Repairs

HRV maintenance

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I recently moved into a house with an HRV positive ventilation system installed. I personally think these are a bit overpriced but comments from people I know with them are generally positive. Ours didn’t appear to be doing much so I had a look in the roof.

Mine has two units daisy-chained together with a single control panel. When set to 100% (“burnt toast” mode) one fan was spinning very slowly and the other not at all.

Pulling the units apart revealed that they are quite simple in construction but generally well-built. Remove two Pozidrive screws then the base simply clips off to access the filter and once the filter is removed the fan is easily accessible inside. The fan itself is German-made by EBM-Papst and an off the shelf part.

Diagnosis revealed both motor run capacitors were significantly degraded, measuring a fraction of their 2uF rating. One motor had overheated and tripped the thermal cut-out attached to it. This is not self-resetting but can be reset by pressing the little black button on it.

Both motors showed similar winding resistance and were visually fine although the overheated motor had noisy bearings. These are easy to replace and are a standard 608ZZ type. I swapped these with genuine NSK replacements.

I’ve replaced both capacitors with a Kemet C27 series. You’ll need to crimp 4.8mm female spade connectors onto the replacement capacitor – do this before climbing into the roof space!

You’ll need to bring a Pozidrive screwdriver, cable ties and side cutters.

On reassembly I made sure the cables were tied down to prevent them moving and possibly being struck by the fan.

After all the repairs the fans run very well indeed and the unit so far seems to to what it claims.

If your HRV unit is underperforming then I’d suggest checking the motor run capacitors in the first instance. I wonder how many units have been discarded and replaced for the sake of a $10 capacitor?

Vertical collapse on Storage System portable PC

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I recently stumbled across a junked ‘Picollo’ portable PC made by Storage System Inc. This unit had a 66MHz 486 Overdrive processor and 10″ Sony Trinitron SVGA display and no doubt cost a fortune in its time.

Unfortunately the chassis had been partly gutted, missing the outer case and a variety of parts. The 10″ monitor still appeared to be salvageable however so I gave it a try.

After connecting everything as it appeared to go I connected a PC and powered on the display. Sadly the it was completely collapsed vertically, although I could make out enough detail in the thin line that appeared to be sure that it was otherwise functional.

The deflection is handled by the board mounted on top of the CRT. After fixing loads of dry joints around the horizontal output stage I decided to focus efforts around the vertical deflection stage.

This is based around an ST TDA1675A IC. I grabbed the datasheet for the IC and discovered that the circuit in the display was quite similar to the test circuit shown in the data sheet. The scope confirmed that the sync input was working, as well as the ramp generator. Unfortunately there was no output.

Google searching for the IC revealed many forum posts identifying this as a common cause of failure, and that it was also sensitive to failed components elsewhere in the vertical deflection circuit. Many people suggested replacing all of the surrounding capacitors and diodes as well as the IC just as a precaution.

TradeTech listed the IC as being on back order which was not ideal. In the mean time I though it would be worth attacking the other likely culprits in case the IC was still OK.

I replaced all of the electrolytics in the vertical stage as a matter of course but it didn’t fix the problem. I also identified a Zener diode (ZD201) with the marking ‘4A3’. After some research I found that this is in fact an HZ4A3, a 3.7V Zener. The closest I could get was a 3.6V 1N4729 so fitted one just in case. No difference.

There was also a standard 1N4004 rectifier diode (D207) mounted under the heatsink. I replaced this and it immediately fixed the problem! The weird part is that the diode appeared to check OK out of the circuit.

The horizontal yoke connector also had some discolouration on the plug due to heating because of a bad connection. I cleaned up the pins in the connector the the PCB header before reassembling it.

So now I have a dinky vintage 10″ Sony Trinitron SVGA display and just need to find a use for it! 🙂

Sharp Carousel microwave LCD repair

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The LCD on my Sharp microwave oven recently failed, beginning with a few segments but quickly getting worse. I’ve seen quite a few similar models with the same problem and a quick Google search proves this. Sharp even offered a recall at one stage.

If your microwave oven suffers a similar fate and you could give this a try. Note that it is not actually an LED display as some seem to think; Sharp have simply done a pretty convincing job of making the LCD look like one. Continue reading →

McIntosh EF-1080I silent output fix

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I recently acquired a McIntosh EF-1080I car amplifier that was almost completely dead. There was a small amount of sound coming from the speakers but it was quite distorted.

With the amp opened up I measured the +/- 26V and +/-15V rails and all were fine. I then concentrated on the muting circuit seeing the problem affected all channels. If you need to open one of these amps, take a note of where each screw goes as replacing them incorrectly can permanently damage the amplifier.

All of the channels in this amplifier model are based around the TDA7295 by ST Microelectronics. This chip features standby mode and muting on pins 9 and 10 respectively. Both must be held high (5v) for the chip to operate.

The power supply section of the amp has a de-thump circuit that pulls these pins low for a small delay at power on and immediately after power off to prevent noises in the speakers. Unfortunately the mute pin was staying tied to ground.

I traced the problem to SMD transistor Q607 that normally switches all of the mute pins. It actually disintegrated while I desoldered it, and replacing it restored the amp to life.

Close-up showing Q607

The transistor bears the marking IY and is  in fact a 2SA1162 general purpose PNP transistor. I replaced it with an identical part, however a friend suggested replacing it with one of a higher current rating such as a FMMT591. The FMMT591 works fine as a direct substitute and has a maximum rating of 500mA so should be a lot more reliable.