across the bench: an electronic menagerie of diagnostic oddities plus photos and design analysis

[sae-7125]

some thermals of the crown.


main board is the only thing that gets warm unless stress testing the outputs.


different color scheme.


idle wattage with 200w big bulb + 40w small bulb on the latest dbt version.


playing some music to exercise the high wattage filament.

since the amp is class a/b +b its output stage gets upset when loaded (speakers connected) and using a reasonable size bulb (40+40+40). power up the amp unloaded or with one speaker isnt too bad when doing safe testing of the system, initial diagnostics.

the sae amp in the past reacted the same way and was also class-b, if i remember correctly.

 

[sae-7125]

mcs 3842 integrated amp, small with a rather large heat management system.


basic cosmetics and what could be a nice looking display. just one problem that will be apparent soon.


modular component systems.

Are MCS Brand vintage Receivers any good?

www.avsforum.com

gremlin hint:

MCS amp. info. needed

I'm searching for a power transformer for an older component stereo amp. It's lasted 30+ years and I'd like to keep it going for nostalgia. Anyways It's an MCS 3842 amp distributed by JCPenny back in the day. The model is 683-3842. The part number of the transformer is SLT5M231. There's lots of...

www.audiokarma.org

 

[sae-7125]

informative repair video about the mcs integrated.

 

[sae-7125]

final batch for the mcs.


note the txf has been unbolted from the frame. easier to inspect and measure the primary + secondary windings.


measuring across the primary and it is open. that would explain zero voltage on the secondary and dc supply.


rear panel photo and the txf has been bolted down to its normal position. trying to source a replacement txf exceeds the repair budget so this will most likely be headed to the recycle pile, depending on what the owner decides.

 

[sae-7125]

yamaha natural sound stereo amplifier, with an issue that will need specialized surgery.


another relatively small integrated, output transistors have decent spacing plus lots of heat dissipation surface area around them.


yamaha ax-500u


the preamp output jumpers will receive some deoxit gold attention.


and the hunt begins, time to corner the gremlin for extraction. in this case identifying a power switch issue. button clicked into "on" position and no conductivity. a simple "cord to the board" diagnostic procedure.

whats the difference? A-S500 VS. AX-500u

ive been considering adding some new equipment to go along with my vintage stuff, but the integrated i have been looking at is the Yamaha A-S500. i already have the older AX-500U and as near as i can tell the specs are virtually the same. can someone shed some light on this. will the newer...

www.audiokarma.org

Yamaha AX-500 specifications Yamaha

Yamaha AX-500 | hifi-wiki.com

hifi-wiki.com

Yamaha AX-500U Integrated Amplifier Repair - Page 1

Yamaha AX-500U Integrated Amplifier Repair - Page 1

www.eevblog.com

 

[sae-7125]

Free Audio Service Manuals - Free download yamaha ax 500

Read or download yamaha ax 500 PDF with 27 pages. No login, no charge and no limit.

www.audioservicemanuals.com

quick reference, dc bias.

 

[w3jn]

A shame about that little MCS and its dead power tranny - looks like a cool little amp.

 

[sae-7125]

more yamaha power switch info.


when pushing the button to its max inward position we have conduction.


power switch removed from its board, which requires removing the entire face plate.


and now we delve into the heart of the matter. the usual spray made no improvements, just made a larger mess when extracting the mechanism.

more surgery photos to follow.

 

[sae-7125]

a multi part series (of photos) highlighting how a simple power switch can be quite time consuming. part number esb82101v and 8d15 might be a form factor reference. have to look up 41-10749miw to see whats out there on the internet, other than:

Yamaha verstärker reparaturen - Ersatzteile und Reparatur Suche

Yamaha verstärker reparaturen

forum.electronicwerkstatt.de

slowly disassembling the switch mech. mini roller wheel makes contact with the copper teeter-totter although that is only the top half of the conduction bar.


close up of the switch roller.


the gunk at the bottom of the switch housing probably isnt helping, lets see how bad the contact patch is on the bar.


side profile of "the bar" and its 2 piece construction. also 2 different metals which have slightly different conduction properties, not that it matters.


yup, more gunk to be cleaned. no spray will handle this type of cleaning.


a mixture of carbon and plastic contamination, plus the spray i tried earlier which only made a larger mess.


never let spray do a razor blades job, waste of spray. conduction bar and contact rivet cleaned up nicely.


inside the switch house, there is an electrical contact down there somewhere.

