Erase problem on 24 track reel to reel

[thebunker]

Ok, well I have the cards loaded in the machine while testing and am following the procedure you describe spamunkulus.

The DC voltage across R138 is lower on the suspect cards than those erasing properly. Around 0.6v on a bad card and between 1.8-2.5 v on a good card.

I can still tune the resonance on C83, but AC voltage on there is around half that of a good card. Waveform at collector of TS43/TS47 is as you suggest, almost square.

If we are looking at a faulty driver circuit which components should I test next, or would you suspect are at fault?

[spamunkulus]

erase driver TR532

Next is to check the supply current, which draws the circuit around TS43/47.
Do the following:

Check the voltage drop across R138.
Unfortunately I don't have my notes with me, but
you can compare it with a working card.

If the current is too low, check if the voltage across D52 reaches 4.3 V.

TS46 forms a constant current source.

The voltage at C76 is interesting as well.
compare that as well.

[thebunker]

Two of the cards have lower DC voltage across R138 (0.6V-0.8V) than a healthy card (1.75V-2.5V).

They both read lower DC voltage across C76 too (7.8V-10.7V) compared to healthy cards around (12V-14V)

I also have a card which reads only 2.02 volts across D52, the rest are all 3.78-4.19 volts DC.

Could C76 be leaky? It looks like a polyester type cap, which I was under the impression were unlikely to go bad.

[spamunkulus]

LYREC AM77 card erase driver

Your measurements confuse me a bit.

Your healty cards have 12 to 14 Volts at the collector of TS46.
This is way too much. Supply Voltage is 15 Volts. So I am wondering
how you get there a reading of 14 Volts considering the drop of 3 Volts across
R138 and P3. Is the supply voltage correct ?(15Volts).

Maybe you meter has a low battery.

Normally the readings are like this(RECORD MODE)

Erase voltage at the Head: 170Vpp , erase current test point 150mVpp

voltage across C76 6.5 to 8.5 V (= Collector of TS46)
Current draw: 1.35 to 1.45 V across R138
Erase Pot is set at 1 o'clock(30Ω)

Voltage at the zener D52 has to be around 4 V. your reading is lower.
This is an indication that something is wrong there.
Is the voltage at TS48 (Collector emitter) almost zero?

Yes...go on

Is the Voltage of the Z-Diode increasing if you turn down the erase current?
(yes = beta of TS 46 might be too low)

Is the voltage of the base to emitter of TS46 around 1.25 V(Darlington)?
No = TS46 is defective,
yes= C82 might have a short or zener might be bad
If C82 is defective you might see a short ramp instead of a slow one of the erase current.
You can pull this point down with another 1..4,7 k Resistor and check if anything changes.
(parallel R142/TS48).
Before I stress the board with a soldering iron, I double check
and try to find a way of sorting out bad and good components.


The erase current should not be at max, The head might get too warm and you record a nasty bump when dropping in.
I record a 1 KHz tone and raise the erase current until the tone disappears in the tape hiss.
This point is around 150mVpp at the eras test point.

cheers, spamunkulus

[spamunkulus]

AM77 component layout

A bigger scan of the AM77 Card layout:

[spamunkulus]

Better concentrate on the card which does not erase.
Check again if it can be tunes correctly.
some pictures of the scope (PDF file):

LYREC-TR532-erase1.pdf

[thebunker]

Thanks for the informative reply. Up until now I've been tuning the erase resonance with an analogue meter as the manual suggests. I'm finding it much easier with a scope though.

As for the other tests:

Erase voltage at the Head: 170Vpp , erase current test point 150mVpp

I can get 170Vpp on the scope at C87. The test point reads 20vpp!!!??? both are sine wave

voltage across C76 6.5 to 8.5 V (= Collector of TS46)

With the erase current at a better level I read 6.43v @ C76

Current draw: 1.35 to 1.45 V across R138

1.23V @ R138

Erase Pot is set at 1 o'clock(30Ω)

Yes

Voltage at the zener D52 has to be around 4 V. your reading is lower.
This is an indication that something is wrong there.

Now reads 4.17V

Is the voltage at TS48 (Collector emitter) almost zero?

Yes, 0.08V

Is the Voltage of the Z-Diode increasing if you turn down the erase current?
(yes = beta of TS 46 might be too low)

No

Is the voltage of the base to emitter of TS46 around 1.25 V(Darlington)?

Yes

I've added photos of the waveforms I get at to compare with the positions on the circuit you have scoped.

[spamunkulus]

20Vpp at the test point!

seems that the 1 Ohm resistor R143 or it's soldering joints are bad.
remove the card from the recorder(to disconnect the erase head and
measure the resistance at the test jack, should be 1 Ohm.


Your waveforms look well so far.

[spamunkulus]

If the 1 Ohm resistor is blown, you can short it temporarily.
No more measuring at the test point though.
4,7 or 10 ohms may work for a test if you have those handy.

[spamunkulus]

sorry, wrong labeling

Your waveforms look well so far.[/QUOTE]

I have made a mistake in my pictures (PDF)


It should be labeled anode of D53/primary of the transformer

instead of Cathode of D53 (this would be the same as collector of the Transistor 47).
Interesting are waveforms on both pins of D53.

[thebunker]

No problem.

R143 is indeed open circuit.

Anode of D53 looks like this

[spamunkulus]

Oh, a very rare fault of the Am77 card.
Did this solve the erase problem?

[thebunker]

It may be a rare fault in the rest of the world, but I have found 4 cards with R143 open! (it's an ex BBC machine, with which I suspect they have been robbing good cards to replace faulty ones in other machines)

All have now been replaced and work well apart from 1 card which I suspect the zener diode D52 to be faulty. The reading across it varies when I adjust the erase current & reads between 1.85-3.33V DC. I have already replaced C82.

The replacement I am looking at is a 4v3 0.11A type. Would that be correct?

Hoping to pick it up and fit it tomorrow.

[spamunkulus]

D52, TS46?

When the voltage of the Zener varies with the erase current,
it is much likely that TS46 is bad.

If TS 46 (Darlington) has lost it's high ß(hfe) of 3000 , maybe if
the collector of the driver has disconnected internally, Vbe is still around 1.5V but hfe will drop to about 70.
The high base current is than too much load for the circuit around D52.

If you have 330 Ohms handy, hold this from the zenerD52 to ground. Voltage across D52 should now raise to 4.3 V.
Or the following test.
Insulate the Base of TS46 (desoldering pump and then adding some thin soldering resin for insulation).
Voltage across D52 should be normal.
In this case replace TS46 . I assume that you checked , that TS48 is switching to ground correctly.

defectice R143, very uncommon. Maybe this resistor ages and I was servicing these machines 20 years ago
and never saw that.
Don't forget to put a tiny drop of oil to the topmost bearing of the capstan,
as written in the manual.

[thebunker]

Insulate the Base of TS46 (desoldering pump and then adding some thin soldering resin for insulation).
Voltage across D52 should be normal.
In this case replace TS46 . I assume that you checked , that TS48 is switching to ground correctly.

You're correct. D52 does show 4.3V with TS46 base disconnected!

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!