• Please note that Audiokarma will be offline briefly for updates early on Monday morning, September 28th.

Rotel Ra-414 Loud(ish) whoomp on power up

G

Gertjan90

Member
Hi guys,

I've got some questions relating to my Rotel Ra-414 I pickup a while ago:

On powering up the unit, I get an audible "whoomp" through the speakers, and this scares me a little.
I've heard that this can be common in amps without a protection relay, but I wonder if it might damage my speakers?
The amp runs fine afterwards, btw.

Also, when powering on, the needles on the VU-meters jump up to about half scale and then go down again.
They refuse to move during playback, however.
The SM says that they are controlled by an IC-Opamp. I guess it's damaged.

Could the same thing that causes the whoomp be responsible for frying the little opamp of my meters?
If so, where should I be looking find the culprit?

I kinda want to fix this amp up, since I think it looks and sounds great and might have payed a bit to much for it..

Any help is appreciated!
 
Register to hide this ad
First up, to avoid damage to spkrs, turn off spkrs before p-on AND p-off.

The fact that the VU meters jumps at p-on suggests they are ok, perhaps they need calibration, refer to service manual.
Download SM from hifiengine, need to register, it's safe, no spam etc...

Is the dc spike "whoomp" on both channels?
Maybe disconnect spkrs then measure dc voltage at power on with multimeter, it will just flash up then collapse.
Repeat a few times, if it's 0-3Vdc then not bad, if it's 8-15Vdc then concern.

Has the amp been refurbished, recapped or still original?
 
Check to make sure it's not Cap coupled 1st. If it IS cap coupled, leave the spkrs connected during the test or else you'll get wacky readings that will have you diving for the power cord!
 
Hi guys,

Thanks for the feedback.

Larryderouin, how do I check if the amp is cap-coupled? I have a Yamaha that is, so I tried compairing them, but as far as I can tell the Rotel isn't cap-coupled.

I have some spare new caps lying around (6800µF, 35V), I'll try putting them in first.
Then I'll measure the DC offset.

The amp hasen't been refurbished, I compaired it to reference pics online and everything matches up.
I have to say, though, the build quality and components used in this old Rotel are not up to spec with the other Japanese companies.. Built in Taiwan, too.
But it looks great from the front :D

I'll report back when I get back from work.
 
It's not a cap-coupled output. I'd follow the others advice - and check for DC on the speaker terminals, during and after power on, before making any component changes. It's better to measure first, and find the fault(s), than just assume something's bad....

If you have it open, it may be worth checking the main dc rail voltages as well - a lot of the caps look to be spec'ed with 35V rated caps, which are very 'close' to the indicated schematic values (+/- 34.7V).
 
Update:

I've replaced the power supply caps. I already took them out yesterday.

The DC spike still happens at power up, but as stated by MBZ I can avoid the thump by switching in the speakers after the amp has stabilized.

DC offset is as follows:

Left: ±28.4 mVDC
Right: ±34.1 mVDC

Not great, but not worrying either.
The spike on power up never goes up higher than 2.48VDC on either channel, so this should be OK?

The sad thing is that the Watt -meters still aren't moving with the music.. I saw the tiniest bit of movement once, so next thing I'm going to try is to adjust the trimpots next to the meter-amp.
Could the trimpots themselves be the issue?

I'm listening to the amp now and it's definitely sounding ok. Sweet features for a lower end amp as well: mag/cer phono IN, high/low filters and stereo/mono/reverse switch.

Goldie, how do I measure the DC rail voltage?
What values should I be looking for?

Thanks in advance.
 
Hmm. Trimpots have no effect on the meters. They seem fine, though, values are on point.
I'm starting to blame the IC meter-amp.
It's an NJM-4558DD, is there any way to see if it's faulty?
 
Re. the main PS caps - I'd just check the voltage across them. The schematic shows them as 35V rated, but with 34.7V across them (the main supply voltages are shown as +/-34.7V) - I'd prefer a bit more margin than that, esp. if the voltage in Belgium varies at all or is higher than 30 - 40 years ago....

For the meters - are you setting them as per the manual (at 10W output) ? checked the IC-603 pin voltages ? I also don't see what TR601 & 602 are - thermistors ?
 
Hi.

The voltages in Belgium are OK :D The unit is rated for 240V, but I see what you mean. I've got some new 85V caps around, I'll swap them when I get everything sorted out.

I've probed around the IC603 and these are the voltages:

1:16.8V
2:17V
3:-7mV
4:-16V
5:0V
6:17V
7:17V
8:17V

Now what they should be:

1: 8mV
2: 1.21mV
3: 0V
4: -16V
5: 0V
6: 8mV
7: 1.20mV
8: 16V

So there's definately something wrong here. I've already ordered a (few) new ones.

Yes, TR601/2 are thermistors. Do these tend to go bad? They say TD5-A150 but that turns up nothing on the Google.
 
Looks like you've found the main problem. IC603 is indeed toast.

On the thermistors, no, I looked up 'TD5-A150' as well, with the same results - just to check what they were. Provided they're not open, I doubt they're a problem. IC603 will probably sort it.
 
Hi guys,

I just wanted to stop by here one more time to say thanks and give a quick update.
The faulty IC603 has been replaced and the Watt-meters are working again! The DC-spike when powering up has gone down a lot as wel.
I'm really glad the amp is back working as intended.
Next time I order some parts, I'll probably do a recap and replace the transistors on the tone board.

Thanks again!

Rotel Ra-414.jpg
 
You must log in or register to reply here.
Back
Top Bottom