 

[sae-7125]

a closer look at the switch housing clean up plus reassemble.


utilizing a slightly used piece of paper towel within arms reach, most of the gunk was absorbed/removed from the contact.


the right tools for the right job.


more than just surgery, preventive maintenance via strategic use of a contact conditioner. less resistance between to metal surfaces yields lower loss/energy/heat across the junction. going for the gold.


reassembled once more although there is an extended story after the first test "attempt" which had no improvement vs original condition.


this result is after taking the switch apart a second time to re-engineer the mechanical design of the teeter-totter conduction bar.

long story short, the hemostats shown earlier were used to bend the lower bar (non-copper piece) slightly upward for a better angle of contact and improved mech action, apparently. the bend point is 1 hemostat nose width below the conduction rivet, where there is an original bend already obvious. a photo would have been easier but too late now.

 

[sae-7125]

a few more things about the yamaha before the next item.


power switch, after successful repair and testing, waiting to be soldered for final installation.


approximate idle wattage, using 40+40w bulbs, most likely closer to 22w but nothing of major concern.


a quick thermal to verify no other issues or thermal asymmetry between the channels.


dark spot on the top cover inside, most likely indicating this system has more than a few hours on it. the spot is directly above some higher wattage resistors, most of which run at 140-150f and surrounded by electrolytics (what could possibly go wrong).


all back together and ready for the out-box rack with the other audio gear waiting for pickup (eventually).

another rotel coming up next, less of a back breaker i.e. under 100 pounds, better yet under 30 pounds (weight not currency).

 

[sae-7125]

a quick side step about the dim bulb system (v4.0) and its ongoing metamorphosis or mutation as it were.


i found a non polarized down angle plug for the breakout modified inline receptacle. in future it would be easier and better to hard wire this junction. the mount wall outlet is not sturdy enough for single handed plug and unplugging of bulbs but it is good enough for a "mounting" proof of concept.


yes, the exposed electrical connections are a safety hazard so a protective housing or a few layers of electrical tape will be needed. the foot screws in this particular variac end up having a couple threads of bite when adding the thickness of the wall outlet mounting frame. good enough for this design experiment but woefully lacking for mechanical longevity.

a future design mod would utilize x2 foot screw anchors for stability plus the hard wire optimization plus a shallow mount wall receptacle box that is properly grounded. currently using the lower plug position for a narrow yet long term 40w bulb. the upper position has a receptacle expander (x3 three prong outlets) that is used for spacing two bulbs next to each other with an open/unusable three prong outlet in between.

the self-inflicted mess continues.


a sneak peek at the upcoming rotel ra-02, much lighter vs the other rotel's in this thread. might snag a few shots of a sansui au-717 with rack mount face plate handles, still undecided but it needs some love regardless hence being at the repair shop.

Heavy duty rack mount handles (AU/TU 517 or 717)?

I have a TU-717 that came with handles. Question.... although they also fit the AU-717 or AU-517 are they identical? The only reason I ask is that some time ago there was an ad for "heavy duty" rack handles on eBay. Can't remember now but he/she may have had another ad for handles without...

audiokarma.org

au-717 example photo (tu-717 makes for a lovely pair) sourced from expired advert:

SANSUI AU-717 AND TU-717 (Includes Rack handles) SEE PHOTOS

Very nice condition. Given a clean bill of health by my tech. Amp is very powerful with that sweet Sansui sound! Tuner is one of the best out there period! I've been using this set up in my main rig now for the past 4 months. Amazing performance!This is a killer set up!I'll separate for...Amp...

www.canuckaudiomart.com

personally i would put the au-717 on top of the rack minus the chassis cover, but thats just me. unless it was an open top rack, top spot for sure.

 

[sae-7125]

first batch of rotel photos, customer said right channel no sound.


rotel ra-02 integrated amp, 40w stereo 8r and rated for 4r for "a" or "b" channel.


reasonable size heat sink for low wattage output rating. is it me or do the "speaker a" terminals look warped?


either its a qc alignment fail at the factory or someone was mechanically stressing the terminals.


clean pcb design and breathing space for the heat sinks. toroid txf mount plate might cause some issues for a few components.


speaker terminals are definitely flexed/warped. a rather odd occurrence regardless of root cause.

https://rotel.com/sites/default/files/product/manuals/RA-02-OM.pdf

Rotel RA-02 specs

www.hifi-review.com

Free Audio Service Manuals - Free download Rotel RA 02 Service Manual

Read or download Rotel RA 02 Service Manual PDF with 11 pages. No login, no charge and no limit.

www.audioservicemanuals.com

 

[sae-7125]

a couple of electrical engineering hints for the upcoming gremlin reveal with the rotel.

