Denon PRA-1500 Help...

[dneu201]

My Denon PRA-1500 Preamp I found at a Goodwill for $20 5 months ago just started having issues yesterday. I hooked it back up after 2 weeks of no use and noticed the right channel was playing awfully quiet. I checked my connections on the back of the preamp and cranked the volume. Sure enough the right channel started working again, but as I experimented and turned the volume down/unit off then back up/on, the right channel went quiet again and would start to crackle and distort at 3/4 volume. I opened up the unit and inspected everything, cleaned the pots again, blew out where I saw some dust, but had no luck. I checked the headphone out and it played perfectly with no issues. I noticed the voltage regulators were getting VERY hot while feeling the different components in the unit and narrowed it down to one specific one causing the heat. I went ahead and ordered two of the three regulators off Mouser, but am struggling to find the last one available that doesn't have to be purchased in bulk.

Here are the the regulators:
1. NJM78M18FA - Purchased off Mouser (18V .5A 3 terminal)
2. NJM79M18FA - Purchased off Mouser (but had to purchase NJM7918FA which is 18V 1.5A instead of .5A)
3. NJM78M15FA - Can't find this one available

Based on the issues I am having, does it seem like these regulators can be the problem and if I was to remove them, how can I easily test them to see if they are faulty?

Thanks.

 

[pw5599]

An old post but the issue outlined above is the relays. This preamp has two small rectangular shaped relays and of course relays do what relays do, the contacts become touchy and need cleaning or replacing. I didn't have much luck cleaning mine so I replaced them and haven't had issues since. Another option is to short out the contacts on the circuit board, if you don't mind a bit of noise when switching between sources and at startup. Very nice preamp btw!

 

[lauritsen40]

An old post but the issue outlined above is the relays. This preamp has two small rectangular shaped relays and of course relays do what relays do, the contacts become touchy and need cleaning or replacing. I didn't have much luck cleaning mine so I replaced them and haven't had issues since. Another option is to short out the contacts on the circuit board, if you don't mind a bit of noise when switching between sources and at startup. Very nice preamp btw!

You mean the blue ones ?

 

[pw5599]

Yes, the blue ones. If I recall correctly my problem was caused by the blue one labelled RL2. That relay switches both L&R channels at the pre output. The other exact relay on the board is RL3 which switches signal to the headphones. To be sure which relay is causing the issues, while you have something playing through the preamp at low volume and listening from signal going through the pre output, tap on the relay lightly with something plastic such as a handle of a screwdriver. If you hear sound cutting out or breaking up when tapping, that's your problem. Replacing is the best way to fix this and I found exact replacements on Ebay, from China I do believe. The relay is a TAKAMISAWA RY12W-K
The relay will need to be unsoldered and the new one soldered in so you need to have that ability, or someone you know does.
A search brought up this: https://octopart.com/ry-12w-k-fujitsu-796064
And an example from ebay: https://www.ebay.com/itm/2pcs-TAKAM...374504&hash=item4b16b61aff:g:7Y8AAMXQgb1RNvlZ

 

[lauritsen40]

thanks for your answer

i have already replace the 3 regulators
before i replace they get very hot, so hot that it burn my finger
after i replace they still get very hot !
do you think its because of these relays they get hot ?

 

[toxcrusadr]

I doubt it. Hot parts means excessive current, and line level relays would not draw excessive current just because their contacts were corroded.

What are these regulators regulating? That's where your problem is.

 

[lauritsen40]

i try to explain !
after turn on and play 5-30 it suddenly just pop out of signal so no signal is active, so i have to push a signalbuttom to get the signal Again.

so i open and noticed these 3 very hot regulators

 

[toxcrusadr]

Reading the thread again, the relays are certainly an explanation for the channel(s) cutting out, but probably completely unrelated to the hot regulators, it seems to me.

I am not smart enough with troubleshooting to diagnose this. Have you been able to locate a schematic for the unit? If so, post it, or a link to it. This would help people tell you what to check.

Are there any obvious signs of bulging or leaking capacitors? Especially in the power supply.

Did the output transistors get hot, or anything else?

I would avoid running this unit as much as possible to avoid more damage until you can find the problem.

