Erase problem on 24 track reel to reel

[thebunker]

Hey folks,

I'm having trouble tracking down a fault with my old Lyrec multitrack. Erase signal is weak at the test point shown on the schematic attached. I have gone through all the test voltages shown on the schematic at the line input section & the sync / repro parts of the circuit. They all read around the values shown.

I have also scoped the HF input at TS 51/ TS 52 on the collector leg. The signal level compares with that of a card which has good erasure level. This level carries all the way up to D50 / D53.

I'm not sure where to go next.

If anyone can give me some help with this it would be great!

Cheers

[soundguruman]

You have to make sure the erase head is aligned. It can be a mechanical problem, not electronics.
You have to make sure your erase bias voltage / frequency is all tweaked up to par.
You have to make sure the tape is wrapped on the head properly, and has correct tension.
Mechanical relays: all have to be closing the contacts, nice and tight...

You have to be using tape, that has only been recorded on ONE side.

So, about half of it, is mechanical. Not electronic.
Check all your mechanical operations.

Is the erase head worn out?
Is your tape lifter dropping the tape to the head, all the way down, or is it stuck?

[thebunker]

Thanks for the reply soundguruman.

Tape travel and the erase head are all good. The problem follows the card if I slot it onto a different track.

It is the erase current, or erase bias voltage measurement that is weak on these particular cards.

It is part of the setup procedure in the recorder manual to check this after having set repro / sync frequency response, record level/eq & bias.

I will check the relays. Anything else on the card that you can think of that might cause this?

[soundguruman]

Clean the contacts where the card plugs in with white a gum eraser.
Remove the old oxide.
Overhaul the circuit on the card. How much to do that, $5?

Parts can test perfectly good.
Then when the power is on, they fail.
It's not common but it happens.

Swap parts from good board to bad board.
eliminate the possible causes.

[km6xz]

Is the record function working correctly? The master oscillator for record amp and erase amp is the same, TS50 and 51. Both phases have to be there or it will be the wrong frequency. TS43 and 47 form a push-pull, transformer coupled amplifier and the secondary has a resonant load. You say the level is weak, how weak? The scope probe on the head will load it since the head is pretty high Z at the frequency of the erase bias. Make sure the collector signals are normal at TS42-47. If they are, the problem is in the secondary. If the collectors do not have supply voltage and the erase signal, the problem is in the control circuit driving TS46.
If the primary looks good, the controlled damper, tank circuit or head in the secondary are the areas to focus on. Can you get erase current at the test jack across R143? If so, can it peak with tuning the tank circuit. If the tank tuning does not make much difference, either of the reactive components( the two caps,one is the trimmer) or the frequency is way off from the master oscillator. The other secondary related possibility is the damper (TS49) is turning on and shorting out the RF signal to ground. That is controlled by the control line on the base of TS50.
Don't worry about alignment or switches, if the signals and voltages described above are not present, you can easily see what is causing it.
NEVER use an eraser on contacts, if there are oxidized, de-oxidize them, if pitted or corroded, replace them. Erasers will ruin signal carrying contracts by scoring them and raising resistance, and greatly accelerating corrosion and oxidation.
Do this logically in just a few measurements to isolate the defect. It is much simpler than random checking and following old wives tales. The erase circuit is very simple and only has a few components that are involved.
Good luck, report back about whether the record function works normally, whether you get amplification in the erase amp (ts43-47) and if the tank tunes to the erase bias frequency.

[thebunker]

Thanks for the reply km6xz,

Is the record function working correctly? One of the cards is recording ok. The other 3 are not.

You say the level is weak, how weak? The erase current (at it's test jack) is as low as 6mV on one card, but interestingly is nearer 6V on another which will not erase completely.

The collector signals on the 4 faulty cards at TS43 / TS47 are all comparable with good cards. Also, the erase current at the test jack does peak when I turn the variable cap C83.

Does this mean I should be looking towards TS49 / TS50 for a fault?

If so, what can I do to test them?

[km6xz]

If the erase tank circuits are tuning and a peak is seen, the problem narrows down to the shunt switch or wiring. Something is loading down the erase RF. Try disconnecting TS50, to show whether it is related to the shunt which turns on to drop erase current in sync with the other tracks when punch out occurs. If TS50 being out of the circuit brings the RF level to normal, I suspect it will, then the problem is either in the control line feeding TS49, which in turn controls TS50. You can try a passive out of circuit test of resistance to ground for TS50, the collector of TS49 but R143 will show a low DC resistance across the shunt triac so the difference between a good card and bad card might be seen in fractions of 1 ohm. Monitor the collector of TS49 when switching in and out of record, you should see it go between high and low when cycling in and out of record. If it does not shift level look at the base of ts49. If no change there when punching in and out, look for the missing control line, where it goes, I do not see on those pages of the schematic, but you probably have the full schematic.

[thebunker]

Thanks for getting back to me again km6xz!

I disconnected TS50 and it did recover the erase signal and bring it up closer to the level of a good card at the erase curr. test point.

Next I checked the signal at M2 of TS49. When punching record in and out. The signal went high and low accordingly on good cards. The faulty cards showed a positive DC voltage here when not recording & were dead when record was engaged.

Then I checked the voltage from the emitter of TS50 onto the gate of the TS49. The DC voltage read the same on good cards and bad cards in record and when idle.

Is the base terminal of the triac TS49 the one connected to earth, or did you mean the base of TS50?

Voltages onto the base of TS50 were also the same across the good and bad cards punching in and out of record. The control signal for this part of the circuit originates at TS31 which is located on the 1st page of the schematic attached to my first post.

I would have thought this might rule out a problem with the control signal. It seems to be constant on all cards.

Does this mean that it is TS49 that is causing the problem? If so, should repair be as simple as just replacing it?

[km6xz]

I do not have the print in front of me but if I remember correctly, TS49 is a triac so the gate is pin two. TS50 is small PNP transistor. The base should be pin 2, collector should be to ground and pin 1 is the emitter. If the transistor flat side is facing you, the emitter should be on the left side, with base in the middle. If your control voltage is switching at the emitter, pin 1 when punching in or out, and it is connected directly to the Triac's Gate, my assumption is the triac is not working.

[thebunker]

Excellent, I'll order the components today & let you know how it goes. Thanks again!

[spamunkulus]

Did you solve the erase problem?
I attach a better scan of the LYREC AM77 channel card

Best, spamunkulus

[thebunker]

Thanks for that spamunkulus,

I've replaced the triacs and all the cards have a good erase current apart from 1 of them. I've still to investigate further, but am recording with the machine ok and happy it's working to a degree.

Anyone got any thoughts on what else might be at fault in the erase circuit other than the triacs and variable capacitor?

[spamunkulus]

Did you check the erase voltage at the high side? (Triac,Head)
This point is easily reachable even without extension card.
the lower pin of the variable capacitor .

I measure 170Vpp (10:1 Probe)

Erase test point: 150 mV pp

sine wave, no visible distortion.

[km6xz]

Current is not going to tell you anything is the load is not present, as in the case of an open head, but the voltage across the head or across the connector for the wires going to the erase head would be very high, if you are using a really hi-z probe.

[spamunkulus]

Missunderstanding:
Of cause I am doing the measurements in the complete circuit, so a hi-Z Probe is an advantage.
Result: I know if the relation of erase-current and voltage across the erase head is ok,
Next I will check the current to the driver (across R138) , which gives you some
information if the output driver is working efficient.
Resonance must be tuned correctly.
Checking the Voltage at the collector TS43 and TS47 is next, then.
looks more like a square, 22...26V pp
If it does not look like a square, D50 or D53 might be defective.