Yamaha CR-820 - Speaker Protection and Amplifier Repair

Pete Ditmars

New Member
Hello, I am new to this forum. I have enjoyed reading many of your previous posts and I have learned a lot in the process, but I am missing some critical info that I need to repair my amplifier.

I have a Yamaha CR-820 from 1977. It worked well for 40 years and then gave a loud POP and no more sound from any source. I would like to repair it myself because a) I want to learn how to do it b) I am nowhere near a shop that could repair it. I have some serious time to devote to the project. I am hoping that someone out there can point me in the right direction.

First thing that I did was look at the fuses: tested all ok. Next I looked for obvious burning on the PCB: Nothing. Checked all the resistors that I could see (did not remove the transformer): no burning. Ditto for the capacitors: no obvious swelling or damage.

Found no voltage at the speakers, so I checked the speaker protection relay. It appears that the speaker protection circuit functioned as designed, since the relay is not sending power to the speakers and one channel of the amplifier has 44.1 vdc on it (measured at the relay). The other channel has .0004 vdc measured at the relay.

I then let the amplifier warm up for 10 minutes and started checking voltages with input = aux, nothing connected to speakers or headphones, vol =zero, tone controls = mid, filters = off.

The transformer output voltages are: 35.9 vac (orange to orange) and 17.8 vac (orange to blue). The main power supply transistor (TR806) measures base 29.3, collector 47.1, emitter 28.7).

The amplifier test points read as follows:

TP1: (44.0 vdc to chassis ground). If this is emitter of TR725, collector is 44.0, base is 36.4
TP2: (44.1 vdc). If this is the emitter of TR723, collector is 44.3, base is 44.0
TP3: (.0006 vdc). If this is the emitter of TR726, collector is 44.0, base is .58
TP4: (.0014 vdc). If this is the emitter of TR724, collector is 44.1, base is .63

Based on what I have read, it is likely that one or more transistors in the right channel amplifier (TP1 and TP2) is toast. There may also be capacitors that need replacing.

So. Now what should I do?

1.) Am I correct in saying that the TP1 and TP2 outputs are a problem and that TP3 and TP4 are reasonable (i.e., left channel ok, right channel screwed)?

2) Does it seem reasonable that I am looking for bad transistors? Or capacitors? Or something else?

3) The circuit diagram lists what I assume are test voltages at various points on the right channel amplifier. The left channel amp is almost (but not exactly) the same as the right amp, but no test voltages are shown Do the same test voltages apply? Partial schematic attached.

4) How can I identify which transistors/capacitors are faulty? When I tested the voltages in circuit at various transistors, I sometimes got readings that matched fairly close (but not exact) to the circuit diagram, and sometimes they were WAY off. But everything is so interconnected I don't know where to look for the root cause of a bad reading, nor do I know how much variance in the readings to allow for. Do I have to remove each component and test it individually?

5) Maybe someone else has had this exact problem and there is a thread that goes into what steps to take to diagnose it?

Thanks for reading this long post, but I wanted to include everything relevant to save time. Looking forward to fixing this sucker!
 

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TP1: (44.0 vdc to chassis ground). If this is emitter of TR725, collector is 44.0, base is 36.4
TP2: (44.1 vdc). If this is the emitter of TR723, collector is 44.3, base is 44.0
2) Does it seem reasonable that I am looking for bad transistors?
Looks like one or more shorted outputs. Remove TR725 and 723 and check for shorted junctions with your multimeter.
 
images


http://audiokarma.org/forums/index.php?threads/testing-transistors.201303/#post-2362176
 
TR723 and TR725 appear to be toast Here are all the readings in ohms with R/B and B/R leads:

TR723 - B/C 5.3 & 5.5
E/C 0L & 0L
E/B 0L & 0L
TR725 - B/C 80.6K & 1.9M
E/C .4 & .5
E/B 1.9M & 79K
TR724 - B/C 0L & 13.6M
E/C .0L & 0L
E/B 13.9M & 0L
TR726 - B/C 0L & 6.6M
E/C .0L & 0L
E/B 8.5M & 0L

Not sure if 724 and 726 are in spec, but since these transistors are readily available and not too pricey, I'm favoring replacing all four of them. The originals appear to be Toshiba 2SD427 and 2SB557. Is there any reason not to purchase Toshiba brand replacements and am I ok buying them off eBay, or should I use some more reputable source?
 
Rather than measuring resistance, better off to use the diode test function on your mm.
All look suspect, suggest redo using diode function, possibly your mm resistance function doesn't push
out enough "amps".

am I ok buying them off eBay, or should I use some more reputable source?
Don't buy silicon of ebay.
 
Sorry. Brain-fart. Of course I should have used diode function. Revised readings:

TR725 - B/C 0L & .527v
E/C 0 (Beep) & 0 (Beep)
B/E 0L & .527v
TR723 - B/C .008 (Beep) & .008 (Beep)
E/C 0L & 0L
B/E 0L & 0L
TR726 - B/C 0L & .535v
E/C .0L & 0L
B/E 0L & .543v
TR724 - B/C .623v & 0L
E/C .0L & 0L
B/E .623v & 0L

These readings make more sense, and seem to confirm that TR723 and TR725 are shorted out.

Where is a reliable place to get replacements and is it reasonable to stay w/ toshiba brand transistors?
 
After a fair bit of searching it looks like my choices are LittleDiode in the UK that has Toshiba 2sd427 and 2sb557. Or several places in the US that have NTE280 and NTE281 which I think cross to the toshiba transistors. NTE281MCP (matched pair) is also available. Does anyone have an opinion on what is my best option? Are there other cross-referenced transistors that I should consider?
 
After a fair bit of searching it looks like my choices are LittleDiode in the UK that has Toshiba 2sd427 and 2sb557. Or several places in the US that have NTE280 and NTE281 which I think cross to the toshiba transistors. NTE281MCP (matched pair) is also available. Does anyone have an opinion on what is my best option? Are there other cross-referenced transistors that I should consider?
Where are you located?
 
I'm in Magdalena NM, about 100 miles from Albuquerque. Looking at the Mouser data, these transistors seem a LOT beefier than to Toshiba versions (PD = 250w vs PD=80w). Do I have to worry about overheating? (forgive me if this is a dumb question...)
 
I'm in Magdalena NM, about 100 miles from Albuquerque. Looking at the Mouser data, these transistors seem a LOT beefier than to Toshiba versions (PD = 250w vs PD=80w). Do I have to worry about overheating? (forgive me if this is a dumb question...)
That they are. As far as I'm concerned they are the best game in town.
 
I'm in Magdalena NM, about 100 miles from Albuquerque. Looking at the Mouser data, these transistors seem a LOT beefier than to Toshiba versions (PD = 250w vs PD=80w). Do I have to worry about overheating? (forgive me if this is a dumb question...)

No. The higher power rating will just insure that the Qs don't even break a sweat. That rating is just what it's CAPABLE of dissipating if driven hard enough.
 
Rather than measuring resistance, better off to use the diode test function on your mm.
All look suspect, suggest redo using diode function, possibly your mm resistance function doesn't push
out enough "amps".


Don't buy silicon of ebay.

Hello, I bought a pair of Toshiba transistors for my CR-820, 2sb557 and 2sd427, from a seller on Ebay before reading your post. Can you explain why you recommend NOT buying from ebay?

Regards,

Benicio
 
You already bought them. Might as well try them.You might get lucky..See post #11 on the 12th of Dec.
 
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