Realistic Audio Power Meter APM-300 Repair - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: March 16, 2020, 11:10:34 pm
content: When I was recently in the middle of repairing a NAD 3150 amplifier, I pulled from storage my Realistic audio power meter I bought in 1983I wanted to use it when I brought the amplifier back to my friend to add some "coolness."Well, I brought it slowly up on the Variac and watched the center green LED illuminate like I'd remembered.But when I hooked it to the amplifier speaker outputs, none of the red LED's would fly back and forth with the tones.Here's the black beauty! She sat on one of my old Mach One speakers way back when. I had a Kenwood KA-9100 integrated amp and my friends could here me streets away...and with the cover off...and the schematicsLike I had mentioned, the center green LED does come on. I put a red dot on the schematics showing where it is located.I see no burn marks on the board or components. I also see electrolytic capacitors speckled around have a brand name of Delcon, made in Korea.Weneedto bring this thing back. It's going to be put on top of a Nakamichi PA-7 in the garage/tech space!

author: floobydust
author_rank: Super Contributor
timestamp: March 16, 2020, 11:29:28 pm
content: Ha I have two of those in my junk collection, I recapped one but didn't find it useful because it has high 200W/low 2W range which only does something at loud levels. And the LED's are fake- two dots is really one LED.Realistic APM-300 Peak/RMS Audio Power Meter 42-2104 uses Sanyo LB1405 bargraph IC's.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 16, 2020, 11:32:31 pm
content: FloobyDust, we're going to fix this thing...the memories are coming back to me now!And how technology has improved, here's the old Delcon 470uf 25v 85c cap next to a new Nichicon UHE 470uf 25v 105c cap:

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2020, 12:35:07 am
content: I remember when this unit came out, it was a popular sell with the Mach 1 15" loudspeakers.I heard Boston Long Time cranked up for the first time in my life asTom Scholzeffects and layering were a new sound.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 17, 2020, 01:01:29 am
content: ...and I used to hide my bag of grass in the horn

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2020, 01:08:22 am
content: Ahhh it's from 1982 and pretty expensive, source:RadioShackCatalogs.comThe Mach 1's for sale in 1976 but I was a kid then.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 17, 2020, 05:10:03 am
content: Thanks for posting those pictures FloobyDust!Tonight, I got as far as testing and replacing the twelve electrolytic capacitors. The only capacitor that was tested having high ESR was C1 (10uf 16v). It looks to be a filter capacitor on the (+) side of the left channel speaker input:When I was done with that, I did a quick restart on the Variac. The power meter still acts the same. The green LED illuminates but none of the red LED's work when attached to the speaker terminals of my Kenwood receiver.So tomorrow night I will start with voltage checks and component testing. There aren't too many in this thingI'll download the spec sheet on those IC's too.*Well*, I decided to stay up a little later tonight to try to understand the schematics. The green LED (D19) is on the power supply rail to all of the IC's and LED's. When LED D19 is lit, it's telling me that Transistor Q1 is good and power should be at pin 4 of all LB1405 IC's. Pin 3 is where the input from the speaker terminals enter the LB1405 IC's. Do I have this right so farIt seems that if this circuit doesn't function, the problem is most probably the IC's, at least, two of them. I brought this power meter out of its 30 year coma slowly on a Variac initially.I'll read the LB1405 data sheet.This is good practice for me. I disconnected the transformer and the two blue output and white center-tap leads test good. The diodes test good. The (+) side of C13 gets continuity to every pin 4 of the four LB1405 IC's too.*Another update*, I removed Q1 (2SD400E) and it checks good on the bench. I also have continuity from the (+) positive L & R speaker terminals to IC 1 and IC 3. So the problem isn't in that section.My next plan of attack is to remove one of the IC's and test it on the bench. I do have my Huntron tracker at the disposal too, but I will need some help discerning where that short or problem could be. I'll post the LB1405 data sheet. I also tested most, if not all, of the resistors.I'll test the to small tantalum caps on the Sencore (C5 & C10) next.Any hints as to where I should go next? Thank you!

author: floobydust
author_rank: Super Contributor
timestamp: March 17, 2020, 11:13:07 pm
content: I used a signal generator for test and calibration, injecting a 1kHz sine wave at a few V. If the +12V rail is OK, I would check pin 6 is at Vref 2.6-3V (zener) and pin 15 is the output of the first op-amp and should have some average DC there. Any shorted small electrolytic will upset the IC.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 18, 2020, 12:44:11 am
content: Thank you FloobyDust!I had just soldered a pin to that 12v rail (+ side of C13) to demonstrate the 222A with its new battery pack. 11.31 VDC.I'll check pin 6 right shortly.Pin 6for the four IC's read: 2.82, 2.74, 2.86, and 2.77 VDCPin 15on IC 1 and IC 2 is 27.4mV, on IC 3 and IC 4 is 60.8mV

author: Smoky
author_rank: Frequent Contributor
timestamp: March 18, 2020, 01:47:39 am
content: You guys are not going to believe this in a million years!When I was setting up to take a picture of the Tektronix 222A oscilloscope with its new battery pack, I hooked up a probe to the APM-300 12v rail so I could show how well it worked. Well, initially, the waveform that showed up on the screen of the 222A looked really awkward. One pulse was taller than the other and a lot of noise was riding on it.It was a minute later when I took the photograph, I had noticed that the waveform looked "cleaner." the pulses were even and the noise was gone from it too.Here's how the waveform looked after:I'm thinking that those new capacitors put into the APM-300 needed some time to come to life. I will, for now on, reform even the smallest capacitors on my Sencore LC102 or LC53 just to charge them up beforehand.But anyway, it's good to see her ready for another twenty years of Rock'n Roll...andFloobyDust, I couldn't do it without ya!