Erase problem on 24 track reel to reel

[spamunkulus]

Ts48

"Ts48 shows around 0.08V between collector / emitter when record is engaged. Is that what you meant by switching to ground?

Thanks for all your help, by the way!" [/QUOTE]

Thanks, You are welcome!
TS48: yes, that's what I mean. Looks good.
It seems that you have to replace the darlington TIP125.
The danish transistor symbol in the schematic does not show a darlington,
maybe over the years a wrong type found it's way onto the AM77 card.

best, spamunkulus

[thebunker]

Well, I eventually got TS48 replaced, it was a darlington type that was fitted & it looked original, but unfortunately the new component has not improved the cards performance. The voltage reading across the zener diode D52 is still only around 2.5v-3v and varying when the erase current pot is adjusted.

So, I have spent a little time scoping round the board and have found that the waveform at the anode of D53 looks like
this



the voltage peak to peak is reading somewhere around 150v! whereas the pic below is from a good card is like this reading around 30v



would the suspect component be TS47? Is there any other testing I can try?

[spamunkulus]

uhh, maybe the erase current is way too high.

at 150 V the core of the might run into saturation already.

Please reduce the current and measure the other voltages,
erase head voltage, erase head current and voltage at center tap and collector TS47

Please use a sweep speed of 5 or 2 µs , the picture of the scope is better readable then.

[thebunker]

I have had a look at the voltage at the erase head & it measures 80volts peak to peak. Adjusting the erase current pot has very little effect on the voltage around +/-10v peak to peak! Erase resonance is tuned correctly.

However, as I mentioned in a previous post the DC voltage across zener diode D52 does shift when the erase current pot is turned. Fully anti-clockwise shows around 3.8v. Fully clockwise around 1.8v.

P3 itself sweeps very smoothly from 0-100 ohms

I would forget the waveforms I posted last night. They have errors in the way I have logged them.



the above are at 20v/5us



this is at 5us/500mV

[spamunkulus]

Uhh,so exchanging TS46 was a mistake.

It looks as if the circuit does not work symmetrically.
strange that the signal at collector of TS47 looks ok.
If Ts43 fails, the signal at TS47 looks bad as well normally.

How does the signal at collector of TS43 look like?
Check signals at the base of Ts43 and 47)

Short both diodes D49 and D51 .
Then go into record and check the voltage at the collectors of TS43, 47
D50, D53 and as well at collector of TS46.
Voltage has to raise considerably. (12 ..14V DC) .

[spamunkulus]

check as well if both ends of the Transformer are connected to the diodes
D50 and D53. Resistance from anode to anode less than 1 Ohm.

[thebunker]

TS43 VDC=14.07
TS47 VDC=14.07
TS46 VDC=14.40 with diodes D49 & D51 removed

I measured the base voltages before removal as well:

TS43 Base VDC= 10.17
TS47 Base VDC = 0.16

Resistance between anodes of D50 & D53 is around 4 ohms !

[spamunkulus]

Seems that TS43 is dead. Base voltage with signal present should be limited at 800mV.
and the DC voltage of 10V is not a good sign either. Is the trace of the emitter ok?
e.g. is the emitter on Ground level - 0Volt ?

(By the way, I think you shorted the Diodes D49 and 591, not removed them
just short them with a clip)

4 Ohms of the Transformer primary is weird, but maybe there are versions with thinner wire
or the leads of your Meter have somewhat higher resistance as usual.
In diode test mode you might test the base/emmitter and Base/Collector
junctions before you touch the board with a soldering iron.

TS47 seems still to work but had certainly a hard time withstanding 150V peaks.
I would exchange both.

[thebunker]

Ok, the base/emitter junction on both TS43 & TS47 seem to be short circuit. I compared the diode test with the same components on a good board.

So, I guess I'll just order them today.

I have removed both D49 & D51 and replaced with a wire link. Could I have shorted them without doing that?

Thanks again for your help. Much appreciated

[spamunkulus]

TS47 is certainly ok (your picture with the scope) so there must be another reason for
the result with the diode tester.
I assume you did the test with the unplugged card.

The base emitter junction of TS43 can't have a short, cause you read10V Dc at the base.
Your results are unfortunately a bit confusing.

When I do repairs, I try to avoid touching the board with a soldering iron too much.
shorting a diode can be easily done with little testclips.
also I use the trick to add a resistor to the circuit just by hand and watch the
result of the now changed "wrong" circuit.

[thebunker]

Sorry for the confusion!

Having removed the diodes D51 & D49 I'm shorting via earth when testing between base to emitter junction.

I'll re-test with the diodes back in the circuit and let you know the result.

Guess that's another good reason for not soldering too soon!

[thebunker]

Yes, the wire links replaced D51/49 with were reading as a short on the diode check. I have soldered both diodes back into circuit.

TS43 checks out as normal ( compared to another boards TS43 ) across Emitter/Base & Base/Collector terminals.

So, I checked the results from the Emitter/Collector junction. It is open when reading both ways on the suspect TS43. The other cards and TS47 appear to read open only with +ve lead on the emitter -ve on collector. If i swap the test leads round I get a reading with the diode test on a good transistor.

Should I suspect TS43 has broken down?

[spamunkulus]

Yes, TS43 can be defective.

Maybe the internal emitter connection is open, as your voltage from the reply 30june is not
clamped to 700mV by the base/emitter diode. But check the trace and soldering point of the emitter.
It can be that a bad connection there is the reason as well.

[thebunker]

Well, it seems lady luck has been on my side here.

I put the card back into the machine to trace the path of the HF input a couple of days ago & when I got to tracing the base terminals on TS43/TS47 they were both the same! So, next I checked the DC voltage over zener diode D52. It was reading 4.4v. I altered the erase current pot and it stayed steady as a rock. I have now been able to set the erase current on the card as normal & it is erasing tracks perfectly. I can only think there may have been a dried out solder joint around D51 which has been cured when I re-soldered it to the board.

Spamunkulus, many, many thanks for sticking through this with me all the way!

I have learned much from you & still have much to learn

[spamunkulus]

That's nice that you got everything working now.
I think it was a bad solder joint at the emitter of TS43.

You used a card extension to work comfortably?
Cause I had to do a lot with these cards I built a jig where
I can test everything on the workbench, which is even more convenient.

cheers, spamunkulus