale. Will probably order up three new caps as I have no other way of testing, also have no way of injecting signal for test.

Also noticed speaker is making noise even when play button is open. Noise levels increase when wire from head is moved. Current is flowing when recorder is supposed to be off, if it's not a faulty switch then would it most likely be a capacitor?
There are mainly electrolytic caps in circuit along with four tantalum. Tried measuring tantalum cap C33 but no reading, have read that they can be very sensitive to spikes and reverse voltages, as it's an old recorder are they likely to have failed?

[Copey84]

Hello again, don't think fault is with caps. Replaced 400mf and 200mf with paralleled equivalent values and no change in circuit.

Think the only option left is to use a signal generator. Have all components to build my own using wiring diagram from maplins website. Only difference is transistors, could a bc5468b be substituted for a bc5468 and still output correctly.

Would also appreciate any advice on what signal is best to use and where in circuit to start.

[Copey84]

Correction to last post, its a bc547b transistor that is used in wiring diagram on maplin website.
I've only got a bc547 and was wanting to know can I use it instead.

[TonyDuell]

I would be VERY surprised if the plain BC547 wouldn't work there.

[Copey84]

Thanks Tony, going to breadboard circuit when I get chance, will check the signals on a oscilloscope first.
Could you advise where best to start fault finding, and what signal to use?
Apologies for ameteur questions, not confident when it comes to fault finding this way.

[TonyDuell]

As you have a 'scope you can answer the question about using the BC547. If you get a good signal out from the signal generator on the 'scope then the transistor is fine.

Use the sine wave (actually a rounded off square wave, not very pure, but it will do) setting.

A good place to inject a signal (in playback mode) is the top (non-earthed) end of the volume control. It's easy to find and about half way along the amplfiier. If that signal gets through to the speaker (put the 'scope across the speaker connections) then the second half off the amplifier is fine, if not then that's where the trouble is.

[Copey84]

From what you said Tony I will inject the signal on the variable side of the pot that controls the volume, while in playback.

If no signal appears at speaker should I move to the base of T4 then T5 and T6, moving closer to speaker until signal appears.

Just to be clear is it ok to apply signal through all parts of the circuit without any risk of damage to transistors?
I assume signal is so small that resistors and capacitors are ok.

Probably silly questions but thought I'd ask just to be sure

[TonyDuell]

Yes. That little signal generator doesn't produce much voltage, and it has a high output impedance so it can't supply much current. This is actually a good thing here, you won't do any damage if you connect it to just about any point in the amplifier.