Yam R-70 ('83) still sounds poor & imbalanced: voltages good after re-cap/reflows

[guy232]

Service manual .pdf hosted:
https://docdro.id/EbfVYvc

I've been doing some work (details listed below) on my R-70 and learning quite a bit but it still sounds rather poor as it initially did, also the left channel is weaker. I've tested the device unloaded and all of the PS voltages are in spec, cleaned pots (concentric pot that has vol/bal), cleaned mechanical switches, a few electrical switches used to control the LCD may have not been cleaned well but I read they dont cause audio issues?

I don't exactly know what a properly working system should sound like. I believe something may be wrong though because the R-70 sounds noticeably worse to even my untrained ear compared to a similar era/spec amp (80s Fisher CA-39, "BPC"). There is no comparison between the units, the CA-39 sounds much better using the same speakers/wires/room/input etc.

I've done the following:

- Replaced all main board caps with Nichicon HE/PW, all matched to uf/v specs except for filter caps (oem:8500uF/55v, replaced w/ 10000uf/55v)
Note: Out of the old caps, four 100uF 6.3v caps were out of spec, 3 of them were in "Power Amp" and 1 was in "Relay"

- Power on, no load tested DC offset & PS voltages to spec

- Power on, load, tapped around on board to discover tapping on relay caused channels to drop out. Desoldered, cleaned contacts deoxit d5+printer paper, resoldered. No longer responds to being tapped.

- Reflowed the speaker terminals, rca inputs, and a couple questionable old joints

Most tasks that I havent done yet require a scope I believe. I've been waiting for a decent deal to pop up. Meanwhile at my current position; is there more I can check on the device or is further diagnosis best done via scope?

I seen that fusible resistors were recommended in a sticky here, planning to desolder and test them shortly -- I'm not sure if those could cause these issues though?

Any advice/links are welcome, will update if I discover anything new meanwhile.

Cheers fellas,
Guy232

 

[rcs16]

It is difficult to determine what is causing your problem because it could be a few suspects. Even for a vet it can be a challenge.
The poor sounds was before you did anything?, then you assumed it was ecaps and went to town, but to no success, yeah it goes that way some times.
The sound is poor on both channels? if so if could be something that is common to both channels, it could even be the speaker protector relay contacts are shot, sometimes it is method of isolation or substitution. You could do a quick test and temp short out the speaker relay contacts to eliminate that part, but make sure you remove the shorted relay contacts after the quick test.
I am not sure if even having a scope will find this fault. have to figure out where in the signal path the signal looses its fidelity, start from inputs and go through to o/p's, there are a number of quad bi-lateral switches that could be an issue. I see STK modules?
TR401,402 muting bjts can cause issues too and the list goes on ...
It is a red LED display? it certainly is not the problem.

 

[guy232]

On the very first power up, not connected to anything, the unit kept rapidly clicking atleast 5+ times (believe this was the relay). It stopped clicking and remained on. I attached phone + y splitter rca to aux and noticed poor performance of audio similar to current situation. Did research and kept seeing capacitor replacement being recommended. I now know that it is often best to troubleshoot prior to recap but I believe I bought all of the proper supplies and used proper technique to rule out causing any further problems; of course I could be wrong though.

I'm kind of glad the ecaps didnt completely solve the problem because i'm now addicted to this.

Thanks for the lead, I will look up how to short the relay, I seen a post awhile back where someone soldered a wire across the points -- will investigate.

Will also read through schematic and check into the different things you mentioned and update.

Yes it uses STK, a variety that has a "Y" suffix to the model. I read in one post that the non "Y" models of the stk do not work and "oscillate", unknown. I believe the STK failure symptom is complete loss of a channel and not distortion?

The balance does need to be turned far left to have equal level on each speaker.

If your question regarding if it is a red LED display: yes the unit uses LED for the tone control display. Here is a short vid shot for documenting the BAL issue; shows the display.

Thanks again for the leads, will report back

 

[avionic]

I believe the STK failure symptom is complete loss of a channel and not distortion?

Urban legend. A bad STK can fail any number of ways. Distorted output,no output, trigger protect relay. etc.

 

[avionic]

The balance does need to be turned far left to have equal level on each speaker

Check the voltages on the left channel STK and compare to the right channel.

 

[rcs16]

On the very first power up, not connected to anything, the unit kept rapidly clicking at least 5+ times (believe this was the relay). It stopped clicking and remained o

This is a clue, need to determine what is causing this, because it is not normal, thinking ... without looking at the schematic, i think the protection ckt is a not an IC based design.
1) relay not getting the proper coil drive at start up
2) something causing the protection ckt to act this way. the ckt itself or it is detecting something fishy going on in the amp.