| Servicing the QRS |
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| Phone: +49/89/333034 | Fax: +49/89/33079-323 | Email: | Design Gurus | Documentation | Repairs | Sales |
|
| Servicing the QRS |
| ||||||
|---|---|---|---|---|---|---|---|
| Phone: +49/89/333034 | Fax: +49/89/33079-323 | Email: | Design Gurus | Documentation | Repairs | Sales |
|
This manual covers most common power supply faults, which appeared in production tests and service.
Please mind that changes in the power supply are critical for safety. Bad service can cause a total destruction of the whole QRS (it happened!) or fire (it happened!) or death by electrical shock (it did not happen...yet).
It is assumed that the QRS will be repaired by individuals familiar with the service of studio equipment at the component level. Attempts to service the QRS by individuals not so qualified are done at their own risk, and may result in danger for health and goods or in a board or unit that is not repairable even by qualified Quantec technicians.
To prevent this please take a look at the following hints:
Good luck
The power supply transforms the line voltage of 120 V AC ( 95 to 130 V AC) or 240 V AC (190 to 250 V AC) to 5 V DC 8 A max.
The +1OV, -1OV and -15V AD-DA converter supply voltages are produced by local DC-DC converters using the 5V logic supply.
The 5V power supply consists of two blocks. The first block with line transformer T1, full wave rectifier D1 and filtering capacitors C1+C2+C3 supplies an unregulated output of about 33V DC 2.5A max also used as power-on signal in the control processor COP.
To minimize AC leakage current, a grounded Faraday shield is employed between the primary and secondary winding of T1.
Burst distortion from line and other external sources are effectively supressed by varistor Y1, a low radiative toroid line tranformer with shield, the two filtering capacitors C6 and C7 and the Metal enclosure of the QRS.
In the second block the high voltage (33V) from the first block is regulated to a lower value (5V 8Amax) by pulse width modulation of the switch Q1.
This concept of DC-DC conversion is known as three-terminal buck regulator.
The output voltage is compared with a reference voltage and the difference is amplified to drive a pulse width modulator which in turn drives the switch. The energy stored in the inductor is determined by the on-off ratio of the switch. Current flows through the inductor during both halves of the switching cycle, either through Q or D.
The regulator circuit U1 controles Q1 via a fast 4OmA current source and voltage booster configurated by Q2 + R9 and the 5V reference included in U1.
Switching frequency depends on the sawtooth generator formed by R9, C11 and the generator section integrated in U1.
Free-run frequency without the TTL signal 'clock 20k' from the control processor COP lies about 18khz and can be increased to 20khz and synchronized via C10 + R15 + R14 and Q3.
The regulator works stable even with no load and handles a short circuit via its built in dynamic current limiter.
For best stability the pulse-width-modulation regulator U1 senses the 5V output voltage at the motherboard MB.
An additional feature of the QRS power supply is overvoltage protection on the output.
This is particulary important for powering TTL logic and computer component circuits which have an absolute maximum supply voltage of 7V, so that if, due to an internal fault condition, the voltage rises above 7V, most of the circuits will be destroyed.
To prevent this an overvoltage crowbar circuit is used consisting of R1 + R2 + Y2 + Z1 (Y2=thyristor=silicon controlled rectifier=SCR).
If the output voltage for any reason exceeds to the 5.6V breakdown voltage of the zener a current flows through R1 and if this current is producing enough voltage to trigger Y2 (0.6V) the SCR turns on, places a short circuit directly across the 33V supply and blows the line fuse F1 thereby protecting the QRS boards.
F2 is an emergency fuse located inside the QRS and blows in case of customers using a stronger Line fuse F1.
F2 (4A SB) is chosen relative strong to assure F1 blowing first. The ORS is also working properly with F2 = 2.5 A.
Please read the following pages for complete information.
If the customer did not cause additional faults by using many or stronger fuses the easiest way to solve the problem is to add a single hardwire connection to short the ground connectors J4 and J5 between PS and MB.
Use copper wire with more than 0.5mm dia.
For precaution it is recommended to add this single wire to every QRS whether J4 and J5 are burned or not.
If QRS works again after adding the wire please accomplish the following visual checks (12 to 14) to assure safe operation.
If J4 and J5 are not burned or the wire is already added and fuse is still blowing go to 12 and replace J4 to J6 if necessary
Placing the removed parts in sequence on a table makes the reinstallment faster and error free.
All boards are electrostatic sensitive.
Do not loose any parts.
Information of parameter memory can be erased if COP is touched in battery powered CMOS memory section.
Take a close look at the PS board / clean it if nessesary / add C13 or C14 if missing / add C14 from pin 7 of U1 to the pin of R6 which is connected to pin 14 of U1.
During measurements allow 10 seconds for stabilizing because of capacitors have to be charged and discharged.
Please mind: If MB is not connected to the PS via J1 to J9, the smallest load will cause overvoltage at the outputs because of the sensing principle applied.
To adjust minimum output voltage of about 4.5V turn R18 counterclockwise.
Clip a test lead between J6 and an external resistor of 33k or 47k.
Touch the left pin of R18 with the free end of the external resistor to connect it in parallel to R12.
Very slowly turn P18 clockwise and watch the output voltage rising. At about 6.2V (6.0 to 6.4V) the fuse at place F2 should blow.
Switch off and reinstall the 4A fuse at F2.
If fuse did not blow the overvoltage protection circuit is not working.
Do never use this PS to power a complete QRS.
A kickback that increases output voltage to 5.5V max is acceptable. Switch off.
If period time is less than 50us C11 lost capacitance. Replace C11.
If upper voltage is higher than 5V replace Q2 and repeat this test.
If one period has more than one falling edge like this:
add C13 and C14 if missing.
If there is still more than one edge replace Q2 and repeat this test.
If screen picture shows soft corners like this:
replace Q2 and repeat this test.
Now you can start reinstalling reverse to removing.
See points 36 to 17 in this manual.
Do not forget the hardwire connection from J4 of PS to J5 of MB.
Make sure to get good thermal contact between heat sink of Q1 and front panel.
Before reinstalling the cover plates take a last look and check if QRS is working.
Thank you
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