Sequerra model 1 FM tuner

Buonasera,

mi è stato dato da riparare  il sintonizzatore Sequerra Model 1 FM tuner.

L'apparecchio è ora funzionante, per fortuna senza grosse difficoltà vista la complessità circuitale, ad eccezione dell'indicatore a display di sintonia (counter).

Chiedo se qualcuno può avere lo schema elettrico del solo modulo counter non presente in Radiomusuem.

Se necessario posso fare alcune foto ed allegarle.

Saluti a tutti

Enrico Puccini

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Buona sera,

lo schema per il contatore di frequenza è ora disponibile per il download, per l'alimentatore troppo. Vorrei anche preparare i documenti ancora mancanti, come matrice di 4 canali, spazzata panoramico, limitatore panoramica e la scheda oscilloscopio. Purtroppo, questi sono solo copie in condizioni mediocri che possiedo, ma schemi migliori per il ricevitore Sequerra Model 1 non sono ottenibili.

Cordiali saluti
Bernhard Nagel

Good evening,
the schematic for the frequency counter is now available for download, for the power supply unit too. I will also prepare the still missing documents, like 4-channel matrix, panoramic sweep, panoramic limiter and the oscilloscope board for upload. Unfortunately, these are only copies in mediocre quality I own, but better schematics for the Sequerra Model 1 are not obtainable.

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Buonasera, Bernhard

ringrazio infinitamente per aver reso disponibile lo schema del counter per me indispensabile per riportare il sintonizzatore al suo completo funzionamento.

Lo schema contiene inoltre ulteriori informazioni utili per la ricerca del guasto e la risoluzione del problema.

La difficoltà sarà nel recuperare eventuali componenti obsoleti.

Prossimamente inizierò l'indagine.

Ringrazio e saluto

Enrico Puccini

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Hello Bernhard

I am actually working on a Sequerra FM1 tuner that has problems with the frequency counter.

The tuner works perfectly but the frequency display is fixed on a single value. As the led display works, I am inclined to believe the trouble is in the freq counter itself.

Please let me know if you have any experience with these counters. Allready studying the schematics, but due to it's complexity I welcome all help I can get.

Best regards

Ricardo Cruz

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Hello Ricardo,

first, welcome to RMorg's forum!

Indeed, the frequency counter of the Sequerras seems to be unreliable. My tuner have had the same problem, the counter "stucks" to a fixed value 169.6 regardless to which frequency the receiver is tuned:

Fig.1 Freezed freqency display due to a non-working counter

At the first time, this happend only after a "warm-up" period (the Sequerra draws a decent amount of 60 watts!), later on the counter started freezed when powered on.

My first attempt to solve this defect was measuring the time base frequency after checking the correct 5 V power supply. This counter has a 1 MHz crystal (the old big case style with 12 mm pin distance). Using a 1/10 dividing probe and a scope doesn't show up any oscillation. So either the TTL I.C. SN7404 (six inverting buffers) or the crystal should have a defect.

Fig. 2 Counter module, cover removed. 1 MHz crystal at the right side.

Next I tried to cool down the crystal using coolant spray which animates some oscillation for a short time. The crystal needs to be replaced!  This can be done without removing the pcb out of its alumnium box. Be careful when unsoldering the leads from the pcb lugs because they are prone to come apart from the epoxy surface. Best is to cut the wires separately near the old crystal and solder the new one to the remaining wire ends. It depends on the working principle of the new crystal (serial or parallel resonance, crystal cut) wether a series capacitor of about 30 pF will be necessary to be inserted in one lead.

Fig. 3 New quartz crystal aligned to 999.992 kHz

Fig.4 hp counter 5345 (1 MΩ input impedance) connected via 1/10 probe to crystal

With my replacement crystal (the only one I had in stock...) this capacitor was inserted for correct frequency reading results. I used first a small variable condenser and aligned to exact 1 MHz. The trimming C was replaced by a small polystyrene film capacitor (33 pF) of the same value.

Fig. 5 Last measurement on the repaired counter unit.

That repair solved the frozen display and brings the counter back to work! Other reasons can also stop the counter from working. If the time base is ok, the incoming signal from the LO buffer amplifier into the counter's preamplifier (MECL) should be traced with an 100 MHz oscilloscope thru the dividing stages. The schematic helps with curve forms/typical patterns to locate the erratic part.

But in most cases the crystal is the delinquent..:) Especially the types with soldered bases seems to be unreliable after a period of some 40 years. The Q of the crystal degrades and if become too low the oscillation will stop.

The better crystals have welded bases and no tin soldered hole at the side. These should last for decades to come...

Bernhard

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Thank you Bernhard

I will test the cristal and post my findings here.

Best

Ricardo

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Finally replaced the 1Mhz crystal and the counter now came to life.

Profited to replace alo the stereo pilot light (got it from an automotive surplus retailer)

This tuner is wondefull and sounds like no other I heard before.

Thank you so much for all the schematics and precious comments.

Regards

Ricrdo

PS: Will post some pics later :)

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My sequerra counter stopped working again.

Maybe I did not use the correct crystal for the repair.

Please let me know what should be the optimum replacemen part.

Best regards

Ricardo

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Finally got it sorted.

Found the ideal quartz oscilator and now it is locked at 1Mhz.

Perfect.

Profited to replace the knob control belt also.

Thank you all for the precious help.

• Counter repair (60 KB)

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Hello Bernhard

I am happy to say the counter issues are now completely solved after replacement of the crystal by a compatible model.

Unfortunately I am now having another problem with this wonderfull machine.

The stereo pilot lits when tunning a stereo station but all emissions are received in mono.

The scope in display tuner vector mode does not spread the image so it only shows a vertical pulsating line.

Hope you can guide me searching for a solution.

Best regards

Ricardo

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