Grundig "monomat" t608

[Jimmyhaflinger]

Hi,

I've been given a hybrid Grundig T608 B/W set from 1965, fitted with the "Monomat" tuner and a "bonded shield" 23" CRT.
This set came straight from the living room of an old lady, even though the set quit working several years ago, she continued to shine it and keep doilies and photo frames on top of it until she passed away a couple of weeks ago.

The set came with a disintegrated mains fuse, i tried to power it up with a series bulb and the bulb lit up brightly, i couldn't find any obvious short, but it must be badly shorted somewhere, the half wave silicon rectifier tests fine and the smoothing cans don't appear to be shorted when tested with a DMM but maybe they may short only when full HT is applied to them?

Then, the Monomat tuner seems to be seized, and i don't even know how it's intricate mechanism is supposed to work, anyone has some experience with this kind of tuner?

Also, the CRT has developed a small air pocket in the bonding material near the lower left corner of the screen, is this normal or maybe the CRT is going to separate from its safety lens?

[Steve_P]

The switch itself is faulty? The Flex cable? If the diodes are OK, then try disconnecting various parts of the set until it stops blowing fuses.

If the power supply is OK, I would first go to the line output stage. If the valves are in series, pull one out and see if it still blows fuses. If in parallel, try pulling each one in turn.

As for the CRT and Tuner, pictures are worth a thousand words.

Cheers,

Steve P.

[Jimmyhaflinger]

The fuse is on the chassis, after the mains switch, so the switch itself can't be the problem.
One side of the mains goes straight to the rectifier bridge, the other side goes straight to the metal chassis frame, then the HT from the diode goes to the first section of a smoothing can, then to a chain of individual dropper resistors.

I'll try to disconnect the cans and the line output valves etc. i'll let you know. and i'll post some pics as soon as possible.

[Jimmyhaflinger]

Here are some pics.

The third pic shows the Monomat preset tuner mechanism

The air pocket in the CRT's armor layer is virtually impossible to capture with my camera thoug, it's actually hard to see it even in full daylight.
Since it's hard to see it's not a cosmetic issue but i'm afraid it could lead to the CRT falling apart.

[Tazman1966]

Looks like quite an advanced design for it's time. Nice to see that the cabinet's been looked after so well.

As far as the CRT's concerned, I wouldn't worry too much although it may detract from the picture quality.

Good luck.

[Jimmyhaflinger]

The air pocket doesn't detract from picture quality or cosmetic appearance, in fact it's only visible when looking at it at an angle under good light.

[ENGLISH VICTOR]

If the silicon diode was O/K and the fault was after the diode the test lamp would only light at half brightness. Is there a small ceramic or polycarbonate cap across the diode or from the mains side of the diode to chassis? Try removing the diode, they can measure O/K on test with a meter but break down at mains voltage. I think it rather unlikely though in this case because it has already tolerated a couple of massive surges. That normally make them go S/C.

Just a thought, how can the mains go to chassis if a bridge rectifier is employed. Both sides of the mains should go to the bridge and one side of the bridge should go to chassis and the other side of the bridge should go to the first electro (reservoir cap), unless it is a very unusual arrangement that derives a negative and positive supply with respect to chassis. If it is a bridge there are four diodes to consider even if they are in one four lead package, check each one.
Regards Victor.

[Jimmyhaflinger]

ow, well, i wrongfully wrote "rectifier bridge" but instead it's actually a half wave rectifier made out of a single diode.

There were two small capacitors connected in parallel straight across the mains input, since they were of the horrible green wima type i replaced them with modern X-rated caps, but the short is still there.

[ENGLISH VICTOR]

Fine about the half wave rectifier, I initially presumed that to be the case, thus my comment re the brightness of the test lamp. It is really most strange to find the mains going straight to a silicon rectifier and cathode of the rectifier going directly to the reservoir cap. that arrangement provides zero surge or inrush limiting. Is that the way the circuit shows it?
Perhaps there is a hidden thermistor or low value WW resistor lurking somewhere before the rectifier or the reservoir cap. If not there should be some current limiting device to protect the switch, rectifier and reservoir cap. It would be nice to see the relevant part of the circuit i.e the power supply. Of course I have neglected the current consumed by the valve heaters so the lamp will not glow at half brightness but certainly less than full when using a 100 or better 150W lamp.
Victor.

[Jimmyhaflinger]

The only component between the mains input and the rectifier diode is a small coil, with a few turns of thick wire over a plastic former with no core, there's absolutely nothing after the rectifier, it just goes straight into the first section of the smoothing can.

That's probably why they sold millions of "saturated core stabilizers" boxes back in the 60s to plug TVs into.

[ENGLISH VICTOR]

A real mystery, I look forward to reading about the cause of the trouble. Good luck.
Victor

[Studio263]

Replace / remove the mains filter capacitor, the types used in German sets of this period are not trustworthy. It may measue OK on a meter but change it anyway.

[Jimmyhaflinger]

Well, i'm going to tackle it tomorrow,

Is it OK to just disconnect the first section of the can and put a separate capacitor in its place? i don't have a replacement can for this set.

[Jimmyhaflinger]

i finally replaced the shorted can with one from a later Grundig chassis, with the new can, the set came up on the variac without blowing the fuse and worked for a few seconds before the other smaller can began to boil and drip its electrolyte out,
Sadly i didn't have a replacement for the smaller can so i finally fitted four separate capacitors in its place.

Anyway, the set is now displaying a nice bright picture but the sound drifts out when the signal is strong, it's not just noisy or off frequency, it's completely muted!
Maybe it's a bad Germaniun transistor in the sound IF strip?

[Tazman1966]

Nice to hear of your success. Are there any wax/paper caps in the IF strip - or anywhere else for that matter?

[Jimmyhaflinger]

There are no wax/paper capacitors anywhere in this set, only polyester or mylar types, but i have since found a tiny electrolytic capacitor hidden behind the EHT cage that looked corroded, i think it was in the frame timebase circuit, i replaced it but a new capacitor apparently made no difference (the frame section was already working properly)

There are still many similar capacitors all over the sets so maybe the ones involved in more delicate signal circuits are causing faults and need to be replaced...

[Steve_P]

If it's FM sound where you are, then it is probably demodulated the same way as in a radio. Look for two diodes and a capacitor which is the wrong way round. Change these and give the sound IF a tweak if you need to.

Cheers,

Steve P.

[Jimmyhaflinger]

We have FM sound here,

our current TV system (625 lines, 5,5 mhz FM sound on both UHF and VHF channels) is actually still basically unchanged since 1949 or so.

I'll check the discriminator, i think it's inside the IF module.