 

[pw5599]

Great advice by toxcrusadr. The relays are not your problem. I wonder if the OP has fixed his. I can't say if these regulators are normally hot, I'd have to check mine. Some do run quite hot. Your preamp may have some power issues or a circuit the regulators supply is drawing too much current. What components did you replace, the number on the board? If all regulators are getting hot, look for something common to them such as the supply to them. You need to start there. Measure the supply voltage, output voltage. First you need a schematic, second you know how to read it. The service manual is available at https://www.hifiengine.com/
You will need to register first. Also check this post: https://www.diyaudio.com/forums/solid-state/249420-denon-pra-1500-input-selector-fixed.html

 

[lauritsen40]

Great advice by toxcrusadr. The relays are not your problem. I wonder if the OP has fixed his. I can't say if these regulators are normally hot, I'd have to check mine. Some do run quite hot. Your preamp may have some power issues or a circuit the regulators supply is drawing too much current. What components did you replace, the number on the board? If all regulators are getting hot, look for something common to them such as the supply to them. You need to start there. Measure the supply voltage, output voltage. First you need a schematic, second you know how to read it. The service manual is available at https://www.hifiengine.com/
You will need to register first. Also check this post: https://www.diyaudio.com/forums/solid-state/249420-denon-pra-1500-input-selector-fixed.html

the last link you post is the one i have follow !
i only have this :

 

[pw5599]

Are you saying you replaced IC14, and that has solved your problem?

 

[lauritsen40]

Are you saying you replaced IC14, and that has solved your problem?

i have replaced all 3 (ic14-ic15-ic16)
and it did not solved the heat problem
after 1-2 minutes it get so hot so i close the preamp

 

[thoresui]

Hi everybody,

I have similar trouble with my Denon PRA-1500. Or at least I think it is something similar.

First during summer (higher room temperature) the problem with "unselected" input happened couple of times (max 3). Like described in this well known post: https://www.diyaudio.com/forums/solid-state/249420-denon-pra-1500-input-selector-fixed.html

Then there were no issues for about 2 years. Recently the problem returned in slightly different form. Some relay in the preamp started to switch randomly. For now I am not sure what triggers it (high temperature, preamp warm up, mains spikes, ...). It just does couple of "switches" from time to time. Not very often. With any long audible signal loss.

It also makes me wonder a little bit why the regulators should fail, they are usually pretty robust. For some reason there was the TO-220F (or FP) packaged used, could it be that simple application of thermal grease under the regulator body would fix the glitches?

Since I am not able reproduce the issue = I cannot instantly test if any single the fix helped and the wiring in/out from my setup is rather annoying I am thinking about following:

- replacing of IC15 -18V regulator (TO-220FP 1A from NTE)
- replacing of IC14 18V regulator (TO-220FP 1A from NTE)
- replacing of IC16 15V regulator (TO-220FP 1A from NTE)
- replacing PSU filter caps C302/C303 (Nichicon UPW, 2200 uF 63V)
- replacing regulator caps C305/C304/C311 (Nichicon ULD, 1uF 400V)

when the previous not helps also the small takamisawa relays in round II.

The reasoning behind the decision is that if one regulator starts to act weird other from the same era can be next. The small caps are near the heatsink and could be out of spec and when I solder them out I can just go for straightforward replacement instead of measuring their parameters. Same goes for the main filter cap which are also near the regulators.

I am just a hobbyist without proper deep knowledge of circuit design and diagnostics and I would appreciate any advice how to tackle this issue.

@lauritsen40 @dneu201 Did you fixed your problems or is the solving still in progress?

Thank you very much.

 

[thoresui]

I have also stumbled upon a complete capacitor set for recap: https://www.audio-high-store.com/product/denon-pra-1500-upgrade-kit-audio-capacitors/

 

[thoresui]

I have finally moved forward little bit with my PRA-1500. Replaced all 3 voltage regulators (18v, -18v, 15V) and all electrolytic caps which were in close range around the heatsinks.
The removed caps looked pretty much ok according to my cheap ESR meter (except maybe one of the main filtering caps, but that could be only bad oxidation on the legs).

Need to run it more but, the first 6 hours of continuous playing were without the "relay switching" issues. The preamp sits on Denon POA-2400 which puts also some heat underneath.

 

[lauritsen40]

do the heatsink still get hot ?

 

[thoresui]

Did not make any exact measurements. I have tested the unit first without cover a checked the heatsinks only by hand. Mine were not too hot before and they were also fine after the replacing. I have used 1A regulators instead of original 0.5A and added thermal paste (it is recommended also for isolated TO-220FP, original installation was without any thermal compound).

 

[thoresui]

So far it looks fixed. No occurence of intermittent relay switching since the partial psu restoration.

 

[Stephen300]

It appears to me that the regulators are labeled wrong in the schematic. For example IC14 is an NJM78M18FA which is a +18V regulator. The pin-out is as follows...
Pin1 = Input, Pin 2 = Ground and pin 3 = Output. The schematic has it labeled as pin 1 = 18V output, pin 2 = ground and pin 3 = 26.9V input. IC15 is an NJM79M18FA which is a -18V regulator. Pin 1 is Ground, pin 2 is Input and pin 3 is the Output. The schematic has pin 1 as the -18V output, pin 2 as -27V input and pin 3 as ground. IC16 is backwards the same way as IC14.

 

[thoresui]

It is wrong only ine the board layout/topology picture. The parts leg description and actual schematic look ok to me. The board layout has all pins labeled the same IN-GND-OUT.