How to calculate fuse value for speaker protection?

How to calculate fuse value for speaker protection? I just got a GFA-555 and will use it to power the Boston A400's. This amp doesn't have speaker protection from DC voltage. So I'm thinking of using an in-line fuse for the tweeters. How do I know what value fuse is needed? I read a...

audiokarma.org

Improved speaker protection circuit

Dear DIY Audio Community, In this thread I like to discuss a amplifier / speaker protection circuit that I designed. Foreword: I once fried a speaker by turning equipment on the wrong order. As a countermeasure I improvised a circuit to delay speaker connection on power on. So far so bad. I...

www.diyaudio.com

 

[Celadon]

A thank you to you -- for your inspiration. Your Yamaha switch repair reminded me of a similar small Yamaha Receiver I have... an R-300.
It has a similar sort of switch, actually 3. Its not worth much, but it lives in a really nice looking faux rosewood; and the tuning pointer does some cool stuff when tuning in stations.
Think the tape mon SW, that kept it dropping one channel, like you say, No spray ever worked. haven't looked at it for many a moon.
Out, and apart it shall come.

I think this thing uses STK modules, -- maybe.
Another project...
It's hiding somewhere, so the Yammie might get to see some power pretty soon. More sentimental than anything but the lil guy sounded pretty good.
Pic taken from the Internet...

 

[sae-7125]

conclusion for the little rotel system.


one of the amp output fuses was open, did its job to protect the transistors. no idea if it was due to an unexpected cable short or overloading the amp (low impedance speakers plus high output volume). either way the circuit design demonstrated its benefit.


thermals look balanced when exercising the system, no other electrical concerns present.


low wattage amp idles nice and low even with the 40+40w bulb active.


other than a crunchy sounding tone control switch, which causes one of the channels to cut out briefly/intermittently, the system is ready to be reassembled.


brief look at the output fuse part of the schematic. document quality is decent especially for zooming in.


dc bias notes for quick reference.

 

[sae-7125]

Your Yamaha switch repair reminded me of a similar small Yamaha Receiver I have... an R-300.

looks like a cutie with a nicely illuminated tuner and needle. some tuner boxes have no backlight which reduces the visual appeal.

Yamaha R-300 receiver

I just purchased a Yamaha Natural Sound Stereo Receiver R-300 for $100.00 It is in excellent cosmetic condition. Receiver is in perfect working condition. I will be getting a service manual for the receiver. Sound quality is excellent. I'm using it with my Sony TAE-5450 preamp.

audiokarma.org

Yamaha R-300 | hifi-wiki.com

hifi-wiki.com

 

[Celadon]

^ If I remember correctly - the pointer which is 2 wide green, with a thin red in between; the intensity of those would change as you dialed in the station -- left vs right green indicator.

My best --

 

[sae-7125]

forte (threshold / nelson pass) model 4a class-a amp.


small footprint amplifier, compared to the average chassis sized gear.


brand logo speaks for itself.


XLR input signifies "a" model variant hence 4a and not 4.

Audiogon Discussion Forum

Before sending my Forte 4a on to its next owner, I've copied the interesting bits of the manual as a public service and in the hope of building at some ...

forum.audiogon.com

Audiogon Discussion Forum

Category: Amplifiers The Forte Model 4a is clearly a gotta have amplifier. It is 50 Watts RMS per side in Pure Class A configuration.Forte was the sist...

forum.audiogon.com

Quick measurements of Forte 1a, a 50wpc class A amp by Nelson Pass

This is an updated set of measurements of Forte 1a. Thanks for everyone's comments on the previous sets of measurements! Changes made for this run: Bias adjusted for class A operation USB to SPDIF/optical converter into Holo Spring DAC feeding the amp (single-ended on RCA) Laptop on battery...

www.audiosciencereview.com

 

[sae-7125]

not much info floating around about the forte amp, like documents or even dc bias value and suggested measurement locations.