Technics SA-700 Right channel Help

[dneu201]

I just received a free technics sa-700 from a very nice couple that owns a record store in Houston. The lady informed me the right channel was out, but they had no time to fix it and didn't want to spend the money. I want to add that the headphone section works fine and I will be testing the preamp section later today.

Knowing this is a unit worth restoring and a full recap I opened it up and noticed there were two badly burnt resistors and a capacitor next to it took some of the heat in the pre-driver section. I removed the three components and was able to identify them as R636 and R640. I am going to start by replacing these first to see if the issue is fixed. Can anyone help me confirm those are 56.7ohms (636) and .6 ohms (640)? Also, any recommendation on other parts to check that could have also been fried?

The resistors are on the very right running up and down.

Thanks.

 

[dneu201]

Just wanted to add a few things I checked. The preamp section works fine. I tested it on a separate headphone amplifier with no issues. I also tested the output transistors I believe on the correct channel with a diode test. Would the right channel be the output transistors closest to me in the receiver or the ones located closest to the back?

Thanks.

 

[slimecity]

Those resistors you identified may still be holding their values - have you checked them on a meter? Regardless they are on either side of TR610 - I assume that's a pre-driver. I would first remove and test all the outputs and then do the same with the pre-drivers as these resistors going suggest that one/some pre-drivers and outputs on the dead channel could be toast.

 

[dneu201]

Those resistors you identified may still be holding their values - have you checked them on a meter? Regardless they are on either side of TR610 - I assume that's a pre-driver. I would first remove and test all the outputs and then do the same with the pre-drivers as these resistors going suggest that one/some pre-drivers and outputs on the dead channel could be toast.

All output transistors on both channels were checked (just to be sure I didn't have the wrong channel) and tested to be good. The resistors I measured and it looks like one is .536 and the other is 14.4. If I read the schematic correctly (which is tough to tell, then 640 should be good at .6 but the other doesn't seem right (should be 56.7). Unfortunately the part list doesn't specify the values and I don't know which resistors I would need to replace them with.

I'll pull the transistor #TR610 to see if it is bad.

 

[dneu201]

I just checked tr610 and it actually appears to be good. This is an NPN (BCE) and I got a reading of .511 and .533 off the base. Any recommendation on what to check next?

Are there fusible resistors I need to check?

Also, I just checked the two fuses on the back of the amplifier (supposed to be 6amps) and someone had put a 15amp and 7.5 amp amp in. That is probably the reason that right channel is blown...

 

[dneu201]

Another check: the resistors on the amp board (R661 and R663) are supposed to be 1.0 ohm. I just tried measuring them and didn't get a reading (OL). These two resistors look different than the other amp board, which means one set was previously replaced.

I checked the other side and they are .82ohms. These are for sure bad. Now I just need to confirm the 2 burnt resistors to in order to replace them with the correct parts. I'm not sure the schematic has the exact measurements and I am unable to use the part numbers from the part list to locate the specs online.

 

[slimecity]

You may be in luck and those resistors got sacrificed to save that pre-driver.

I would also pull and test the other three pre-drivers (but thats just me).

From what I can see of the schematic - my guess is that the resistors on the emitters and collectors of all the pre-drivers may well all be the same values.

See the resistor on the Collector of TR611 has the value of 3.3 ohm (1/4 watt). Can you reproduce and post the part of the schematic that is cut off, showing the other two pre-drivers?

If you can see a visual pattern on the PCB, whereby the resistors on the E and C of all the remaining three pre-drivers have the same colour codes - then it would be a safe bet those are the values that you need to get for the toasted tranny.

 

[dneu201]

Good call. I just checked the parts list and it DOES tell me the opposite channel resistor. I just checked the left channel and off of TR 609 the values are 82ohms and 18ohms. One of the resistors measured 14ohms and the other was less than .543ohms. I would guess that resistor is also toast and it would need replaced.

I'll start by buying:

1. Two 6 amp fuses for each channel to replace the incorrect fuses placed in the amp
2. Two replacement .82 ohm fusible resistors
3. One 18 ohm 1/4 watt carbon film resistor
4. One 82 ohm 1/4 watt carbon film resistor

I'll check the other pre-drivers tomorrow to be on the safe side. Can you recommend or send me a link of fusible resistors you recommend? This is the first receiver I have had to replace them in. I may try to buy these parts locally to get the receiver working; is it recommended I buy higher quality resistors? Or can I get by with cheaper components?

Thanks.

 

[dneu201]

If you can see a visual pattern on the PCB, whereby the resistors on the E and C of all the remaining three pre-drivers have the same colour codes - then it would be a safe bet those are the values that you need to get for the toasted tranny.

I can test this off of tr609.

 

[slimecity]

I've always replaced fusibles with 1/4 watt metal film however if you want fusibles recommend you search Mouser or Element 14 or another big US supplier.

 

[dneu201]

$6 later. I have a working receiver....

Replaced the fuses with 6 amp, replaced the 2 blown fusible resistors, changed out the capacitor that was burned next to R640, and replaced R640 and R636 resistors. It took me just 10 minutes to solder everything back together. Sounds really good on some Mach Ones, but the DC offset is pretty high at 64mv on one channel and almost 70mv on the other. There are no trim pots to lower this, so I may have to replace the differential pair. Could you help me locate these on the schematic? I still don't know how to determine which are the correct transistors. I'll check the bias later tonight.

Since this was free and such a cheap fix I may go ahead and recap it and make it my main receiver for some Dynaco A25's.

Thanks.

 

[chefg1]

Just fyi i have one of these and the output trouble can come from dirty moniter switches and volume control on this model and also pull the pre-amp ,power amp connecters and use rca jacks those metal connecters somtime become damaged.

 

[dneu201]

Yea. It's working fine. I just need to clean it and put it all back together. The guy who gave it to me already cleaned the pots and switches, I just need to clean one that's a bit scratchy.